Co-reporter:Wesley Sattler, Aaron A. Rachford, Paul J. LaBeaume, Suzanne M. Coley, James W. Thackeray, James F. Cameron, Astrid M. Müller, Jay R. Winkler, and Harry B. Gray
The Journal of Physical Chemistry A October 12, 2017 Volume 121(Issue 40) pp:7572-7572
Publication Date(Web):September 28, 2017
DOI:10.1021/acs.jpca.7b07777
We report the rates of electron transfer (ET) reactions of electronically excited [Ir(COD)(μ-Me2pz)]2 with onium salt photoacid generators (PAGs). The reduction potentials of the PAGs span a large electrochemical window that allows determination of the driving force dependence of the ET reactions. Rate constants of ET from electronically excited [Ir(COD)(μ-Me2pz)]2 to onium PAGs are determined by the reaction driving force until the diffusion limit in acetonitrile is reached.
Co-reporter:Maraia E. Ener, Harry B. Gray, and Jay R. Winkler
Biochemistry July 18, 2017 Volume 56(Issue 28) pp:3531-3531
Publication Date(Web):July 9, 2017
DOI:10.1021/acs.biochem.7b00432
Electron-transfer kinetics have been measured in four conjugates of cytochrome P450 with surface-bound Ru-photosensitizers. The conjugates are constructed with enzymes from Bacillus megaterium (CYP102A1) and Sulfolobus acidocaldarius (CYP119). A W96 residue lies in the path between Ru and the heme in CYP102A1, whereas H76 is present at the analogous location in CYP119. Two additional conjugates have been prepared with (CYP102A1)W96H and (CYP119)H76W mutant enzymes. Heme oxidation by photochemically generated Ru3+ leads to P450 compound II formation when a tryptophan residue is in the path between Ru and the heme; no heme oxidation is observed when histidine occupies this position. The data indicate that heme oxidation proceeds via two-step tunneling through a tryptophan radical intermediate. In contrast, heme reduction by photochemically generated Ru+ proceeds in a single electron tunneling step with closely similar rate constants for all four conjugates.
Co-reporter:Martin Pižl, Bryan M. Hunter, Gregory M. Greetham, Michael Towrie, Stanislav Záliš, Harry B. Gray, and Antonín Vlček
The Journal of Physical Chemistry A December 7, 2017 Volume 121(Issue 48) pp:9275-9275
Publication Date(Web):November 13, 2017
DOI:10.1021/acs.jpca.7b10215
Binuclear complexes of d8 metals (PtII, IrI, RhI,) exhibit diverse photonic behavior, including dual emission from relatively long-lived singlet and triplet excited states, as well as photochemical energy, electron, and atom transfer. Time-resolved optical spectroscopic and X-ray studies have revealed the behavior of the dimetallic core, confirming that M–M bonding is strengthened upon dσ* → pσ excitation. We report the bridging ligand dynamics of Ir2(1,8-diisocyanomenthane)42+ (Ir(dimen)), investigated by fs–ns time-resolved IR spectroscopy (TRIR) in the region of C≡N stretching vibrations, ν(C≡N), 2000–2300 cm–1. The ν(C≡N) IR band of the singlet and triplet dσ*pσ excited states is shifted by −22 and −16 cm–1 relative to the ground state due to delocalization of the pσ LUMO over the bridging ligands. Ultrafast relaxation dynamics of the 1dσ*pσ state depend on the initially excited Franck–Condon molecular geometry, whereby the same relaxed singlet excited state is populated by two different pathways depending on the starting point at the excited-state potential energy surface. Exciting the long/eclipsed isomer triggers two-stage structural relaxation: 0.5 ps large-scale Ir–Ir contraction and 5 ps Ir–Ir contraction/intramolecular rotation. Exciting the short/twisted isomer induces a ∼5 ps bond shortening combined with vibrational cooling. Intersystem crossing (70 ps) follows, populating a 3dσ*pσ state that lives for hundreds of nanoseconds. During the first 2 ps, the ν(C≡N) IR bandwidth oscillates with the frequency of the ν(Ir–Ir) wave packet, ca. 80 cm–1, indicating that the dephasing time of the high-frequency (16 fs)−1 C≡N stretch responds to much slower (∼400 fs)−1 Ir–Ir coherent oscillations. We conclude that the bonding and dynamics of bridging di-isocyanide ligands are coupled to the dynamics of the metal–metal unit and that the coherent Ir–Ir motion induced by ultrafast excitation drives vibrational dephasing processes over the entire binuclear cation.
Co-reporter:Kana Takematsu;Sara A. M. Wehlin;Wesley Sattler;Jay R. Winkler
Dalton Transactions 2017 vol. 46(Issue 39) pp:13188-13193
Publication Date(Web):2017/10/10
DOI:10.1039/C7DT02632C
The two-photon absorption (TPA) cross sections (δ) for tungsten(0) arylisocyanides (W(CNAr)6) were determined in the 800–1000 nm region using two-photon luminescence (TPL) spectroscopy. The complexes have high TPA cross sections, in the range 1000–2000 GM at 811.8 nm. In comparison, the cross section at 811.8 nm for tris-(2,2′-bipyridine)ruthenium(II), [Ru(bpy)3]2+, is 7 GM. All measurements were performed using a nanosecond-pulsed laser system.
Co-reporter:Harry B. Gray, Stanislav Záliš, Antonín Vlček
Coordination Chemistry Reviews 2017 Volume 345(Volume 345) pp:
Publication Date(Web):15 August 2017
DOI:10.1016/j.ccr.2017.01.008
•Excitation, reduction, and oxidation strengthen d8-d8 metal-metal bonding interactions.•Long-lived singlet and triplet excited states and dual emission are frequent features of d8-d8 complexes.•Femtosecond optical excitation triggers coherent motions of the metal-metal unit.•Higher excited states have mixed-spin, with some charge-transfer character.•Two-electron superreduced complexes feature (pσ)2 metal-metal bonds combined with electron delocalization over the ligands.Research on d8-d8 complexes is being actively pursued, owing, in part, to newly developed time-resolved optical, IR, and X-ray methods that directly interrogate bonding changes upon excitation. Our review covers work on the ground- and electronic excited states, as well as the oxidized and reduced forms, of these complexes. Recent experimental and theoretical results add a new chapter to the rich history of d8-d8 spectroscopic and chemical behavior.Download high-res image (169KB)Download full-size image
Co-reporter:Stanislav Záliš, Bryan M. Hunter, Harry B. Gray, and Antonín Vlček
Inorganic Chemistry 2017 Volume 56(Issue 5) pp:
Publication Date(Web):February 20, 2017
DOI:10.1021/acs.inorgchem.6b03001
Molecular and electronic structures of Ir2(1,8-diisocyanomenthane)4n+ (Ir(dimen)n+) complexes have been investigated by DFT for n = 2, 1, 0 (abbreviated 2+, 1+, 0). Calculations reproduced the experimental structure of 2+, ν(C≡N) IR, and visible absorption spectra of all three oxidation states, as well as the EPR spectrum of 1+. We have shown that the two reduction steps correspond to successive filling of the Ir–Ir pσ orbital. Complexes 2+ and 1+ have very similar structures with 1+ having a shorter Ir–Ir distance. The unpaired electron density in 1+ is delocalized along the Ir–Ir axis and over N atoms of the eight C≡N– ligands. The second reduction step 1+ → 0 changes the Ir(CN−)4 coordination geometry at each Ir site from approximately planar to seesaw whereby one −N≡C–Ir–C≡N– moiety is linear and the other bent at the Ir (137°) as well as N (146°) atoms. Although complex 0 is another example of a rare (pσ)2 dimetallic species (after [Pt2(μ-P2O5(BF2)2)4]6–, J. Am. Chem. Soc. 2016, 138, 5699), the redistribution of lower lying occupied molecular orbitals increases electron density predominantly at the bent C≡N– ligands whose N atoms are predicted to be nucleophilic reaction centers.
Co-reporter:Ruijie D. Teo, Jae Youn Hwang, John Termini, Zeev Gross, and Harry B. Gray
Chemical Reviews 2017 Volume 117(Issue 4) pp:
Publication Date(Web):October 19, 2016
DOI:10.1021/acs.chemrev.6b00400
Corroles are exceptionally promising platforms for the development of agents for simultaneous cancer-targeting imaging and therapy. Depending on the element chelated by the corrole, these theranostic agents may be tuned primarily for diagnostic or therapeutic function. Versatile synthetic methodologies allow for the preparation of amphipolar derivatives, which form stable noncovalent conjugates with targeting biomolecules. These conjugates can be engineered for imaging and targeting as well as therapeutic function within one theranostic assembly. In this review, we begin with a brief outline of corrole chemistry that has been uniquely useful in designing corrole-based anticancer agents. Then we turn attention to the early literature regarding corrole anticancer activity, which commenced one year after the first scalable synthesis was reported (1999–2000). In 2001, a major advance was made with the introduction of negatively charged corroles, as these molecules, being amphipolar, form stable conjugates with many proteins. More recently, both cellular uptake and intracellular trafficking of metallocorroles have been documented in experimental investigations employing advanced optical spectroscopic as well as magnetic resonance imaging techniques. Key results from work on both cellular and animal models are reviewed, with emphasis on those that have shed new light on the mechanisms associated with anticancer activity. In closing, we predict a very bright future for corrole anticancer research, as it is experiencing exponential growth, taking full advantage of recently developed imaging and therapeutic modalities.
Co-reporter:Bryan M. Hunter, Harry B. Gray, and Astrid M. Müller
Chemical Reviews 2016 Volume 116(Issue 22) pp:14120-14136
Publication Date(Web):October 31, 2016
DOI:10.1021/acs.chemrev.6b00398
Water oxidation is a key chemical transformation for the conversion of solar energy into chemical fuels. Our review focuses on recent work on robust earth-abundant heterogeneous catalysts for the oxygen-evolving reaction (OER). We point out that improvements in the performance of OER catalysts will depend critically on the success of work aimed at understanding reaction barriers based on atomic-level mechanisms. We highlight the challenge of obtaining acid-stable OER catalysts, with proposals for elements that could be employed to reach this goal. We suggest that future advances in solar fuels science will be accelerated by the development of new methods for materials synthesis and characterization, along with in-depth investigations of redox mechanisms at catalytic surfaces.
Co-reporter:Tania V. Darnton; Bryan M. Hunter; Michael G. Hill; Stanislav Záliš; Antonín VlčekJr.
Journal of the American Chemical Society 2016 Volume 138(Issue 17) pp:5699-5705
Publication Date(Web):April 11, 2016
DOI:10.1021/jacs.6b02559
A d8–d8 complex [Pt2(μ-P2O5(BF2)4]4– (abbreviated Pt(pop-BF2)4–) undergoes two 1e– reductions at E1/2 = −1.68 and Ep = −2.46 V (vs Fc+/Fc) producing reduced Pt(pop-BF2)5– and superreduced Pt(pop-BF2)6– species, respectively. The EPR spectrum of Pt(pop-BF2)5– and UV–vis spectra of both the reduced and the superreduced complexes, together with TD-DFT calculations, reveal successive filling of the 6pσ orbital accompanied by gradual strengthening of Pt–Pt bonding interactions and, because of 6pσ delocalization, of Pt–P bonds in the course of the two reductions. Mayer–Millikan Pt–Pt bond orders of 0.173, 0.268, and 0.340 were calculated for the parent, reduced, and superreduced complexes, respectively. The second (5–/6−) reduction is accompanied by a structural distortion that is experimentally manifested by electrochemical irreversibility. Both reduction steps proceed without changing either d8 Pt electronic configuration, making the superreduced Pt(pop-BF2)6– a very rare 6p2 σ-bonded binuclear complex. However, the Pt–Pt σ bonding interaction is limited by the relatively long bridging-ligand-imposed Pt–Pt distance accompanied by repulsive electronic congestion. Pt(pop-BF2)4– is predicted to be a very strong photooxidant (potentials of +1.57 and +0.86 V are estimated for the singlet and triplet dσ*pσ excited states, respectively).
Co-reporter:Brendon J. McNicholas, James D. Blakemore, Alice B. Chang, Christopher M. Bates, Wesley W. Kramer, Robert H. Grubbs, and Harry B. Gray
Journal of the American Chemical Society 2016 Volume 138(Issue 35) pp:11160-11163
Publication Date(Web):August 25, 2016
DOI:10.1021/jacs.6b08795
The electrochemical characterization of brush polymer ion gels containing embedded small-molecule redox-active species is reported. Gels comprising PS–PEO–PS triblock brush polymer, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI), and some combination of ferrocene (Fc), cobaltocenium (CoCp2+), and Re(bpy)(CO)3Cl (1) exhibit diffusion-controlled redox processes with diffusion coefficients approximately one-fifth of those observed in neat BMIm-TFSI. Notably, 1 dissolves homogeneously in the interpenetrating matrix domain of the ion gel and displays electrocatalytic CO2 reduction to CO in the gel. The catalytic wave exhibits a positive shift versus Fc+/0 compared with analogous nonaqueous solvents with a reduction potential 450 mV positive of onset and 90% Faradaic efficiency for CO production. These materials provide a promising and alternative approach to immobilized electrocatalysis, creating numerous opportunities for application in solid-state devices.
Co-reporter:Ruijie D. Teo; John Termini
Journal of Medicinal Chemistry 2016 Volume 59(Issue 13) pp:6012-6024
Publication Date(Web):February 10, 2016
DOI:10.1021/acs.jmedchem.5b01975
Lanthanide complexes are of increasing importance in cancer diagnosis and therapy, owing to the versatile chemical and magnetic properties of the lanthanide-ion 4f electronic configuration. Following the first implementation of gadolinium(III)-based contrast agents in magnetic resonance imaging in the 1980s, lanthanide-based small molecules and nanomaterials have been investigated as cytotoxic agents and inhibitors, in photodynamic therapy, radiation therapy, drug/gene delivery, biosensing, and bioimaging. As the potential utility of lanthanides in these areas continues to increase, this timely review of current applications will be useful to medicinal chemists and other investigators interested in the latest developments and trends in this emerging field.
Co-reporter:James R. McKone, Debbie C. Crans, Cheryl Martin, John Turner, Anil R. Duggal, and Harry B. Gray
Inorganic Chemistry 2016 Volume 55(Issue 18) pp:9131-9143
Publication Date(Web):September 8, 2016
DOI:10.1021/acs.inorgchem.6b01097
A clear challenge for the coming decades is decreasing the carbon intensity of the global energy supply while simultaneously accommodating a rapid worldwide increase in power demand. Meeting this challenge of providing abundant, clean energy undoubtedly requires synergistic efforts between basic and applied researchers in the chemical sciences to develop and deploy new technologies. Among the available options, solar energy is one of the promising targets because of the high abundance of solar photons over much of the globe. Similarly, decarbonization of the global energy supply will require clean sources of hydrogen to use as reducing equivalents for fuel and chemical feedstocks. In this report, we discuss the importance of translational research—defined as work that explicitly targets basic discovery as well as technology development—in the context of photovoltaics and solar fuels. We focus on three representative research programs encompassing translational research in government, industry, and academia. We then discuss more broadly the benefits and challenges of translational research models and offer recommendations for research programs that address societal challenges in the energy sector and beyond.
Co-reporter:Oliver S. Shafaat;Dr. Jay R. Winkler;Dr. Harry B. Gray;Dr. Dennis A. Dougherty
ChemBioChem 2016 Volume 17( Issue 14) pp:1323-1327
Publication Date(Web):
DOI:10.1002/cbic.201600230
Abstract
We describe the reversible photoactivation of the acid sensitive ligand-gated ion channel ASIC2a, a mammalian channel found throughout the central and peripheral nervous systems that is associated with vision and pain. We also show the activation of GLIC, an acid-sensitive prokaryotic homologue of the nicotinic acetylcholine receptor. Photoactivation was achieved by using visible light irradiation of a newly synthesized water-soluble merocyanine photoacid, 1, which was designed to remove adverse channel blocking effects of a related system. Activation of ASIC2a and GLIC occurs reversibly, in a benign manner, and only upon irradiation. Further studies using transient absorption spectroscopy showed that protonation of a colorimetric base occurred rapidly (ca. 100 μs) after excitation of 1. These results demonstrate that irradiation of 1 can induce rapid, local pH changes that can be used to investigate both biological and chemical proton transfer reactions.
Co-reporter:Melanie Pribisko;John Termini;Punnajit Lim;Robert H. Grubbs;Joshua Palmer
PNAS 2016 Volume 113 (Issue 16 ) pp:E2258-E2266
Publication Date(Web):2016-04-19
DOI:10.1073/pnas.1517402113
We report derivatives of gallium(III) tris(pentafluorophenyl)corrole, 1 [Ga(tpfc)], with either sulfonic (2) or carboxylic
acids (3, 4) as macrocyclic ring substituents: the aminocaproate derivative, 3 [Ga(ACtpfc)], demonstrated high cytotoxic activity
against all NCI60 cell lines derived from nine tumor types and confirmed very high toxicity against melanoma cells, specifically
the LOX IMVI and SK-MEL-28 cell lines. The toxicities of 1, 2, 3, and 4 [Ga(3-ctpfc)] toward prostate (DU-145), melanoma (SK-MEL-28),
breast (MDA-MB-231), and ovarian (OVCAR-3) cancer cells revealed a dependence on the ring substituent: IC50 values ranged from 4.8 to >200 µM; and they correlated with the rates of uptake, extent of intracellular accumulation, and
lipophilicity. Carboxylated corroles 3 and 4, which exhibited about 10-fold lower IC50 values (<20 µM) relative to previous analogs against all four cancer cell lines, displayed high efficacy (Emax = 0). Confocal fluorescence imaging revealed facile uptake of functionalized gallium corroles by all human cancer cells that
followed the order: 4 >> 3 > 2 >> 1 (intracellular accumulation of gallium corroles was fastest in melanoma cells). We conclude
that carboxylated gallium corroles are promising chemotherapeutics with the advantage that they also can be used for tumor
imaging.
Co-reporter:Wesley Sattler; Lawrence M. Henling; Jay R. Winkler
Journal of the American Chemical Society 2015 Volume 137(Issue 3) pp:1198-1205
Publication Date(Web):January 16, 2015
DOI:10.1021/ja510973h
Modular syntheses of oligoarylisocyanide ligands that are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have been developed; tungsten complexes incorporating these oligoarylisocyanide ligands exhibit intense metal-to-ligand charge-transfer visible absorptions that are red-shifted and more intense than those of the parent W(CNdipp)6 complex. Additionally, these W(CNAr)6 complexes have enhanced excited-state properties, including longer lifetimes and very high quantum yields. The decay kinetics of electronically excited W(CNAr)6 complexes (*W(CNAr)6) show solvent dependences; faster decay is observed in higher dielectric solvents. *W(CNAr)6 lifetimes are temperature dependent, suggestive of a strong coupling nonradiative decay mechanism that promotes repopulation of the ground state. Notably, *W(CNAr)6 complexes are exceptionally strong reductants: [W(CNAr)6]+/*W(CNAr)6 potentials are more negative than −2.7 V vs [Cp2Fe]+/Cp2Fe.
Co-reporter:Stanislav Záliš; Yan-Choi Lam; Harry B. Gray;Antonín Vlček
Inorganic Chemistry 2015 Volume 54(Issue 7) pp:3491-3500
Publication Date(Web):March 16, 2015
DOI:10.1021/acs.inorgchem.5b00063
[Pt2(μ-P2O5H2)4]4– (Pt(pop)) and its perfluoroborated derivative [Pt2(μ-P2O5(BF2)2)4]4– (Pt(pop-BF2)) are d8–d8 complexes whose electronic excited states can drive reductions and oxidations of relatively inert substrates. We performed spin–orbit (SO) TDDFT calculations on these complexes that account for their absorption spectra across the entire UV–vis spectral region. The complexes exhibit both fluorescence and phosphorescence attributable, respectively, to singlet and triplet excited states of dσ*pσ origin. These features are energetically isolated from each other (∼7000 cm–1 for (Pt(pop-BF2)) as well as from higher-lying states (5800 cm–1). The lowest 3dσ*pσ state is split into three SO states by interactions with higher-lying singlet states with dπpσ and, to a lesser extent, pπpσ contributions. The spectroscopically allowed dσ*pσ SO state has ∼96% singlet character with small admixtures of higher triplets of partial dπpσ and pπpσ characters that also mix with 3dσ*pσ, resulting in a second-order 1dσ*pσ–3dσ*pσ SO interaction that facilitates intersystem crossing (ISC). All SO interactions involving the dσ*pσ states are weak because of large energy gaps to higher interacting states. The spectroscopically allowed dσ*pσ SO state is followed by a dense manifold of ligand-to-metal–metal charge transfer states, some with pπpσ (at lower energies) or dπpσ contributions (at higher energies). Spectroscopically active higher states are strongly spin-mixed. The electronic structure, state ordering, and relative energies are minimally perturbed when the calculation is performed at the optimized geometries of the 1dσ*pσ and 3dσ*pσ excited states (rather than the ground state). Results obtained for Pt(pop) are very similar, showing slightly smaller energy gaps and, possibly, an additional 1dσ*pσ – 3dσ*pσ second order SO interaction involving higher 1dπpσ* states that could account in part for the much faster ISC. It also appears that 1dσ*pσ → 3dσ*pσ ISC requires a structural distortion that has a lower barrier for Pt(pop) than for the more rigid Pt(pop-BF2).
Co-reporter:Hana Kvapilová; Wesley Sattler; Aaron Sattler; Igor V. Sazanovich; Ian P. Clark; Michael Towrie; Harry B. Gray; Stanislav Záliš;Antonín Vlček
Inorganic Chemistry 2015 Volume 54(Issue 17) pp:8518-8528
Publication Date(Web):August 12, 2015
DOI:10.1021/acs.inorgchem.5b01203
W(CNAryl)6 complexes containing 2,6-diisopropylphenyl isocyanide (CNdipp) are powerful photoreductants with strongly emissive long-lived excited states. These properties are enhanced upon appending another aryl ring, e.g., W(CNdippPhOMe2)6; CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide (Sattler et al. J. Am. Chem. Soc. 2015, 137, 1198−1205). Electronic transitions and low-lying excited states of these complexes were investigated by time-dependent density functional theory (TDDFT); the lowest triplet state was characterized by time-resolved infrared spectroscopy (TRIR) supported by density functional theory (DFT). The intense absorption band of W(CNdipp)6 at 460 nm and that of W(CNdippPhOMe2)6 at 500 nm originate from transitions of mixed ππ*(C≡N–C)/MLCT(W → Aryl) character, whereby W is depopulated by ca. 0.4 e– and the electron-density changes are predominantly localized along two equatorial molecular axes. The red shift and intensity rise on going from W(CNdipp)6 to W(CNdippPhOMe2)6 are attributable to more extensive delocalization of the MLCT component. The complexes also exhibit absorptions in the 300–320 nm region, owing to W → C≡N MLCT transitions. Electronic absorptions in the spectrum of W(CNXy)6 (Xy = 2,6-dimethylphenyl), a complex with orthogonal aryl orientation, have similar characteristics, although shifted to higher energies. The relaxed lowest W(CNAryl)6 triplet state combines ππ* excitation of a trans pair of C≡N–C moieties with MLCT (0.21 e–) and ligand-to-ligand charge transfer (LLCT, 0.24–0.27 e–) from the other four CNAryl ligands to the axial aryl and, less, to C≡N groups; the spin density is localized along a single Aryl–N≡C–W–C≡N–Aryl axis. Delocalization of excited electron density on outer aryl rings in W(CNdippPhOMe2)6 likely promotes photoinduced electron-transfer reactions to acceptor molecules. TRIR spectra show an intense broad bleach due to ν(C≡N), a prominent transient upshifted by 60–65 cm–1, and a weak down-shifted feature due to antisymmetric C≡N stretch along the axis of high spin density. The TRIR spectral pattern remains unchanged on the femtosecond-nanosecond time scale, indicating that intersystem crossing and electron-density localization are ultrafast (<100 fs).
Co-reporter:Christopher W. Roske; Eric J. Popczun; Brian Seger; Carlos G. Read; Thomas Pedersen; Ole Hansen; Peter C. K. Vesborg; Bruce S. Brunschwig; Raymond E. Schaak; Ib Chorkendorff; Harry B. Gray;Nathan S. Lewis
The Journal of Physical Chemistry Letters 2015 Volume 6(Issue 9) pp:1679-1683
Publication Date(Web):April 20, 2015
DOI:10.1021/acs.jpclett.5b00495
The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n+p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H2SO4(aq). The CoP-coated (2.0 mg cm–2) n+p-Si MW photocathodes were stable for over 12 h of continuous operation and produced an open-circuit photovoltage (Voc) of 0.48 V, a light-limited photocurrent density (Jph) of 17 mA cm–2, a fill factor (ff) of 0.24, and an ideal regenerative cell efficiency (ηIRC) of 1.9% under simulated 1 Sun illumination. Pt-coated (0.5 mg cm–2) n+p-Si MW-array photocathodes produced Voc = 0.44 V, Jph = 14 mA cm–2, ff = 0.46, and η = 2.9% under identical conditions. Thus, the MW geometry allows the fabrication of photocathodes entirely comprised of earth-abundant materials that exhibit performance comparable to that of devices that contain Pt.
Co-reporter:Harry B. Gray;Jay R. Winkler
PNAS 2015 Volume 112 (Issue 35 ) pp:10920-10925
Publication Date(Web):2015-09-01
DOI:10.1073/pnas.1512704112
Living organisms have adapted to atmospheric dioxygen by exploiting its oxidizing power while protecting themselves against
toxic side effects. Reactive oxygen and nitrogen species formed during oxidative stress, as well as high-potential reactive
intermediates formed during enzymatic catalysis, could rapidly and irreversibly damage polypeptides were protective mechanisms
not available. Chains of redox-active tyrosine and tryptophan residues can transport potentially damaging oxidizing equivalents
(holes) away from fragile active sites and toward protein surfaces where they can be scavenged by cellular reductants. Precise
positioning of these chains is required to provide effective protection without inhibiting normal function. A search of the
structural database reveals that about one third of all proteins contain Tyr/Trp chains composed of three or more residues.
Although these chains are distributed among all enzyme classes, they appear with greatest frequency in the oxidoreductases
and hydrolases. Consistent with a redox-protective role, approximately half of the dioxygen-using oxidoreductases have Tyr/Trp
chain lengths ≥3 residues. Among the hydrolases, long Tyr/Trp chains appear almost exclusively in the glycoside hydrolases.
These chains likely are important for substrate binding and positioning, but a secondary redox role also is a possibility.
Co-reporter:Jay R. Winkler and Harry B. Gray
Chemical Reviews 2014 Volume 114(Issue 7) pp:3369
Publication Date(Web):November 27, 2013
DOI:10.1021/cr4004715
Co-reporter:Peter Agbo ; James R. Heath
Journal of the American Chemical Society 2014 Volume 136(Issue 39) pp:13882-13887
Publication Date(Web):September 4, 2014
DOI:10.1021/ja5077519
We report a general kinetics model for catalytic dioxygen reduction on multicopper oxidase (MCO) cathodes. Our rate equation combines Butler–Volmer (BV) electrode kinetics and the Michaelis–Menten (MM) formalism for enzymatic catalysis, with the BV model accounting for interfacial electron transfer (ET) between the electrode surface and the MCO type 1 copper site. Extending the principles of MM kinetics to this system produced an analytical expression incorporating the effects of subsequent intramolecular ET and dioxygen binding to the trinuclear copper cluster into the cumulative model. We employed experimental electrochemical data on Thermus thermophilus laccase as benchmarks to validate our model, which we suggest will aid in the design of more efficient MCO cathodes. In addition, we demonstrate the model’s utility in determining estimates for both the electronic coupling and average distance between the laccase type-1 active site and the cathode substrate.
Co-reporter:Jay R. Winkler
Journal of the American Chemical Society 2014 Volume 136(Issue 8) pp:2930-2939
Publication Date(Web):February 5, 2014
DOI:10.1021/ja500215j
Electrons have so little mass that in less than a second they can tunnel through potential energy barriers that are several electron-volts high and several nanometers wide. Electron tunneling is a critical functional element in a broad spectrum of applications, ranging from semiconductor diodes to the photosynthetic and respiratory charge transport chains. Prior to the 1970s, chemists generally believed that reactants had to collide in order to effect a transformation. Experimental demonstrations that electrons can transfer between reactants separated by several nanometers led to a revision of the chemical reaction paradigm. Experimental investigations of electron exchange between redox partners separated by molecular bridges have elucidated many fundamental properties of these reactions, particularly the variation of rate constants with distance. Theoretical work has provided critical insights into the superexchange mechanism of electronic coupling between distant redox centers. Kinetics measurements have shown that electrons can tunnel about 2.5 nm through proteins on biologically relevant time scales. Longer-distance biological charge flow requires multiple electron tunneling steps through chains of redox cofactors. The range of phenomena that depends on long-range electron tunneling continues to expand, providing new challenges for both theory and experiment.
Co-reporter:James R. McKone, Smaranda C. Marinescu, Bruce S. Brunschwig, Jay R. Winkler and Harry B. Gray
Chemical Science 2014 vol. 5(Issue 3) pp:865-878
Publication Date(Web):2013/11/05
DOI:10.1039/C3SC51711J
Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.
Co-reporter:Ruijie D. Teo, Harry B. Gray, Punnajit Lim, John Termini, Elena Domeshek and Zeev Gross
Chemical Communications 2014 vol. 50(Issue 89) pp:13789-13792
Publication Date(Web):05 Sep 2014
DOI:10.1039/C4CC06577H
We have synthesized and characterized a water-soluble gold(III) corrole (1-Au) that is highly toxic to cisplatin-resistant cancer cells. Relative to its 1-Ga analogue, axial ligands bind only weakly to 1-Au, which likely accounts for its lower affinity for human serum albumin (HSA). We suggest that the cytotoxicity of 1-Au may be related to this lower HSA affinity.
Co-reporter:Stephanie M. Laga, James D. Blakemore, Lawrence M. Henling, Bruce S. Brunschwig, and Harry B. Gray
Inorganic Chemistry 2014 Volume 53(Issue 24) pp:12668-12670
Publication Date(Web):November 19, 2014
DOI:10.1021/ic501804h
We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO4] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO4] moiety. Using electrochemical methods, the dicobalt complex was found to be an electrocatalyst for proton reduction in acetonitrile solution. Notably, the overpotential for proton reduction (954 mV) was found to be higher than in the cases of two analogous single-site cobalt glyoximes under virtually identical conditions.
Co-reporter:Judith R. C. Lattimer, James D. Blakemore, Wesley Sattler, Sheraz Gul, Ruchira Chatterjee, Vittal K. Yachandra, Junko Yano, Bruce S. Brunschwig, Nathan S. Lewis and Harry B. Gray
Dalton Transactions 2014 vol. 43(Issue 40) pp:15004-15012
Publication Date(Web):17 Jul 2014
DOI:10.1039/C4DT01149J
Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2′-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2′-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.
Co-reporter:Petr Pospíšil, Katja E. Luxem, Maraia Ener, Jan Sýkora, Jana Kocábová, Harry B. Gray, Antonín Vlček Jr., and Martin Hof
The Journal of Physical Chemistry B 2014 Volume 118(Issue 34) pp:10085-10091
Publication Date(Web):July 31, 2014
DOI:10.1021/jp504625d
Fluorescence of 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes Badan and Prodan is quenched by tryptophan in Brij 58 micelles as well as in two cytochrome P450 proteins (CYP102, CYP119) with Badan covalently attached to a cysteine residue. Formation of nonemissive complexes between a dye molecule and tryptophan accounts for about 76% of the fluorescence intensity quenching in micelles, the rest is due to diffusive encounters. In the absence of tryptophan, fluorescence of Badan-labeled cytochromes decays with triexponential kinetics characterized by lifetimes of about 100 ps, 700–800 ps, and 3 ns. Site mutation of a histidine residue in the vicinity of the Badan label by tryptophan results in shortening of all three decay lifetimes. The relative amplitude of the fastest component increases at the expense of the two slower ones. The average quenching rate constants are 4.5 × 108 s–1 (CYP102) and 3.7 × 108 s–1 (CYP119), at 288 K. Cyclic voltammetry of Prodan in MeCN shows a reversible reduction peak at −1.85 V vs NHE that becomes chemically irreversible and shifts positively upon addition of water. A quasireversible reduction at −0.88 V was observed in an aqueous buffer (pH 7.3). The excited-state reduction potential of Prodan (and Badan) is estimated to vary from about +0.6 V (vs NHE) in polar aprotic media (MeCN) to approximately +1.6 V in water. Tryptophan quenching of Badan/Prodan fluorescence in CYPs and Brij 58 micelles is exergonic by ≤0.5 V and involves tryptophan oxidation by excited Badan/Prodan, coupled with a fast reaction between the reduced dye and water. Photoreduction is a new quenching mechanism for 2-(N,N-dimethylamino)-6-propionylnaphthalene dyes that are often used as solvatochromic polarity probes, FRET donors and acceptors, as well as reporters of solvation dynamics.
Co-reporter:James D. Blakemore, Emilia S. Hernandez, Wesley Sattler, Bryan M. Hunter, Lawrence M. Henling, Bruce S. Brunschwig, Harry B. Gray
Polyhedron 2014 84() pp: 14-18
Publication Date(Web):
DOI:10.1016/j.poly.2014.05.022
Co-reporter:Carl M. Blumenfeld, Bryce F. Sadtler, G. Esteban Fernandez, Lily Dara, Cathie Nguyen, Felix Alonso-Valenteen, Lali Medina-Kauwe, Rex A. Moats, Nathan S. Lewis, Robert H. Grubbs, Harry B. Gray, Karn Sorasaenee
Journal of Inorganic Biochemistry 2014 140() pp: 39-44
Publication Date(Web):
DOI:10.1016/j.jinorgbio.2014.06.015
Co-reporter:Qixi Mi, Robert H. Coridan, Bruce S. Brunschwig, Harry B. Gray and Nathan S. Lewis
Energy & Environmental Science 2013 vol. 6(Issue 9) pp:2646-2653
Publication Date(Web):18 Jun 2013
DOI:10.1039/C3EE40712H
The behavior of WO3 photoanodes has been investigated in contact with a combination of four anions (Cl−, CH3SO3−, HSO4−, and ClO4−) and three solvents (water, acetonitrile, and propylene carbonate), to elucidate the role of the semiconductor surface, the electrolyte, and redox kinetics on the current density vs. potential properties of n-type WO3. In 1.0 M aqueous strong acids, although the flat-band potential (Efb) of WO3 was dominated by electrochemical intercalation of protons into WO3, the nature of the electrolyte influenced the onset potential (Eon) of the anodic photocurrent. In aprotic solvents, the electrolyte anion shifted both Efb and Eon, but did not significantly alter the overall profile of the voltammetric data. For 0.50 M tetra(n-butyl)ammonium perchlorate in propylene carbonate, the internal quantum yield exceeded unity at excitation wavelengths of 300–390 nm, indicative of current doubling. A regenerative photoelectrochemical cell based on the reversible redox couple B10Br10˙−/2− in acetonitrile, with a solution potential of ∼1.7 V vs. the normal hydrogen electrode, exhibited an open-circuit photovoltage of 1.32 V under 100 mW cm−2 of simulated Air Mass 1.5 global illumination.
Co-reporter:Morgan L. Cable, James P. Kirby, Harry B. Gray, and Adrian Ponce
Accounts of Chemical Research 2013 Volume 46(Issue 11) pp:2576
Publication Date(Web):September 16, 2013
DOI:10.1021/ar400050t
In the design of molecular sensors, researchers exploit binding interactions that are usually defined in terms of topology and charge complementarity. The formation of complementary arrays of highly cooperative, noncovalent bonding networks facilitates protein-ligand binding, leading to motifs such as the “lock-and-key”. Synthetic molecular sensors often employ metal complexes as key design elements as a way to construct a binding site with the desired shape and charge to achieve target selectivity. In transition metal complexes, coordination number, structure and ligand dynamics are governed primarily by a combination of inner-sphere covalent and outer-sphere noncovalent interactions. These interactions provide a rich variable space that researchers can use to tune structure, stability, and dynamics.In contrast, lanthanide(III)-ligand complex formation and ligand-exchange dynamics are dominated by reversible electrostatic and steric interactions, because the unfilled f shell is shielded by the larger, filled d shell. Luminescent lanthanides such as terbium, europium, dysprosium, and samarium display many photophysical properties that make them excellent candidates for molecular sensor applications. Complexes of lanthanide ions act as receptors that exhibit a detectable change in metal-based luminescence upon binding of an anion. In our work on sensors for detection of dipicolinate, the unique biomarker of bacterial spores, we discovered that the incorporation of an ancillary ligand (AL) can enhance binding constants of target anions to lanthanide ions by as much as two orders of magnitude.In this Account, we show that selected ALs in lanthanide/anion systems greatly improve sensor performance for medical, planetary science, and biodefense applications. We suggest that the observed anion binding enhancement could result from an AL-induced increase in positive charge at the lanthanide ion binding site. This effect depends on lanthanide polarizability, which can be established from the ionization energy of Ln3+ → Ln4+. These results account for the order Tb3+ > Dy3+ > Eu3+ ≈ Sm3+. As with many lanthanide properties, ranging from hydration enthalpy to vaporization energy, this AL-induced enhancement shows a large discrepancy between Tb3+ and Eu3+ despite their similarity in size, a phenomenon known as the “gadolinium break”. This discrepancy, based on the unusual stabilities of the Eu2+ and Tb4+ oxidation states, results from the half-shell effect, as both of these ions have half-filled 4f-shells. The high polarizability of Tb3+ explains the extraordinarily large increase in the binding affinity of anions for terbium compared to other lanthanides.We recommend that researchers consider this AL-induced enhancement when designing lanthanide-macrocycle optical sensors. Ancillary ligands also can reduce the impact of interfering species such as phosphate commonly found in environmental and physiological samples.
Co-reporter:Kana Takematsu ; Heather Williamson ; Ana María Blanco-Rodríguez ; Lucie Sokolová ; Pavle Nikolovski ; Jens T. Kaiser ; Michael Towrie ; Ian P. Clark ; Antonín Vlček ; Jr.; Jay R. Winkler
Journal of the American Chemical Society 2013 Volume 135(Issue 41) pp:15515-15525
Publication Date(Web):September 13, 2013
DOI:10.1021/ja406830d
We report a new metallolabeled blue copper protein, Re126W122CuI Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: ReI(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt CuI oxidation (<50 ns), followed by slow back ET to regenerate CuI and ground-state ReI with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122CuI)2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←CuI, where // denotes a protein–protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the CuII form shows two Re126W122CuII molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122CuI)2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes.
Co-reporter:Jeffrey J. Warren ; Nadia Herrera ; Michael G. Hill ; Jay R. Winkler
Journal of the American Chemical Society 2013 Volume 135(Issue 30) pp:11151-11158
Publication Date(Web):July 16, 2013
DOI:10.1021/ja403734n
We have designed ruthenium-modified Pseudomonas aeruginosa azurins that incorporate 3-nitrotyrosine (NO2YOH) between Ru(2,2′-bipyridine)2(imidazole)(histidine) and Cu redox centers in electron transfer (ET) pathways. We investigated the structures and reactivities of three different systems: RuH107NO2YOH109, RuH124NO2YOH122, and RuH126NO2YOH122. RuH107NO2YOH109, unlabeled H124NO2YOH122, and unlabeled H126NO2YOH122 were structurally characterized. The pKa’s of NO2YOH at positions 122 and 109 are 7.2 and 6.0, respectively. Reduction potentials of 3-nitrotyrosinate (NO2YO–)-modified azurins were estimated from cyclic and differential pulse voltammetry data: oxidation of NO2YO–122 occurs near 1.1 versus NHE; oxidation of NO2YO–109 is near 1.2 V. Our analysis of transient optical spectroscopic experiments indicates that hopping via NO2YO– enhances CuI oxidation rates over single-step ET by factors of 32 (RuH107NO2YO–109), 46 (RuH126NO2YO–122), and 13 (RuH124NO2YO–122).
Co-reporter:Wesley Sattler ; Maraia E. Ener ; James D. Blakemore ; Aaron A. Rachford ; Paul J. LaBeaume ; James W. Thackeray ; James F. Cameron ; Jay R. Winkler
Journal of the American Chemical Society 2013 Volume 135(Issue 29) pp:10614-10617
Publication Date(Web):July 15, 2013
DOI:10.1021/ja4047119
The homoleptic arylisocyanide tungsten complexes, W(CNXy)6 and W(CNIph)6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400–550 nm). MLCT emission (λmax ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bun4N][PF6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6]+ and [anth]•–. ET from *W to benzophenone and cobalticenium also is observed in [Bun4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6]+/*W couple is −2.8 V vs Cp2Fe+/0, establishing W(CNIph)6 as one of the most powerful photoreductants that has been generated with visible light.
Co-reporter:James D. Blakemore ; Ayush Gupta ; Jeffrey J. Warren ; Bruce S. Brunschwig
Journal of the American Chemical Society 2013 Volume 135(Issue 49) pp:18288-18291
Publication Date(Web):November 18, 2013
DOI:10.1021/ja4099609
We show that molecular catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. A pyrene-appended bipyridine ligand (P) serves as the linker between each complex and the surface. Immobilization of a rhodium proton-reduction catalyst, [Cp*Rh(P)Cl]Cl (1), and a rhenium CO2-reduction catalyst, Re(P)(CO)3Cl (2), afford electrocatalytically active assemblies. X-ray photoelectron spectroscopy and electrochemistry confirm catalyst immobilization. Reduction of 1 in the presence of p-toluenesulfonic acid results in catalytic H2 production, while reduction of 2 in the presence of CO2 results in catalytic CO production.
Co-reporter:Jeffrey J. Warren, Jay R. Winkler, Harry B. Gray
Coordination Chemistry Reviews 2013 Volume 257(Issue 1) pp:165-170
Publication Date(Web):1 January 2013
DOI:10.1016/j.ccr.2012.07.002
Photosynthetic reaction centers (PRCs) employ multiple-step tunneling (hopping) to separate electrons and holes that ultimately drive the chemistry required for metabolism. We recently developed hopping maps that can be used to interpret the rates and energetics of electron/hole hopping in three-site (donor–intermediate–acceptor) tunneling reactions, including those in PRCs. Here we analyze several key ET reactions in PRCs, including forward ET in the L-branch, and hopping that could involve thermodynamically uphill intermediates in the M-branch, which is ET-inactive in vivo. We also explore charge recombination reactions, which could involve hopping. Our hopping maps support the view that electron flow in PRCs involves strong electronic coupling between cofactors and reorganization energies that are among the lowest in biology (≤0.4 eV).Highlights► Photosynthetic reaction centers (PRCs) use hopping to separate electrons and holes. ► We employ hopping maps to interpret electron flow in PRCs. ► Charge-separation and charge-recombination reactions in PRCS are discussed.
Co-reporter:James R. McKone, Bryce F. Sadtler, Caroline A. Werlang, Nathan S. Lewis, and Harry B. Gray
ACS Catalysis 2013 Volume 3(Issue 2) pp:166
Publication Date(Web):December 3, 2012
DOI:10.1021/cs300691m
Earth-abundant metals are attractive alternatives to the noble metal composite catalysts that are used in water electrolyzers based on proton-exchange membrane technology. Ni–Mo alloys have been previously developed for the hydrogen evolution reaction (HER), but synthesis methods to date have been limited to formation of catalyst coatings directly on a substrate. We report a method for generating unsupported nanopowders of Ni–Mo, which can be suspended in common solvents and cast onto arbitrary substrates. The mass-specific catalytic activity under alkaline conditions approaches that of the most active reported non-noble HER catalysts, and the coatings display good stability under alkaline conditions. We have also estimated turnover frequencies per surface atom at various overpotentials and conclude that the activity enhancement for Ni–Mo relative to pure Ni is due to a combination of increased surface area and increased fundamental catalytic activity.Keywords: cathode; electrolysis; HER; hydrogen evolution; molybdenum; nickel; Ni−Mo; stability
Co-reporter:Alec C. Durrell;Megan N. Jackson;Nilay Hazari
European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 7) pp:1134-1137
Publication Date(Web):
DOI:10.1002/ejic.201201498
Abstract
[(Benzo[h]quinolinyl)PdII(μ-OAc)]2 and [(2-phenylpyridinyl)PdII(μ-OAc)]2 undergo one-electron oxidation to afford mixed-valent (PdII–PdIII) species. Electrochemical oxidation of the PdII–PdII complexes in the presence of chloride at the PdIII–PdII/PdII–PdII potential results in a two-electron loss with addition of two chlorides to form [(benzo[h]quinolinyl)PdIIICl(μ-OAc)]2 and [(2-phenylpyridinyl)PdIIICl(μ-OAc)]2, respectively. When both excess benzo[h]quinoline and chloride are present, [(benzo[h]quinolinyl)PdII(μ-OAc)]2 electrocatalyzes the chlorination of the substrate to afford 10-chlorobenzo[h]quinoline with high chemical and Faradaic yields.
Co-reporter:Peter Agbo, James R. Heath, and Harry B. Gray
The Journal of Physical Chemistry B 2013 Volume 117(Issue 2) pp:527-534
Publication Date(Web):November 19, 2012
DOI:10.1021/jp309759g
We present electrochemical analyses of the catalysis of dioxygen reduction by Thermus thermophilus strain HB27 laccase on ketjen black substrates. Our cathodes reliably produce 0.56 mA cm–2 at 0.0 V vs Ag|AgCl reference at 30 °C in air-saturated buffer, under conditions of nonlimiting O2 flux. We report the electrochemical activity of this laccase as a function of temperature, pH, time, and the efficiency of its conversion of dioxygen to water. We have measured the surface concentration of electrochemically active species, permitting the extraction of electron transfer rates at the enzyme-electrode interface: 1 s–1 for this process at zero driving force at 30 °C and a limiting rate of 23 s–1 at 240 mV overpotential at 50 °C.
Co-reporter:Bahar Bingöl, Alec C. Durrell, Gretchen E. Keller, Joshua H. Palmer, Robert H. Grubbs, and Harry B. Gray
The Journal of Physical Chemistry B 2013 Volume 117(Issue 16) pp:4177-4182
Publication Date(Web):April 25, 2012
DOI:10.1021/jp3010053
We have investigated excited-state electron transfer in a donor-bridge-acceptor complex containing phenothiazine (PTZ) linked via tris(meta-phenylene-ethynylene) to a tricarbonyl(bipyridine)(pyridine)Re(I) unit. Time-resolved luminescence experiments reveal two excited-state (*Re) decay regimes, a multiexponential component with a mean lifetime of 2.7 ns and a longer monoexponential component of 530 ns in dichloromethane solution. The faster decay is attributed to PTZ → *Re electron transfer in a C-shaped PTZ-bridge-Re conformer (PTZ–Re ≈ 7.5 Å). We assign the longer lifetime, which is virtually identical to that of free *Re, to an extended conformer (PTZ–Re > 20 Å). The observed biexponential *Re decay requires that interconversion of PTZ-bridge-Re conformers be slower than 106 s–1.
Co-reporter:Nicole D. Bouley Ford, Dong-Woo Shin, Harry B. Gray, and Jay R. Winkler
The Journal of Physical Chemistry B 2013 Volume 117(Issue 42) pp:13206-13211
Publication Date(Web):August 30, 2013
DOI:10.1021/jp403234h
We have investigated intrachain contact dynamics in unfolded cytochrome cb562 by monitoring heme quenching of excited ruthenium photosensitizers covalently bound to residues along the polypeptide. Intrachain diffusion for chemically denatured proteins proceeds on the microsecond time scale with an upper limit of 0.1 μs. The rate constants exhibit a power-law dependence on the number of peptide bonds between the heme and Ru complex. The power-law exponent of −1.5 is consistent with theoretical models for freely jointed Gaussian chains, but its magnitude is smaller than that reported for several synthetic polypeptides. Contact formation within a stable loop was examined in a His63-heme ligated form of the protein under denaturing conditions. Loop formation accelerated contact kinetics for the Ru66 labeling site, owing to reduction in the length of the peptide separating redox sites. For other labeling sites within the stable loop, quenching rates were modestly reduced compared to the open chain polymer.
Co-reporter:Charlotte A. Whited, Jeffrey J. Warren, Katherine D. Lavoie, Jay R. Winkler, Harry B. Gray
Polyhedron 2013 Volume 58() pp:134-138
Publication Date(Web):13 July 2013
DOI:10.1016/j.poly.2012.08.079
We report the kinetics of CO rebinding to the heme in His134Ser, Ile223Val and His134Ser/Ile223Ser mutants of Geobacillus stearothermophilus nitric oxide synthase (gsNOS). The amplitudes of the two observed kinetics phases, which are insensitive to CO concentration, depend on enzyme concentration. We suggest that two forms of gsNOS are in equilibrium under the conditions employed (6.1–27 μM gsNOS with 20 or 100% CO atmosphere). The kinetics of CO rebinding to the heme do not depend on the identity of the NO-gate residues at positions 134 and 223.
Co-reporter:Michael F. Lichterman, Matthew R. Shaner, Sheila G. Handler, Bruce S. Brunschwig, Harry B. Gray, Nathan S. Lewis, and Joshua M. Spurgeon
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 23) pp:4188-4191
Publication Date(Web):November 21, 2013
DOI:10.1021/jz4022415
Atomic-layer deposition (ALD) of thin layers of cobalt oxide on n-type BiVO4 produced photoanodes capable of water oxidation with essentially 100% faradaic efficiency in alkaline, pH = 13 electrolytes. By contrast, under the same operating conditions, BiVO4 photoanodes without the Co oxide catalytic layer exhibited lower faradaic yields, of ca. 70%, for O2 evolution and were unstable, becoming rapidly photopassivated. High numbers (>25) of ALD cycles of Co oxide deposition gave electrodes that displayed poor photoelectrochemical behavior, but 15–20 ALD cycles produced Co oxide overlayers ∼1 nm in thickness, with the resulting photoelectrodes exhibiting a stable photocurrent density of 1.49 mA cm–2 at the oxygen-evolution potential and an open-circuit potential of 0.404 V versus the reversible hydrogen electrode, under 100 mW cm–2 of simulated air mass 1.5 illumination.Keywords: catalysis; interfaces; surfaces;
Co-reporter:Jeffrey J. Warren, Artur R. Menzeleev, Joshua S. Kretchmer, Thomas F. Miller III, Harry B. Gray, and James M. Mayer
The Journal of Physical Chemistry Letters 2013 Volume 4(Issue 3) pp:519-523
Publication Date(Web):January 14, 2013
DOI:10.1021/jz400029w
Concerted proton–electron transfer (CPET) reactions in iron carboxytetraphenylporphyrin complexes have been investigated using both experimental and theoretical methods. Synthetic heme models abstract H+ and e– from the hydroxylamine TEMPOH or an ascorbate derivative, and the kinetics of the TEMPOH reaction indicate concerted transfer of H+ and e–. Phenylene linker domains vary the electron donor/acceptor separation by approximately 4 Å. The rate data and extensive molecular simulations show that the electronic coupling decay constant (β) depends on conformational flexibility and solvation associated with the linker domain. Our best estimate of β is 0.23 ± 0.07 Å–1, a value that is near the low end of the range (0.2–0.5 Å–1) established for electron-transfer reactions involving related linkers. This is the first analysis of β for a CPET reaction.Keywords: concerted proton−electron transfer; distance dependence; electronic coupling; molecular dynamics; proton-coupled electron transfer;
Co-reporter:Seiji Yamada;Nicole D. Bouley Ford;Gretchen E. Keller;William C. Ford;Jay R. Winkler
PNAS 2013 110 (5 ) pp:1606-1610
Publication Date(Web):
DOI:10.1073/pnas.1221832110
Co-reporter:Seiji Yamada;Nicole D. Bouley Ford;Gretchen E. Keller;William C. Ford;Jay R. Winkler
PNAS 2013 110 (5 ) pp:1606-1610
Publication Date(Web):2013-01-29
DOI:10.1073/pnas.1221832110
We have investigated the folding dynamics of Thermus thermophilus cytochrome c552 by time-resolved fluorescence energy transfer between the heme and each of seven site-specific fluorescent probes. We have
found both an equilibrium unfolding intermediate and a distinct refolding intermediate from kinetics studies. Depending on
the protein region monitored, we observed either two-state or three-state denaturation transitions. The unfolding intermediate
associated with three-state folding exhibited native contacts in β-sheet and C-terminal helix regions. We probed the formation
of a refolding intermediate by time-resolved fluorescence energy transfer between residue 110 and the heme using a continuous
flow mixer. The intermediate ensemble, a heterogeneous mixture of compact and extended polypeptides, forms in a millisecond,
substantially slower than the ∼100-μs formation of a burst-phase intermediate in cytochrome c. The surprising finding is that, unlike for cytochrome c, there is an observable folding intermediate, but no microsecond burst phase in the folding kinetics of the structurally
related thermostable protein.
Co-reporter:Qixi Mi, Almagul Zhanaidarova, Bruce S. Brunschwig, Harry B. Gray and Nathan S. Lewis
Energy & Environmental Science 2012 vol. 5(Issue 2) pp:5694-5700
Publication Date(Web):03 Jan 2012
DOI:10.1039/C2EE02929D
The faradaic efficiency for O2(g) evolution at thin-film WO3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H2SO4, persulfate was the predominant photoelectrochemical oxidation product, and no O2 was detected unless catalytic quantities of Ag+(aq) were added to the electrolyte. In contact with 1.0 M HClO4, dissolved O2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N-oxide, completely suppressed O2 formation. In 1.0 M HCl, Cl2(g) was the primary oxidation product. These results indicate that at WO3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H2SO4, and HClO4, respectively.
Co-reporter:James R. McKone ; Adam P. Pieterick ; Harry B. Gray ;Nathan S. Lewis
Journal of the American Chemical Society 2012 Volume 135(Issue 1) pp:223-231
Publication Date(Web):November 30, 2012
DOI:10.1021/ja308581g
Crystalline p-type WSe2 has been grown by a chemical vapor transport method. After deposition of noble metal catalysts, p-WSe2 photocathodes exhibited thermodynamically based photoelectrode energy-conversion efficiencies of >7% for the hydrogen evolution reaction under mildly acidic conditions, and were stable under cathodic conditions for at least 2 h in acidic as well as in alkaline electrolytes. The open circuit potentials of the photoelectrodes in contact with the H+/H2 redox couple were very close to the bulk recombination/diffusion limit predicted from the Shockley diode equation. Only crystals with a prevalence of surface step edges exhibited a shift in flat-band potential as the pH was varied. Spectral response data indicated effective minority-carrier diffusion lengths of ∼1 μm, which limited the attainable photocurrent densities in the samples to ∼15 mA cm–2 under 100 mW cm–2 of Air Mass 1.5G illumination.
Co-reporter:Alec C. Durrell ; Gretchen E. Keller ; Yan-Choi Lam ; Jan Sýkora ; Antonín Vlček ; Jr.
Journal of the American Chemical Society 2012 Volume 134(Issue 34) pp:14201-14207
Publication Date(Web):August 6, 2012
DOI:10.1021/ja305666b
Analysis of variable-temperature fluorescence quantum yield and lifetime data for per(difluoroboro)tetrakis(pyrophosphito)diplatinate(II) ([Pt2(μ-P2O5(BF2)2)4]4–, abbreviated Pt(pop-BF2)), yields a radiative decay rate (kr = 1.7 × 108 s–1) an order of magnitude greater than that of the parent complex, Pt(pop). Its temperature-independent and activated intersystem crossing (ISC) pathways are at least 18 and 142 times slower than those of Pt(pop) [ISC activation energies: 2230 cm–1 for Pt(pop-BF2); 1190 cm–1 for Pt(pop)]. The slowdown in the temperature-independent ISC channel is attributed to two factors: (1) reduced spin–orbit coupling between the 1A2u state and the mediating triplet(s), owing to increases of LMCT energies relative to the excited singlet; and (2) diminished access to solvent, which for Pt(pop) facilitates dissipation of the excess energy into solvent vibrational modes. The dramatic increase in Ea is attributed to increased P-O-P framework rigidity, which impedes symmetry-lowering distortions, in particular asymmetric vibrations in the Pt2(P-O-P)4 core that would allow direct 1A2u–3A2u spin–orbit coupling.
Co-reporter:Kyle M. Lancaster ; María-Eugenia Zaballa ; Stephen Sproules ; Mahesh Sundararajan ; Serena DeBeer ; John H. Richards ; Alejandro J. Vila ; Frank Neese
Journal of the American Chemical Society 2012 Volume 134(Issue 19) pp:8241-8253
Publication Date(Web):May 7, 2012
DOI:10.1021/ja302190r
Bioinorganic canon states that active-site thiolate coordination promotes rapid electron transfer (ET) to and from type 1 copper proteins. In recent work, we have found that copper ET sites in proteins also can be constructed without thiolate ligation (called “type zero” sites). Here we report multifrequency electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), and nuclear magnetic resonance (NMR) spectroscopic data together with density functional theory (DFT) and spectroscopy-oriented configuration interaction (SORCI) calculations for type zero Pseudomonas aeruginosa azurin variants. Wild-type (type 1) and type zero copper centers experience virtually identical ligand fields. Moreover, O-donor covalency is enhanced in type zero centers relative that in the C112D (type 2) protein. At the same time, N-donor covalency is reduced in a similar fashion to type 1 centers. QM/MM and SORCI calculations show that the electronic structures of type zero and type 2 are intimately linked to the orientation and coordination mode of the carboxylate ligand, which in turn is influenced by outer-sphere hydrogen bonding.
Co-reporter:Qixi Mi ; Yuan Ping ; Yan Li ; Bingfei Cao ; Bruce S. Brunschwig ; Peter G. Khalifah ; Giulia A. Galli ; Harry B. Gray ;Nathan S. Lewis
Journal of the American Chemical Society 2012 Volume 134(Issue 44) pp:18318-18324
Publication Date(Web):September 28, 2012
DOI:10.1021/ja3067622
We describe stable intercalation compounds of the composition xN2·WO3 (x = 0.034–0.039), formed by trapping N2 in WO3. The incorporation of N2 significantly reduced the absorption threshold of WO3; notably, 0.039N2·WO3 anodes exhibited photocurrent under illumination at wavelengths ≤640 nm with a faradaic efficiency for O2 evolution in 1.0 M HClO4(aq) of nearly unity. Spectroscopic and computational results indicated that deformation of the WO3 host lattice, as well as weak electronic interactions between trapped N2 and the WO3 matrix, contributed to the observed red shift in optical absorption. Noble-gas-intercalated WO3 materials similar to xN2·WO3 are predicted to function as photoanodes that are responsive to visible light.
Co-reporter:Jeffrey J. Warren, Maraia E. Ener, Antonín Vlček Jr., Jay R. Winkler, Harry B. Gray
Coordination Chemistry Reviews 2012 Volume 256(21–22) pp:2478-2487
Publication Date(Web):November 2012
DOI:10.1016/j.ccr.2012.03.032
Biological redox machines require efficient transfer of electrons and holes for function. Reactions involving multiple tunneling steps, termed “hopping,” often promote charge separation within and between proteins that is essential for energy storage and conversion. Here we show how semiclassical electron transfer theory can be extended to include hopping reactions: graphical representations (called hopping maps) of the dependence of calculated two-step reaction rate constants on driving force are employed to account for flow in a rhenium-labeled azurin mutant as well as in two structurally characterized redox enzymes, DNA photolyase and MauG. Analysis of the 35 Å radical propagation in ribonucleotide reductases using hopping maps shows that all tyrosines and tryptophans on the radical pathway likely are involved in function. We suggest that hopping maps can facilitate the design and construction of artificial photosynthetic systems for the production of fuels and other chemicals.Highlights► Semiclassical electron transfer theory is extended to multistep tunneling. ► “Hopping maps” show calculated two-step ET rate constants vs. driving force. ► ET reactions in characterized proteins are interpreted using hopping maps.
Co-reporter:Jae Youn Hwang, David J. Lubow, David Chu, Jessica Sims, Felix Alonso-Valenteen, Harry B. Gray, Zeev Gross, Daniel L. Farkas, Lali K. Medina-Kauwe
Journal of Controlled Release 2012 Volume 163(Issue 3) pp:368-373
Publication Date(Web):10 November 2012
DOI:10.1016/j.jconrel.2012.09.015
The tumor-targeted corrole particle, HerGa, displays preferential toxicity to tumors in vivo and can be tracked via fluorescence for simultaneous detection, imaging, and treatment. We have recently uncovered an additional feature of HerGa in that its cytotoxicity is enhanced by light irradiation. In the present study, we have elucidated the cellular mechanisms for HerGa photoexcitation-mediated cell damage using fluorescence optical imaging. In particular, we found that light irradiation of HerGa produces singlet oxygen, causing mitochondrial damage and cytochrome c release, thus promoting apoptotic cell death. An understanding of the mechanisms of cell death induced by HerGa, particularly under conditions of light-mediated excitation, may direct future efforts in further customizing this nanoparticle for additional therapeutic applications and enhanced potency.
Co-reporter:Bryan M. Hunter, Randy M. Villahermosa, Christopher L. Exstrom, Michael G. Hill, Kent R. Mann, and Harry B. Gray
Inorganic Chemistry 2012 Volume 51(Issue 12) pp:6898-6905
Publication Date(Web):May 23, 2012
DOI:10.1021/ic300716q
Isomers of Ir2(dimen)42+ (dimen = 1,8-diisocyanomenthane) exhibit different Ir–Ir bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter Ir–Ir distance is favored at room temperature [K = ∼8; ΔH° = −0.8 kcal/mol; ΔS° = 1.44 cal mol–1 K–1]. We report calculations that shed light on M2(dimen)42+ (M = Rh, Ir) structural differences: (1) metal–metal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A2u) distortion promotes twisting of the ligand backbone at short metal–metal separations. Calculated potential-energy surfaces reveal a double minimum for Ir2(dimen)42+ (∼4.1 Å Ir–Ir with 0° twist angle and ∼3.6 Å Ir–Ir with ±12° twist angle) but not for the rhodium analogue (∼4.5 Å Rh–Rh with no twisting). Because both the ligand strain and A2u distortional energy are virtually identical for the two complexes, the strength of the metal–metal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the Ir–Ir coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir2(dimen)42+ (Re,Ir–Ir = 2.87 Å; FIr–Ir = 0.99 mdyn Å–1); (2) a single-minimum, anharmonic surface for the ground state of Rh2(dimen)42+ (Re,Rh–Rh = 3.23 Å; FRh–Rh = 0.09 mdyn Å–1); (3) a double-minimum (along the Ir–Ir coordinate) surface for the ground state of Ir2(dimen)42+ (Re,Ir–Ir = 3.23 Å; FIr–Ir = 0.16 mdyn Å–1).
Co-reporter:Michael J. Rose ; Jay R. Winkler
Inorganic Chemistry 2012 Volume 51(Issue 4) pp:1980-1982
Publication Date(Web):February 9, 2012
DOI:10.1021/ic202253v
We report the syntheses, X-ray structures, and reductive electrochemistry of the FeII complexes [(dmgBF2)2Fe(MeCN)2] (1; dmg = dimethylglyoxime, MeCN = acetonitrile) and [(dmgBF2)Fe(tBuiNC)2] (2; tBuiNC = tert-butylisocyanide). The reaction of 1 with Na/Hg amalgam led to isolation and the X-ray structure of [(dmgBF2)2Fe(glyIm)] (3; glyIm = glyimine), wherein the (dmgBF2)2 macrocyclic frame is bent to accommodate the binding of a bidentate apical ligand. We also report the X-ray structure of a rare mixed-valence Fe4 cluster with supporting dmg-type ligands. In the structure of [(dmg2BF2)3Fe3(1/2dmg)3Fe(O)6] (4), the (dmgBF2)2 macrocycle has been cleaved, eliminating BF2 groups. Density functional theory calculations and electron paramagnetic resonance data are in accordance with a central FeIII ion surrounded by three formally FeIIdmg2BF2 units.
Co-reporter:Michael J. Rose, Donatela E. Bellone, Angel J. Di Bilio and Harry B. Gray
Dalton Transactions 2012 vol. 41(Issue 38) pp:11788-11797
Publication Date(Web):20 Aug 2012
DOI:10.1039/C2DT31229H
Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI2 spontaneously generates a one-electron reduced complex, [(PhP3)CoI(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co–I bond distance of ∼2.52 Å. CoII/I redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M−1 cm−1) and a d–d feature at 850 nm (ε = 840 M−1 cm−1). Two more d–d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm−1 (ε = 430 M−1 cm−1). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20–300 K) give μeff = 2.99(6) μB, consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm−1. DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0–15000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g ≈ 4 that arises from a (Δm = 2) transition within the MS = <+1,−1> manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D| ≈ 8 cm−1) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin CoI in trigonal ligation.
Co-reporter:Punnajit Lim, Atif Mahammed, Zoya Okun, Irena Saltsman, Zeev Gross, Harry B. Gray, and John Termini
Chemical Research in Toxicology 2012 Volume 25(Issue 2) pp:400
Publication Date(Web):December 20, 2011
DOI:10.1021/tx200452w
A gallium(III)-substituted amphiphilic corrole noncovalently associated with a targeting protein was previously found by us to confer promising cytotoxic and antitumor activities against a breast cancer cell line and a mouse xenograft breast cancer model. To further explore potential anticancer applications, the cytostatic and cytotoxic properties of six nontargeted metallocorroles were evaluated against seven human cancer cell lines. Results indicated that toxicity toward human cancer cells depended on the metal ion as well as corrole functional group substitution. Ga(III)-substituted metallocorrole 1-Ga inhibited proliferation of breast (MDA-MB-231), melanoma (SK-MEL-28), and ovarian (OVCAR-3) cancer cells primarily by arrest of DNA replication, whereas 2-Mn displayed both cytostatic and cytotoxic properties. Confocal microscopy revealed extensive uptake of 1-Ga into the cytoplasm of melanoma and ovarian cancer cells, while prostate cancer cells (DU-145) displayed extensive nuclear localization. The localization of 1-Ga to the nucleus in DU-145 cells was exploited to achieve a 3-fold enhancement in the IC50 of doxorubicin upon coadministration. Time–course studies showed that over 90% of melanoma cells incubated with 30 μM 1-Ga internalized metallocorrole after 15 min. Cellular uptake of 1-Ga and 1-Al was fastest and most efficient in melanoma, followed by prostate and ovarian cancer cells. Cell cycle analyses revealed that bis-sulfonated corroles containing Al(III), Ga(III), and Mn(III) induced late M phase arrest in several different cancer cell lines, a feature that could be developed for potential therapeutic benefit.
Co-reporter:Jeffrey J. Warren, Kyle M. Lancaster, John H. Richards, Harry B. Gray
Journal of Inorganic Biochemistry 2012 Volume 115() pp:119-126
Publication Date(Web):October 2012
DOI:10.1016/j.jinorgbio.2012.05.002
Blue copper proteins (BCPs) comprise classic cases of Nature's profound control over the electronic structures and chemical reactivity of transition metal ions. Early studies of BCPs focused on their inner coordination spheres, that is, residues that directly coordinate Cu. Equally important are the electronic and geometric perturbations to these ligands provided by the outer coordination sphere. In this tribute to Hans Freeman, we review investigations that have advanced the understanding of how inner-sphere and outer-sphere coordination affects biological Cu properties.In this tribute to Hans Freeman, we review investigations of how inner-sphere and outer-sphere coordination affects blue Cu properties, such as in Pseudomonas aeruginosa azurin (shown).Highlights► We present a focused review of metal coordination in blue copper proteins. ► Inner- and outer-sphere interactions tune properties of copper sites. ► Electronic structure and chemical reactivity of blue copper centers are discussed.
Co-reporter:Smaranda C. Marinescu;Jay R. Winkler
PNAS 2012 Volume 109 (Issue 38 ) pp:
Publication Date(Web):2012-09-18
DOI:10.1073/pnas.1213442109
Several cobalt complexes catalyze the evolution of hydrogen from acidic solutions, both homogeneously and at electrodes. The
detailed molecular mechanisms of these transformations remain unresolved, largely owing to the fact that key reactive intermediates
have eluded detection. One method of stabilizing reactive intermediates involves minimizing the overall reaction free-energy
change. Here, we report a new cobalt(I) complex that reacts with tosylic acid to evolve hydrogen with a driving force of just
30 meV/Co. Protonation of CoI produces a transient CoIII-H complex that was characterized by nuclear magnetic resonance spectroscopy. The CoIII-H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics
suggests that CoIII-H produces hydrogen by two competing pathways: a slower homolytic route involving two CoIII-H species and a dominant heterolytic channel in which a highly reactive CoII-H transient is generated by CoI reduction of CoIII-H.
Co-reporter:Carolyn N. Valdez;Bruce S. Brunschwig;Jay R. Winkler;Jillian L. Dempsey
PNAS 2012 Volume 109 (Issue 39 ) pp:
Publication Date(Web):2012-09-25
DOI:10.1073/pnas.1118329109
A dicobaloxime in which monomeric Co(III) units are linked by an octamethylene bis(glyoxime) catalyzes the reduction of protons
from p-toluenesulfonic acid as evidenced by electrocatalytic waves at -0.4 V vs. the saturated calomel electrode (SCE) in acetonitrile
solutions. Rates of hydrogen evolution were determined from catalytic current peak heights (kapp = 1100 ± 70 M-1 s-1). Electrochemical experiments reveal no significant enhancement in the rate of H2 evolution from that of a monomeric analogue: The experimental rate law is first order in catalyst and acid consistent with
previous findings for similar mononuclear cobaloximes. Our work suggests that H2 evolution likely occurs by protonation of reductively generated CoIIH rather than homolysis of two CoIIIH units.
Co-reporter:Leif Hammarström;Jay R. Winkler;Stenbjörn Styring
Science 2011 Vol 333(6040) pp:288
Publication Date(Web):15 Jul 2011
DOI:10.1126/science.333.6040.288-a
Co-reporter:Harry Gray;Jay Labinger
Science 2011 Vol 331(6023) pp:1365
Publication Date(Web):18 Mar 2011
DOI:10.1126/science.1204757
Summary
Throughout 2011, nations are celebrating the International Year of Chemistry. This worldwide recognition of the importance of chemistry is somewhat unusual. It is true that chemistry has been called “the central science,” not only by chemists but even in Wikipedia (of course, that article may have been written by a chemist), perhaps as a metaphor for its role in connections between the fundamental concepts of physics and the practical problems of biology. Furthermore, it is the discipline that will continue to drive the discoveries that tackle today's most vexing challenges: solving the energy problem, developing and producing new treatments for diseases, devising advanced materials for a host of applications, and many more. It seems most appropriate to talk of chemistry as overlapping, rather than bridging, other disciplines. And most certainly it is time to celebrate the creative future of chemistry, which lies in myriad directions.
Co-reporter:James R. McKone, Emily L. Warren, Matthew J. Bierman, Shannon W. Boettcher, Bruce S. Brunschwig, Nathan S. Lewis and Harry B. Gray
Energy & Environmental Science 2011 vol. 4(Issue 9) pp:3573-3583
Publication Date(Web):01 Aug 2011
DOI:10.1039/C1EE01488A
The dark electrocatalytic and light photocathodic hydrogen evolution properties of Ni, Ni–Mo alloys, and Pt on Si electrodes have been measured, to assess the viability of earth-abundant electrocatalysts for integrated, semiconductor coupled fuel formation. In the dark, the activities of these catalysts deposited on degenerately doped p+-Si electrodes increased in the order Ni < Ni–Mo ≤ Pt. Ni–Mo deposited on degenerately doped Si microwires exhibited activity that was very similar to that of Pt deposited by metal evaporation on planar Si electrodes. Under 100 mW cm−2 of Air Mass 1.5 solar simulation, the energy conversion efficiencies of p-type Si/catalyst photoelectrodes ranged from 0.2–1%, and increased in the order Ni ≈ Ni–Mo < Pt, due to somewhat lower photovoltages and photocurrents for p-Si/Ni–Mo relative to p-Si/Ni and p-Si/Pt photoelectrodes. Deposition of the catalysts onto microwire arrays resulted in higher apparent catalytic activities and similar photoelectrode efficiencies than were observed on planar p-Si photocathodes, despite lower light absorption by p-Si in the microwire structures.
Co-reporter:Kyle M. Lancaster ; Ole Farver ; Scot Wherland ; Edward J. Crane ; III; John H. Richards ; Israel Pecht
Journal of the American Chemical Society 2011 Volume 133(Issue 13) pp:4865-4873
Publication Date(Web):March 15, 2011
DOI:10.1021/ja1093919
Type zero copper is a hard-ligand analogue of the classical type 1 or blue site in copper proteins that function as electron transfer (ET) agents in photosynthesis and other biological processes. The EPR spectroscopic features of type zero CuII are very similar to those of blue copper, although lacking the deep blue color, due to the absence of thiolate ligation. We have measured the rates of intramolecular ET from the pulse radiolytically generated C3−C26 disulfide radical anion to the CuII in both type zero C112D/M121L and type 2 C112D Pseudomonas aeruginosa azurins in pH 7.0 aqueous solutions between 8 and 45 °C. We also have obtained rate/temperature (10−30 °C) profiles for ET reactions between these mutants and the wild-type azurin. Analysis of the rates and activation parameters for both intramolecular and intermolecular ET reactions indicates that the type zero copper reorganization energy falls in a range (0.9−1.1 eV) slightly above that for type 1 (0.7−0.8 eV), but substantially smaller than that for type 2 (>2 eV), consistent with XAS and EXAFS data that reveal minimal type zero site reorientation during redox cycling.
Co-reporter:Jenya Vestfrid ; Mark Botoshansky ; Joshua H. Palmer ; Alec C. Durrell ; Harry B. Gray ;Zeev Gross
Journal of the American Chemical Society 2011 Volume 133(Issue 33) pp:12899-12901
Publication Date(Web):July 27, 2011
DOI:10.1021/ja202692b
The first reported iodination of a corrole leads to selective functionalization of the four C–H bonds on one pole of the macrocycle. An aluminum(III) complex of the tetraiodinated corrole, which exhibits red fluorescence, possesses a long-lived triplet excited state.
Co-reporter:Charlotte A. Whited ; Jeffrey J. Warren ; Katherine D. Lavoie ; Emily E. Weinert ; Theodor Agapie ; Jay R. Winkler
Journal of the American Chemical Society 2011 Volume 134(Issue 1) pp:27-30
Publication Date(Web):December 7, 2011
DOI:10.1021/ja2069533
We have investigated the kinetics of NO escape from Geobacillus stearothermophilus nitric oxide synthase (gsNOS). Previous work indicated that NO release was gated at position 223 in mammalian enzymes; our kinetics experiments include mutants at that position along with measurements on the wild type enzyme. Employing stopped-flow UV–vis methods, reactions were triggered by mixing a reduced enzyme/N-hydroxy-l-arginine complex with an aerated buffer solution. NO release kinetics were obtained for wt NOS and three mutants (H134S, I223V, H134S/I223V). We have confirmed that wt gsNOS has the lowest NO release rate of known NOS enzymes, whether bacterial or mammalian. We also have found that steric clashes at positions 223 and 134 hinder NO escape, as judged by enhanced rates in the single mutants. The empirical rate of NO release from the gsNOS double mutant (H134/I223V) is nearly as rapid as that of the fastest mammalian enzymes, demonstrating that both positions 223 and 134 function as gates for escape of the product diatomic molecule.
Co-reporter:Bryan D. Stubbert ; Jonas C. Peters
Journal of the American Chemical Society 2011 Volume 133(Issue 45) pp:18070-18073
Publication Date(Web):October 24, 2011
DOI:10.1021/ja2078015
A cobalt bis(iminopyridine) complex is a highly active electrocatalyst for water reduction, with an estimated apparent second order rate constant kapp ≤ 107 M–1s–1 over a range of buffer/salt concentrations. Scan rate dependence data are consistent with freely diffusing electroactive species over pH 4–9 at room temperature for each of two catalytic reduction events, one of which is believed to be ligand based. Faradaic H2 yields up to 87 ± 10% measured in constant potential electrolyses (−1.4 V vs SCE) confirm high reactivity and high fidelity in a catalyst supported by the noninnocent bis(iminopyridine) ligand. A mechanism involving initial reduction of Co2+ and subsequent protonation is proposed.
Co-reporter:Sijia S. Dong ; Robert J. Nielsen ; Joshua H. Palmer ; Harry B. Gray ; Zeev Gross ; Siddharth Dasgupta ;William A. Goddard ; III
Inorganic Chemistry 2011 Volume 50(Issue 3) pp:764-770
Publication Date(Web):January 7, 2011
DOI:10.1021/ic1005902
The electronic structures of metallocorroles (tpfc)M(NH3)2 and (tfc)M(NH3)2 (tpfc is the trianion of 5,10,15-(tris)pentafluorophenylcorrole, tfc is the trianion of 5,10,15-trifluorocorrole, and M = Co, Rh, Ir) have been computed using first principles quantum mechanics [B3LYP flavor of Density Functional Theory (DFT) with Poisson−Boltzmann continuum solvation]. The geometry was optimized for both the neutral systems (formal MIII oxidation state) and the one-electron oxidized systems (formally MIV). As expected, the MIII systems have a closed shell d6 configuration; for all three metals, the one-electron oxidation was calculated to occur from a ligand-based orbital (highest occupied molecular orbital (HOMO) of B1 symmetry). The ground state of the formal MIV system has MIII-Cπ character, indicating that the metal remains d6, with the hole in the corrole π system. As a result the calculated MIV/III reduction potentials are quite similar (0.64, 0.67, and 0.56 V vs SCE for M = Ir, Rh and Co, respectively), whereas the differences would have been large for purely metal-based oxidations. Vertically excited states with substantial metal character are well separated from the ground state in one-electron-oxidized cobalt (0.27 eV) and rhodium (0.24 eV) corroles, but become closer in energy in the iridium (0.15 eV) analogues. The exact splittings depend on the chosen functional and basis set combination and vary by ∼0.1 eV.
Co-reporter:Richard Eisenberg and Harry B. Gray
Inorganic Chemistry 2011 Volume 50(Issue 20) pp:9741-9751
Publication Date(Web):September 13, 2011
DOI:10.1021/ic2011748
Noninnocence in inorganic chemistry traces its roots back half a century to work that was done on metal complexes containing unsaturated dithiolate ligands. In a flurry of activity in the early 1960s by three different research groups, homoleptic bis and tris complexes of these ligands, which came to be known as dithiolenes, were synthesized, and their structural, electrochemical, spectroscopic, and magnetic properties were investigated. The complexes were notable for facile one-electron transfers and intense colors in solution, and conventional oxidation-state descriptions could not account for their electronic structures. The bis complexes were, in general, found to be square-planar, including the first examples of this geometry for paramagnetic complexes and different formal dn configurations. Several of the neutral and monoanionic tris complexes were found to have trigonal-prismatic coordination, the first time that this geometry had been observed in molecular metal complexes. Electronic structural calculations employing extended Hückel and other semiempirical computational methods revealed extensive ligand–metal mixing in the frontier orbitals of these systems, including the observation of structures in which filled metal-based orbitals were more stable than ligand-based orbitals of the same type, suggesting that the one-electron changes upon oxidation or reduction were occurring on the ligand rather than on the metal center. A summary of this early work is followed with a brief section on the current interpretations of these systems based on more advanced spectroscopic and computational methods. The take home message is that the early work did indeed provide a solid foundation for what was to follow in investigations of metal complexes containing redox-active ligands.
Co-reporter:Jillian L. Dempsey, Jay R. Winkler and Harry B. Gray
Dalton Transactions 2011 vol. 40(Issue 40) pp:10633-10636
Publication Date(Web):02 Sep 2011
DOI:10.1039/C1DT11138H
Powerful reductants [OsII(NH3)5L]2+ (L = OH2, CH3CN) can be generated upon ultraviolet excitation of relatively inert [OsII(NH3)5(N2)]2+ in aqueous and acetonitrile solutions. Reactions of photogenerated Os(II) complexes with methyl viologen to form methyl viologen radical cation and [OsIII(NH3)5L]3+ were monitored by transient absorption spectroscopy. Rate constants range from 4.9 × 104 M−1 s−1 in acetonitrile solution to 3.2 × 107 (pH 3) and 2.5 × 108 M−1 s−1 (pH 12) in aqueous media. Photogeneration of five-coordinate Os(II) complexes opens the way for mechanistic investigations of activation/reduction of CO2 and other relatively inert molecules.
Co-reporter:Lucie Sokolová, Heather Williamson, Jan Sýkora, Martin Hof, Harry B. Gray, Bernd Brutschy, and Antonín Vlček Jr.
The Journal of Physical Chemistry B 2011 Volume 115(Issue 16) pp:4790-4800
Publication Date(Web):March 31, 2011
DOI:10.1021/jp110460k
We have employed laser-induced liquid bead ion desorption mass spectroscopy (LILBID MS) to study the solution behavior of Pseudomonas aeruginosa azurin as well as two mutants and corresponding Re-labeled derivatives containing a Re(CO)3(4,7-dimethyl-1,10-phenanthroline)+ chromophore appended to a surface histidine. LILBID spectra show broad oligomer distributions whose particular patterns depend on the solution composition (pure H2O, 20−30 mM NaCl, 20 and 50 mM NaPi or NH4Pi at pH = 7). The distribution maximum shifts to smaller oligomers upon decreasing the azurin concentration and increasing the buffer concentration. Oligomerization is less extensive for native azurin than its mutants. The oligomerization propensities of unlabeled and Re-labeled proteins are generally comparable, and only Re126 shows some preference for the dimer that persists even in highly diluted solutions. Peak shifts to higher masses and broadening in 20−50 mM NaPi confirm strong azurin association with buffer ions and solvation. We have found that LILBID MS reveals the solution behavior of weakly bound nonspecific protein oligomers, clearly distinguishing individual components of the oligomer distribution. Independently, average data on oligomerization and the dependence on solution composition were obtained by time-resolved anisotropy of the Re-label photoluminescence that confirmed relatively long rotation correlation times, 6−30 ns, depending on Re−azurin and solution composition. Labeling proteins with Re-chromophores that have long-lived phosphorescence extends the time scale of anisotropy measurements to hundreds of nanoseconds, thereby opening the way for investigations of large oligomers with long rotation times.
Co-reporter:Dr. Ana María Blanco-Rodríguez;Dr. Angel J. DiBilio;Dr. Crystal Shih;Dr. Anna Katrine Museth;Dr. Ian P. Clark;Dr. Michael Towrie;Dr. Andrea Cannizzo;Dr. Jawahar Sudhamsu;Dr. Brian R. Crane;Dr. Jan Sýkora;Dr. Jay R. Winkler;Dr. Harry B. Gray;Dr. Stanislav Záli&x161;;Dr. Antonín Vl&x10d;ek Jr.
Chemistry - A European Journal 2011 Volume 17( Issue 19) pp:5350-5361
Publication Date(Web):
DOI:10.1002/chem.201002162
Abstract
The [ReI(CO)3(4,7-dimethyl-1,10-phenanthroline)(histidine-124)(tryptophan-122)] complex, denoted [ReI(dmp)(W122)], of Pseudomonas aeruginosa azurin behaves as a single photoactive unit that triggers very fast electron transfer (ET) from a distant (2 nm) CuI center in the protein. Analysis of time-resolved (ps–μs) IR spectroscopic and kinetics data collected on [ReI(dmp)(W122)AzM] (in which M=ZnII, CuII, CuI; Az=azurin) and position-122 tyrosine (Y), phenylalanine (F), and lysine (K) mutants, together with excited-state DFT/time-dependent (TD)DFT calculations and X-ray structural characterization, reveal the character, energetics, and dynamics of the relevant electronic states of the [ReI(dmp)(W122)] unit and a cascade of photoinduced ET and relaxation steps in the corresponding Re–azurins. Optical population of [ReI(imidazole-H124)(CO)3]dmp 1CT states (CT=charge transfer) is followed by around 110 fs intersystem crossing and about 600 ps structural relaxation to a 3CT state. The IR spectrum indicates a mixed ReI(CO)3,Admp/ππ*(dmp) character for aromatic amino acids A122 (A=W, Y, F) and ReI(CO)3dmp metal–ligand charge transfer (MLCT) for [ReI(dmp)(K122)AzCuII]. In a few ns, the 3CT state of [ReI(dmp)(W122)AzM] establishes an equilibrium with the [ReI(dmp.−)(W122.+)AzM] charge-separated state, 3CS, whereas the 3CT state of the other Y, F, and K122 proteins decays to the ground state. In addition to this main pathway, 3CS is populated by fs- and ps-W(indole)ReII ET from 1CT and the initially “hot” 3CT states, respectively. The 3CS state undergoes a tens-of-ns dmp.−W122.+ ET recombination leading to the ground state or, in the case of the CuI azurin, a competitively fast (≈30 ns over 1.12 nm) CuIW.+ ET, to give [ReI(dmp.−)(W122)AzCuII]. The overall photoinduced CuIRe(dmp) ET through [ReI(dmp)(W122)AzCuI] occurs over a 2 nm distance in <50 ns after excitation, with the intervening fast 3CT–3CS equilibrium being the principal accelerating factor. No reaction was observed for the three Y, F, and K122 analogues. Although the presence of [Re(dmp)(W122)AzCuII] oligomers in solution was documented by mass spectrometry and phosphorescence anisotropy, the kinetics data do not indicate any significant interference from the intermolecular ET steps. The ground-state dmp–indole π–π interaction together with well-matched W/W.+ and excited-state [ReII(CO)3(dmp.−)]/[ReI(CO)3(dmp.−)] potentials that result in very rapid electron interchange and 3CT–3CS energetic proximity, are the main factors responsible for the unique ET behavior of [ReI(dmp)(W122)]-containing azurins.
Co-reporter:Jillian L. Dempsey, Jay R. Winkler, and Harry B. Gray
Chemical Reviews 2010 Volume 110(Issue 12) pp:7024
Publication Date(Web):November 17, 2010
DOI:10.1021/cr100182b
Co-reporter:Jillian L. Dempsey ; Jay R. Winkler
Journal of the American Chemical Society 2010 Volume 132(Issue 47) pp:16774-16776
Publication Date(Web):November 10, 2010
DOI:10.1021/ja109351h
Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged CoI-diglyoxime complex, forming CoIIIH, was monitored via transient absorption. The second-order rate constant for CoIIIH formation is in the range (3.5−4.7) × 109 M−1 s−1, with proton transfer coupled to excited-state deactivation of the photoacid. CoIIIH is subsequently reduced by excess CoI-diglyoxime in solution to produce CoIIH (kred = 9.2 × 106 M−1 s−1), which is then protonated to yield CoII-diglyoxime and H2.
Co-reporter:Kyle M. Lancaster ; Stephen Sproules ; Joshua H. Palmer ; John H. Richards
Journal of the American Chemical Society 2010 Volume 132(Issue 41) pp:14590-14595
Publication Date(Web):September 29, 2010
DOI:10.1021/ja105731x
Redox and spectroscopic (electronic absorption, multifrequency electron paramagnetic resonance (EPR), and X-ray absorption) properties together with X-ray crystal structures are reported for the type 2 CuII C112D/M121E variant of Pseudomonas aeruginosa azurin. The results suggest that CuII is constrained from interaction with the proximal glutamate; this structural frustration implies a “rack” mechanism for the 290 mV (vs NHE) reduction potential measured at neutral pH. At high pH (∼9), hydrogen bonding in the outer coordination sphere is perturbed to allow axial glutamate ligation to CuII, with a decrease in potential to 119 mV. These results highlight the role played by outer-sphere interactions, and the structural constraints they impose, in determining the redox behavior of transition metal protein cofactors.
Co-reporter:Joshua H. Palmer ; Alec C. Durrell ; Zeev Gross ; Jay R. Winkler
Journal of the American Chemical Society 2010 Volume 132(Issue 27) pp:9230-9231
Publication Date(Web):June 22, 2010
DOI:10.1021/ja101647t
The photophysical properties of Ir(III) corroles differ from those of phosphorescent porphyrin complexes, cyclometalated and polyimine Ir(III) compounds, and other luminescent metallocorroles. Ir(III) corrole phosphorescence is observed at ambient temperature at wavelengths much longer (>800 nm) than those of most Ir(III) phosphors. The solvatochromic behavior of Ir(III)-corrole Soret and Q absorption bands suggests that the lowest singlet excited states (S2 and S1) are substantially more polar than the ground state.
Co-reporter:Matthew R. Hartings, Igor V. Kurnikov, Alexander R. Dunn, Jay R. Winkler, Harry B. Gray, Mark A. Ratner
Coordination Chemistry Reviews 2010 Volume 254(3–4) pp:248-253
Publication Date(Web):February 2010
DOI:10.1016/j.ccr.2009.08.008
We report a quantitative theoretical analysis of long-range electron transfer through sensitizer wires bound in the active-site channel of cytochrome P450cam. Each sensitizer wire consists of a substrate group with high binding affinity for the enzyme active site connected to a ruthenium-diimine through a bridging aliphatic or aromatic chain. Experiments have revealed a dramatic dependence of electron transfer rates on the chemical composition of both the bridging group and the substrate. Using combined molecular dynamics simulations and electronic coupling calculations, we show that electron tunneling through perfluorinated aromatic bridges is promoted by enhanced superexchange coupling through virtual reduced states. In contrast, electron flow through aliphatic bridges occurs by hole-mediated superexchange. We have found that a small number of wire conformations with strong donor–acceptor couplings can account for the observed electron tunneling rates for sensitizer wires terminated with either ethylbenzene or adamantane. In these instances, the rate is dependent not only on electronic coupling of the donor and acceptor but also on the nuclear motion of the sensitizer wire, necessitating the calculation of average rates over the course of a molecular dynamics simulation. These calculations along with related recent findings have made it possible to analyze the results of many other sensitizer-wire experiments that in turn point to new directions in our attempts to observe reactive intermediates in the catalytic cycles of P450 and other heme enzymes.
Co-reporter:Kyle M. Lancaster, James B. Gerken, Alec C. Durrell, Joshua H. Palmer, Harry B. Gray
Coordination Chemistry Reviews 2010 Volume 254(15–16) pp:1803-1811
Publication Date(Web):August 2010
DOI:10.1016/j.ccr.2010.04.005
The properties of RuII complexes involving the imidazole moiety are discussed. Complexes [Ru(bpy)2(L)]2+ [bpy = 2,2′-bipyridine, L = 2-(2′-pyridyl)imidazole (2-pimH) and 4-(2′-pyridyl)imidazole (4-pimH)] have been synthesized and fully characterized. Reduction potentials are 0.76 V vs. Fc+/Fc0 for both complexes in acetonitrile solution, and the deprotonated complexes undergo irreversible electrochemical oxidation at 0.38 V vs. Fc+/Fc0. Density functional theory (DFT) calculations suggest that oxidation of the protonated complexes is primarily metal-based and that of the deprotonated complexes is ligand-centered. The pKa of the 4-pimH complex was found to be 9.7 ± 0.2; the pKa of the 2-pimH complex is 7.9 ± 0.2. Luminescence lifetimes (L = 4-pimH, 277 ns; 2-pimH, 224 ns; 4pim−, 40 ns; 2pim−, 34 ns in 5% methanol/water solution) combined with quantum yield data and acid–base behavior suggest that the non-coordinated imidazole nitrogen tunes deactivation pathways.
Co-reporter:John E. Bercaw ; Alec C. Durrell ; Harry B. Gray ; Jennifer C. Green ; Nilay Hazari ; Jay A. Labinger ;Jay R. Winkler
Inorganic Chemistry 2010 Volume 49(Issue 4) pp:1801-1810
Publication Date(Web):January 21, 2010
DOI:10.1021/ic902189g
The PdII dimers [(2-phenylpyridine)Pd(μ-X)]2 and [(2-p-tolylpyridine)Pd(μ-X)]2 (X = OAc or TFA) do not exhibit the expected planar geometry (of approximate D2h symmetry) but instead resemble an open “clamshell” in which the acetate ligands are perpendicular to the plane containing the Pd atoms and 2-arylpyridine ligands, with the Pd atoms brought quite close to one another (approximate distance 2.85 Å). The molecules adopt this unusual geometry in part because of a d8−d8 bonding interaction between the two Pd centers. The Pd−Pd dimers exhibit two successive one-electron oxidations: PdII−PdII to PdII−PdIII to PdIII−PdIII. Photophysical measurements reveal clear differences in the UV−visible and low-temperature fluorescence spectra between the clamshell dimers and related planar dimeric [(2-phenylpyridine)Pd(μ-Cl)]2 and monomeric [(2-phenylpyridine)Pd(en)][Cl] (en = ethylenediamine) complexes that do not have any close Pd−Pd contacts. Density functional theory and atoms in molecules analyses confirm the presence of a Pd−Pd bonding interaction in [(2-phenylpyridine)Pd(μ-X)]2 and show that the highest occupied molecular orbital is a dz2 σ* Pd−Pd antibonding orbital, while the lowest unoccupied molecular orbital and close-lying empty orbitals are mainly located on the 2-phenylpyridine rings. Computational analyses of other PdII−PdII dimers that have short Pd−Pd distances yield an orbital ordering similar to that of [(2-phenylpyridine)Pd(μ-X)]2, but quite different from that found for d8−d8 dimers of Rh, Ir, and Pt. This difference in orbital ordering arises because of the unusually large energy gap between the 4d and 5p orbitals in Pd and may explain why Pd d8−d8 dimers do not exhibit the distinctive photophysical properties of related Rh, Ir, and Pt species.
Co-reporter:Maraia E. Ener;Young-Tae Lee;Jay R. Winkler;Lionel Cheruzel
PNAS 2010 Volume 107 (Issue 44 ) pp:18783-18786
Publication Date(Web):2010-11-02
DOI:10.1073/pnas.1012381107
High-valent iron-oxo species are thought to be intermediates in the catalytic cycles of oxygenases and peroxidases. An attractive
route to these iron-oxo intermediates involves laser flash-quench oxidation of ferric hemes, as demonstrated by our work on
the ferryl (compound II) and ferryl porphyrin radical cation (compound I) intermediates of horseradish peroxidase. Extension
of this work to include cytochrome P450-BM3 (CYP102A1) has required covalent attachment of a RuII photosensitizer to a nonnative cysteine near the heme (), in order to promote electron transfer from the FeIII porphyrin to photogenerated RuIII. The conjugate was structurally characterized by X-ray crystallography (2.4 Å resolution; Ru-Fe distance, 24 Å). Flash-quench
oxidation of the ferric-aquo heme produces an FeIV-hydroxide species (compound II) within 2 ms. Difference spectra for three singly oxidized P450-BM3 intermediates were obtained
from kinetics modeling of the transient absorption data in combination with generalized singular value decomposition analysis
and multiexponential fitting.
Co-reporter:Jillian L. Dempsey, Bruce S. Brunschwig, Jay R. Winkler and Harry B. Gray
Accounts of Chemical Research 2009 Volume 42(Issue 12) pp:1995
Publication Date(Web):November 23, 2009
DOI:10.1021/ar900253e
Natural photosynthesis uses sunlight to drive the conversion of energy-poor molecules (H2O, CO2) to energy-rich ones (O2, (CH2O)n). Scientists are working hard to develop efficient artificial photosynthetic systems toward the “Holy Grail” of solar-driven water splitting. High on the list of challenges is the discovery of molecules that efficiently catalyze the reduction of protons to H2. In this Account, we report on one promising class of molecules: cobalt complexes with diglyoxime ligands (cobaloximes). Chemical, electrochemical, and photochemical methods all have been utilized to explore proton reduction catalysis by cobaloxime complexes. Reduction of a CoII-diglyoxime generates a CoI species that reacts with a proton source to produce a CoIII-hydride. Then, in a homolytic pathway, two CoIII-hydrides react in a bimolecular step to eliminate H2. Alternatively, in a heterolytic pathway, protonation of the CoIII-hydride produces H2 and CoIII. A thermodynamic analysis of H2 evolution pathways sheds new light on the barriers and driving forces of the elementary reaction steps involved in proton reduction by CoI-diglyoximes. In combination with experimental results, this analysis shows that the barriers to H2 evolution along the heterolytic pathway are, in most cases, substantially greater than those of the homolytic route. In particular, a formidable barrier is associated with CoIII-diglyoxime formation along the heterolytic pathway. Our investigations of cobaloxime-catalyzed H2 evolution, coupled with the thermodynamic preference for a homolytic route, suggest that the rate-limiting step is associated with formation of the hydride. An efficient water splitting device may require the tethering of catalysts to an electrode surface in a fashion that does not inhibit association of CoIII-hydrides.
Co-reporter:Jillian L. Dempsey ; Jay R. Winkler
Journal of the American Chemical Society 2009 Volume 132(Issue 3) pp:1060-1065
Publication Date(Web):December 31, 2009
DOI:10.1021/ja9080259
Co−diglyoxime complexes catalyze H2 evolution from protic solutions at modest overpotentials. Upon reduction to CoI, a CoIII-hydride is formed by reaction with a proton donor. Two pathways for H2 production are analyzed: one is a heterolytic route involving protonation of the hydride to release H2 and generate CoIII; the other is a homoytic pathway requiring association of two CoIII-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash−quench kinetics experiments (CoIII−CoII self-exchange k = 9.5 × 10−8 − 2.6 × 10−5 M−1 s−1; λ = 3.9 (±0.3) eV: CoII−CoI self-exchange k = 1.2 (±0.5) × 105 M−1 s−1; λ = 1.4 (±0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (CoIIIH + CoIIIH → 2 CoII + H2) is favored over the route that goes through a CoIII intermediate (CoIIIH + H+ → CoIII + H2).
Co-reporter:Ana María Blanco-Rodríguez ; Michael Busby ; Kate Ronayne ; Michael Towrie ; Cristian Grădinaru ; Jawahar Sudhamsu ; Jan Sýkora ; Martin Hof ; Stanislav Záliš ; Angel J. Di Bilio ; Brian R. Crane ; Harry B. Gray ;Antonín Vlček ; Jr.
Journal of the American Chemical Society 2009 Volume 131(Issue 33) pp:11788-11800
Publication Date(Web):July 29, 2009
DOI:10.1021/ja902744s
Photoinduced relaxation processes of five structurally characterized Pseudomonas aeruginosa ReI(CO)3(α-diimine)(HisX) (X = 83, 107, 109, 124, 126)CuII azurins have been investigated by time-resolved (ps−ns) IR spectroscopy and emission spectroscopy. Crystal structures reveal the presence of Re-azurin dimers and trimers that in two cases (X = 107, 124) involve van der Waals interactions between interdigitated diimine aromatic rings. Time-dependent emission anisotropy measurements confirm that the proteins aggregate in mM solutions (D2O, KPi buffer, pD = 7.1). Excited-state DFT calculations show that extensive charge redistribution in the ReI(CO)3 → diimine 3MLCT state occurs: excitation of this 3MLCT state triggers several relaxation processes in Re-azurins whose kinetics strongly depend on the location of the metallolabel on the protein surface. Relaxation is manifested by dynamic blue shifts of excited-state ν(CO) IR bands that occur with triexponential kinetics: intramolecular vibrational redistribution together with vibrational and solvent relaxation give rise to subps, ∼2, and 8−20 ps components, while the ∼102 ps kinetics are attributed to displacement (reorientation) of the ReI(CO)3(phen)(im) unit relative to the peptide chain, which optimizes Coulombic interactions of the ReI excited-state electron density with solvated peptide groups. Evidence also suggests that additional segmental movements of Re-bearing β-strands occur without perturbing the reaction field or interactions with the peptide. Our work demonstrates that time-resolved IR spectroscopy and emission anisotropy of ReI carbonyl−diimine complexes are powerful probes of molecular dynamics at or around the surfaces of proteins and protein−protein interfacial regions.
Co-reporter:Morgan L. Cable ; James P. Kirby ; Dana J. Levine ; Micah J. Manary ; Harry B. Gray ;Adrian Ponce
Journal of the American Chemical Society 2009 Volume 131(Issue 27) pp:9562-9570
Publication Date(Web):June 19, 2009
DOI:10.1021/ja902291v
The detection of bacterial spores via dipicolinate-triggered lanthanide luminescence has been improved in terms of detection limit, stability, and susceptibility to interferents by use of lanthanide−macrocycle binary complexes. Specifically, we compared the effectiveness of Sm, Eu, Tb, and Dy complexes with the macrocycle 1,4,7,10-tetraazacyclododecane-1,7-diacetate (DO2A) to the corresponding lanthanide aquo ions. The Ln(DO2A)+ binary complexes bind dipicolinic acid (DPA), a major constituent of bacterial spores, with greater affinity and demonstrate significant improvement in bacterial spore detection. Of the four luminescent lanthanides studied, the terbium complex exhibits the greatest dipicolinate binding affinity (100-fold greater than Tb3+ alone, and 10-fold greater than other Ln(DO2A)+ complexes) and highest quantum yield. Moreover, the inclusion of DO2A extends the pH range over which Tb−DPA coordination is stable, reduces the interference of calcium ions nearly 5-fold, and mitigates phosphate interference 1000-fold compared to free terbium alone. In addition, detection of Bacillus atrophaeus bacterial spores was improved by the use of Tb(DO2A)+, yielding a 3-fold increase in the signal-to-noise ratio over Tb3+. Out of the eight cases investigated, the Tb(DO2A)+ binary complex is best for the detection of bacterial spores.
Co-reporter:Joshua H. Palmer, Atif Mahammed, Kyle M. Lancaster, Zeev Gross and Harry B. Gray
Inorganic Chemistry 2009 Volume 48(Issue 19) pp:9308-9315
Publication Date(Web):September 8, 2009
DOI:10.1021/ic901164r
Group 9 metallocorroles 1-M(PPh3) and 1-M(py)2 [M = Co(III), Rh(III), Ir(III); 1 denotes the trianion of 5,10,15-tris-pentafluorophenylcorrole] have been fully characterized by structural, spectroscopic, and electrochemical methods. Crystal structure analyses reveal that average metal−N(pyrrole) bond lengths of the bis-pyridine metal(III) complexes increase from Co (1.886 Å) to Rh (1.957 Å)/Ir (1.963 Å); and the average metal−N(pyridine) bond lengths also increase from Co (1.995 Å) to Rh (2.065 Å)/Ir (2.059 Å). Ligand affinities for 1-M(PPh3) axial coordination sites increase dramatically in the order 1-Co(PPh3) < 1-Rh(PPh3) < 1-Ir(PPh3). There is a surprising invariance in the M(+/0) reduction potentials within the five- and six-coordinate corrole series, and even between them; the average M(+/0) potential of 1-M(PPh3) is 0.78 V vs Ag/AgCl in CH2Cl2 solution, whereas that of 1-M(py)2 is 0.70 V under the same conditions. Electronic structures of one-electron-oxidized 1-M(py)2 complexes have been assigned by analysis of electron paramagnetic resonance spectroscopic measurements: oxidation is corrole-centered for 1-Co(py)2 (g = 2.008) and 1-Rh(py)2 (g = 2.003), and metal-centered for 1-Ir(tma)2 (gzz = 2.489, gyy = 2.010, gxx = 1.884, gav = 2.128) and 1-Ir(py)2 (gzz = 2.401, gyy = 2.000, gxx = 1.937, gav = 2.113).
Co-reporter:Kyle M. Lancaster ; Keiko Yokoyama ; John H. Richards ; Jay R. Winkler
Inorganic Chemistry 2009 Volume 48(Issue 4) pp:1278-1280
Publication Date(Web):December 29, 2008
DOI:10.1021/ic802322e
Site-directed mutagenesis of Pseudomonas aeruginosa azurin C112D at the M121 position has afforded a series of proteins with elevated CuII/I reduction potentials relative to the CuII aquo ion. The high potential and low axial hyperfine splitting (CuII electron paramagnetic resonance A∥) of the C112D/M121L protein are remarkably similar to features normally associated with type 1 copper centers.
Co-reporter:Charlotte A. Whited, Wendy Belliston-Bittner, Alexander R. Dunn, Jay R. Winkler, Harry B. Gray
Journal of Inorganic Biochemistry 2009 Volume 103(Issue 6) pp:906-911
Publication Date(Web):June 2009
DOI:10.1016/j.jinorgbio.2009.04.001
A Ru-diimine wire, [(4,4′,5,5′-tetramethylbipyridine)2Ru(F9bp)]2+ (tmRu-F9bp, where F9bp is 4-methyl-4′-methylperfluorobiphenylbipyridine), binds tightly to the oxidase domain of inducible nitric oxide synthase (iNOSoxy). The binding of tmRu-F9bp is independent of tetrahydrobiopterin, arginine, and imidazole, indicating that the wire resides on the surface of the enzyme, distant from the active-site heme. Photoreduction of an imidazole-bound active-site heme iron in the enzyme-wire conjugate (kET = 2(1) × 107 s−1) is fully seven orders of magnitude faster than the in vivo process.
Co-reporter:Harry B. Gray, Jay R. Winkler
Chemical Physics Letters 2009 Volume 483(1–3) pp:1-9
Publication Date(Web):24 November 2009
DOI:10.1016/j.cplett.2009.10.051
Electron transfers in photosynthesis and respiration commonly occur between metal-containing cofactors that are separated by large molecular distances. Employing laser flash-quench triggering methods, we have shown that 20-Å, coupling-limited FeII–RuIII and CuI–RuIII electron tunneling in Ru-modified cytochromes and blue copper proteins can occur on the microsecond timescale both in solutions and crystals. Redox equivalents can be transferred even longer distances by multistep tunneling, often called hopping, through intervening amino acid side chains. Our work has established that 20-Å hole hopping through an intervening tryptophan is two orders of magnitude faster than single-step electron tunneling in a Re-modified blue copper protein.Pathways of photoinduced electron (red) and hole (blue) separation and long-range transport in Re-modified azurin.
Co-reporter:Tetsunari Kimura;Jennifer C. Lee;Jay R. Winkler
PNAS 2009 Volume 106 (Issue 19 ) pp:7834-7839
Publication Date(Web):2009-05-12
DOI:10.1073/pnas.0902562106
Cytochrome cb562 is a variant of an Escherichia coli four-helix bundle b-type heme protein in which the porphyrin prosthetic group is covalently ligated to the polypeptide near the terminus of helix
4. Studies from other laboratories have shown that the apoprotein folds rapidly without the formation of intermediates, whereas
the holoprotein loses heme before native structure can be attained. Time-resolved fluorescence energy transfer (TRFET) measurements
of cytochrome cb562 refolding triggered using an ultrafast continuous-flow mixer (150 μs dead time) reveal that heme attachment to the polypeptide
does not interfere with rapid formation of the native structure. Analyses of the TRFET data produce distributions of Trp-59–heme
distances in the protein before, during, and after refolding. Characterization of the moments and time evolution of these
distributions provides compelling evidence for a refolding mechanism that does not involve significant populations of intermediates.
These observations suggest that the cytochrome b562 folding energy landscape is minimally frustrated and able to tolerate the introduction of substantial perturbations (i.e.,
the heme prosthetic group) without the formation of deep misfolded traps.
Co-reporter:Harry B. Gray;Patrick Weinkam;Jay R. Winkler;Ekaterina V. Pletneva;Peter G. Wolynes
PNAS 2009 Volume 106 (Issue 6 ) pp:1796-1801
Publication Date(Web):2009-02-10
DOI:10.1073/pnas.0813120106
The denatured state of proteins is heterogeneous and susceptible to general hydrophobic and electrostatic forces, but to what
extent does the funneled nature of protein energy landscapes play a role in the unfolded ensemble? We simulate the denatured
ensemble of cytochrome c using a series of models. The models pinpoint the efficacy of incorporating energetic funnels toward the native state in
contrast with models having no native structure-seeking tendency. These models also contain varying strengths of electrostatic
effects and hydrophobic collapse. The simulations based on these models are compared with experimental distributions for the
distances between a fluorescent donor and the heme acceptor that were extracted from time-resolved fluorescence energy transfer
experiments on cytochrome c. Comparing simulations to detailed experimental data on several labeling sites allows us to quantify the dominant forces
in denatured protein ensembles.
Co-reporter:Jun Ma;Altan Rentsendorj;Hasmik Agadjanian;Jae Youn Hwang;Vinod Valluripalli;Atif Mahammed;Daniel L. Farkas;Zeev Gross;Lali K. Medina-Kauwe
PNAS 2009 Volume 106 (Issue 15 ) pp:6105-6110
Publication Date(Web):2009-04-14
DOI:10.1073/pnas.0901531106
Sulfonated gallium(III) corroles are intensely fluorescent macrocyclic compounds that spontaneously assemble with carrier
proteins to undergo cell entry. We report in vivo imaging and therapeutic efficacy of a tumor-targeted corrole noncovalently
assembled with a heregulin-modified protein directed at the human epidermal growth factor receptor (HER). Systemic delivery
of this protein-corrole complex results in tumor accumulation, which can be visualized in vivo owing to intensely red corrole
fluorescence. Targeted delivery in vivo leads to tumor cell death while normal tissue is spared. These findings contrast with
the effects of doxorubicin, which can elicit cardiac damage during therapy and required direct intratumoral injection to yield
similar levels of tumor shrinkage compared with the systemically delivered corrole. The targeted complex ablated tumors at
>5 times a lower dose than untargeted systemic doxorubicin, and the corrole did not damage heart tissue. Complexes remained
intact in serum and the carrier protein elicited no detectable immunogenicity. The sulfonated gallium(III) corrole functions
both for tumor detection and intervention with safety and targeting advantages over standard chemotherapeutic agents.
Co-reporter:Kristopher G. Urie, David Angulo, Jennifer C. Lee, John J. Kozak, Harry B. Gray and Jay R. Winkler
The Journal of Physical Chemistry B 2009 Volume 113(Issue 2) pp:522-530
Publication Date(Web):December 19, 2008
DOI:10.1021/jp806727e
α-Synuclein (α-syn) is an intrinsically unstructured 140-residue neuronal protein of uncertain function that is implicated in the etiology of Parkinson’s disease. Tertiary contact formation rate constants in α-syn, determined from diffusion-limited electron-transfer kinetics measurements, are poorly approximated by simple random polymer theory. One source of the discrepancy between theory and experiment may be that interior-loop formation rates are not well approximated by end-to-end contact dynamics models. We have addressed this issue with Monte Carlo simulations to model asynchronous and synchronous motion of contacting sites in a random polymer. These simulations suggest that a dynamical drag effect may slow interior-loop formation rates by about a factor of 2 in comparison to end-to-end loops of comparable size. The additional deviations from random coil behavior in α-syn likely arise from clustering of hydrophobic residues in the disordered polypeptide.
Co-reporter:Richard Eisenberg
Inorganic Chemistry 2008 Volume 47(Issue 6) pp:1697-1699
Publication Date(Web):March 10, 2008
DOI:10.1021/ic800155g
Co-reporter:Keiko Yokoyama, Brian S. Leigh, Yuling Sheng, Katsumi Niki, Nobuhumi Nakamura, Hiroyuki Ohno, Jay R. Winkler, Harry B. Gray, John H. Richards
Inorganica Chimica Acta 2008 Volume 361(Issue 4) pp:1095-1099
Publication Date(Web):3 March 2008
DOI:10.1016/j.ica.2007.08.022
Robust voltammetric responses were obtained for wild-type and Y72F/H83Q/Q107H/Y108F azurins adsorbed on CH3(CH2)nSH:HO(CH2)mSH (n = m = 4, 6, 8, 11; n = 13, 15 m = 11) self-assembled-monolayer (SAM) gold electrodes in acidic solution (pH 4.6) at high ionic strengths. Electron-transfer (ET) rates do not vary substantially with ionic strength, suggesting that the SAM methyl headgroup binds to azurin by hydrophobic interactions. The voltammetric responses for both proteins at higher pH values (>4.6–11) also were strong. A binding model in which the SAM hydroxyl headgroup interacts with the Asn47 carboxamide accounts for the relatively strong coupling to the copper center that can be inferred from the ET rates. Of particular interest is the finding that rate constants for electron tunneling through n = 8, 13 SAMs are higher at pH 11 than those at pH 4.6, possibly owing to enhanced coupling of the SAM to Asn47 caused by deprotonation of nearby surface residues.Robust voltammetric responses were obtained for wild-type and Y72F/H83Q/Q107H/Y108F azurins adsorbed on H3(CH2)nSH:HO(CH2)mSH (n = m = 4, 6, 8, 11; n = 13, 15 m = 11) self-assembled-monolayer (SAM) gold electrodes in acidic solution (pH 4.6) at high ionic strengths. The voltammetric responses for both proteins at higher pH values (>4.6–11) also were strong.
Co-reporter:Patrick Hummel;Jay R. Winkler
Theoretical Chemistry Accounts 2008 Volume 119( Issue 1-3) pp:35-38
Publication Date(Web):2008 January
DOI:10.1007/s00214-006-0236-8
The standard oxidation states of central metal atoms in C4v nitrido ([M(N)(L)5]z) complexes are four units higher than those in corresponding nitrosyls ([M(NO)(L)5]z) (L=CN: z = 3−, M = Mn, Tc, Re; z = 2−, M = Fe, Ru, Os; L = NH3: z = 2+, M = Mn, Tc, Re; z = 3+, M = Fe, Ru, Os). Recent work has suggested that [Mn(NO)(CN)5]3− behaves electronically much closer to Mn(V)[b2(xy)]2, the ground state of [Mn(N)(CN)5]3−, than to Mn(I)[b2(xy)]2[e(xz,yz)]4. We have employed density functional theory and time-dependent density functional theory to calculate the properties of the ground states and lowest-lying excitations of [M(N)(L)5]z and [M(NO)(L)5]z. Our results show that [M(N)(L)5]z and [M(NO)(L)5]z complexes with the same z value have strikingly similar electronic structures.
Co-reporter:Crystal Shih;Anna Katrine Museth;Malin Abrahamsson;Ana Maria Blanco-Rodriguez;Angel J. Di Bilio;Jawahar Sudhamsu;Brian R. Crane;Kate L. Ronayne;Mike Towrie;Antonín Vlček Jr.;John H. Richards;Jay R. Winkler
Science 2008 Volume 320(Issue 5884) pp:1760-1762
Publication Date(Web):27 Jun 2008
DOI:10.1126/science.1158241
Abstract
Energy flow in biological structures often requires submillisecond charge transport over long molecular distances. Kinetics modeling suggests that charge-transfer rates can be greatly enhanced by multistep electron tunneling in which redox-active amino acid side chains act as intermediate donors or acceptors. We report transient optical and infrared spectroscopic experiments that quantify the extent to which an intervening tryptophan residue can facilitate electron transfer between distant metal redox centers in a mutant Pseudomonas aeruginosa azurin. CuI oxidation by a photoexcited ReI-diimine at position 124 on a histidine(124)-glycine(123)-tryptophan(122)-methionine(121) β strand occurs in a few nanoseconds, fully two orders of magnitude faster than documented for single-step electron tunneling at a 19 angstrom donor-acceptor distance.
Co-reporter:Gunnar F. Kaufmann;Erik W. Debler;Andreas Heine;Michael M. Meijler;Jenny M. Mee;Goran Pljevaljčić;Angel J. Di Bilio;Peter G. Schultz;Kim D. Janda;Ian A. Wilson;David P. Millar;Richard A. Lerner
Science 2008 Volume 319(Issue 5867) pp:1232-1235
Publication Date(Web):29 Feb 2008
DOI:10.1126/science.1153445
Abstract
The blue-emissive antibody EP2-19G2 that has been elicited against trans-stilbene has unprecedented ability to produce bright luminescence and has been used as a biosensor in various applications. We show that the prolonged luminescence is not stilbene fluorescence. Instead, the emissive species is a charge-transfer excited complex of an anionic stilbene and a cationic, parallel π-stacked tryptophan. Upon charge recombination, this complex generates exceptionally bright blue light. Complex formation is enabled by a deeply penetrating ligand-binding pocket, which in turn results from a noncanonical interface between the two variable domains of the antibody.
Co-reporter:Ekaterina V. Pletneva, Ziqing Zhao, Tetsunari Kimura, Krastina V. Petrova, Harry B. Gray, Jay R. Winkler
Journal of Inorganic Biochemistry 2007 Volume 101(11–12) pp:1768-1775
Publication Date(Web):November 2007
DOI:10.1016/j.jinorgbio.2007.06.019
The folding kinetics of R. palustris cytochrome c′ (cyt c′) have been monitored by heme absorption and native Trp72 fluorescence at pH 5. The Trp72 fluorescence burst signal suggests early compaction of the polypeptide ensemble. Analysis of heme transient absorption spectra reveals deviations from two-state behavior, including a prominent slow phase that is accelerated by the prolyl isomerase cyclophilin. A nonnative proline configuration (Pro21) likely interferes with the formation of the helical bundle surrounding the heme.
Co-reporter:John H. Dawson, Harry B. Gray
Journal of Inorganic Biochemistry 2007 Volume 101(11–12) pp:1543
Publication Date(Web):November 2007
DOI:10.1016/j.jinorgbio.2007.08.004
Co-reporter:Tetsunari Kimura;Jennifer C. Lee;Jay R. Winkler;
Proceedings of the National Academy of Sciences 2007 104(1) pp:117-122
Publication Date(Web):December 19, 2006
DOI:10.1073/pnas.0609413103
The evolution of tryptophan-to-heme (W/heme) distance distributions extracted from analysis of fluorescence energy transfer
kinetics during the refolding of Rhodopseudomonas palustris cytochrome c′ reveals dramatic differences between two variants [W32 (Q1A/F32W/W72F) and W72 (Q1A)]. Both W32/heme and W72/heme distance
distributions measured at the earliest time point attainable with a continuous-flow mixer (150 μs) confirm that the polypeptide
ensemble is not uniformly collapsed and that native structure is not formed. Time-resolved fluorescence spectra indicate that
W32 is sequestered from the aqueous solution during the first 700 μs of folding, whereas W72 remains exposed to solvent. The
first moment of the W32/heme distance distribution evolves to its native value faster than that of W72, suggesting that the
approach of W32 to the heme precedes that of W72.
Co-reporter:Patrick Hummel, Jay R. Winkler and Harry B. Gray
Dalton Transactions 2006 (Issue 1) pp:168-171
Publication Date(Web):04 Nov 2005
DOI:10.1039/B512299F
We have employed computational methods based on density functional theory to elucidate the effects of equatorial ligands on the electronic structures of trans-dioxometal complexes. In complexes with ammine (σ-only) equatorial donors, the 1A1 g(b2 g)2
→
1Eg(b2 g)1(eg)1 excitation energy increases with metal oxidation state: Mo(IV) < Tc(V) < Ru(VI) and W(IV) < Re(V) < Os(VI). Increasing transition energies are attributed to enhanced oxometal π-donor interactions in the higher valent central metals. But in complexes with cyanide equatorial donors, the 1A1 g(b2 g)2
→
1Eg(b2 g)1(eg)1 energy remains roughly independent of metal oxidation state, likely owing to the compensating increased π-donation from the π(CN) orbitals to the metal dxy orbitals as the oxidation state of the metal increases.
Co-reporter:Oliver S. Wenger;Brian S. Leigh;Randy M. Villahermosa;Jay R. Winkler
Science 2005 Vol 307(5706) pp:99-102
Publication Date(Web):07 Jan 2005
DOI:10.1126/science.1103818
Abstract
Reaction rates extracted from measurements of donor luminescence quenching by randomly dispersed electron acceptors reveal an exponential decay constant of 1.23 per angstrom for electron tunneling through a frozen toluene glass (with a barrier to tunneling of 1.4 electron volts). The decay constant is 1.62 per angstrom (the barrier, 2.6 electron volts) in a frozen 2-methyl-tetrahydrofuran glass. Comparison to decay constants for tunneling across covalently linked xylyl (0.76 per angstrom) and alkyl (1.0 per angstrom) bridges leads to the conclusion that tunneling between solvent molecules separated by ∼2 angstroms (van der Waals contact) is 20 to 50 times slower than tunneling through a comparable length of a covalently bonded bridge. Our results provide experimental confirmation that covalently bonded pathways can facilitate electron flow through folded polypeptide structures.
Co-reporter:Jasmin Faraone-Mennella;Jay R. Winkler;
Proceedings of the National Academy of Sciences 2005 102(18) pp:6315-6319
Publication Date(Web):April 20, 2005
DOI:10.1073/pnas.0502301102
Topologically homologous four-helix-bundle heme proteins exhibit striking diversity in their refolding kinetics. Cytochrome
b
562 has been reported to fold on a submillisecond time scale, whereas cytochrome c′ refolding requires 10 s or more to complete. Heme dissociation in cytochrome b
562 interferes with studies of folding kinetics, so a variant of cytochrome b
562 (cytochrome c-b
562) with a covalent c-type linkage to the heme has been expressed in Escherichia coli. Early events in the electron transfer-triggered folding of FeII-cytochrome c-b
562, along with those of FeII-cytochrome c
556, have been examined by using time-resolved absorption spectroscopy. Coordination of S(Met) to FeII occurs within 10 μs after reduction of the denatured FeIII-cytochromes, and shortly thereafter (100 μs) the heme spectra are indistinguishable from those of the folded proteins. Under
denaturing conditions, carbon monoxide binds to the FeII-hemes in ≈15 ms. By contrast, CO binding cannot compete with refolding in the FeII-cytochromes, thereby confirming that the polypeptide encapsulates the heme in <10 ms. We suggest that Fe-S(Met) ligation
facilitates refolding in these four-helix-bundle heme proteins by reducing the conformational freedom of the polypeptide chain.
Co-reporter:Jay R. Winkler
PNAS 2005 Volume 102 (Issue 10 ) pp:3534-3539
Publication Date(Web):2005-03-08
DOI:10.1073/pnas.0408029102
Recent investigations have shed much light on the nuclear and electronic factors that control the rates of long-range electron
tunneling through molecules in aqueous and organic glasses as well as through bonds in donor–bridge–acceptor complexes. Couplings
through covalent and hydrogen bonds are much stronger than those across van der Waals gaps, and these differences in coupling
between bonded and nonbonded atoms account for the dependence of tunneling rates on the structure of the media between redox
sites in Ru-modified proteins and protein–protein complexes.
Co-reporter:Gregory A. Juda;David B. Langley;J. Mitchell Guss;Alexander R. Dunn;Stephen M. Contakes;Hans C. Freeman;David M. Dooley;Anthony P. Duff;Nicholas W. Halpern-Manners
PNAS 2005 Volume 102 (Issue 38 ) pp:13451-13456
Publication Date(Web):2005-09-20
DOI:10.1073/pnas.0506336102
Molecular wires comprising a Ru(II)- or Re(I)-complex head group, an aromatic tail group, and an alkane linker reversibly
inhibit the activity of the copper amine oxidase from Arthrobacter globiformis (AGAO), with K
i values between 6 μM and 37 nM. In the crystal structure of a Ru(II)-wire:AGAO conjugate, the wire occupies the AGAO active-site
substrate access channel, the trihydroxyphenylalanine quinone cofactor is ordered in the “off-Cu” position with its reactive
carbonyl oriented toward the inhibitor, and the “gate” residue, Tyr-296, is in the “open” position. Head groups, tail-group
substituents, and linker lengths all influence wire-binding interactions with the enzyme.
Co-reporter:Ekaterina V. Pletneva;Jay R. Winkler
PNAS 2005 102 (51 ) pp:18397-18402
Publication Date(Web):2005-12-20
DOI:10.1073/pnas.0509076102
Dansyl-to-heme distance distributions [P(r)] during folding have been determined in five variants of Saccharomyces cerevisiae iso-1 ferricytochrome c (labeled at mutant Cys residues 4, 39, 50, 66, and 99) by analysis of fluorescence energy-transfer kinetics. Moment analysis
of the P(r) distributions clearly indicates that cytochrome c refolding is not a simple two-state process. After 1 ms of folding, the polypeptide ensemble is not uniformly collapsed and
there are site variations in the relative populations of collapsed structures. P(r) distributions reveal structural features of the multiple intermediate species and evolution of the polypeptide ensemble.
Co-reporter:Zeev Gross;Harry B. Gray
Advanced Synthesis & Catalysis 2004 Volume 346(Issue 2-3) pp:
Publication Date(Web):29 MAR 2004
DOI:10.1002/adsc.200303145
Metallocorroles, in particular those containing chromium, manganese, and iron, have been found to be efficient catalysts for oxidation reactions. This review deals with work on hydrocarbon oxidations after the 1999 advance made in corrole synthesis that sparked the Technion-Caltech collaborative program.
Co-reporter:Jay R. Winkler;Julia G. Lyubovitsky
Israel Journal of Chemistry 2004 Volume 44(Issue 1‐3) pp:263-269
Publication Date(Web):8 MAR 2010
DOI:10.1560/JWJ4-M90E-HTUY-GGQN
Employing fluorescence energy transfer kinetics, we have shown that substantial populations of compact molecules are present in GuHCl-denatured CX-dansyl-labeled (X = 39, 85, 102) S. cerevisiae iso-1 cytochrome c. Fully 40% of unfolded molecules are in compact conformations in the C39 protein.
Co-reporter:Jennifer C. Lee;Patrick A. Hummel;Ralf Langen;Jay R. Winkler
PNAS 2004 Volume 101 (Issue 47 ) pp:16466-16471
Publication Date(Web):2004-11-23
DOI:10.1073/pnas.0407307101
Parkinson's disease is associated with the deposition and accumulation of α-synuclein fibrils in the brain. A30P and A53T
mutations have been linked to the early-onset familial disease state. Time-resolved tryptophan fluorescence energy-transfer
measurements have been used to probe the structures of pseudo-wild-type and mutant (A30P) α-synucleins at physiological pH
(7.4), in acidic pH (4.4) solutions, and in the presence of SDS micelles, a membrane mimic. Fluorescent donor–energy acceptor
(DA) distance distributions for six different tryptophan/3-nitro-tyrosine pairs reveal the presence of compact, intermediate,
and extended conformations of the protein. CD spectra indicate that the protein develops substantial helical structure in
the presence of SDS micelles. DA distributions show that micelles induce compaction in the N-terminal region and expansion
of the acidic C terminus. In acidic solutions, there is an increased population of collapsed structures in the C-terminal
region. Energy-transfer measurements demonstrate that the average DA distances for the W4–Y19 and Y19–W39 pairs are longer
in one of the two disease-related mutants (A30P).
Co-reporter:Harry B. Gray;
Proceedings of the National Academy of Sciences 2003 100(7) pp:3563-3568
Publication Date(Web):March 25, 2003
DOI:10.1073/pnas.0730378100
Advances in bioinorganic chemistry since the 1970s have been driven by three factors: rapid determination of high-resolution
structures of proteins and other biomolecules, utilization of powerful spectroscopic tools for studies of both structures
and dynamics, and the widespread use of macromolecular engineering to create new biologically relevant structures. Today,
very large molecules can be manipulated at will, with the result that certain proteins and nucleic acids themselves have become
versatile model systems for elucidating biological function.
Co-reporter:I-Jy Chang;Jennifer C. Lee;Jay R. Winkler;
Proceedings of the National Academy of Sciences 2003 100(7) pp:3838-3840
Publication Date(Web):March 19, 2003
DOI:10.1073/pnas.0637283100
The kinetics of electron transfer from the triplet-excited Zn-porphyrin to a Ru(NH3)5(His-33)3+ complex have been measured in Zn-substituted ruthenium-modified cytochrome c under denaturing conditions. In the folded protein, the electron-tunneling rate constant is 7.5 × 105 s−1. As the protein is denatured with guanidine hydrochloride, a faster adiabatic electron-transfer reaction appears (4.0 × 106 s−1, [guanidine hydrochloride] = 5.4 M) that is limited by the rate of intrachain diffusion to bring the Zn-porphyrin and Ru
complex into contact. The 250-ns contact time for formation of a 15-residue loop in denatured cytochrome c is in accord with a statistical model developed by Camacho and Thirumalai [Camacho, C. J. & Thirumalai, D. (1995) Proc. Natl. Acad. Sci. USA 92, 1277–1281] that predicts that the most probable transient loops formed in denatured proteins are comprised of 10 amino
acids. Extrapolation of the cytochrome c contact time to a 10-residue loop sets the folding speed limit at ≈107 s−1.
Co-reporter:F. Akif Tezcan;William M. Findley;Brian R. Crane;Scott A. Ross;Julia G. Lyubovitsky;Jay R. Winkler
PNAS 2002 Volume 99 (Issue 13 ) pp:8626-8630
Publication Date(Web):2002-06-25
DOI:10.1073/pnas.132254499
Replacement of iron with cobalt(III) selectively introduces a deep trap in the folding-energy landscape of the heme protein
cytochrome c. Remarkably, neither the protein structure nor the folding thermodynamics is perturbed by this metal–ion substitution, as
shown by data from spectroscopic and x-ray diffraction experiments. Through kinetics measurements, we have found parallel
folding pathways involving several different misligated Co(III) species, and, as these folding intermediates persist for several
hours under certain conditions, we have been able to elucidate fully their spectroscopic properties. The results, along with
an analysis of the fluorescence energy-transfer kinetics during refolding, show that rapidly equilibrating populations of
compact and extended polypeptide conformations are present until all molecules have reached the native structure. These measurements
provide direct evidence that collapsed denatured structures are not substantially more stable than extended conformations
of cytochrome c.
Co-reporter:F. Akif Tezcan;Brian R. Crane;Jay R. Winkler
PNAS 2001 Volume 98 (Issue 9 ) pp:5002-5006
Publication Date(Web):2001-04-24
DOI:10.1073/pnas.081072898
The current understanding of electron tunneling
through proteins has come from work on systems where
donors and acceptors are held at fixed distances and orientations. The
factors that control electron flow between proteins are
less well understood, owing to uncertainties in the relative
orientations and structures of the reactants during the very short time
that tunneling occurs. As we report here, the way around such
structural ambiguity is to examine oxidation–reduction reactions in
protein crystals. Accordingly, we have measured and analyzed the
kinetics of electron transfer between native and Zn-substituted tuna
cytochrome c (cyt c) molecules in
crystals of known structure. Electron transfer rates [(320
s−1 for *Zn-cyt c → Fe(III)-cyt
c; 2000 s−1 for Fe(II)-cyt c
→ Zn-cyt c+)] over a Zn–Fe distance of
24.1 Å closely match those for intraprotein electron tunneling over
similar donor–acceptor separations. Our results indicate that van der
Waals interactions and water-mediated hydrogen bonds are effective
coupling elements for tunneling across a protein–protein interface.
Co-reporter:Ally Aukauloo, Harry B. Gray
Comptes Rendus Chimie (March 2017) Volume 20(Issue 3) pp:207
Publication Date(Web):March 2017
DOI:10.1016/j.crci.2016.03.016
Co-reporter:Charlotte A. Whited, Wendy Belliston-Bittner, Alexander R. Dunn, Jay R. Winkler, Harry B. Gray
Journal of Inorganic Biochemistry (June 2009) Volume 103(Issue 6) pp:906-911
Publication Date(Web):1 June 2009
DOI:10.1016/j.jinorgbio.2009.04.001
A Ru-diimine wire, [(4,4′,5,5′-tetramethylbipyridine)2Ru(F9bp)]2+ (tmRu-F9bp, where F9bp is 4-methyl-4′-methylperfluorobiphenylbipyridine), binds tightly to the oxidase domain of inducible nitric oxide synthase (iNOSoxy). The binding of tmRu-F9bp is independent of tetrahydrobiopterin, arginine, and imidazole, indicating that the wire resides on the surface of the enzyme, distant from the active-site heme. Photoreduction of an imidazole-bound active-site heme iron in the enzyme-wire conjugate (kET = 2(1) × 107 s−1) is fully seven orders of magnitude faster than the in vivo process.
Co-reporter:Jeffrey J. Warren, Jay R. Winkler, Harry B. Gray
FEBS Letters (9 March 2012) Volume 586(Issue 5) pp:596-602
Publication Date(Web):9 March 2012
DOI:10.1016/j.febslet.2011.12.014
Redox reactions of tyrosine play key roles in many biological processes, including water oxidation and DNA synthesis. We first review the redox properties of tyrosine (and other phenols) in small molecules and related polypeptides, then report work on (H20)/(Y48)-modified Pseudomonas aeruginosa azurin. The crystal structure of this protein (1.18 Å resolution) shows that H20 is strongly hydrogen bonded to Y48 (2.7–2.8 Å tyrosine-O to histidine-N distance). A firm conclusion is that proper tuning of the tyrosine potential by a proton-accepting base is critical for biological redox functions.
Co-reporter:Ruijie D. Teo, Harry B. Gray, Punnajit Lim, John Termini, Elena Domeshek and Zeev Gross
Chemical Communications 2014 - vol. 50(Issue 89) pp:NaN13792-13792
Publication Date(Web):2014/09/05
DOI:10.1039/C4CC06577H
We have synthesized and characterized a water-soluble gold(III) corrole (1-Au) that is highly toxic to cisplatin-resistant cancer cells. Relative to its 1-Ga analogue, axial ligands bind only weakly to 1-Au, which likely accounts for its lower affinity for human serum albumin (HSA). We suggest that the cytotoxicity of 1-Au may be related to this lower HSA affinity.
Co-reporter:James R. McKone, Smaranda C. Marinescu, Bruce S. Brunschwig, Jay R. Winkler and Harry B. Gray
Chemical Science (2010-Present) 2014 - vol. 5(Issue 3) pp:NaN878-878
Publication Date(Web):2013/11/05
DOI:10.1039/C3SC51711J
Splitting water to hydrogen and oxygen is a promising approach for storing energy from intermittent renewables, such as solar power. Efficient, scalable solar-driven electrolysis devices require active electrocatalysts made from earth-abundant elements. In this mini-review, we discuss recent investigations of homogeneous and heterogeneous hydrogen evolution electrocatalysts, with emphasis on our own work on cobalt and iron complexes and nickel-molybdenum alloys.
Co-reporter:Judith R. C. Lattimer, James D. Blakemore, Wesley Sattler, Sheraz Gul, Ruchira Chatterjee, Vittal K. Yachandra, Junko Yano, Bruce S. Brunschwig, Nathan S. Lewis and Harry B. Gray
Dalton Transactions 2014 - vol. 43(Issue 40) pp:NaN15012-15012
Publication Date(Web):2014/07/17
DOI:10.1039/C4DT01149J
Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2′-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate). For the surface prepared with Ir, X-ray absorption spectroscopy at the Ir LIII edge showed an edge energy as well as post-edge features that were essentially identical with those observed on a powder sample of [Cp*Ir(bpy)Cl]Cl (bpy is 2,2′-bipyridyl). Charge-carrier lifetime measurements confirmed that the silicon surfaces retain their highly favorable photoelectronic properties upon assembly of the metal complexes. Electrochemical data for surfaces prepared on highly doped, n-type Si(111) electrodes showed that the assembled molecular complexes were redox active. However the stability of the molecular complexes on the surfaces was limited to several cycles of voltammetry.
Co-reporter:Jillian L. Dempsey, Jay R. Winkler and Harry B. Gray
Dalton Transactions 2011 - vol. 40(Issue 40) pp:NaN10636-10636
Publication Date(Web):2011/09/02
DOI:10.1039/C1DT11138H
Powerful reductants [OsII(NH3)5L]2+ (L = OH2, CH3CN) can be generated upon ultraviolet excitation of relatively inert [OsII(NH3)5(N2)]2+ in aqueous and acetonitrile solutions. Reactions of photogenerated Os(II) complexes with methyl viologen to form methyl viologen radical cation and [OsIII(NH3)5L]3+ were monitored by transient absorption spectroscopy. Rate constants range from 4.9 × 104 M−1 s−1 in acetonitrile solution to 3.2 × 107 (pH 3) and 2.5 × 108 M−1 s−1 (pH 12) in aqueous media. Photogeneration of five-coordinate Os(II) complexes opens the way for mechanistic investigations of activation/reduction of CO2 and other relatively inert molecules.
Co-reporter:Michael J. Rose, Donatela E. Bellone, Angel J. Di Bilio and Harry B. Gray
Dalton Transactions 2012 - vol. 41(Issue 38) pp:NaN11797-11797
Publication Date(Web):2012/08/20
DOI:10.1039/C2DT31229H
Reaction of the tripodal phosphine ligand 1,1,1-tris((diphenylphosphino)phenyl)ethane (PhP3) with CoI2 spontaneously generates a one-electron reduced complex, [(PhP3)CoI(I)] (1). The crystal structure of 1 reveals a distorted tetrahedral environment, with an apical Co–I bond distance of ∼2.52 Å. CoII/I redox occurs at an unusually high potential (+0.38 V vs. SCE). The electronic absorption spectrum of 1 exhibits an MLCT peak at 320 nm (ε = 8790 M−1 cm−1) and a d–d feature at 850 nm (ε = 840 M−1 cm−1). Two more d–d bands are observed in the NIR region, 8650 (ε = 450) and 7950 cm−1 (ε = 430 M−1 cm−1). Temperature dependent magnetic measurements (SQUID) on 1 (solid state, 20–300 K) give μeff = 2.99(6) μB, consistent with an S = 1 ground state. Magnetic susceptibilities below 20 K are consistent with a zero field splitting (zfs) |D| = 8 cm−1. DFT calculations also support a spin-triplet ground state for 1, as optimized (6-31G*/PW91) geometries (S = 1) closely match the X-ray structure. EPR measurements performed in parallel mode (X-band; 0–15000 G, 15 K) on polycrystalline 1 or frozen solutions of 1 (THF/toluene) exhibit a feature at g ≈ 4 that arises from a (Δm = 2) transition within the MS = <+1,−1> manifold. Below 10 K, the EPR signal decreases significantly, consistent with a solution zfs parameter (|D| ≈ 8 cm−1) similar to that obtained from SQUID measurements. Our work provides an EPR signature for high-spin CoI in trigonal ligation.
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