Visible and ultraviolet resonance Raman (RR) spectra are reported for FeIII(NO) adducts of myoglobin variants with altered polarity in the distal heme pockets. The stretching frequencies of the FeIII−NO and N−O bonds, νFeN and νNO, are negatively correlated, consistent with backbonding. However, the correlation shifts to lower νNO for variants lacking a distal histidine. DFT modeling reproduces the shifted correlations and shows the shift to be associated with the loss of a lone-pair donor interaction from the distal histidine that selectively strengthens the N−O bond. However, when the model contains strongly electron-withdrawing substituents at the heme β-positions, νFeN and νNO become positively correlated. This effect results from FeIII−N−O bending, which is induced by lone-pair donation to the NNO atom. Other mechanisms for bending are discussed, which likewise lead to a positive νFeN/νNO correlation, including thiolate ligation in heme proteins and electron-donating meso-substituents in heme models. The νFeN/νNO data for the Fe(III) complexes are reporters of heme pocket polarity and the accessibility of lone pair, Lewis base donors. Implications for biologically important processes, including NO binding, reductive nitrosylation, and NO reduction, are discussed.

We have used low-temperature (77 K) resonance Raman (RR) spectroscopy as a probe of the electronic and molecular structure to investigate weak π–π interactions between the metal ion-coordinated His imidazoles and aromatic side chains in the second coordination sphere of blue copper proteins. For this purpose, the RR spectra of Met16 mutants of Achromobacter cycloclastes pseudoazurin (AcPAz) with aromatic (Met16Tyr, Met16Trp, and Met16Phe) and aliphatic (Met16Ala, Met16Val, Met16Leu, and Met16Ile) amino acid side chains have been obtained and analyzed over the 100–500 cm−1 spectral region. Subtle strengthening of the Cu(II)–S(Cys) interaction on replacing Met16 with Tyr, Trp, and Phe is indicated by the upshifted (0.3–0.8 cm−1) RR bands involving ν(Cu–S)Cys stretching modes. In contrast, the RR spectra of Met16 mutants with aliphatic amino acids revealed larger (0.2–1.8 cm−1) shifts of the ν(Cu–S)Cys stretching modes to a lower frequency region, which indicate a weakening of the Cu(II)–S(Cys) bond. Comparisons of the predominantly ν(Cu–S)Cys stretching RR peaks of the Met16X = Tyr, Trp, and Phe variants, with the molar absorptivity ratio ε1/ε2 of σ(∼455 nm)/π(∼595 nm) (Cys)S → Cu(II) charge-transfer bands in the optical spectrum and the axial/rhombic EPR signals, revealed a slightly more trigonal disposition of ligands about the copper(II) ion. In contrast, the RR spectra of Met16Z = Ala, Val, Leu, and Ile variants with aliphatic amino acid side chains show a more tetrahedral perturbation of the copper active site, as judged by the lower frequencies of the ν(Cu–S)Cys stretching modes, much larger values of the ε1/ε2 ratio, and the increased rhombicity of the EPR spectra.

Resonance Raman (RR) spectroscopy and density functional theory (DFT) calculations of oxochromium(IV,V) derivatives of 5,10,15-tris(pentafluorophenyl)corrole (tpfpc) are shown to provide useful information about the relative strength of the metal−oxo bond in high-valent CrIV versus CrV corroles. Isotope labeling of the terminal oxo group with 18O revealed that the CrV−oxo (perchromyl) stretch of (tpfpc)CrVO vibrates at a frequency of 986 cm−1 in carbon disulfide, consistent with a triply bonded CrV≡O unit. In contrast, an acetonitrile solution produced RR scattering that rapidly changed with the number of scans collected and eventually became dominated by an 18O-sensitive vibration at a significantly higher frequency of 1002 cm−1. On the basis of DFT calculations and the observed 18/16O isotopic shift, we assigned this new RR band at 1002 cm−1 in acetonitrile as the CrIV−oxo (chromyl) stretch of the autoreduced [(tpfpc)CrIVO]− product, which previously has been shown to form only during the course of the oxygen atom transfer (OAT) reaction with triphenylphosphine in acetonitrile or in the presence of a reducing chemical (cobaltocene) and electrochemical agents in other solvents. Consequently, RR observations indicate that the π-bonding character of the chromyl bond is actually increased relative to that of the perchromyl bond, which is of interest if the beneficial role of acetonitrile in OAT catalysis by high-valent oxochromium(IV,V) corroles is to be elucidated.

Metal-substituted blue copper proteins (cupredoxins) have been successfully used to study the effect of metal-ion identity on their active-site properties, specifically the coordination geometry and metal–ligand bond strengths. In this work, low-temperature (77 K) resonance Raman (RR) spectra of the blue copper protein Alcaligenes xylosoxidans azurin I and its Ni(II) derivative are reported. A detailed analysis of all observed bands is presented and responsiveness to metal substitution is discussed in terms of structural and bonding changes. The native cupric site exhibits a RR spectrum characteristic of a primarily trigonal planar (type 1) coordination geometry, identified by the ν(Cu–S)Cys markers at 373, 399, 409, and 430 cm−1. Replacement of Cu(II) with Ni(II) results in optical and RR spectra that reveal (1) a large hypsochromic shift in the main (Cys)S → M(II) charge-transfer absorption from 622 to 440 nm, (2) greatly reduced metal–thiolate bonding interaction, indicated by substantially lower ν(Ni–S)Cys stretching frequencies, (3) elevation of the cysteine ν(Cβ–S) stretching, amide III, and ρs(CβH2) scissors vibrational modes, and (4) primarily four-coordinated, trigonally distorted tetrahedral geometry of the Ni(II) site that is marked by characteristic ν(Ni–S)Cys stretching RR bands at 347, 364, and 391 cm−1. Comparisons of the electronic and vibrational properties between A. xylosoxidans azurin I and its closely structurally related azurin from Pseudomonas aeruginosa are made and discussed. For cupric azurins, the intensity-weighted average M(II)–S(Cys) stretching frequencies are calculated to be ν(Cu–S)iwa = 406.3 and 407.6 cm−1, respectively. These values decreased to ν(Ni–S)iwa = 359.3 and 365.5 cm−1, respectively, after Ni(II) → Cu(II) exchange, suggesting that the metal–thiolate interactions are similar in the two native proteins but are much less alike in their Ni(II)-substituted forms.

We report density functional theory (DFT) calculations at the B3LYP/6-31G(d) level on nickel(II) complexes of the geochemically significant cycloalkanoporphyrins, Ni(CAP5–7), that contain a five-, six-, or seven-membered alicyclic ring (E) fused to the pyrrole-β and methine bridge aromatic carbon atoms. The structure of each Ni(CAP) is optimized, with no symmetry constraints, by considering different orientations of the peripheral ethyl groups. The ground state structures demonstrate that the shortening of the nickel–nitrogen bonds is accommodated by out-of-plane ruffling of the porphinato skeleton, in good agreement with the experimental data available. Our results reveal Ni(CAP5) to be slightly ruffled, with an angle between the planes of opposite pyrrole rings of 7.7°, but Ni(CAP6) and Ni(CAP7) have a severe ruffling of the macrocycle, with the corresponding angle of respectively 35.1° and 39.7°. The DFT and scaled quantum mechanical force-field method (DFT–SQM) have been used to calculate the vibrational frequencies of Ni(CAP5–7) in the gas phase. The trends in the DFT–SQM vibrational shifts of the porphyrin structure-sensitive modes (1400–1700 cm−1) upon varying the length of the meso,β-alkano chain are analyzed and compared with those observed previously by us for the solution resonance Raman spectra of Ni(CAP5–7). We are able to verify by theory that the observed frequency downshifts with ring E size are driven by the increasing out-of-plane ruffling distortion.

Nickel(II) complexes of the geochemically significant four regioisomers of benzoetioporphyrin, Ni(BP–A–D), which contain a benzene ring fused onto the Cβ atoms of a pyrrole ring, have been synthesized and structurally characterized by resonance Raman spectroscopy. Laser excitations in resonance with the porphyrin Soret (406.7 nm) and Q (530.9 and 568.2 nm) electronic absorption bands exposed nearly all Raman active vibrations in the fingerprint region (100–1700 cm−1). The porphyrin skeletal vibrations above 1300 cm–1 are largely unaffected by the different location of the β,β-benzo exocyclic ring, but their frequencies indicate slightly more planar structures in solution for Ni(BP) porphyrins relative to nickel tetrahydrobenzo- and etioporphyrins. Several unique marker bands are also found for vibrations of the β,β-benzo substituent, especially in the Soret-band resonant spectra. Alkyl substituent and porphyrin skeletal vibrations in the low- (350–550 cm−1) and mid-frequency (750–1300 cm−1) regions show striking sensitivity to small conformational changes in the porphyrin, allowing the four Ni(BP) regioisomers to be readily distinguished.

The isolation and structural characterization are reported for two new unsymmetrical (μ-oxo)bis(μ-carboxylato)dimanganese(III) complexes, [Mn2O(O2CC2H5)2(H2O)(Cr2O7)(bpy)2] (1) and [Mn2O(O2CCH2Cl)2(H2O)(CH2OH)(bpy)2]2− (2) (bpy = 2,2′-bipyridine) that mimic end-on substrate and/or inhibitor coordination to only one manganese atom of the dimeric active site in bacterial manganese catalases. Crystals of 1 were self assembled spontaneously in aqueous medium following treatment of equimolar Mn(II) and bpy with K2Cr2O, in propionic acid rich solution. Complex 2 was synthesized by oxidation of Mn(II) with KMnO4 in pure methanol in the presence of bpy, chloroacetic acid, and sodium chloroacetate. X-ray crystallographic studied of 1 and 2 establish the presence of a similar triply bridged dimanganese core with distinct manganese sites in which one manganese atom contains an axial water molecule and the other is bonded to a terminal oxygen atom of dichromate anion (1), or to the oxygen atom of methanol (2), a neutral substrate. Complex 1 represents the first structurally characterized instance in which the dichromate acts as a monodentate ligand, the MnO(dichromate) bond of 2.132(23) A indicating strong anion coordination. Isolation of these model complexes and their structural characteristics suggest a preference of the [Mn2O(O2CR)2(H2O)2]2− core towards a replacement of one water molecule only a feature that is suspected to operate in hydroperoxide binding to a dimanganese site of catalases. The resonance Raman signatures with visible excitation (406.7–501.7 nm) were obtained for a series of catalase model complexes containing dimanganese cores bridged by one or two μ-oxo groups, including (μ-oxo)bis(μ-carboxylato) Mn(III.III), and mixed-valent bis (μ-oxo) and bis (μ-oxo)mono(μ-carboxylato)Mn(III.IV) dimers. The two structural types, which model the oxidized Mn(III,III) and superoxidized Mn(III,IV) forms of catalase, are distinguished readily by the uniquely resonance-enhanced μ-18O-sensitive MnO(oxo) stretching bands at ≈ 560 and ≈ 700 cm, respectively.