Conversion of carbon monoxide (CO), a major one-carbon product of carbon dioxide (CO2) reduction, into value-added multicarbon species is a challenge to addressing global energy demands and climate change. Here we report a modular synthetic approach for aqueous electrochemical CO reduction to carbon–carbon coupled products via self-assembly of supramolecular cages at molecular–materials interfaces. Heterobimetallic cavities formed by face-to-face coordination of thiol-terminated metalloporphyrins to copper electrodes through varying organic struts convert CO to C2 products with high faradaic efficiency (FE = 83% total with 57% to ethanol) and current density (1.34 mA/cm2) at a potential of −0.40 V vs RHE. The cage-functionalized electrodes offer an order of magnitude improvement in both selectivity and activity for electrocatalytic carbon fixation compared to parent copper surfaces or copper functionalized with porphyrins in an edge-on orientation.

Formaldehyde (FA) is a major reactive carbonyl species (RCS) that is naturally produced in living systems through a diverse array of cellular pathways that span from epigenetic regulation to the metabolic processing of endogenous metabolites. At the same time, however, aberrant elevations in FA levels contribute to pathologies ranging from cancer and diabetes to heart, liver, and neurodegenerative diseases. Disentangling the complex interplay between FA physiology and pathology motivates the development of chemical tools that can enable the selective detection of this RCS in biological environments with spatial and temporal fidelity. We report the design, synthesis, and biological evaluation of ratiometric formaldehyde probe (RFAP) indicators for the excitation-ratiometric fluorescence imaging of formaldehyde production in living systems. RFAP-1 and RFAP-2 utilize FA-dependent aza-Cope reactivity to convert an alkylamine-functionalized coumarin platform into its aldehyde congener with a ca. 50 nm shift in the excitation wavelength. The probes exhibit visible excitation and emission profiles, and high selectivity for FA over a variety of RCS and related reactive biological analytes, including acetaldehyde, with up to a 6-fold change in the fluorescence ratio. The RFAP indicators can be used to monitor changes in FA levels in biological samples by live-cell imaging and/or flow cytometry. Moreover, RFAP-2 is capable of visualizing differences in the resting FA levels between wild-type cells and models with a gene knockout of ADH5, a major FA-metabolizing enzyme, establishing the utility of this ratiometric detection platform for identifying and probing sources of FA fluxes in biology.

•Fluorescent probes give sensitive and selective detection of formaldehyde.•Formaldehyde detection utilizes aza-Cope, formimine, and aminal reaction-based methods.•Probes successfully image formaldehyde in cells, tissue, and organisms.Formaldehyde (FA) is a common environmental toxin but is also endogenously produced through a diverse array of essential biological processes, including mitochondrial one-carbon metabolism, metabolite oxidation, and nuclear epigenetic modifications. Its high electrophilicity enables reactivity with a wide variety of biological nucleophiles, which can be beneficial or detrimental to cellular function depending on the context. New methods that enable detection of FA in living systems can help disentangle the signal/stress dichotomy of this simplest reactive carbonyl species (RCS), and fluorescent probes for FA with high selectivity and sensitivity have emerged as promising chemical tools in this regard.

Copper is an essential nutrient for life, but at the same time, hyperaccumulation of this redox-active metal in biological fluids and tissues is a hallmark of pathologies such as Wilson’s and Menkes diseases, various neurodegenerative diseases, and toxic environmental exposure. Diseases characterized by copper hyperaccumulation are currently challenging to identify due to costly diagnostic tools that involve extensive technical workup. Motivated to create simple yet highly selective and sensitive diagnostic tools, we have initiated a program to develop new materials that can enable monitoring of copper levels in biological fluid samples without complex and expensive instrumentation. Herein, we report the design, synthesis, and properties of PAF-1-SMe, a robust three-dimensional porous aromatic framework (PAF) densely functionalized with thioether groups for selective capture and concentration of copper from biofluids as well as aqueous samples. PAF-1-SMe exhibits a high selectivity for copper over other biologically relevant metals, with a saturation capacity reaching over 600 mg/g. Moreover, the combination of PAF-1-SMe as a material for capture and concentration of copper from biological samples with 8-hydroxyquinoline as a colorimetric indicator affords a method for identifying aberrant elevations of copper in urine samples from mice with Wilson’s disease and also tracing exogenously added copper in serum. This divide-and-conquer sensing strategy, where functional and robust porous materials serve as molecular recognition elements that can be used to capture and concentrate analytes in conjunction with molecular indicators for signal readouts, establishes a valuable starting point for the use of porous polymeric materials in noninvasive diagnostic applications.

Iron is essential for sustaining life, as its ability to cycle between multiple oxidation states is critical for catalyzing chemical transformations in biological systems. However, without proper regulation, this same redox capacity can trigger oxidative stress events that contribute to aging along with diseases ranging from cancer to cardiovascular and neurodegenerative disorders. Despite its importance, methods for monitoring biological iron bound weakly to cellular ligands−the labile iron pool−to generate a response that preserves spatial and temporal information remain limited, owing to the potent fluorescence quenching ability of iron. We report the design, synthesis, and biological evaluation of FRET Iron Probe 1 (FIP-1), a reactivity-based probe that enables ratiometric fluorescence imaging of labile iron pools in living systems. Inspired by antimalarial natural products and related therapeutics, FIP-1 links two fluorophores (fluorescein and Cy3) through an Fe(II)-cleavable endoperoxide bridge, where Fe(II)-triggered peroxide cleavage leads to a decrease in fluorescence resonance energy transfer (FRET) from the fluorescein donor to Cy3 acceptor by splitting these two dyes into separate fragments. FIP-1 responds to Fe(II) in aqueous buffer with selectivity over competing metal ions and is capable of detecting changes in labile iron pools within living cells with iron supplementation and/or depletion. Moreover, application of FIP-1 to a model of ferroptosis reveals a change in labile iron pools during this form of cell death, providing a starting point to study iron signaling in living systems.

Conversion of the greenhouse gas carbon dioxide (CO2) to value-added products is an important challenge for sustainable energy research, and nanomaterials offer a broad class of heterogeneous catalysts for such transformations. Here we report a molecular surface functionalization approach to tuning gold nanoparticle (Au NP) electrocatalysts for reduction of CO2 to CO. The N-heterocyclic (NHC) carbene-functionalized Au NP catalyst exhibits improved faradaic efficiency (FE = 83%) for reduction of CO2 to CO in water at neutral pH at an overpotential of 0.46 V with a 7.6-fold increase in current density compared to that of the parent Au NP (FE = 53%). Tafel plots of the NHC carbene-functionalized Au NP (72 mV/decade) vs parent Au NP (138 mV/decade) systems further show that the molecular ligand influences mechanistic pathways for CO2 reduction. The results establish molecular surface functionalization as a complementary approach to size, shape, composition, and defect control for nanoparticle catalyst design.

Formaldehyde (FA) is a reactive carbonyl species (RCS) that plays a broad spectrum of roles in epigenetics, toxicology, and progression of diseases ranging from cancer to diabetes to neurodegeneration, motivating the development of translatable technologies for FA imaging. Here we report formaldehyde-caged-[18F]fluorodeoxyglucose-1 ([18F]FAC-FDG-1), an aza-Cope-based reactivity probe for in vivo FA imaging using positron emission tomography (PET). [18F]FAC-FDG-1 reacts selectively with FA over potentially competing analytes to generate [18F]FDG, allowing its FA-dependent uptake and retention in cell culture as well as in animal models. The relative uptake of [18F]FAC-FDG-1 was evaluated using FA-treated PC3 prostate cancer and U87-MG glioblastoma cells demonstrating a dose-dependent response to exogenously added FA. Moreover, [18F]FAC-FDG-1 is capable of FA detection in vivo using a PC3 tumor xenograft model. In addition to providing a unique tool for monitoring FA in living animals, these data establish a general approach for translatable detection of FA and other reactive biological analytes in vivo by exploiting the widely-available clinical [18F]FDG tracer as a masked aldehyde that can be caged by analyte-responsive triggers.

We present the synthesis and characterization of alkali metal salts of the new tetraanionic, tetrapodal ligand 2,2′-(pyridine-2,6-diyl)bis(2-methylmalonate) (A4[PY(CO2)4], A = Li+, Na+, K+, and Cs+), via deprotection of the neutral tetrapodal ligand tetraethyl 2,2′-(pyridine-2,6-diyl)bis(2-methylmalonate) (PY(CO2Et)4). The [PY(CO2)4]4– ligand is composed of an axial pyridine and four equatorial carboxylate groups and must be kept at or below 0 °C to prevent decomposition. Exposing it to a number of divalent first-row transition metals cleanly forms complexes to give the series K2[(PY(CO2)4)M(H2O)] (M = Mn2+, Fe2+, Co2+, Ni2+, Zn2+). The metal complexes were comprehensively characterized via single-crystal X-ray diffraction, 1H NMR and UV–vis absorption spectroscopy, and cyclic voltammetry. Crystal structures reveal that [PY(CO2)4]4– coordinates in a pentadentate fashion to allow for a nearly ideal octahedral coordination geometry upon binding an exogenous water ligand. Additionally, depending on the nature of the charge-balancing countercation (Li+, Na+, or K+), the [(PY(CO2)4)M(H2O)]2– complexes can assemble in the solid state to form one-dimensional channels filled with water molecules. Aqueous electrochemistry performed on [(PY(CO2)4)M(H2O)]2– suggested accessible trivalent oxidation states for the Fe, Co, and Ni complexes, and the trivalent Co3+ species [(PY(CO2)4)Co(OH)]2– could be isolated via chemical oxidation. The successful synthesis of the [PY(CO2)4]4– ligand and its transition metal complexes illustrates the still-untapped versatility within the tetrapodal ligand family, which may yet hold promise for the isolation of more reactive and higher-valent metal complexes.

Copper is a required metal nutrient for life, but global or local alterations in its homeostasis are linked to diseases spanning genetic and metabolic disorders to cancer and neurodegeneration. Technologies that enable longitudinal in vivo monitoring of dynamic copper pools can help meet the need to study the complex interplay between copper status, health, and disease in the same living organism over time. Here, we present the synthesis, characterization, and in vivo imaging applications of Copper-Caged Luciferin-1 (CCL-1), a bioluminescent reporter for tissue-specific copper visualization in living animals. CCL-1 uses a selective copper(I)-dependent oxidative cleavage reaction to release d-luciferin for subsequent bioluminescent reaction with firefly luciferase. The probe can detect physiological changes in labile Cu+ levels in live cells and mice under situations of copper deficiency or overload. Application of CCL-1 to mice with liver-specific luciferase expression in a diet-induced model of nonalcoholic fatty liver disease reveals onset of hepatic copper deficiency and altered expression levels of central copper trafficking proteins that accompany symptoms of glucose intolerance and weight gain. The data connect copper dysregulation to metabolic liver disease and provide a starting point for expanding the toolbox of reactivity-based chemical reporters for cell- and tissue-specific in vivo imaging.

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The potent redox activity of copper is required for sustaining life. Mismanagement of its cellular pools, however, can result in oxidative stress and damage connected to aging, neurodegenerative diseases, and metabolic disorders. Therefore, copper homeostasis is tightly regulated by cells and tissues. Whereas copper and other transition metal ions are commonly thought of as static cofactors buried within protein active sites, emerging data points to the presence of additional loosely bound, labile pools that can participate in dynamic signalling pathways. Against this backdrop, we review advances in sensing labile copper pools and understanding their functions using synthetic fluorescent indicators. Following brief introductions to cellular copper homeostasis and considerations in sensor design, we survey available fluorescent copper probes and evaluate their properties in the context of their utility as effective biological screening tools. We emphasize the need for combined chemical and biological evaluation of these reagents, as well as the value of complementing probe data with other techniques for characterizing the different pools of metal ions in biological systems. This holistic approach will maximize the exciting opportunities for these and related chemical technologies in the study and discovery of novel biology of metals.

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Hydrogen sulfide (H2S), a gaseous species produced by both bacteria and higher eukaryotic organisms, including mammalian vertebrates, has attracted attention in recent years for its contributions to human health and disease. H2S has been proposed as a cytoprotectant and gasotransmitter in many tissue types, including mediating vascular tone in blood vessels as well as neuromodulation in the brain. The molecular mechanisms dictating how H2S affects cellular signaling and other physiological events remain insufficiently understood. Furthermore, the involvement of H2S in metal-binding interactions and formation of related RSS such as sulfane sulfur may contribute to other distinct signaling pathways. Owing to its widespread biological roles and unique chemical properties, H2S is an appealing target for chemical biology approaches to elucidate its production, trafficking, and downstream function. In this context, reaction-based fluorescent probes offer a versatile set of screening tools to visualize H2S pools in living systems. Three main strategies used in molecular probe development for H2S detection include azide and nitro group reduction, nucleophilic attack, and CuS precipitation. Each of these approaches exploits the strong nucleophilicity and reducing potency of H2S to achieve selectivity over other biothiols. In addition, a variety of methods have been developed for the detection of other reactive sulfur species (RSS), including sulfite and bisulfite, as well as sulfane sulfur species and related modifications such as S-nitrosothiols. Access to this growing chemical toolbox of new molecular probes for H2S and related RSS sets the stage for applying these developing technologies to probe reactive sulfur biology in living systems.

Metals are essential for life, playing critical roles in all aspects of the central dogma of biology (e.g., the transcription and translation of nucleic acids and synthesis of proteins). Redox-inactive alkali, alkaline earth, and transition metals such as sodium, potassium, calcium, and zinc are widely recognized as dynamic signals, whereas redox-active transition metals such as copper and iron are traditionally thought of as sequestered by protein ligands, including as static enzyme cofactors, in part because of their potential to trigger oxidative stress and damage via Fenton chemistry. Metals in biology can be broadly categorized into two pools: static and labile. In the former, proteins and other macromolecules tightly bind metals; in the latter, metals are bound relatively weakly to cellular ligands, including proteins and low molecular weight ligands. Fluorescent probes can be useful tools for studying the roles of transition metals in their labile forms. Probes for imaging transition metal dynamics in living systems must meet several stringent criteria. In addition to exhibiting desirable photophysical properties and biocompatibility, they must be selective and show a fluorescence turn-on response to the metal of interest. To meet this challenge, we have pursued two general strategies for metal detection, termed “recognition” and “reactivity”. Our design of transition metal probes makes use of a recognition-based approach for copper and nickel and a reactivity-based approach for cobalt and iron. This Account summarizes progress in our laboratory on both the development and application of fluorescent probes to identify and study the signaling roles of transition metals in biology. In conjunction with complementary methods for direct metal detection and genetic and/or pharmacological manipulations, fluorescent probes for transition metals have helped reveal a number of principles underlying transition metal dynamics. In this Account, we give three recent examples from our laboratory and collaborations in which applications of chemical probes reveal that labile copper contributes to various physiologies. The first example shows that copper is an endogenous regulator of neuronal activity, the second illustrates cellular prioritization of mitochondrial copper homeostasis, and the third identifies the “cuprosome” as a new copper storage compartment in Chlamydomonas reinhardtii green algae. Indeed, recognition- and reactivity-based fluorescent probes have helped to uncover new biological roles for labile transition metals, and the further development of fluorescent probes, including ones with varied Kd values and new reaction triggers and recognition receptors, will continue to reveal exciting and new biological roles for labile transition metals.

Climate change, rising global energy demand, and energy security concerns motivate research into alternative, sustainable energy sources. In principle, solar energy can meet the world’s energy needs, but the intermittent nature of solar illumination means that it is temporally and spatially separated from its consumption. Developing systems that promote solar-to-fuel conversion, such as via reduction of protons to hydrogen, could bridge this production–consumption gap, but this effort requires invention of catalysts that are cheap, robust, and efficient and that use earth-abundant elements. In this context, catalysts that utilize water as both an earth-abundant, environmentally benign substrate and a solvent for proton reduction are highly desirable. This Account summarizes our studies of molecular metal–polypyridyl catalysts for electrochemical and photochemical reduction of protons to hydrogen. Inspired by concept transfer from biological and materials catalysts, these scaffolds are remarkably resistant to decomposition in water, with fast and selective electrocatalytic and photocatalytic conversions that are sustainable for several days. Their modular nature offers a broad range of opportunities for tuning reactivity by molecular design, including altering ancillary ligand electronics, denticity, and/or incorporating redox-active elements. Our first-generation complex, [(PY4)Co(CH3CN)2]2+, catalyzes the reduction of protons from a strong organic acid to hydrogen in 50% water. Subsequent investigations with the pentapyridyl ligand PY5Me2 furnished molybdenum and cobalt complexes capable of catalyzing the reduction of water in fully aqueous electrolyte with 100% Faradaic efficiency. Of particular note, the complex [(PY5Me2)MoO]2+ possesses extremely high activity and durability in neutral water, with turnover frequencies at least 8500 mol of H2 per mole of catalyst per hour and turnover numbers over 600 000 mol of H2 per mole of catalyst over 3 days at an overpotential of 1.0 V, without apparent loss in activity. Replacing the oxo moiety with a disulfide affords [(PY5Me2)MoS2]2+, which bears a molecular MoS2 triangle that structurally and functionally mimics bulk molybdenum disulfide, improving the catalytic activity for water reduction. In water buffered to pH 3, catalysis by [(PY5Me2)MoS2]2+ onsets at 400 mV of overpotential, whereas [(PY5Me2)MoO]2+ requires an additional 300 mV of driving force to operate at the same current density. Metalation of the PY5Me2 ligand with an appropriate Co(ii) source also furnishes electrocatalysts that are active in water. Importantly, the onset of catalysis by the [(PY5Me2)Co(H2O)]2+ series is anodically shifted by introducing electron-withdrawing functional groups on the ligand. With the [(bpy2PYMe)Co(CF3SO3)]1+ system, we showed that introducing a redox-active moiety can facilitate the electro- and photochemical reduction of protons from weak acids such as acetic acid or water. Using a high-throughput photochemical reactor, we examined the structure–reactivity relationship of a series of cobalt(ii) complexes. Taken together, these findings set the stage for the broader application of polypyridyl systems to catalysis under environmentally benign aqueous conditions.

Direct solar-powered production of value-added chemicals from CO2 and H2O, a process that mimics natural photosynthesis, is of fundamental and practical interest. In natural photosynthesis, CO2 is first reduced to common biochemical building blocks using solar energy, which are subsequently used for the synthesis of the complex mixture of molecular products that form biomass. Here we report an artificial photosynthetic scheme that functions via a similar two-step process by developing a biocompatible light-capturing nanowire array that enables a direct interface with microbial systems. As a proof of principle, we demonstrate that a hybrid semiconductor nanowire–bacteria system can reduce CO2 at neutral pH to a wide array of chemical targets, such as fuels, polymers, and complex pharmaceutical precursors, using only solar energy input. The high-surface-area silicon nanowire array harvests light energy to provide reducing equivalents to the anaerobic bacterium, Sporomusa ovata, for the photoelectrochemical production of acetic acid under aerobic conditions (21% O2) with low overpotential (η < 200 mV), high Faradaic efficiency (up to 90%), and long-term stability (up to 200 h). The resulting acetate (∼6 g/L) can be activated to acetyl coenzyme A (acetyl-CoA) by genetically engineered Escherichia coli and used as a building block for a variety of value-added chemicals, such as n-butanol, polyhydroxybutyrate (PHB) polymer, and three different isoprenoid natural products. As such, interfacing biocompatible solid-state nanodevices with living systems provides a starting point for developing a programmable system of chemical synthesis entirely powered by sunlight.

Formaldehyde (FA) is a reactive carbonyl species (RCS) produced in living systems that has been implicated in epigenetics as well as in the pathologies of various cancers, diabetes, and heart, liver, and neurodegenerative diseases. Traditional methods for biological FA detection rely on sample destruction and/or extensive processing, resulting in a loss of spatiotemporal information. To help address this technological gap, we present the design, synthesis, and biological evaluation of a fluorescent probe for live-cell FA imaging that relies on a FA-induced aza-Cope rearrangement. Formaldehyde probe-1 (FAP-1) is capable of detecting physiologically relevant concentrations of FA in aqueous buffer and in live cells with high selectivity over potentially competing biological analytes. Moreover, FAP-1 can visualize endogenous FA produced by lysine-specific demethylase 1 in a breast cancer cell model, presaging the potential utility of this chemical approach to probe RCS biology.

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(II) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron–hole reservoirs for multielectron transformations.

We report the photochemical generation and study of a family of water-soluble iron(IV)-oxo complexes supported by pentapyridine PY5Me2-X ligands (PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine; X = CF3, H, Me, or NMe2), in which the oxidative reactivity of these ferryl species correlates with the electronic properties of the axial pyridine ligand. Synthesis of a systematic series of [FeII(L)(PY5Me2-X)]2+ complexes, where L = CH3CN or H2O, and characterizations by several methods, including X-ray crystallography, cyclic voltammetry, and Mössbauer spectroscopy, show that increasing the electron-donating ability of the axial pyridine ligand tracks with less positive Fe(III)/Fe(II) reduction potentials and quadrupole splitting parameters. The FeII precursors are readily oxidized to their Fe(IV)-oxo counterparts using either chemical outer-sphere oxidants such as CAN (ceric ammonium nitrate) or flash-quench photochemical oxidation with [Ru(bpy)3]2+ as a photosensitizer and K2S2O8 as a quencher. The Fe(IV)-oxo complexes are capable of oxidizing the C–H bonds of alkane (4-ethylbenzenesulfonate) and alcohol (benzyl alcohol) substrates via hydrogen atom transfer (HAT) and an olefin (4-styrenesulfonate) substrate by oxygen atom transfer (OAT). The [FeIV(O)(PY5Me2-X)]2+ derivatives with electron-poor axial ligands show faster rates of HAT and OAT compared to their counterparts supported by electron-rich axial donors, but the magnitudes of these differences are relatively modest.

Natural photosynthesis harnesses solar energy to convert CO2 and water to value-added chemical products for sustaining life. We present a hybrid bioinorganic approach to solar-to-chemical conversion in which sustainable electrical and/or solar input drives production of hydrogen from water splitting using biocompatible inorganic catalysts. The hydrogen is then used by living cells as a source of reducing equivalents for conversion of CO2 to the value-added chemical product methane. Using platinum or an earth-abundant substitute, α-NiS, as biocompatible hydrogen evolution reaction (HER) electrocatalysts and Methanosarcina barkeri as a biocatalyst for CO2 fixation, we demonstrate robust and efficient electrochemical CO2 to CH4 conversion at up to 86% overall Faradaic efficiency for ≥7 d. Introduction of indium phosphide photocathodes and titanium dioxide photoanodes affords a fully solar-driven system for methane generation from water and CO2, establishing that compatible inorganic and biological components can synergistically couple light-harvesting and catalytic functions for solar-to-chemical conversion.

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Reactive oxygen species (ROS) play important roles in the development and progression of cancer and other diseases, motivating the development of translatable technologies for biological ROS imaging. Here we report Peroxy-Caged-[18F]Fluorodeoxy thymidine-1 (PC-FLT-1), an oxidatively immolative positron emission tomography (PET) probe for H2O2 detection. PC-FLT-1 reacts with H2O2 to generate [18F]FLT, allowing its peroxide-dependent uptake and retention in proliferating cells. The relative uptake of PC-FLT-1 was evaluated using H2O2-treated UOK262 renal carcinoma cells and a paraquat-induced oxidative stress cell model, demonstrating ROS-dependent tracer accumulation. The data suggest that PC-FLT-1 possesses promising characteristics for translatable ROS detection and provide a general approach to PET imaging that can be expanded to the in vivo study of other biologically relevant analytes.

For reasons that remain insufficiently understood, the brain requires among the highest levels of metals in the body for normal function. The traditional paradigm for this organ and others is that fluxes of alkali and alkaline earth metals are required for signaling, but transition metals are maintained in static, tightly bound reservoirs for metabolism and protection against oxidative stress. Here we show that copper is an endogenous modulator of spontaneous activity, a property of functional neural circuitry. Using Copper Fluor-3 (CF3), a new fluorescent Cu+ sensor for one- and two-photon imaging, we show that neurons and neural tissue maintain basal stores of loosely bound copper that can be attenuated by chelation, which define a labile copper pool. Targeted disruption of these labile copper stores by acute chelation or genetic knockdown of the CTR1 (copper transporter 1) copper channel alters the spatiotemporal properties of spontaneous activity in developing hippocampal and retinal circuits. The data identify an essential role for copper neuronal function and suggest broader contributions of this transition metal to cell signaling.

Growing global energy demands and climate change motivate the development of new renewable energy technologies. In this context, water splitting using sustainable energy sources has emerged as an attractive process for carbon-neutral fuel cycles. A key scientific challenge to achieving this overall goal is the invention of new catalysts for the reductive and oxidative conversions of water to hydrogen and oxygen, respectively. This review article will highlight progress in molecular electrochemical approaches for catalytic reduction of protons to hydrogen, focusing on complexes of earth-abundant metals that can function in pure aqueous or mixed aqueous–organic media. The use of water as a reaction medium has dual benefits of maintaining high substrate concentration as well as minimizing the environmental impact from organic additives and by-products.

A cobalt-sulfide (Co–S) film prepared via electrochemical deposition on conductive substrates is shown to behave as an efficient and robust catalyst for electrochemical and photoelectrochemical hydrogen generation from neutral pH water. Electrochemical experiments demonstrate that the film exhibits a low catalytic onset overpotential (η) of 43 mV, a Tafel slope of 93 mV/dec, and near 100% Faradaic efficiency in pH 7 phosphate buffer. Catalytic current densities can approach 50 mA/cm2 and activity is maintained for at least 40 h. The catalyst can also be electrochemically coated on silicon, rendering a water-compatible photoelectrochemical system for hydrogen production under simulated 1 sun illumination. The facile preparation of this Co–S film, along with its low overpotential, high activity, and long-term aqueous stability, offer promising features for potential use in solar energy applications.

Iron is an essential metal for living organisms, but misregulation of its homeostasis at the cellular level can trigger detrimental oxidative and/or nitrosative stress and damage events. Motivated to help study the physiological and pathological consequences of biological iron regulation, we now report a reaction-based strategy for monitoring labile Fe2+ pools in aqueous solution and living cells. Iron Probe 1 (IP1) exploits a bioinspired, iron-mediated oxidative C–O bond cleavage reaction to achieve a selective turn-on response to Fe2+ over a range of cellular metal ions in their bioavailable forms. We show that this first-generation chemical tool for fluorescence Fe2+ detection can visualize changes in exchangeable iron stores in living cells upon iron supplementation or depletion, including labile iron pools at endogenous, basal levels. Moreover, IP1 can be used to identify reversible expansion of labile iron pools by stimulation with vitamin C or the iron regulatory hormone hepcidin, providing a starting point for further investigations of iron signaling and stress events in living systems as well as future probe development.

The solar-driven reduction of carbon dioxide to value-added chemical fuels is a longstanding challenge in the fields of catalysis, energy science, and green chemistry. In order to develop effective CO2 fixation, several key considerations must be balanced, including (1) catalyst selectivity for promoting CO2 reduction over competing hydrogen generation from proton reduction, (2) visible-light harvesting that matches the solar spectrum, and (3) the use of cheap and earth-abundant catalytic components. In this report, we present the synthesis and characterization of a new family of earth-abundant nickel complexes supported by N-heterocyclic carbene–amine ligands that exhibit high selectivity and activity for the electrocatalytic and photocatalytic conversion of CO2 to CO. Systematic changes in the carbene and amine donors of the ligand have been surveyed, and [Ni(Prbimiq1)]2+ (1c, where Prbimiq1 = bis(3-(imidazolyl)isoquinolinyl)propane) emerges as a catalyst for electrochemical reduction of CO2 with the lowest cathodic onset potential (Ecat = −1.2 V vs SCE). Using this earth-abundant catalyst with Ir(ppy)3 (where ppy = 2-phenylpyridine) and an electron donor, we have developed a visible-light photoredox system for the catalytic conversion of CO2 to CO that proceeds with high selectivity and activity and achieves turnover numbers and turnover frequencies reaching 98,000 and 3.9 s–1, respectively. Further studies reveal that the overall efficiency of this solar-to-fuel cycle may be limited by the formation of the active Ni catalyst and/or the chemical reduction of CO2 to CO at the reduced nickel center and provide a starting point for improved photoredox systems for sustainable carbon-neutral energy conversion.

In vivo molecular imaging holds promise for understanding the underlying mechanisms of health, injury, aging, and disease, as it can detect distinct biochemical processes such as enzymatic activity, reactive small-molecule fluxes, or post-translational modifications. Current imaging techniques often detect only a single biochemical process, but, within whole organisms, multiple types of biochemical events contribute to physiological and pathological phenotypes. In this report, we present a general strategy for dual-analyte detection in living animals that employs in situ formation of firefly luciferin from two complementary caged precursors that can be unmasked by different biochemical processes. To establish this approach, we have developed Peroxy Caged Luciferin-2 (PCL-2), a H2O2-responsive boronic acid probe that releases 6-hydroxy-2-cyanobenzothiazole (HCBT) upon reacting with this reactive oxygen species, as well as a peptide-based probe, z-Ile-Glu-ThrAsp-d-Cys (IETDC), which releases d-cysteine in the presence of active caspase 8. Once released, HCBT and d-cysteine form firefly luciferin in situ, giving rise to a bioluminescent signal if and only if both chemical triggers proceed. This system thus constitutes an AND-type molecular logic gate that reports on the simultaneous presence of H2O2 and caspase 8 activity. Using these probes, chemoselective imaging of either H2O2 or caspase 8 activity was performed in vitro and in vivo. Moreover, concomitant use of PCL-2 and IETDC in vivo establishes a concurrent increase in both H2O2 and caspase 8 activity during acute inflammation in living mice. Taken together, this method offers a potentially powerful new chemical tool for studying simultaneous oxidative stress and inflammation processes in living animals during injury, aging, and disease, as well as a versatile approach for concurrent monitoring of multiple analytes using luciferin-based bioluminescence imaging technologies.

Recently, a family of cobalt pentapyridine complexes of the type [(R-PY5Me2)Co(H2O)])(CF3SO3)2, (R = CF3, H, or NMe2; PY5Me2 = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine) were shown to catalyze the electrochemical generation of hydrogen from neutral aqueous solutions using a mercury electrode. We now report that the CF3 derivative of this series, [(CF3PY5Me2)Co(H2O)](CF3SO3)2 (1), can also operate in neutral water as an electrocatalyst for hydrogen generation under soluble, diffusion-limited conditions on a glassy carbon electrode, as well as a photocatalyst for hydrogen production using either molecular or semiconductor nanowire photosensitizers. Owing to its relatively low overpotential compared to other members of the PY5 family, complex 1 exhibits multiple redox features on glassy carbon, including a one-proton, one-electron coupled oxidative wave. Further, rotating disk electrode voltammetry measurements reveal the efficacy of 1 as a competent hydrogen evolution catalyst under soluble, diffusion-limited conditions. In addition, we establish that 1 can also generate hydrogen from neutral water under photocatalytic conditions with visible light irradiation (λirr ≥ 455 nm), using [Ru(bpy)3]2+ as a molecular inorganic chromophore and ascorbic acid as a sacrificial donor. Dynamic light scattering measurements show no evidence for nanoparticle formation for the duration of the photolytic hydrogen evolution experiments. Finally, we demonstrate that 1 is also able to enhance the hydrogen photolysis yield of GaP nanowires in water, showing that this catalyst is compatible with solid-state photosensitizers. Taken together, these data establish that the well-defined cobalt pentapyridine complex [(CF3PY5Me2)Co(H2O)]2+ is a versatile catalyst for hydrogen production from pure aqueous solutions using either solar or electrical input, providing a starting point for integrating molecular systems into sustainable energy generation devices.

The pentapyridine cobalt complex [Co(PY5Me2)]2+ and its congeners have been shown to catalyze proton reduction to hydrogen in aqueous solution over a wide pH range using electrical or solar energy input. Here, we employ electrochemical and spectroscopic studies to examine the mechanisms of proton reduction by this parent complex under soluble, diffusion-limited conditions in acetonitrile with acetic acid as the proton donor. Two pathways for proton reduction are identified via cyclic voltammetry: one pathway occurring from an acetonitrile-bound CoII/I couple and the other pathway operating from an acetate-bound CoII/I couple. Kinetics studies support protonation of a CoI species as the rate-determining step for both processes, and additional electrochemical measurements further suggest that the onset of catalysis from the acetonitrile-bound CoII/I couple is highly affected by catalyst electronics. Taken together, this work not only establishes the CoPY5Me2 unit as a unique molecular platform that catalyzes the reduction of protons under soluble, diffusion-limited conditions in both aqueous and organic media, but also highlights the participation of anation processes that are likely relevant for a wide range of hydrogen-producing and related catalytic systems.

Hydrogen sulfide (H2S) is a reactive small molecule generated in the body that can be beneficial or toxic owing to its potent redox activity. In living systems, disentangling the pathways responsible for H2S production and their physiological and pathological consequences remains a challenge in part due to a lack of methods for monitoring changes in endogenous H2S fluxes. The development of fluorescent probes with appropriate selectivity and sensitivity for monitoring production of H2S at biologically relevant signaling levels offers opportunities to explore its roles in a variety of systems. Here we report the design, synthesis, and application of a family of azide-based fluorescent H2S indicators, Sulfidefluor-4, Sulfidefluor-5 acetoxymethyl ester, and Sulfidefluor-7 acetoxymethyl ester, which offer the unique capability to image H2S generated at physiological signaling levels. These probes are optimized for cellular imaging and feature enhanced sensitivity and cellular retention compared with our previously reported molecules. In particular, Sulfidefluor-7 acetoxymethyl ester allows for direct, real-time visualization of endogenous H2S produced in live human umbilical vein endothelial cells upon stimulation with vascular endothelial growth factor (VEGF). Moreover, we show that H2S production is dependent on NADPH oxidase–derived hydrogen peroxide (H2O2), which attenuates VEGF receptor 2 phosphorylation and establishes a link for H2S/H2O2 crosstalk.

Hydrogen sulfide (H2S) is a reactive small molecule generated in the body that can be beneficial or toxic owing to its potent redox activity. In living systems, disentangling the pathways responsible for H2S production and their physiological and pathological consequences remains a challenge in part due to a lack of methods for monitoring changes in endogenous H2S fluxes. The development of fluorescent probes with appropriate selectivity and sensitivity for monitoring production of H2S at biologically relevant signaling levels offers opportunities to explore its roles in a variety of systems. Here we report the design, synthesis, and application of a family of azide-based fluorescent H2S indicators, Sulfidefluor-4, Sulfidefluor-5 acetoxymethyl ester, and Sulfidefluor-7 acetoxymethyl ester, which offer the unique capability to image H2S generated at physiological signaling levels. These probes are optimized for cellular imaging and feature enhanced sensitivity and cellular retention compared with our previously reported molecules. In particular, Sulfidefluor-7 acetoxymethyl ester allows for direct, real-time visualization of endogenous H2S produced in live human umbilical vein endothelial cells upon stimulation with vascular endothelial growth factor (VEGF). Moreover, we show that H2S production is dependent on NADPH oxidase–derived hydrogen peroxide (H2O2), which attenuates VEGF receptor 2 phosphorylation and establishes a link for H2S/H2O2 crosstalk.

We recently reported the catalytic generation of hydrogen from water mediated through the in situ reduction of the molybdenum(IV)–oxo complex [(PY5Me2)MoO]2+ (1; PY5Me2 = 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine) at a mercury electrode. To gain further insight into this unique molecular motif for hydrogen production, we have now examined the competence of this complex for the catalytic reduction of protons on an alternative electrode material. Herein, we demonstrate the ability of the molybdenum–oxo complex 1 to reduce protons at a glassy carbon electrode in acidic organic media, where the active catalyst is shown to be diffusing freely in solution. Cyclic and rotating disk voltammetry experiments reveal that three reductive electrochemical processes precede the catalytic generation of hydrogen, which occurs at potentials more negative than −1.25 V vs. SHE. Gas chromatographic analysis of the bulk electrolysis cell headspace confirms that hydrogen is generated at a Faradaic efficiency of 99%. Under pseudo-first order conditions with an acid-to-catalyst ratio of >290, a rate constant of 385 s−1 is calculated for the reduction of acetic acid in acetonitrile. Taken together, these data show that metal–oxo complex 1 is a competent molecular motif for catalytic generation of hydrogen from protons under soluble and diffusion-limited conditions.

Carbon monoxide is a member of the gasotransmitter family, which also includes NO and H2S, and has been implicated in a variety of pathological and physiological conditions. Whereas exogenous therapeutic additions of CO to tissues and whole animals have been well-studied, the real-time spatial and temporal tracking of CO at the cellular level remains an open challenge. Here we report a new type of turn-on fluorescent probe for selective CO detection based on palladium-mediated carbonylation reactivity. CO Probe 1 (COP-1) is capable of detecting CO both in aqueous buffer and in live cells with high selectivity over a range of biologically relevant reactive small molecules, providing a potentially powerful approach for interrogating its chemistry in biological systems.

We report the generation and characterization of a new high-spin iron(IV)–oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpaMes)FeII]− (tpaAr = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)–alkoxide product [(tpaMes2MesO)FeIII]− resulting from intramolecular C–H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpaPh)FeII]− with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)–oxo complex [(tpaPh)FeIV(O)]− that has been characterized by a suite of techniques, including mass spectrometry as well as UV–vis, FTIR, Mössbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron–oxo chromophore, and Mössbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)–oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpaPh complex is capable of mediating intermolecular C–H oxidation as well as oxygen-atom transfer chemistry.

We present the synthesis and characterization of octaarginine-conjugated Copper-Gad-2 (Arg8CG2), a new copper-responsive magnetic resonance imaging (MRI) contrast agent that combines a Gd3+-DO3A scaffold with a thioether-rich receptor for copper recognition. The inclusion of a polyarginine appendage leads to a marked increase in cellular uptake compared to previously reported MRI-based copper sensors of the CG family. Arg8CG2 exhibits a 220% increase in relaxivity (r1 = 3.9 to 12.5 mM−1 s−1) upon 1:1 binding with Cu+, with a highly selective response to Cu+ over other biologically relevant metal ions. Moreover, Arg8CG2 accumulates in cells at nine-fold greater concentrations than the parent CG2 lacking the polyarginine functionality and is retained well in the cell after washing. In cellulo relaxivity measurements and T1-weighted phantom images using a Menkes disease model cell line demonstrate the utility of Arg8CG2 to report on biological perturbations of exchangeable copper pools.

Hydrogen sulfide (H2S) has long been recognized as a toxic molecule in biological systems. However, emerging studies now link controlled fluxes of this reactive sulfur species to cellular regulation and signaling events akin to other small molecule messengers, such as nitric oxide, hydrogen peroxide, and carbon monoxide. Progress in the development of fluorescent small-molecule indicators with high selectivity for hydrogen sulfide offers a promising approach for studying its production, trafficking, and downstream physiological and/or pathological effects.Highlights► Hydrogen sulfide (H2S) can mediate physiology and disease pathology in mammalian systems. ► The development of molecular probes for H2S detection allows H2S to be studied in intact cells, tissues, or whole organisms. ► Current probes employ a variety of reaction-based strategies to achieve selectivity for H2S over other biological thiols.

A reaction-based strategy exploiting cobalt-mediated oxidative C–O bond cleavage affords a selective turn-on fluorescent probe for paramagnetic Co2+ in water and in living cells.

A pseudotetrahedral cobalt(II) complex with a positive axial zero-field splitting parameter of D = 12.7 cm−1, as determined by high-field EPR spectroscopy, is shown to exhibit slow magnetic relaxation under an applied dc field.

We report the synthesis and structural characterization of a family of well-defined organoazide complexes supported by a three-fold-symmetric pyrrolide scaffold and their conversion to the corresponding terminal imido congeners. Kinetic measurements on a series of structurally homologous but electronically distinct vanadium organoazide complexes reveal that the azido-to-imido transformations proceed via a process that is first-order in the metal complex and has a positive entropy of activation. Further studies suggest that these reactions may involve metal-mediated generation and capture of nitrene fragments.

Copper is an essential metal nutrient that is tightly regulated in the body because loss of its homeostasis is connected to severe diseases such as Menkes and Wilson diseases, Alzheimer’s disease, prion disorders, and amyotrophic lateral sclerosis. The complex relationships between copper status and various stages of health and disease remain challenging to elucidate, in part due to a lack of methods for monitoring dynamic changes in copper pools in whole living organisms. Here we present the synthesis, spectroscopy, and in vivo imaging applications of Coppersensor 790, a first-generation fluorescent sensor for visualizing labile copper pools in living animals. Coppersensor 790 combines a near-infrared emitting cyanine dye with a sulfur-rich receptor to provide a selective and sensitive turn-on response to copper. This probe is capable of monitoring fluctuations in exchangeable copper stores in living cells and mice under basal conditions, as well as in situations of copper overload or deficiency. Moreover, we demonstrate the utility of this unique chemical tool to detect aberrant increases in labile copper levels in a murine model of Wilson disease, a genetic disorder that is characterized by accumulation of excess copper. The ability to monitor real-time copper fluxes in living animals offers potentially rich opportunities to examine copper physiology in health and disease.

Reactive oxygen species (ROS), such as hydrogen peroxide, are important products of oxygen metabolism that, when misregulated, can accumulate and cause oxidative stress inside cells. Accordingly, organisms have evolved molecular systems, including antioxidant metalloenzymes (such as superoxide dismutase and catalase) and an array of thiol-based redox couples, to neutralize this threat to the cell when it occurs. On the other hand, emerging evidence shows that the controlled generation of ROS, particularly H2O2, is necessary to maintain cellular fitness. The identification of NADPH oxidase enzymes, which generate specific ROS and reside in virtually all cell types throughout the body, is a prime example. Indeed, a growing body of work shows that H2O2 and other ROS have essential functions in healthy physiological signaling pathways.The signal–stress dichotomy of H2O2 serves as a source of motivation for disentangling its beneficial from its detrimental effects on living systems. Molecular imaging of this oxygen metabolite with reaction-based probes is a powerful approach for real-time, noninvasive monitoring of H2O2 chemistry in biological specimens, but two key challenges to studying H2O2 in this way are chemoselectivity and bioorthogonality of probe molecules. Chemoselectivity is problematic because traditional methods for ROS detection suffer from nonspecific reactivity with other ROS. Moreover, some methods require enzymatic additives not compatible with live-cell or live-animal specimens. Additionally, bioorthogonality requires that the reactions must not compete with or disturb intrinsic cellular chemistry; this requirement is particularly critical with thiol- or metal-based couples mediating the major redox events within the cell.Chemoselective bioorthogonal reactions, such as alkyne–azide cycloadditions and related click reactions, the Staudinger–Bertozzi ligation, and the transformations used in various reaction-based molecular probes, have found widespread application in the modification, labeling, and detection of biological molecules and processes. In this Account, we summarize H2O2 studies from our laboratory using the H2O2-mediated oxidation of aryl boronates to phenols as a bioorthogonal approach to detect fluxes of this important ROS in living systems. We have installed this versatile switch onto organic and inorganic scaffolds to serve as “turn-on” probes for visible and near-infrared (NIR) fluorescence, ratiometric fluorescence, time-gated lanthanide luminescence, and in vivo bioluminescence detection of H2O2 in living cells and animals. Further chemical and genetic manipulations target these probes to specific organelles and other subcellular locales and can also allow them to be trapped intracellularly, enhancing their sensitivity. These novel chemical tools have revealed fundamental new biological insights into the production, localization, trafficking, and in vivo roles of H2O2 in a wide variety of living systems, including immune, cancer, stem, and neural cell models.

Hydrogen sulfide (H2S) is emerging as an important mediator of human physiology and pathology but remains difficult to study, in large part because of the lack of methods for selective monitoring of this small signaling molecule in live biological specimens. We now report a pair of new reaction-based fluorescent probes for selective imaging of H2S in living cells that exploit the H2S-mediated reduction of azides to fluorescent amines. Sulfidefluor-1 (SF1) and Sulfidefluor-2 (SF2) respond to H2S by a turn-on fluorescence signal enhancement and display high selectivity for H2S over other biologically relevant reactive sulfur, oxygen, and nitrogen species. In addition, SF1 and SF2 can be used to detect H2S in both water and live cells, providing a potentially powerful approach for probing H2S chemistry in biological systems.

We present the design, synthesis, spectroscopy, and biological applications of Mitochondrial Coppersensor-1 (Mito-CS1), a new type of targetable fluorescent sensor for imaging exchangeable mitochondrial copper pools in living cells. Mito-CS1 is a bifunctional reporter that combines a Cu+-responsive fluorescent platform with a mitochondrial-targeting triphenylphosphonium moiety for localizing the probe to this organelle. Molecular imaging with Mito-CS1 establishes that this new chemical tool can detect changes in labile mitochondrial Cu+ in a model HEK 293T cell line as well as in human fibroblasts. Moreover, we utilized Mito-CS1 in a combined imaging and biochemical study in fibroblasts derived from patients with mutations in the two synthesis of cytochrome c oxidase 1 and 2 proteins (SCO1 and SCO2), each of which is required for assembly and metalation of functionally active cytochrome c oxidase (COX). Interestingly, we observe that although defects in these mitochondrial metallochaperones lead to a global copper deficiency at the whole cell level, total copper and exchangeable mitochondrial Cu+ pools in SCO1 and SCO2 patient fibroblasts are largely unaltered relative to wild-type controls. Our findings reveal that the cell maintains copper homeostasis in mitochondria even in situations of copper deficiency and mitochondrial metallochaperone malfunction, illustrating the importance of regulating copper stores in this energy-producing organelle.

We report a new reaction-based approach for the detection of hydrogen peroxide (H2O2) using hyperpolarized 13C magnetic resonance imaging (13C MRI) and the H2O2-mediated oxidation of α-ketoacids to carboxylic acids. 13C-Benzoylformic acid reacts selectively with H2O2 over other reactive oxygen species to generate 13C-benzoic acid and can be hyperpolarized using dynamic nuclear polarization, providing a method for dual-frequency detection of H2O2. Phantom images collected using frequency-specific imaging sequences demonstrate the efficacy of this responsive contrast agent to monitor H2O2 at pre-clinical field strengths. The combination of reaction-based detection chemistry and hyperpolarized 13C MRI provides a potentially powerful new methodology for non-invasive multi-analyte imaging in living systems.

Nitrous oxide (N2O), a widespread greenhouse gas, is a thermodynamically potent and environmentally green oxidant that is an attractive target for activation by metal centers. However, N2O remains underutilized owing to its high kinetic stability, and the poor ligand properties of this molecule have made well-characterized metal−N2O complexes a rarity. We now report a vanadium−pyrrolide system that reversibly binds N2O at room temperature and provide the first single-crystal X-ray structure of such a complex. Further characterization by vibrational spectroscopy and DFT calculations strongly favor assignment as a linear, N-bound metal−N2O complex.

A set of robust molecular cobalt catalysts for the generation of hydrogen from water is reported. The cobalt complex supported by the parent pentadentate polypyridyl ligand PY5Me2 features high stability and activity and 100% Faradaic efficiency for the electrocatalytic production of hydrogen from neutral water, with a turnover number reaching 5.5 × 104 mol of H2 per mole of catalyst with no loss in activity over 60 h. Control experiments establish that simple Co(II) salts, the free PY5Me2 ligand, and an isostructural PY5Me2 complex containing redox-inactive Zn(II) are all ineffective for this reaction. Further experiments demonstrate that the overpotential for H2 evolution can be tuned by systematic substitutions on the ancillary PY5Me2 scaffold, presaging opportunities to further optimize this first-generation platform by molecular design.

We present a modular dendrimer-based platform for simultaneous dual fluorescence imaging of hydrogen peroxide (H2O2) and pH gradients produced in living cells. Acetyl-capped G5 PAMAM dendrimers functionalized with boronate-caged Peroxyfluor-1 (PF1) fluorophores for H2O2 detection and semi-naphthorhodafluor (SNARF2) dyes for pH sensing provide a single probe system that is capable of sensing multiple analytes at a time by multicolor fluorescence imaging. Spectroscopic measurements of the doubly-labeled dendrimer conjugates establish their ability to simultaneously monitor changes in both H2O2 and pH using different excitation/emission profiles. Moreover, this dual-probe platform allows for selective discrimination between H2O2 and pH changes in live RAW 264.7 macrophage cells when stimulated by an immune insult. Further imaging experiments show that pharmacological inhibition of NADPH oxidase (Nox) proteins triggers a decrease in both oxidative burst and in pH regulation within phagocytic compartments and leads to disruptions of endocytic activity, suggesting that Nox-derived H2O2 signaling is critical to the maintenance of multiple components of the immune response. This work establishes a general molecular platform for simultaneous, real-time imaging of multiple analytes associated with redox biology in living systems and should be applicable to a wide range of chemosensor constructs.

We report a homologous series of nickel(II) complexes supported by N-heterocyclic carbene–pyridine (Rbimpy, R = Me, Et, Pr) ligands that exhibit high selectivity for reducing carbon dioxide over water under electrocatalytic conditions.

We report a two-photon fluorescent probe (PN1) that can be excited by 750 nm femto-second pulses, shows high photostability and negligible toxicity, and can visualize H2O2 distribution in live cells and tissue by two-photon microscopy.

Hydrogen peroxide (H2O2) can serve as a beneficial signaling agent or toxin depending on its concentration and location within a cell or organism. Methods to measure the localized accumulation of H2O2 in living specimens remain limited. Motivated to meet this need, we have developed a nuclear-localized fluorescent probe for H2O2, Nuclear Peroxy Emerald 1 (NucPE1), to selectively interrogate ROS fluxes within this sensitive organelle. NucPE1 selectively accumulates in the nuclei of a variety of mammalian cell lines as well as in whole model organisms like Caenorhabditis elegans, where it can respond to subcellular changes in H2O2 fluxes. Moreover, in vivo NucPE1 imaging reveals a reduction in nuclear H2O2 levels in worms overexpressing sir-2.1 compared with wild-type congeners, supporting a link between this longevity-promoting sirtuin protein and enhanced regulation of nuclear ROS pools.Graphical AbstractFigure optionsDownload full-size imageDownload high-quality image (211 K)Download as PowerPoint slideHighlights► NucPE1 selectively responds to H2O2 with a turn-on fluorescent response ► NucPE1 can detect H2O2 in the nuclei of a variety of common cell lines ► NucPE1 can detect H2O2 in vivo within the nuclei of C. elegans ► C. elegans overexpressing sir-2.1 have enhanced nuclear protection against H2O2

We present a new structurally determined seven-coordinate iron platform supported by the tris(2-picolyl)amine ligand 6,6′-(pyridin-2-ylmethylazanediyl)bis(methylene)bis(N-tert-butylpicolinamide) (TPA2C(O)NHtBu, 3) and its reactivity with oxo and nitrene transfer agents. Oxidation of the pentagonal bipyramidal, seven-coordinate iron(II)–triflate complex [TPA2C(O)NHtBuFeII(OTf)][OTf] (4) with PhIO produces the corresponding diiron(III) μ-oxo complex [(TPA2C(O)NHtBuFeIII)2(O)][OTf]4 (5). Mössbauer and magnetic measurements on 5 in the solid-state establish antiferromagnetic coupling between its two Fe(III) centers. Reactions of 4 with the nitrene transfer agents PhINTs (Ts = p-MeC6H4SO2) and PhINNs (Ns = p-NO2C6H4SO2) provide the corresponding iron(III)-amide congeners [TPA2C(O)NHtBuFeIII(NHTs)][OTf]2 (6) and [TPA2C(O)NHtBuFeIII(NHNs)][OTf]2 (7), respectively, affording a rare pair of isolable Fe(III)-amide compounds formed from nitrene transfer. By characterizing well-defined products in the crystalline form, derived from atom and group transfer to seven-coordinate iron, the collective data provide a starting point for the exploration of high-valent and metal–ligand multiply bonded species supported by approximate pentagonal-type ligand fields.Graphical abstractWe present a new seven-coordinate iron(II) platform and its reactivity with oxo and nitrene transfer agents to give diiron(III) μ-oxo and iron(III) amido complexes characterized by single-crystal X-ray diffraction and Mössbauer spectroscopy. By obtaining well-defined products in crystalline form, derived from group transfer to seven-coordinate iron, the collective data provide a starting point for the exploration of high-valent and metal-ligand multiply bonded species supported by approximate pentagonal-type ligand fields with different donor sets in the equatorial plane.Research highlights► A pentagonal-enforcing ligand gives a new seven-coordinate iron(II) platform. ► Reactivity with oxo and nitrene transfer agents gives diiron(III) μ-oxo and iron(III) amido complexes. ► Mössbauer spectroscopy establishes the electronic structures of the iron(III) products.

The pentagonal bipyramidal high-spin iron(II) complex, [(TPA2C(O)NHtBu)Fe(CF3SO3)]+, is shown to exhibit a high-anisotropy ground state, with fits to dc magnetization data providing an axial zero-field splitting parameter of D = − 7.9 cm−1. The utility of this compound as a building unit is demonstrated, as its reaction with [ReCl4(CN)2]2− affords the cyano-bridged dinuclear cluster (TPA2C(O)NHtBu)FeReCl4(CN)2. dc magnetic susceptibility measurements reveal intracluster ferromagnetic exchange interactions between FeII and ReIV centers, with J = +3.0 cm−1, giving rise to a spin ground state of S = 7/2. Moreover, fits to dc magnetization data obtained for the FeRe cluster show the presence of strong axial anisotropy, with D = −2.3 cm−1. Finally, variable-frequency ac susceptibility measurements reveal the onset of slow magnetic relaxation at low temperature, suggesting that the FeRe cluster is a single-molecule magnet.Graphical abstractThe pentagonal bipyramidal high-spin iron(II) complex, [(TPA2C(O)NHtBu)FeII(CF3SO3)]+, which exhibits strong uniaxial magnetic anisotropy, reacts with [ReCl4(CN)2]2− to generate the cyano-bridged dinuclear coordination cluster (TPA2C(O)NHtBu)FeReCl4(CN)2. Ferromagnetic exchange coupling between FeII and ReIV ions within the cluster leads to an overall spin ground state of S = 7/2. Uniaxial zero-field splitting of the spin ground state, induced by the single-ion anisotropy of the constituent metal centers, leads to slow magnetic relaxation in the cluster at low temperature.Research highlights► The complex [(TPA2C(O)NHtBu)FeII(CF3SO3)]+ exhibits strong uniaxial anisotropy. ► The Fe complex reacts with [ReCl4(CN)2]2− to form an FeRe cluster. ► Ferromagnetic coupling in the FeRe cluster gives rise to an S = 7/2 ground state. ► The FeRe cluster exhibits uniaxial anisotropy and slow magnetic relaxation.

Dynamic fluxes of s-block metals like potassium, sodium, and calcium are of broad importance in cell signaling. In contrast, the concept of mobile transition metals triggered by cell activation remains insufficiently explored, in large part because metals like copper and iron are typically studied as static cellular nutrients and there are a lack of direct, selective methods for monitoring their distributions in living cells. To help meet this need, we now report Coppersensor-3 (CS3), a bright small-molecule fluorescent probe that offers the unique capability to image labile copper pools in living cells at endogenous, basal levels. We use this chemical tool in conjunction with synchotron-based microprobe X-ray fluorescence microscopy (XRFM) to discover that neuronal cells move significant pools of copper from their cell bodies to peripheral processes upon their activation. Moreover, further CS3 and XRFM imaging experiments show that these dynamic copper redistributions are dependent on calcium release, establishing a link between mobile copper and major cell signaling pathways. By providing a small-molecule fluorophore that is selective and sensitive enough to image labile copper pools in living cells under basal conditions, CS3 opens opportunities for discovering and elucidating functions of copper in living systems.

This tutorial review highlights progress in the development of responsive magnetic resonance imaging (MRI) contrast agents for detecting and sensing biologically relevant metal ions. Molecular imaging with bioactivatable MRI indicators offers a potentially powerful methodology for studying the physiology and pathology of metals by capturing dynamic three-dimensional images of living systems for research and clinical applications. This emerging area at the interface of inorganic chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from coordination chemistry and spectroscopy to supramolecular chemistry and molecular recognition to metals in biology and medicine.

We present a new family of fluorescent probes with varying emission colors for selectively imaging hydrogen peroxide (H2O2) generated at physiological cell signaling levels. This structurally homologous series of fluorescein- and rhodol-based reporters relies on a chemospecific boronate-to-phenol switch to respond to H2O2 over a panel of biologically relevant reactive oxygen species (ROS) with tunable excitation and emission maxima and sensitivity to endogenously produced H2O2 signals, as shown by studies in RAW264.7 macrophages during the phagocytic respiratory burst and A431 cells in response to EGF stimulation. We further demonstrate the utility of these reagents in multicolor imaging experiments by using one of the new H2O2-specific probes, Peroxy Orange 1 (PO1), in conjunction with the green-fluorescent highly reactive oxygen species (hROS) probe, APF. This dual-probe approach allows for selective discrimination between changes in H2O2 and hypochlorous acid (HOCl) levels in live RAW264.7 macrophages. Moreover, when macrophages labeled with both PO1 and APF were stimulated to induce an immune response, we discovered three distinct types of phagosomes: those that generated mainly hROS, those that produced mainly H2O2, and those that possessed both types of ROS. The ability to monitor multiple ROS fluxes simultaneously using a palette of different colored fluorescent probes opens new opporunities to disentangle the complex contributions of oxidation biology to living systems by molecular imaging.

We present the synthesis, properties, and biological applications of Ratio-Coppersensor-1 (RCS1), a new water-soluble fluorescent sensor for ratiometric imaging of copper in living cells. RCS1 combines an asymmetric BODIPY reporter and thioether-based ligand receptor to provide high selectivity and sensitivity for Cu+ over other biologically relevant metal ions, including Cu2+ and Zn2+, a ca. 20-fold fluorescence ratio change upon Cu+ binding, and visible excitation and emission profiles compatible with standard fluorescence microscopy filter sets. Live-cell confocal microscopy experiments show that RCS1 is membrane-permeable and can sense changes in the levels of labile Cu+ pools within living cells by ratiometric imaging, including expansion of endogenous stores of exchangeable intracellular Cu+ triggered by ascorbate stimulation in kidney and brain cells.

Hydrogen peroxide (H2O2) can exert diverse signaling and stress responses within living systems depending on its spatial and temporal dynamics. Here we report a new small-molecule probe for producing H2O2 on demand upon photoactivation and its application for optical regulation of cofilin-actin rod formation in living cells. This chemical method offers many potential opportunities for dissecting biological roles for H2O2 as well as remote control of cell behavior via H2O2-mediated pathways.

Hydrogen peroxide (H2O2) is a potent small-molecule oxidant that can exert a diverse array of physiological and/or pathological effects within living systems depending on the timing and location of its production, accumulation, trafficking, and consumption. To help study the chemistry and biology of this reactive oxygen species (ROS) in its native cellular context, we now present a new method for monitoring local, subcellular changes in H2O2 levels by fluorescence imaging. Specifically, we have exploited the versatility of the SNAP-tag technology for site-specific protein labeling with small molecules on the surface or interior of living cells with the use of boronate-capped dyes to selectively visualize H2O2. The resulting SNAP-Peroxy-Green (SNAP-PG) probes consist of appropriately derivatized boronates bioconjugated to SNAP-tag fusion proteins. Spectroscopic measurements of the SNAP-PG constructs confirm their ability to detect H2O2 with specificity over other biologically relevant ROS. Moreover, these hybrid small-molecule/protein reporters can be used in live mammalian cells expressing SNAP-tag fusion proteins directed to the plasma membrane, nucleus, mitochondria, and endoplasmic reticulum. Imaging experiments using scanning confocal microscopy establish organelle-specific localization of the SNAP-tag probes and their fluorescence turn-on in response to changes in local H2O2 levels. This work provides a general molecular imaging platform for assaying H2O2 chemistry in living cells with subcellular resolution.

We present a family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)n][(tpaR)Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, Mössbauer spectroscopy, and high-field EPR spectroscopy. Expanding on the discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, this homologous series of high-spin iron(II) complexes enables an initial probe of how the ligand field influences the static and dynamic magnetic behavior. Magnetization experiments reveal large, uniaxial zero-field splitting parameters of D = −48, −44, −30, −26, and −6.2 cm−1 for 1−5, respectively, demonstrating that the strength of axial magnetic anisotropy scales with increasing ligand field strength at the iron(II) center. In the case of 2,6-difluorophenyl substituted 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameter (D = −4.397(9) cm−1) that is in reasonable agreement with that obtained from fits to magnetization data. Ac magnetic susceptibility measurements indicate field-dependent, thermally activated spin reversal barriers in complexes 1, 2, and 4 of Ueff = 65, 42, and 25 cm−1, respectively, with the barrier of 1 constituting the highest relaxation barrier yet observed for a mononuclear transition metal complex. In addition, in the case of 1, the large range of temperatures in which slow relaxation is observed has enabled us to fit the entire Arrhenius curve simultaneously to three distinct relaxation processes. Finally, zero-field Mössbauer spectra collected for 1 and 4 also reveal the presence of slow magnetic relaxation, with two independent relaxation barriers in 4 corresponding to the barrier obtained from ac susceptibility data and to the 3D energy gap between the MS = ±2 and ±1 levels, respectively.

Slow magnetic relaxation is observed for [(tpaMes)Fe]−, a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = −39.6 cm−1) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of Ueff = 42 cm−1 and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at ∼1500 Oe.

As the primary consumers of oxygen within all aerobic organisms, mitochondria are a major source of cellular reactive oxygen species (ROS) derived from the in vivo chemistry of oxygen metabolism. Mitochondrial ROS have been traditionally implicated in aging and in a variety of pathologies, including cancer, neurodegeneration, and diabetes, but recent studies also link controlled mitochondrial ROS fluxes to cell regulation and signaling events. Progress in the development of mitochondrial-targeted fluorescent small-molecule indicators that detect specific ROS with high selectivity offers a promising approach for interrogating mitochondrial ROS production, trafficking, and downstream biological effects.

A cobalt(II) complex supported by the new tetradentate polypyridyl ligand PY4 is an electrocatalyst for the reduction of protons to hydrogen and can operate in 50% aqueous media.

Lanthanide-based luminescent probes TPR1 and TPR2 were developed for the detection of hydrogen peroxide (H2O2) in living systems. The chemoselective reaction of these boronate-protected probes with H2O2 resulted in an enhanced lanthanide sensitization and a 6-fold increase in luminescent intensity. TPR2 was utilized to measure the endogenous production of H2O2 in RAW 264.7 macrophages using time-gated luminescent spectroscopy.

We present the synthesis and characterization of Copper-Gad-7 (CG7), a new copper-activated magnetic resonance imaging (MRI) contrast agent that possesses a Gd3+–DO3A scaffold with an appended thioether-rich receptor for copper recognition. Installation of additional carboxylate groups on the periphery of the CG scaffold affords a practical strategy to increase the absolute relaxivity of these types of copper-responsive MRI sensors as well as reduce their sensitivity to biologically abundant anions. Due in large part to restricted access of inner-sphere water molecules to the paramagnetic Gd3+ core, in the absence of copper ions, CG7 exhibits a relatively low relaxivity value of r1 = 2.6 mM−1 s−1; addition of Cu+ triggers a 340% enhancement in relaxivity to r1 = 11.4 mM−1 s−1. For comparison, the relaxivity of the analogous CG2 sensor without peripheral carboxylates increases from r1 = 1.5 to 6.9 mM−1 s−1 upon Cu+ binding. CG7 features high selectivity for Cu+ over a range of biologically relevant metal ions, including the cellular abundant alkali and alkaline earth cations and d-block ions Zn2+ and Cu2+. Moreover, the Cu+-response of the CG7 sensor is not significantly affected by bicarbonate, phosphate, citrate, and lactate anions at cellular levels. 17O NMR dysprosium-induced shift (DIS) and nuclear magnetic relaxation dispersion (NMRD) experiments suggest that the origin of the improved anion compatibility of CG7 is a reduced q modulation compared to previous members of the CG family, and T1-weighted phantom images confirm that CG7 can monitor changes in copper levels by MRI at clinically relevant field strengths.

Hydrogen peroxide (H2O2) produced by cell-surface NADPH Oxidase (Nox) enzymes is emerging as an important signaling molecule for growth, differentiation, and migration processes. However, how cells spatially regulate H2O2 to achieve physiological redox signaling over nonspecific oxidative stress pathways is insufficiently understood. Here we report that the water channel Aquaporin-3 (AQP3) can facilitate the uptake of H2O2 into mammalian cells and mediate downstream intracellular signaling. Molecular imaging with Peroxy Yellow 1 Methyl-Ester (PY1-ME), a new chemoselective fluorescent indicator for H2O2, directly demonstrates that aquaporin isoforms AQP3 and AQP8, but not AQP1, can promote uptake of H2O2 specifically through membranes in mammalian cells. Moreover, we show that intracellular H2O2 accumulation can be modulated up or down based on endogenous AQP3 expression, which in turn can influence downstream cell signaling cascades. Finally, we establish that AQP3 is required for Nox-derived H2O2 signaling upon growth factor stimulation. Taken together, our findings demonstrate that the downstream intracellular effects of H2O2 can be regulated across biological barriers, a discovery that has broad implications for the controlled use of this potentially toxic small molecule for beneficial physiological functions.

Living organisms produce hydrogen peroxide (H2O2) to kill invading pathogens and for cellular signaling, but aberrant generation of this reactive oxygen species is a hallmark of oxidative stress and inflammation in aging, injury, and disease. The effects of H2O2 on the overall health of living animals remain elusive, in part owing to a dearth of methods for studying this transient small molecule in vivo. Here we report the design, synthesis, and in vivo applications of Peroxy Caged Luciferin-1 (PCL-1), a chemoselective bioluminescent probe for the real-time detection of H2O2 within living animals. PCL-1 is a boronic acid-caged firefly luciferin molecule that selectively reacts with H2O2 to release firefly luciferin, which triggers a bioluminescent response in the presence of firefly luciferase. The high sensitivity and selectivity of PCL-1 for H2O2, combined with the favorable properties of bioluminescence for in vivo imaging, afford a unique technology for real-time detection of basal levels of H2O2 generated in healthy, living mice. Moreover, we demonstrate the efficacy of PCL-1 for monitoring physiological fluctuations in H2O2 levels by directly imaging elevations in H2O2 within testosterone-stimulated tumor xenografts in vivo. The ability to chemoselectively monitor H2O2 fluxes in real time in living animals offers opportunities to dissect H2O2’s disparate contributions to health, aging, and disease.

We present the synthesis and properties of Nickelsensor-1 (NS1), a new water-soluble, turn-on fluorescent sensor that is capable of selectively responding to Ni2+ in aqueous solution and in living cells. NS1 combines a BODIPY chromophore and a mixed N/O/S receptor to provide good selectivity for Ni2+ over a range of biologically abundant metal ions in aqueous solution. In addition to these characteristics, confocal microscopy experiments further show that NS1 can be delivered into living cells and report changes in intracellular Ni2+ levels in a respiratory cell model.

The design, synthesis, and evaluation of the Copper-Gad (CG) family, a new class of copper-activated magnetic resonance imaging (MRI) contrast agents, are presented. These indicators comprise a Gd3+−DO3A core coupled to various thioether-rich receptors for copper-induced relaxivity switching. In the absence of copper ions, inner-sphere water binding to the Gd3+ chelate is restricted, resulting in low longitudinal relaxivity values (r1 = 1.2−2.2 mM−1 s−1 measured at 60 MHz). Addition of Cu+ to CG2, CG3, CG4, and CG5 and either Cu+ or Cu2+ to CG6 triggers marked enhancements in relaxivity (r1 = 2.3−6.9 mM−1 s−1). CG2 and CG3 exhibit the greatest turn-on responses, going from r1 = 1.5 mM−1 s−1 in the absence of Cu+ to r1 = 6.9 mM−1 s−1 upon Cu+ binding (a 360% increase). The CG sensors are highly selective for Cu+ and/or Cu2+ over competing metal ions at cellular concentrations, including Zn2+ at 10-fold higher concentrations. 17O NMR dysprosium-induced shift and nuclear magnetic relaxation dispersion measurements support a mechanism in which copper-induced changes in the coordination environment of the Gd3+ core result in increases in q and r1. T1-weighted phantom images establish that the CG sensors are capable of visualizing changes in copper levels by MRI at clinical field strengths.

We report a hydrogen-bond functionalized N4Py ligand platform (N,N-bis(2-R-6-pyridylmethyl)-N-bis(2-pyridyl)methylamine; R = neopentyl-NH, N4Py2NpNH, 9; R = phenyl-NH, N4Py2PhNH, 10) and the ability of its iron(II)-triflate [N4Py2RFeII(OTf)][OTf] complexes (R = NpNH, 11; R = PhNH, 12) to activate and reduce dioxygen in a synthetic cycle by coupled proton and electron transfer. A pair of iron(III)-hydroxide [N4Py2RFeIII(OH)][OTf]2 complexes (R = NpNH, 13; R = PhNH, 14) are isolated and structurally and spectroscopically characterized after exposure of the iron(II)-triflate precursors to 1 atm of O2 at ambient temperature. The stability of this system to acids and bases allows regeneration of the labile iron(II)-triflate starting materials by sequential electron and proton transfer with cobaltocene and triflic acid, respectively, or through direct proton-coupled reduction with ascorbic acid. In the stepwise process, reduction of the iron(III)-hydroxide complexes with cobaltocene gives structurally homologous iron(II)-hydroxide [N4Py2RFeII(OH)][OTf] congeners (R = NpNH, 15; R = PhNH, 16) that can be prepared independently from 11 and 12 with 20% aq. NaOH. Additions of triflic acid to complexes 15 and 16 furnish the starting compounds 11 and 12, respectively, to complete the synthetic cycle. The combined data establish a synthetic cycle for O2 reduction by an iron platform that manages proton and electron transfer through its first and second coordination spheres.

Monoamine Oxidase Reporters 1 and 2 (MR1 and MR2) are substrate-tethered activity probes that utilize a tandem amine oxidation/β-elimination mechanism to detect MAO chemistry in vitro and in living cells directly and specifically without the need for additional enzymes or other activating reagents.

A reaction-based strategy exploiting cobalt-mediated oxidative C–O bond cleavage affords a selective turn-on fluorescent probe for paramagnetic Co2+ in water and in living cells.

We report a homologous series of nickel(II) complexes supported by N-heterocyclic carbene–pyridine (Rbimpy, R = Me, Et, Pr) ligands that exhibit high selectivity for reducing carbon dioxide over water under electrocatalytic conditions.

A cobalt(II) complex supported by the new tetradentate polypyridyl ligand PY4 is an electrocatalyst for the reduction of protons to hydrogen and can operate in 50% aqueous media.

Lanthanide-based luminescent probes TPR1 and TPR2 were developed for the detection of hydrogen peroxide (H2O2) in living systems. The chemoselective reaction of these boronate-protected probes with H2O2 resulted in an enhanced lanthanide sensitization and a 6-fold increase in luminescent intensity. TPR2 was utilized to measure the endogenous production of H2O2 in RAW 264.7 macrophages using time-gated luminescent spectroscopy.

Monoamine Oxidase Reporters 1 and 2 (MR1 and MR2) are substrate-tethered activity probes that utilize a tandem amine oxidation/β-elimination mechanism to detect MAO chemistry in vitro and in living cells directly and specifically without the need for additional enzymes or other activating reagents.

Formaldehyde (FA) is a reactive carbonyl species (RCS) that plays a broad spectrum of roles in epigenetics, toxicology, and progression of diseases ranging from cancer to diabetes to neurodegeneration, motivating the development of translatable technologies for FA imaging. Here we report formaldehyde-caged-[18F]fluorodeoxyglucose-1 ([18F]FAC-FDG-1), an aza-Cope-based reactivity probe for in vivo FA imaging using positron emission tomography (PET). [18F]FAC-FDG-1 reacts selectively with FA over potentially competing analytes to generate [18F]FDG, allowing its FA-dependent uptake and retention in cell culture as well as in animal models. The relative uptake of [18F]FAC-FDG-1 was evaluated using FA-treated PC3 prostate cancer and U87-MG glioblastoma cells demonstrating a dose-dependent response to exogenously added FA. Moreover, [18F]FAC-FDG-1 is capable of FA detection in vivo using a PC3 tumor xenograft model. In addition to providing a unique tool for monitoring FA in living animals, these data establish a general approach for translatable detection of FA and other reactive biological analytes in vivo by exploiting the widely-available clinical [18F]FDG tracer as a masked aldehyde that can be caged by analyte-responsive triggers.

Recently, a family of cobalt pentapyridine complexes of the type [(R-PY5Me2)Co(H2O)])(CF3SO3)2, (R = CF3, H, or NMe2; PY5Me2 = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine) were shown to catalyze the electrochemical generation of hydrogen from neutral aqueous solutions using a mercury electrode. We now report that the CF3 derivative of this series, [(CF3PY5Me2)Co(H2O)](CF3SO3)2 (1), can also operate in neutral water as an electrocatalyst for hydrogen generation under soluble, diffusion-limited conditions on a glassy carbon electrode, as well as a photocatalyst for hydrogen production using either molecular or semiconductor nanowire photosensitizers. Owing to its relatively low overpotential compared to other members of the PY5 family, complex 1 exhibits multiple redox features on glassy carbon, including a one-proton, one-electron coupled oxidative wave. Further, rotating disk electrode voltammetry measurements reveal the efficacy of 1 as a competent hydrogen evolution catalyst under soluble, diffusion-limited conditions. In addition, we establish that 1 can also generate hydrogen from neutral water under photocatalytic conditions with visible light irradiation (λirr ≥ 455 nm), using [Ru(bpy)3]2+ as a molecular inorganic chromophore and ascorbic acid as a sacrificial donor. Dynamic light scattering measurements show no evidence for nanoparticle formation for the duration of the photolytic hydrogen evolution experiments. Finally, we demonstrate that 1 is also able to enhance the hydrogen photolysis yield of GaP nanowires in water, showing that this catalyst is compatible with solid-state photosensitizers. Taken together, these data establish that the well-defined cobalt pentapyridine complex [(CF3PY5Me2)Co(H2O)]2+ is a versatile catalyst for hydrogen production from pure aqueous solutions using either solar or electrical input, providing a starting point for integrating molecular systems into sustainable energy generation devices.

The pentapyridine cobalt complex [Co(PY5Me2)]2+ and its congeners have been shown to catalyze proton reduction to hydrogen in aqueous solution over a wide pH range using electrical or solar energy input. Here, we employ electrochemical and spectroscopic studies to examine the mechanisms of proton reduction by this parent complex under soluble, diffusion-limited conditions in acetonitrile with acetic acid as the proton donor. Two pathways for proton reduction are identified via cyclic voltammetry: one pathway occurring from an acetonitrile-bound CoII/I couple and the other pathway operating from an acetate-bound CoII/I couple. Kinetics studies support protonation of a CoI species as the rate-determining step for both processes, and additional electrochemical measurements further suggest that the onset of catalysis from the acetonitrile-bound CoII/I couple is highly affected by catalyst electronics. Taken together, this work not only establishes the CoPY5Me2 unit as a unique molecular platform that catalyzes the reduction of protons under soluble, diffusion-limited conditions in both aqueous and organic media, but also highlights the participation of anation processes that are likely relevant for a wide range of hydrogen-producing and related catalytic systems.

Mononuclear metalloenzymes in nature can function in cooperation with precisely positioned redox-active organic cofactors in order to carry out multielectron catalysis. Inspired by the finely tuned redox management of these bioinorganic systems, we present the design, synthesis, and experimental and theoretical characterization of a homologous series of cobalt complexes bearing redox-active pyrazines. These donor moieties are locked into key positions within a pentadentate ligand scaffold in order to evaluate the effects of positioning redox non-innocent ligands on hydrogen evolution catalysis. Both metal- and ligand-centered redox features are observed in organic as well as aqueous solutions over a range of pH values, and comparison with analogs bearing redox-inactive zinc(II) allows for assignments of ligand-based redox events. Varying the geometric placement of redox non-innocent pyrazine donors on isostructural pentadentate ligand platforms results in marked effects on observed cobalt-catalyzed proton reduction activity. Electrocatalytic hydrogen evolution from weak acids in acetonitrile solution, under diffusion-limited conditions, reveals that the pyrazine donor of axial isomer 1-Co behaves as an unproductive electron sink, resulting in high overpotentials for proton reduction, whereas the equatorial pyrazine isomer complex 2-Co is significantly more active for hydrogen generation at lower voltages. Addition of a second equatorial pyrazine in complex 3-Co further minimizes overpotentials required for catalysis. The equatorial derivative 2-Co is also superior to its axial 1-Co congener for electrocatalytic and visible-light photocatalytic hydrogen generation in biologically relevant, neutral pH aqueous media. Density functional theory calculations (B3LYP-D2) indicate that the first reduction of catalyst isomers 1-Co, 2-Co, and 3-Co is largely metal-centered while the second reduction occurs at pyrazine. Taken together, the data establish that proper positioning of non-innocent pyrazine ligands on a single cobalt center is indeed critical for promoting efficient hydrogen catalysis in aqueous media, akin to optimally positioned redox-active cofactors in metalloenzymes. In a broader sense, these findings highlight the significance of electronic structure considerations in the design of effective electron–hole reservoirs for multielectron transformations.

We present the synthesis and characterization of octaarginine-conjugated Copper-Gad-2 (Arg8CG2), a new copper-responsive magnetic resonance imaging (MRI) contrast agent that combines a Gd3+-DO3A scaffold with a thioether-rich receptor for copper recognition. The inclusion of a polyarginine appendage leads to a marked increase in cellular uptake compared to previously reported MRI-based copper sensors of the CG family. Arg8CG2 exhibits a 220% increase in relaxivity (r1 = 3.9 to 12.5 mM−1 s−1) upon 1:1 binding with Cu+, with a highly selective response to Cu+ over other biologically relevant metal ions. Moreover, Arg8CG2 accumulates in cells at nine-fold greater concentrations than the parent CG2 lacking the polyarginine functionality and is retained well in the cell after washing. In cellulo relaxivity measurements and T1-weighted phantom images using a Menkes disease model cell line demonstrate the utility of Arg8CG2 to report on biological perturbations of exchangeable copper pools.

We present a modular dendrimer-based platform for simultaneous dual fluorescence imaging of hydrogen peroxide (H2O2) and pH gradients produced in living cells. Acetyl-capped G5 PAMAM dendrimers functionalized with boronate-caged Peroxyfluor-1 (PF1) fluorophores for H2O2 detection and semi-naphthorhodafluor (SNARF2) dyes for pH sensing provide a single probe system that is capable of sensing multiple analytes at a time by multicolor fluorescence imaging. Spectroscopic measurements of the doubly-labeled dendrimer conjugates establish their ability to simultaneously monitor changes in both H2O2 and pH using different excitation/emission profiles. Moreover, this dual-probe platform allows for selective discrimination between H2O2 and pH changes in live RAW 264.7 macrophage cells when stimulated by an immune insult. Further imaging experiments show that pharmacological inhibition of NADPH oxidase (Nox) proteins triggers a decrease in both oxidative burst and in pH regulation within phagocytic compartments and leads to disruptions of endocytic activity, suggesting that Nox-derived H2O2 signaling is critical to the maintenance of multiple components of the immune response. This work establishes a general molecular platform for simultaneous, real-time imaging of multiple analytes associated with redox biology in living systems and should be applicable to a wide range of chemosensor constructs.

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The potent redox activity of copper is required for sustaining life. Mismanagement of its cellular pools, however, can result in oxidative stress and damage connected to aging, neurodegenerative diseases, and metabolic disorders. Therefore, copper homeostasis is tightly regulated by cells and tissues. Whereas copper and other transition metal ions are commonly thought of as static cofactors buried within protein active sites, emerging data points to the presence of additional loosely bound, labile pools that can participate in dynamic signalling pathways. Against this backdrop, we review advances in sensing labile copper pools and understanding their functions using synthetic fluorescent indicators. Following brief introductions to cellular copper homeostasis and considerations in sensor design, we survey available fluorescent copper probes and evaluate their properties in the context of their utility as effective biological screening tools. We emphasize the need for combined chemical and biological evaluation of these reagents, as well as the value of complementing probe data with other techniques for characterizing the different pools of metal ions in biological systems. This holistic approach will maximize the exciting opportunities for these and related chemical technologies in the study and discovery of novel biology of metals.

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Hydrogen sulfide (H2S), a gaseous species produced by both bacteria and higher eukaryotic organisms, including mammalian vertebrates, has attracted attention in recent years for its contributions to human health and disease. H2S has been proposed as a cytoprotectant and gasotransmitter in many tissue types, including mediating vascular tone in blood vessels as well as neuromodulation in the brain. The molecular mechanisms dictating how H2S affects cellular signaling and other physiological events remain insufficiently understood. Furthermore, the involvement of H2S in metal-binding interactions and formation of related RSS such as sulfane sulfur may contribute to other distinct signaling pathways. Owing to its widespread biological roles and unique chemical properties, H2S is an appealing target for chemical biology approaches to elucidate its production, trafficking, and downstream function. In this context, reaction-based fluorescent probes offer a versatile set of screening tools to visualize H2S pools in living systems. Three main strategies used in molecular probe development for H2S detection include azide and nitro group reduction, nucleophilic attack, and CuS precipitation. Each of these approaches exploits the strong nucleophilicity and reducing potency of H2S to achieve selectivity over other biothiols. In addition, a variety of methods have been developed for the detection of other reactive sulfur species (RSS), including sulfite and bisulfite, as well as sulfane sulfur species and related modifications such as S-nitrosothiols. Access to this growing chemical toolbox of new molecular probes for H2S and related RSS sets the stage for applying these developing technologies to probe reactive sulfur biology in living systems.

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This tutorial review highlights progress in the development of responsive magnetic resonance imaging (MRI) contrast agents for detecting and sensing biologically relevant metal ions. Molecular imaging with bioactivatable MRI indicators offers a potentially powerful methodology for studying the physiology and pathology of metals by capturing dynamic three-dimensional images of living systems for research and clinical applications. This emerging area at the interface of inorganic chemistry and the life sciences offers a broad palette of opportunities for researchers with interests ranging from coordination chemistry and spectroscopy to supramolecular chemistry and molecular recognition to metals in biology and medicine.

We present the synthesis and characterization of Copper-Gad-7 (CG7), a new copper-activated magnetic resonance imaging (MRI) contrast agent that possesses a Gd3+–DO3A scaffold with an appended thioether-rich receptor for copper recognition. Installation of additional carboxylate groups on the periphery of the CG scaffold affords a practical strategy to increase the absolute relaxivity of these types of copper-responsive MRI sensors as well as reduce their sensitivity to biologically abundant anions. Due in large part to restricted access of inner-sphere water molecules to the paramagnetic Gd3+ core, in the absence of copper ions, CG7 exhibits a relatively low relaxivity value of r1 = 2.6 mM−1 s−1; addition of Cu+ triggers a 340% enhancement in relaxivity to r1 = 11.4 mM−1 s−1. For comparison, the relaxivity of the analogous CG2 sensor without peripheral carboxylates increases from r1 = 1.5 to 6.9 mM−1 s−1 upon Cu+ binding. CG7 features high selectivity for Cu+ over a range of biologically relevant metal ions, including the cellular abundant alkali and alkaline earth cations and d-block ions Zn2+ and Cu2+. Moreover, the Cu+-response of the CG7 sensor is not significantly affected by bicarbonate, phosphate, citrate, and lactate anions at cellular levels. 17O NMR dysprosium-induced shift (DIS) and nuclear magnetic relaxation dispersion (NMRD) experiments suggest that the origin of the improved anion compatibility of CG7 is a reduced q modulation compared to previous members of the CG family, and T1-weighted phantom images confirm that CG7 can monitor changes in copper levels by MRI at clinically relevant field strengths.

Growing global energy demands and climate change motivate the development of new renewable energy technologies. In this context, water splitting using sustainable energy sources has emerged as an attractive process for carbon-neutral fuel cycles. A key scientific challenge to achieving this overall goal is the invention of new catalysts for the reductive and oxidative conversions of water to hydrogen and oxygen, respectively. This review article will highlight progress in molecular electrochemical approaches for catalytic reduction of protons to hydrogen, focusing on complexes of earth-abundant metals that can function in pure aqueous or mixed aqueous–organic media. The use of water as a reaction medium has dual benefits of maintaining high substrate concentration as well as minimizing the environmental impact from organic additives and by-products.

We report a two-photon fluorescent probe (PN1) that can be excited by 750 nm femto-second pulses, shows high photostability and negligible toxicity, and can visualize H2O2 distribution in live cells and tissue by two-photon microscopy.

A pseudotetrahedral cobalt(II) complex with a positive axial zero-field splitting parameter of D = 12.7 cm−1, as determined by high-field EPR spectroscopy, is shown to exhibit slow magnetic relaxation under an applied dc field.