Heterometallic Transition Metal Oxides Containing Lewis Acids as Molecular Catalysts for the Reduction of Carbon Dioxide to Carbon Monoxide with Bimodal Activity.

Electrocatalytic CO2 reduction (e-CO2RR) to CO is replete with challenges including the need to carry out e-CO2RR at low overpotentials. Previously, a tricopper-substituted polyoxometalate was shown to reduce CO2 to CO with a very high faradaic efficiency albeit at -2.5 V versus Fc/Fc+. It is now demonstrated that introducing a nonredox metal Lewis acid, preferably GaIII, as a binding site for CO2 in the first coordination sphere of the polyoxometalate, forming heterometallic polyoxometalates, e.g., [SiCuIIFeIIIGaIII(H2O)3W9O37]8-, leads to bimodal activity optimal both at -2.5 and -1.5 V versus Fc/Fc+; reactivity at -1.5 V being at an overpotential of ∼150 mV. These results were observed by cyclic voltammetry and quantitative controlled potential electrolysis where high faradaic efficiency and chemoselectivity were obtained at -2.5 and -1.5 V. A reaction with 13CO2 revealed that CO2 disproportionation did not occur at -1.5 V. EPR spectroscopy showed reduction, first of CuII to CuI and FeIII to FeII and then reduction of a tungsten atom (WVI to WV) in the polyoxometalate framework. IR spectroscopy showed that CO2 binds to [SiCuIIFeIIIGaIII(H2O)3W9O37]8- before reduction. In situ electrochemical attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) with pulsed potential modulated excitation revealed different observable intermediate species at -2.5 and -1.5 V. DFT calculations explained the CV, the formation of possible activated CO2 species at both -2.5 and -1.5 V through series of electron transfer, proton-coupled electron transfer, protonation and CO2 binding steps, the active site for reduction, and the role of protons in facilitating the reactions.

Sonodynamic Therapy for HER2+ Breast Cancer with Iodinated Heptamethine Cyanine-Trastuzumab Conjugate.

The first example of sonodynamic therapy (SDT) with a cyanine dye-antibody conjugate is reported. The aim of this study was to evaluate the sonodynamic efficacy of a trastuzumab-guided diiodinated heptamethine cyanine-based sensitizer, 2ICy7-Ab, versus its non-iodinated counterpart, Cy7-Ab, in a human epidermal growth factor receptor 2-positive (HER2+) xenograft model. In addition, the combined sonodynamic and photodynamic (PDT) effects were investigated. A single intravenous injection of 2ICy7-Ab followed by sonication or combined sonication and photoirradiation in mice resulted in complete tumor growth suppression compared with the nontreated control and showed no detectable toxicity to off-target tissues. In contrast, Cy7-Ab provided only a moderate therapeutic effect (~1.4-1.6-fold suppression). SDT with 2ICy7-Ab resulted in a 3.5-fold reduction in tumor volume within 45 days and exhibited 13-fold greater tumor suppression than PDT alone. In addition, 2ICy7-Ab showed more durable sonostability than photostability. The sonotoxicity of the iodinated versus noniodinated counterparts is attributed to the increased generation of hydroxyl radicals, superoxide, and singlet oxygen. We observed no significant contribution of PDT to the efficacy of the combined SDT and PDT, indicating that SDT with 2ICy7-Ab is superior to PDT alone. These new findings set the stage for the application of cyanine-antibody conjugates for fluorescently monitored targeted sonodynamic treatment of cancer.

Composition and Structure of the solid electrolyte interphase on Na-Ion Anodes Revealed by Exo- and Endogenous Dynamic Nuclear Polarization─NMR Spectroscopy.

Sodium ion batteries (SIB) are among the most promising devices for large scale energy storage. Their stable and long-term performance depends on the formation of the solid electrolyte interphase (SEI), a nanosized, heterogeneous and disordered layer, formed due to degradation of the electrolyte at the anode surface. The chemical and structural properties of the SEI control the charge transfer process at the electrode-electrolyte interface, thus, there is great interest in determining these properties for understanding, and ultimately controlling, SEI functionality. However, the study of the SEI is notoriously challenging due to its heterogeneous nature and minute quantity. In this work, we present a powerful approach for probing the SEI based on solid state NMR spectroscopy with increased sensitivity from dynamic nuclear polarization (DNP). Utilizing exogenous (organic radicals) and endogenous (paramagnetic metal ion dopants) DNP sources, we obtain not only a detailed compositional map of the SEI but also, for the first time for the native SEI, determine the spatial distribution of its constituent phases. Using this approach, we perform a thorough investigation of the SEI formed on Li4Ti5O12 used as a SIB anode. We identify a compositional gradient, from organic phases at the electrolyte interface to inorganic phases toward the anode surface. We find that the use of fluoroethylene carbonate as an electrolyte additive leads to performance degradation which can be attributed to formation of a thicker SEI, rich in NaF and carbonates. We expect that this methodology can be extended to examine other titanate anodes and new electrolyte compositions, offering a unique tool for SEI investigations to enable the development of effective and long-lasting SIBs.

Cryogenic and Dissolution DNP NMR on γ-Irradiated Organic Molecules.

Nuclear magnetic resonance (NMR) plays a central role in the elucidation of chemical structures but is often limited by low sensitivity. Dissolution dynamic nuclear polarization (dDNP) emerges as a transformative methodology for both solution-state NMR and metabolic NMR imaging, which could overcome this limitation. Typically, dDNP relies on combining a stable radical with the analyte within a uniform glass under cryogenic conditions. The electron polarization is then transferred through microwave irradiation to the nuclei. The present study explores the use of radicals introduced via γ-irradiation, as bearers of the electron spins that will enhance 1H or 13C nuclides. 1H solid-state NMR spectra of γ-irradiated powders at 1-5 K revealed, upon microwave irradiation, signal enhancements that, in general, were higher than those achieved through conventional glass-based DNP. Transfer of these samples to a solution-state NMR spectrometer via a rapid dissolution driven by a superheated water provided significant enhancements of solution-state 1H NMR signals. Enhancements of 13C signals in the γ-irradiated solids were more modest, as a combined consequence of a low radical concentration and of the dilute concentration of 13C in the natural abundant samples examined. Nevertheless, ca. 700-800-fold enhancements in 13C solution NMR spectra of certain sites recorded at 11.7 T could still be achieved. A total disappearance of the radicals upon performing a dDNP-like aqueous dissolution and a high stability of the samples were found. Overall, the study showcases the advantages and limitations of γ-irradiated radicals as candidates for advancing spectroscopic dDNP-enhanced NMR.

Charge-Transfer Complexes: Halogen-Doped Anthracene as a Case of Study.

Charge transfer (CT) crystals exhibit unique electronic and magnetic properties with interesting applications. We present a rational and easy guide which allows to foresee the effective charge transfer co-crystal production and that is based on the comparison of the frontier molecular orbital (MO) energies of a donor and acceptor couple. For the sake of comparison, theoretical calculations have been carried out by using the cheap and fast PM6 semiempirical Hamiltonian and pure HF/cc-pVTZ level of the theory. The results are then compared with experimental results obtained both by chemical (bromine and iodine were used as the acceptor) and electrochemical doping (exploiting an original experimental set-up by this laboratory: the electrochemical transistor). Infra-red vibrational experimental results and theoretically calculated spectra are compared to assess both the effective donor-acceptor (D/A) charge-transfer and transport mechanism (giant IRAV polaron signature). XPS spectra have been collected (carbon (1 s) and iodine (3d5/2)) signals, yielding further evidence of the effective formation of the CT anthracene:iodine complex.

Secondary Organic Aerosol Generated from Biomass Burning Emitted Phenolic Compounds: Oxidative Potential, Reactive Oxygen Species, and Cytotoxicity.

Phenolic compounds are largely emitted from biomass burning (BB) and have a significant potential to form SOA (Phc-SOA). However, the toxicological properties of Phc-SOA remain unclear. In this study, phenol and guaiacol were chosen as two representative phenolic gases in BB plumes, and the toxicological properties of water-soluble components of their SOA generated under different photochemical ages and NOx levels were investigated. Phenolic compounds contribute greatly to the oxidative potential (OP) of biomass-burning SOA. OH-adducts of guaiacol (e.g., 2-methoxyhydroquinone) were identified as components of guaiacol SOA (GSOA) with high OP. The addition of nitro groups to 2,5-dimethyl-1,4-benzoquinone, a surrogate quinone compound in Phc-SOA, increased its OP. The toxicity of both phenol SOA (PSOA) and GSOA in vitro in human alveolar epithelial cells decreased with aging in terms of both cell death and cellular reactive oxygen species (ROS), possibly due to more ring-opening products with relatively low toxicity. The influence of NOx was consistent between cell death and cellular ROS for GSOA but not for PSOA, indicating that cellular ROS production does not necessarily represent all processes contributing to cell death caused by PSOA. Combining different acellular and cellular assays can provide a comprehensive understanding of aerosol toxicological properties.

Endogenous metal-ion dynamic nuclear polarization for NMR signal enhancement in metal organic frameworks.

Rational design of metal-organic framework (MOF)-based materials for catalysis, gas capture and storage, requires deep understanding of the host-guest interactions between the MOF and the adsorbed molecules. Solid-State NMR spectroscopy is an established tool for obtaining such structural information, however its low sensitivity limits its application. This limitation can be overcome with dynamic nuclear polarization (DNP) which is based on polarization transfer from unpaired electrons to the nuclei of interest and, as a result, enhancement of the NMR signal. Typically, DNP is achieved by impregnating or wetting the MOF material with a solution of nitroxide biradicals, which prevents or interferes with the study of host-guest interactions. Here we demonstrate how Gd(iii) ions doped into the MOF structure, LaBTB (BTB = 4,4',4''-benzene-1,3,5-triyl-trisbenzoate), can be employed as an efficient polarization agent, yielding up to 30-fold 13C signal enhancement for the MOF linkers, while leaving the pores empty for potential guests. Furthermore, we demonstrate that ethylene glycol, loaded into the MOF as a guest, can also be polarized using our approach. We identify specific challenges in DNP studies of MOFs, associated with residual oxygen trapped within the MOF pores and the dynamics of the framework and its guests, even at cryogenic temperatures. To address these, we describe optimal conditions for carrying out and maximizing the enhancement achieved in DNP-NMR experiments. The approach presented here can be expanded to other porous materials which are currently the state-of-the-art in energy and sustainability research.

Aerobic bacteria produce nitric oxide via denitrification and promote algal population collapse.

Microbial interactions govern marine biogeochemistry. These interactions are generally considered to rely on exchange of organic molecules. Here we report on a novel inorganic route of microbial communication, showing that algal-bacterial interactions between Phaeobacter inhibens bacteria and Gephyrocapsa huxleyi algae are mediated through inorganic nitrogen exchange. Under oxygen-rich conditions, aerobic bacteria reduce algal-secreted nitrite to nitric oxide (NO) through denitrification, a well-studied anaerobic respiratory mechanism. The bacterial NO is involved in triggering a cascade in algae akin to programmed cell death. During death, algae further generate NO, thereby propagating the signal in the algal population. Eventually, the algal population collapses, similar to the sudden demise of oceanic algal blooms. Our study suggests that the exchange of inorganic nitrogen species in oxygenated environments is a potentially significant route of microbial communication within and across kingdoms.

The Effect of Disorder on Endogenous MAS-DNP: Study of Silicate Glasses and Crystals.

In dynamic nuclear polarization nuclear magnetic resonance (DNP-NMR) experiments, the large Boltzmann polarization of unpaired electrons is transferred to surrounding nuclei, leading to a significant increase in the sensitivity of the NMR signal. In order to obtain large polarization gains in the bulk of inorganic samples, paramagnetic metal ions are introduced as minor dopants acting as polarizing agents. While this approach has been shown to be very efficient in crystalline inorganic oxides, significantly lower enhancements have been reported when applying this approach to oxide glasses. In order to rationalize the origin of the difference in the efficiency of DNP in amorphous and crystalline inorganic matrices, we performed a detailed comparison in terms of their magnetic resonance properties. To diminish differences in the DNP performance arising from distinct nuclear interactions, glass and crystal systems of similar compositions were chosen, Li2OCaO·2SiO2 and Li2CaSiO4, respectively. Using Gd(III) as polarizing agent, DNP provided signal enhancements in the range of 100 for the crystalline sample, while only up to around factor 5 in the glass, for both 6Li and 29Si nuclei. We find that the drop in enhancement in glasses can be attributed to three main factors: shorter nuclear and electron relaxation times as well as the dielectric properties of glass and crystal. The amorphous nature of the glass sample is responsible for a high dielectric loss, leading to efficient microwave absorption and consequently lower effective microwave power and an increase in sample temperature which leads to further reduction of the electron relaxation time. These results help rationalize the observed sensitivity enhancements and provide guidance in identifying materials that could benefit from the DNP approach.

Selective multi-electron aggregation at a hypervalent iodine center by sequential disproportionation.

We demonstrate that sequential disproportionation reactions can enable selective aggregation of two- or four electron-holes at a hypervalent iodine center. Disproportionation of an anodically generated iodanyl radical affords an iodosylbenzene derivative. Subsequent iodosylbenzene disproportionation can be triggered to provide access to an iodoxybenzene. These results demonstrate multielectron oxidation at the one-electron potential by selective and sequential disproportionation chemistry.

Deciphering the ground state of a C3-symmetrical Blatter-type triradical by CW and pulse EPR spectroscopy.

3,3',3''-(Benzene-1,3,5-triyl)tris(1-phenyl-1H-benzo[e][1,2,4]triazin-4-yl) (1) is a C3-symmetrical triradical comprised of three Blatter radical units connected at the 1, 3, 5 positions of a central trimethylenebenzene core. This triradical has an excellent air, moisture, and thermal stability. Single-crystal XRD indicates that triradical 1 adopts a propeller-like geometry with the benzotriazinyl moieties twisted by 174.1(2)° and packs in 1D chains along the c axis to form an extensive network of weak intermolecular interactions. Frozen solution continuous wave (CW) EPR spectra and variable-temperature field-sweep echo-detected (FSED) spectra revealed an intramolecular ferromagnetic exchange within the spin system, supporting a quartet S = 3/2 ground state. DFT calculations further supported these experimental findings.

Aptamer-Functionalized Ce4+-Ion-Modified C-Dots: Peroxidase Mimicking Aptananozymes for the Oxidation of Dopamine and Cytotoxic Effects toward Cancer Cells.

Aptamer-functionalized Ce4+-ion-modified C-dots act as catalytic hybrid systems, aptananozymes, catalyzing the H2O2 oxidation of dopamine. A series of aptananozymes functionalized with different configurations of the dopamine binding aptamer, DBA, are introduced. All aptananozymes reveal substantially enhanced catalytic activities as compared to the separated Ce4+-ion-modified C-dots and aptamer constituents, and structure-catalytic functions between the structure and binding modes of the aptamers linked to the C-dots are demonstrated. The enhanced catalytic functions of the aptananozymes are attributed to the aptamer-induced concentration of the reaction substrates in spatial proximity to the Ce4+-ion-modified C-dots catalytic sites. The oxidation processes driven by the Ce4+-ion-modified C-dots involve the formation of reactive oxygen species (•OH radicals). Accordingly, Ce4+-ion-modified C-dots with the AS1411 aptamer or MUC1 aptamer, recognizing specific biomarkers associated with cancer cells, are employed as targeted catalytic agents for chemodynamic treatment of cancer cells. Treatment of MDA-MB-231 breast cancer cells and MCF-10A epithelial breast cells, as control, with the AS1411 aptamer- or MUC1 aptamer-modified Ce4+-ion-modified C-dots reveals selective cytotoxicity toward the cancer cells. In vivo experiments reveal that the aptamer-functionalized nanoparticles inhibit MDA-MB-231 tumor growth.

Aptamer-Modified Au Nanoparticles: Functional Nanozyme Bioreactors for Cascaded Catalysis and Catalysts for Chemodynamic Treatment of Cancer Cells.

Polyadenine-stabilized Au nanoparticles (pA-AuNPs) reveal dual nanozyme catalytic activities toward the H2O2-mediated oxidation of dopamine to aminochrome and toward the aerobic oxidation of glucose to gluconic acid and H2O2. The conjugation of a dopamine-binding aptamer (DBA) to the pA-AuNPs yields aptananozyme structures catalyzing simultaneously the H2O2-mediated oxidation of dopamine to aminochrome through the aerobic oxidation of glucose. A set of aptananozymes consisting of DBA conjugated through the 5'- or 3'-end directly or spacer bridges to pA-AuNPs were synthesized. The set of aptananozymes revealed enhanced catalytic activities toward the H2O2-catalyzed oxidation of dopamine to dopachrome, as compared to the separated pA-AuNPs and DBA constituents, and structure-function relationships within the series of aptananozymes were demonstrated. The enhanced catalytic function of the aptananozymes was attributed to the concentration of the dopamine at the catalytic interfaces by means of aptamer-dopamine complexes. The dual catalytic activities of aptananozymes were further applied to design bioreactors catalyzing the effective aerobic oxidation of dopamine in the presence of glucose. Mechanistic studies demonstrated that the aptananozymes generate reactive oxygen species. Accordingly, the AS1411 aptamer, recognizing the nucleolin receptor associated with cancer cells, was conjugated to the pA-AuNPs, yielding a nanozyme for the chemodynamic treatment of cancer cells. The AS1411 aptamer targets the aptananozyme to the cancer cells and facilitates the selective permeation of the nanozyme into the cells. Selective cytotoxicity toward MDA-MB-231 breast cancer cells (ca. 70% cell death) as compared to MCF-10A epithelial cells (ca. 2% cell death) is demonstrated.

A Macrocyclic Furan with Accessible Oxidation States: Switching Between Aromatic and Antiaromatic Global Ring Currents.

Macrocyclic furans are predicted to switch between global aromaticity and antiaromaticity, depending on their oxidation states. However, the macrocyclic furans reported to date are stabilized by electron withdrawing groups, which result in inaccessible oxidation states. To circumvent this problem, a post-macrocyclization approach was applied to introduce methylene-substituted macrocyclic furans, which display an extremely low oxidation potential of -0.23 vs. Fc/Fc+ , and are partially oxidized in ambient conditions. Additional oxidation to the dication results in aromaticity switching to a global 30πe- aromatic state, as indicated by the formation of a strong diatropic current observed in the 1 H NMR spectrum. NICS and ACID calculations support this trend and provide evidence for a different pathway for the global current in the neutral and dicationic states. According to these findings, macrocyclic furans can be rendered as promising p-type materials with stable oxidation states.

Atmospheric-Pressure Conversion of CO2 to Cyclic Carbonates over Constrained Dinuclear Iron Catalysts.

The conversion of CO2 and epoxides to cyclic carbonates over a silica-supported di-iron(III) complex having a reduced Robson macrocycle ligand system is shown to proceed at 1 atm and 80 °C, exclusively producing the cis-cyclohexene carbonate from cyclohexene oxide. We examine the effect of immobilization configuration to show that the complex grafted in a semirigid configuration catalytically outperforms the rigid, flexible configurations and even the homogeneous counterparts. Using the semirigid catalyst, we are able to obtain a TON of up to 800 and a TOF of up to 37 h-1 under 1 atm CO2. The catalyst is shown to be recyclable with only minor leaching and no change to product selectivity. We further examine a range of epoxides with varying electron-withdrawing/donating properties. This work highlights the benefit arising from the constraining effect of a solid surface, akin to the role of hydrogen bonds in enzyme catalysts, and the importance of correctly balancing it.

Iodine-Iodine Cooperation Enables Metal-Free C-N Bond-Forming Electrocatalysis via Isolable Iodanyl Radicals.

Small molecule redox mediators convey interfacial electron transfer events into bulk solution and can enable diverse substrate activation mechanisms in synthetic electrocatalysis. Here, we report that 1,2-diiodo-4,5-dimethoxybenzene is an efficient electrocatalyst for C-H/E-H coupling that operates at as low as 0.5 mol % catalyst loading. Spectroscopic, crystallographic, and computational results indicate a critical role for a three-electron I-I bonding interaction in stabilizing an iodanyl radical intermediate (i.e., formally I(II) species). As a result, the optimized catalyst operates at more than 100 mV lower potential than the related monoiodide catalyst 4-iodoanisole, which results in improved product yield, higher Faradaic efficiency, and expanded substrate scope. The isolated iodanyl radical is chemically competent in C-N bond formation. These results represent the first examples of substrate functionalization at a well-defined I(II) derivative and bona fide iodanyl radical catalysis and demonstrate one-electron pathways as a mechanistic alternative to canonical two-electron hypervalent iodine mechanisms. The observation establishes I-I redox cooperation as a new design concept for the development of metal-free redox mediators.

Monitoring electron spin fluctuations with paramagnetic relaxation enhancement.

The magnetic interactions between the spin of an unpaired electron and the surrounding nuclear spins can be exploited to gain structural information, to reduce nuclear relaxation times as well as to create nuclear hyperpolarization via dynamic nuclear polarization (DNP). A central aspect that determines how these interactions manifest from the point of view of NMR is the timescale of the fluctuations of the magnetic moment of the electron spins. These fluctuations, however, are elusive, particularly when electron relaxation times are short or interactions among electronic spins are strong. Here we map the fluctuations by analyzing the ratio between longitudinal and transverse nuclear relaxation times T1/T2, a quantity which depends uniquely on the rate of the electron fluctuations and the Larmor frequency of the involved nuclei. This analysis enables rationalizing the evolution of NMR lineshapes, signal quenching as well as DNP enhancements as a function of the concentration of the paramagnetic species and the temperature, demonstrated here for LiMg1-xMnxPO4 and Fe(III) doped Li4Ti5O12, respectively. For the latter, we observe a linear dependence of the DNP enhancement and the electron relaxation time within a temperature range between 100 and 300 K.

Extensive Redox Non-Innocence in Iron Bipyridine-Diimine Complexes: a Combined Spectroscopic and Computational Study.

Metal-ligand cooperation is an important aspect in earth-abundant metal catalysis. Utilizing ligands as electron reservoirs to supplement the redox chemistry of the metal has resulted in many new exciting discoveries. Here, we demonstrate that iron bipyridine-diimine (BDI) complexes exhibit an extensive electron-transfer series that spans a total of five oxidation states, ranging from the trication [Fe(BDI)]3+ to the monoanion [Fe(BDI]-1. Structural characterization by X-ray crystallography revealed the multifaceted redox noninnocence of the BDI ligand, while spectroscopic (e.g., 57Fe Mössbauer and EPR spectroscopy) and computational studies were employed to elucidate the electronic structure of the isolated complexes, which are further discussed in this report.

Aptamer-Modified Cu2+-Functionalized C-Dots: Versatile Means to Improve Nanozyme Activities-"Aptananozymes".

The covalent linkage of aptamer binding sites to nanoparticle nanozymes is introduced as a versatile method to improve the catalytic activity of nanozymes by concentrating the reaction substrates at the catalytic nanozyme core, thereby emulating the binding and catalytic active-site functions of native enzymes. The concept is exemplified with the synthesis of Cu2+ ion-functionalized carbon dots (C-dots), modified with the dopamine binding aptamer (DBA) or the tyrosinamide binding aptamer (TBA), for the catalyzed oxidation of dopamine to aminochrome by H2O2 or the oxygenation of l-tyrosinamide to the catechol product, which is subsequently oxidized to amidodopachrome, in the presence of H2O2/ascorbate mixture. Sets of structurally functionalized DBA-modified Cu2+ ion-functionalized C-dots or sets of structurally functionalized TBA-modified Cu2+ ion-functionalized C-dots are introduced as nanozymes of superior catalytic activities (aptananozymes) toward the oxidation of dopamine or the oxygenation of l-tyrosinamide, respectively. The aptananozymes reveal enhanced catalytic activities as compared to the separated catalyst and respective aptamer constituents. The catalytic functions of the aptananozymes are controlled by the structure of the aptamer units linked to the Cu2+ ion-functionalized C-dots. In addition, the aptananozyme shows chiroselective catalytic functions demonstrated by the chiroselective-catalyzed oxidation of l/d-DOPA to l/d-dopachrome. Binding studies of the substrates to the different aptananozymes and mechanistic studies associated with the catalytic transformations are discussed.

Temperature Dependence of Charge and Spin Transfer in Azurin.

The steady-state charge and spin transfer yields were measured for three different Ru-modified azurin derivatives in protein films on silver electrodes. While the charge-transfer yields exhibit weak temperature dependences, consistent with operation of a near activation-less mechanism, the spin selectivity of the electron transfer improves as temperature increases. This enhancement of spin selectivity with temperature is explained by a vibrationally induced spin exchange interaction between the Cu(II) and its chiral ligands. These results indicate that distinct mechanisms control charge and spin transfer within proteins. As with electron charge transfer, proteins deliver polarized electron spins with a yield that depends on the protein's structure. This finding suggests a new role for protein structure in biochemical redox processes.