Proton-Coupled Electron Flow in Protein Redox Machines

Electron transfer (ET) reactions are fundamental steps in biological redox processes. Respiration is a case in point: at least 15 ET reactions are required to take reducing equivalents from NADH, deposit them in O_2, and generate the electrochemical proton gradient that drives ATP synthesis. Most of these reactions involve quantum tunneling between weakly coupled redox cofactors (ET distances > 10 Å) embedded in the interiors of folded proteins. Here we review experimental findings that have shed light on the factors controlling these distant ET events. We also review work on a sensitizer-modified copper protein photosystem in which multistep electron tunneling (hopping) through an intervening tryptophan is orders of magnitude faster than the corresponding single-step ET reaction.If proton transfers are coupled to ET events, we refer to the processes as proton coupled ET, or PCET, a term introduced by Huynh and Meyer in 1981. Here we focus on two protein redox machines, photosystem II and ribonucleotide reductase, where PCET processes involving tyrosines are believed to be critical for function. Relevant tyrosine model systems also will be discussed

Mechanism of H_2 Evolution from a Photogenerated Hydridocobaloxime

Proton transfer from the triplet excited state of brominated naphthol to a difluoroboryl bridged Co^I-diglyoxime complex, forming Co^(III)H, was monitored via transient absorption. The second-order rate constant for Co^(III)H formation is in the range (3.5−4.7) × 10^9 M^(−1) s^(−1), with proton transfer coupled to excited-state deactivation of the photoacid. Co^(III)H is subsequently reduced by excess Co^I-diglyoxime in solution to produce Co^(II)H (k_(red) = 9.2 × 10^6 M^(−1) s^(−1)), which is then protonated to yield Co^(II)-diglyoxime and H_2

When Electrochemistry Met Methane: Rapid Catalyst Oxidation Fuels Hydrocarbon Functionalization

An electrochemical strategy for rapid generation of the highly reactive species necessary for C−H bond functionalization may enable improved technology for methane conversion

Qualitative extension of the EC′ Zone Diagram to a molecular catalyst for a multi-electron, multi-substrate electrochemical reaction

Traverse the EC′ Zone Diagram with a molecular H 2 -evolving electrocatalyst through systematic variation of the acid p K a , scan rate, acid concentration and catalyst concentration

Electrode initiated proton-coupled electron transfer to promote degradation of a nickel( ii ) coordination complex

Electrochemical analysis of a nickel compound that degrades permitted a peek into the decomposition mechanism

Catalytic hydrogen evolution from a covalently linked dicobaloxime

A dicobaloxime in which monomeric Co(III) units are linked by an octamethylene bis(glyoxime) catalyzes the reduction of protons from p-toluenesulfonic acid as evidenced by electrocatalytic waves at -0.4 V vs. the saturated calomel electrode (SCE) in acetonitrile solutions. Rates of hydrogen evolution were determined from catalytic current peak heights (k_(app) = 1100 ± 70 M^(-1) s^(-1)). Electrochemical experiments reveal no significant enhancement in the rate of H2 evolution from that of a monomeric analogue: The experimental rate law is first order in catalyst and acid consistent with previous findings for similar mononuclear cobaloximes. Our work suggests that H_2 evolution likely occurs by protonation of reductively generated Co^(II)H rather than homolysis of two Co^(III)H units

Kinetics of Electron Transfer Reactions of H_2-Evolving Cobalt Diglyoxime Catalysts

Co−diglyoxime complexes catalyze H_2 evolution from protic solutions at modest overpotentials. Upon reduction to Co^I, a Co^(III)-hydride is formed by reaction with a proton donor. Two pathways for H_2 production are analyzed: one is a heterolytic route involving protonation of the hydride to release H_2 and generate Co^(III); the other is a homoytic pathway requiring association of two Co^(III)-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash−quench kinetics experiments (Co^(III)−Co^(II) self-exchange k = 9.5 × 10^(−8) − 2.6 × 10^(−5) M^(−1) s^(−1); λ = 3.9 (±0.3) eV: Co^(II)−Co^(I) self-exchange k = 1.2 (±0.5) × 10^5 M^(−1) s^(−1); λ = 1.4 (±0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (Co^(III)H + Co^(III)H → 2 Co^(II) + H_2) is favored over the route that goes through a Co^(III) intermediate (Co^(III)H + H+ → Co^(III) + H_2)

Theoretical Modeling of Low-Energy Electronic Absorption Bands in Reduced Cobaloximes

The reduced Co^I states of cobaloximes are powerful nucleophiles that play an important role in the hydrogen-evolving catalytic activity of these species. In this work we analyze the low-energy electronic absorption bands of two cobaloxime systems experimentally and use a variety of density functional theory and molecular orbital ab initio quantum chemical approaches. Overall we find a reasonable qualitative understanding of the electronic excitation spectra of these compounds but show that obtaining quantitative results remains a challenging task