1. History 3422. Activation barriers 3432.1 Redox potentials 3442.2 Reorganization energy 3443. Electronic coupling 3454. Ru-modified proteins 3484.1 Reorganization energy 3494.1.1 Cyt c 3494.1.2 Azurin 3504.2 Tunneling timetables 3525. Multistep tunneling 3576. Protein–protein reactions 3596.1 Hemoglobin (Hb) hybrids 3596.2 Cyt c/cyt b₅ complexes 3606.3 Cyt c/cyt c peroxidase complexes 3606.4 Zn–cyt c/Fe–cyt c crystals 3617. Photosynthesis and respiration 3627.1 Photosynthetic reaction centers (PRCs) 3627.2 Cyt c oxidase (CcO) 3648. Concluding remarks 3659. Acknowledgments 36610. References 366Electron transfer processes are vital elements of energy transduction pathways in living cells. More than a half century of research has produced a remarkably detailed understanding of the factors that regulate these ‘currents of life’. We review investigations of Ru-modified proteins that have delineated the distance- and driving-force dependences of intra-protein electron-transfer rates. We also discuss electron transfer across protein–protein interfaces that has been probed both in solution and in structurally characterized crystals. It is now clear that electrons tunnel between sites in biological redox chains, and that protein structures tune thermodynamic properties and electronic coupling interactions to facilitate these reactions. Our work has produced an experimentally validated timetable for electron tunneling across specified distances in proteins. Many electron tunneling rates in cytochrome c oxidase and photosynthetic reaction centers agree well with timetable predictions, indicating that the natural reactions are highly optimized, both in terms of thermodynamics and electronic coupling. The rates of some reactions, however, significantly exceed timetable predictions; it is likely that multistep tunneling is responsible for these anomalously rapid charge transfer events.

Electrons have so little mass that in less than a second they can tunnel through potential energy barriers that are several electron-volts high and several nanometers wide. Electron tunneling is a critical functional element in a broad spectrum of applications, ranging from semiconductor diodes to the photosynthetic and respiratory charge transport chains. Prior to the 1970s, chemists generally believed that reactants had to collide in order to effect a transformation. Experimental demonstrations that electrons can transfer between reactants separated by several nanometers led to a revision of the chemical reaction paradigm. Experimental investigations of electron exchange between redox partners separated by molecular bridges have elucidated many fundamental properties of these reactions, particularly the variation of rate constants with distance. Theoretical work has provided critical insights into the superexchange mechanism of electronic coupling between distant redox centers. Kinetics measurements have shown that electrons can tunnel about 2.5 nm through proteins on biologically relevant time scales. Longer-distance biological charge flow requires multiple electron tunneling steps through chains of redox cofactors. The range of phenomena that depends on long-range electron tunneling continues to expand, providing new challenges for both theory and experiment.

Several cobalt complexes catalyze the evolution of hydrogen from acidic solutions, both homogeneously and at electrodes. The detailed molecular mechanisms of these transformations remain unresolved, largely owing to the fact that key reactive intermediates have eluded detection. One method of stabilizing reactive intermediates involves minimizing the overall reaction free-energy change. Here, we report a new cobalt(I) complex that reacts with tosylic acid to evolve hydrogen with a driving force of just 30 meV/Co. Protonation of Co ^I produces a transient Co ^III -H complex that was characterized by nuclear magnetic resonance spectroscopy. The Co ^III -H intermediate decays by second-order kinetics with an inverse dependence on acid concentration. Analysis of the kinetics suggests that Co ^III -H produces hydrogen by two competing pathways: a slower homolytic route involving two Co ^III -H species and a dominant heterolytic channel in which a highly reactive Co ^II -H transient is generated by Co ^I reduction of Co ^III -H.