Proton-coupled electron transfer: a unifying mechanism for biological charge transport, amino acid radical initiation and propagation, and bond making/breaking reactions of water and oxygen

Redox-driven proton pumps, radical initiation and propagation in biology, and small-molecule activation processes all involve the coupling of electron transfer to proton transport. A mechanistic framework in which to interpret these processes is being developed by examining proton-coupled electron transfer (PCET) in model and natural systems. Specifically, PCET investigations are underway on the following three fronts: (1) the elucidation of the PCET reaction mechanism by time-resolved laser spectroscopy of electron donors and acceptors juxtaposed by a proton transfer interface; (2) the role of amino acid radicals in biological catalysis with the radical initiation and transport processes of E. coli ribonucleotide reductase (RNR) as a focal point; and (3) the application of PCET towards small-molecule activation with emphasis on biologically relevant bond-breaking and bond-making processes involving oxygen and water. A review of recent developments in each of these areas is discussed.