Proton-Coupled Electron Transfer in Solution, Proteins, and Electrochemistry

Abstract

Recent advances in the theoretical treatment of proton-coupled electron transfer (PCET) reactions are reviewed. These reactions play an important role in a wide range of biological processes, as well as in fuel cells, solar cells, chemical sensors, and electrochemical devices. A unified theoretical framework has been developed to describe both sequential and concerted PCET, as well as hydrogen atom transfer (HAT). A quantitative diagnostic has been proposed to differentiate between HAT and PCET in terms of the degree of electronic nonadiabaticity, where HAT corresponds to electronically adiabatic proton transfer and PCET corresponds to electronically nonadiabatic proton transfer. In both cases, the overall reaction is typically vibronically nonadiabatic. A series of rate constant expressions have been derived in various limits by describing the PCET reactions in terms of nonadiabatic transitions between electron-proton vibronic states. These expressions account for the solvent response to both electron and proton transfer and the effects of the proton donor-acceptor vibrational motion. The solvent and protein environment can be represented by a dielectric continuum or described with explicit molecular dynamics. These theoretical treatments have been applied to numerous PCET reactions in solution and proteins. Expressions for heterogeneous rate constants and current densities for electrochemical PCET have also been derived and applied to model systems.

Modifications of Protein Environment of the [2Fe-2S] Cluster of the bc1 Complex

“…This bond therefore provides a pathway for transfer of both the electron and the proton. Consideration of plausible thermodynamic squares strongly suggested either a PT/ET model (14) or a concerted process (33,72,80). We strongly favored the PT/ET model because it accounted for the slow rate observed for the first electron transfer without ad hoc additions.…”

Substrate positioning controls the partition between halogenation and hydroxylation in the aliphatic halogenase, SyrB2

“…Third, the large D-KIEs observed in the halogenase reactions (15, 16) had established that H • abstraction involves quantum mechanical tunneling. Theoretical studies indicate that hydrogen-tunneling reactions are exquisitely sensitive to donor-acceptor distance (23). Although the lack of a structure of any halogenase bound to its aminoacyl-carrier protein substrate has thus far precluded direct assessment of the disposition of the substrate side chain relative to the iron cofactor, it seemed likely that the factor overriding the influence of BDE might be proximity of the donor to the oxo acceptor.…”

Proton-Coupled Electron Transfer in Organic Synthesis: Fundamentals, Applications, and Opportunities

“…Accordingly, he suggests HAT to be more broadly defined as "concerted transfer of [a proton] and [an electron] from a single donor to a single acceptor." While this definition suggests that classical HAT is a specific subset of concerted PCET reactions, subtleties between the two elementary steps preclude these terms from being truly interchangeable (or, in many cases, from being readily distinguished) [39,[41][42]. In light of these problems in both terminology and identification, we present examples pertinent to PCET-type mechanisms and have adopted the nomenclature presented by the original authors.…”