Photoinduced proton-coupled electron transfers in biorelevant phenolic systems

Abstract

Proton-coupled electron transfer (PCET) reactions have received much attention over the past 10 years, from an experimental as well as from a theoretical point of view. At the heart of many chemical and biological processes, such reactions are of particular interest in energy conversion and enzymatic processes. Among the numerous examples of PCET reactions, photosynthesis and particularly reactions inside the Photosystem II (PSII) subunit, involving a global four electrons and four protons process to perform water oxidation and respiration, is the most emblematic one. This review focuses on the photochemical approaches of PCET reactions involving phenolic molecules. Indeed, a significant part of photochemical PCET studies were conducted on tyrosine or phenol relevant to PSII and charge transport in enzymes. The mechanisms of these reactions, sequential or concerted, with particular emphasis on the influence of pH, temperature, solvent nature and H-bonding pattern are presented based on photochemical techniques and related theoretical analysis. Proton-coupled electron transfer reactions (PCET) are ubiquitous in nature as well as in catalysis, e.g. in the activation of small molecules (water, dioxygen, dihydrogen, carbon dioxide). The review focuses on the use of photochemical techniques to study the mechanism of these PCET reactions, with particular emphasis on biorelevant phenolic molecules. Such compounds are widely encountered as cofactors in many enzymatic reactions and are also good model compounds. The various mechanisms (concerted vs stepwise) are presented and discussed, in particular the concerted reactions which provide the advantage avoiding the formation of high-energy intermediates, thus leading to efficient processes. © 2011 Wiley Periodicals, Inc.