Bimolecular proton-coupled electron transfer from tryptophan with water as the proton acceptor

Abstract

Proton-coupled electron transfer (PCET) from amino acids with water as the primary proton acceptor is a fundamental model reaction of wide relevance. We have investigated the kinetics and mechanism of PCET from tryptophan derivatives to laser flash-generated [Ru(bpy)₃]³⁺ oxidants in aqueous solutions. The PCET reaction followed either a concerted electron–proton transfer (CEP) mechanism or a step-wise electron transfer–proton transfer (ETPT) mechanism, depending on pH and the identity of the Ru^III oxidant. The CEP reaction showed a pH-dependent rate constant, with an increase in log k_CEP of 0.3–0.5 per pH unit, and a significant kinetic isotope effect (KIE = 2–4), both in intramolecular and bimolecular model systems. Our results suggest that CEP with water as the proton acceptor is a general feature of amino acid oxidation, and provide further experimental observations towards a detailed understanding of the PCET process in water.