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 RuIII oxidant. The CEP reaction showed a pH-dependent rate constant, with an increase in log kCEP 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.
ASJC Scopus subject areas
- Environmental Chemistry
- Renewable Energy, Sustainability and the Environment
- Nuclear Energy and Engineering