Buffer Effects in the Kinetics of Concerted Proton-Coupled Electron Transfer: The Electrochemical Oxidation of Glutathione Mediated by [IrCl₆]^2–at Variable Buffer p<i>K</i>_a and Concentration

Abstract

The oxidation of glutathione (GSH) by the electrogenerated mediator [IrCl6]2- in the presence of bases (B) to produce the radical GS•, [IrCl6]3- and BH+, was studied by cyclic voltammetry in buffered and unbuffered solution using glassy carbon electrodes. This proton-coupled electron transfer (PCET) is the first step in the oxidation of GSH to produce the disulfide GSSG. The reaction exhibits a slight acceleration of the rate when the phosphate buffer (PB) concentration is raised at constant pH (5.0 or 7.0). The evidence indicates that proton acceptors whose conjugate acids have a pKa lower than GSH (8.7) but higher than GSH•+ can catalyze the reaction even though direct deprotonation of GSH is thermodynamically unfavorable. The value of kobs at 0.005 M of PBpH = 7.0 was equal to 4.3 ± 0.7 × 104 M–1 s–1 whereas at 0.200 M, was 2.56 ± 0.03 × 105 M–1 s–1. A kinetic isotope effect (KIE) dependent on [PB] was observed at pD 7.0, confirming that the S–H bond breaks during the rate-determining step. The predominance of a concerted PCET is postulated based on the compliance with the libido rule of general base catalysis, the KIE observed and the finding that stepwise pathways occur in lesser degree. Digital simulations were used to evaluate this mechanism.