Molecular Basis of the Mechanism of Thiol Oxidation by Hydrogen Peroxide in Aqueous Solution: Challenging the S_N2 Paradigm

The oxidation of cellular thiol-containing compounds, such as glutathione and protein Cys residues, is considered to play an important role in many biological processes. Among possible oxidants, hydrogen peroxide (H₂O₂) is known to be produced in many cell types as a response to a variety of extracellular stimuli and could work as an intracellular messenger. This reaction has been reported to proceed through a S_N2 mechanism, but despite its importance, the reaction is not completely understood at the atomic level. In this work, we elucidate the reaction mechanism of thiol oxidation by H₂O₂ for a model methanethiolate system using state of the art hybrid quantum-classical (QM-MM) molecular dynamics simulations. Our results show that the solvent plays a key role in positioning the reactants, that there is a significant charge redistribution in the first stages of the reaction, and that there is a hydrogen transfer process between H₂O₂ oxygen atoms that occurs after reaching the transition state. These observations challenge the S_N2 mechanism hypothesis for this reaction. Specifically, our results indicate that the reaction is driven by a tendency of the slightly charged peroxidatic oxygen to become even more negative in the product via an electrophilic attack on the negative sulfur atom. This is inconsistent with the S_N2 mechanism, which predicts a protonated sulfenic acid and hydroxyl anion as stable intermediates. These intermediates are not found. Instead, the reaction proceeds directly to unprotonated sulfenic acid and water.