OH oxidation of methionine in the presence of discrete water molecules: DFT, QTAIM and valence bond analyses

The first steps of the oxidation process of amino acid methionine (Met, CAS 63-68-3 ) by • OH radicals, leading to Met-OH • adduct and then to Met radical cation, were investigated theoretically over the last few years considering the aqueous environment as a continuum. In this work, following the same procedure that we used for the oxidation of dimethyl sulfide as reported by Domin et al. (J Phys Chem B, 121:9321), discrete water molecules, as well as relative positions, of the • OH radical to Met were taken from molecular dynamics calculations. The presence of water molecules strongly modifies the relative energies of Met-OH adducts and cations when water is properly modeled. Depending on the terminal functional groups and on the position of the • OH radical, several stable structures were found; however, the most stable radical is the N-centered or the S∴N radical cation. QTAIM analysis and valence bond (VB) treatment allowed for the characterization of the 2c∴3e nature of S∴N and S∴OH bonds. VB analysis estimated the probability of the heterolytic rupture of the • OH adduct that is modified by the presence of water molecules. Graphical abstract Oxidation of amino acid methionine by • OH radicals in the presence of discrete water molecules.