Quantum chemistry calculations have been performed to investigate the kinetic and mechanism of cysteine oxidation using the hydrogen peroxide. For this reaction, four major possible mechanisms have been proposed including concerted, non-concerted, ionic and radical mechanisms. All mechanisms have been investigated in the gas phase and different solvents such as water, THF and dioxane. According to obtained results, the ionic path is preferred from the energy point of view. The effect of the solvent has been investigated with explicit solvent and ONIOM models in water. Better agreement with the experimental data was obtained using this method showing the importance of the solvent effects on this type of reaction. Finally, QTAIM analysis confirmed the covalent nature of S-O bond at the transition state of the reaction.

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