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Hydrogen peroxide is a natural oxidantthat can oxidize proteinthiols (RSH) via sulfenic acid (RSOH) and sulfinic acid (RSO2H) to sulfonic acid (RSO3H). In this paper, we study thecomplete anionic and neutral oxidation pathway from thiol to sulfonicacid. Reaction barriers and reaction free energies for all three oxidationsteps are computed, both for the isolated substrates and for the substratesin the presence of different model ligands (CH4, H2O, NH3) mimicking the enzymatic environment. Wefound for all three barriers that the anionic thiolate is more reactivethan the neutral thiol. However, the assistance of the environmentin the neutral pathway in a solvent-assisted proton-exchange (SAPE)mechanism can lower the reaction barrier noticeably. Polar ligandscan decrease the reaction barriers, whereas apolar ligands do notinfluence the barrier heights. The same holds for the reaction energies:they decrease (become more negative) in the presence of polar ligandswhereas apolar ligands do not have an influence. The consistentlynegative consecutive reaction energies for the oxidation in the anionicpathway when going from thiolate over sulfenic and sulfinic acid tosulfonic acid are in agreement with biological reversibility. [ABSTRACT FROM AUTHOR]