ZOHREH A B E D~E H ,JLJL ARROUB, and MOMQUE GARD~S-ALBERT. Can. J. Chem. 72,2102Chem. 72, (1994.The oxidation kinetics of N-acetylcysteine (RSH) by hydrogen eroxide has been studied at neutral pH at different concentration ratios from 0.2 to 20 (4 x lo4 rnol L-I B [RSH], r 2 x 10-' mol L-I, lo4 rnol L-I 5 [H202], 5 lo-' rnol L-I). In all the cases studied, N-acetylcystine (RSSR) is the only oxidized product formed. Our kinetic data have focused on the importance of the concentration ratio to reach the stoichiometric oxidation of N-acetylcysteine by hydrogen peroxide. Indeed non-stoichiometric oxidation of RSH occurs at relatively low concentration ratios (R < 2.5) whereas stoichiometric oxidation is observed when R > 2.5. Moreover, it has been shown that in the first minutes of the reaction there is the formation of a complex between RSH and H202, the stoichiometry of the complex being RSH concentration-dependent for a given R (R > 2

The complexation of N-acetylcysteine (RSH) with hydrogen peroxide has been studied experimentally and theoretically. Experimentally we have measured the evolution of RSH, H2O2, and RSSR (N-acetylcystine) as a function of time. Surprisingly, H2O2 decays by a biphasic process, which is not the case for RSH and RSSR. In the first stage of the kinetics, H2O2 disappears without oxidizing the thiol function of RSH. By analogy with glutathione (GSH), the formation of a complex between RSH and H2O2 has been proposed. The thermodynamic equilibrium constant of complex formation has been determined. Theoretical calculations were performed within the SIBFA method to pinpoint the sites of complexation in isolated and hydrated states. A mixed "discrete–continuum" model was used to evaluate the solvent effect. The two stable complexes found in isolated state have different behaviour under the influence of the solvent. Comparison with complexed GSH is discussed.