ON N-ACETYLCYSTEINE .2. OXIDATION OF N-ACETYLCYSTEINE BY HYDROGEN-PEROXIDE - KINETIC-STUDY OF THE OVERALL PROCESS

The oxidation kinetics of N-acetylcysteine (RSH) by hydrogen peroxide has been studied at neutral pH at different concentration ratios from 0.2 to 20 (4 x 10(-4) mol L(-1) less than or equal to [RSH](0) less than or equal to 2 x 10(-2) mol L(-1), 10(-4) mol L(-1) less than or equal to [H2O2](0) less than or equal to 10(-2) mol L(-1)). 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 H2O2, the stoichiometry of the complex being RSH concentration-dependent for a given R (R > 2.5). Reaction mechanisms have been quantitatively established and the k values of each step determined.