Mechanism of copper-catalyzed autoxidation of cysteine

The kinetics of copper-catalyzed autoxidation of cysteine and its derivatives were investigated using oxygen consumption, spectroscopy and hydroxyl radical detection by fluorescence of a coumarin probe. The process has complex two-phase kinetics. During the first phase a stoichiometric amount of oxygen (0.25 moles per mole of thiol) is consumed without production of hydroxyl radicals. In the second reaction phase excess oxygen is consumed in a hydrogen peroxide-mediated process with significant ·OH production. The reaction rate in the second phase is decreased for cysteine derivatives with a free aminogroup and increased for compounds with a modified aminogroup. The kinetic data suggest the catalytic action of copper in the form of a cysteine complex. The reaction mechanism consists of two simultaneous reactions (superoxide-dependent and peroxide-dependent) in the first phase, and peroxide-dependent in the second phase. The second reaction phase begins after oxidation of free thiol. This consists of a Fenton-type reaction between cuprous-cysteinyl complex and following oxidation of cysteinyl radical to sulfonate with the consumption of excessive oxygen and significant production of hydroxyl radicals.