Mechanism of copper-catalyzed autoxidation of cysteine

The kinetics of copper-catalyzed autoxidation of cysteine and its derivativ es were investigated using oxygen consumption, spectroscopy and hydroxyl ra dical detection by fluorescence of a coumarin probe. The process has comple x two-phase kinetics. During the first phase a stoichiometric amount of oxy gen (0.25 moles per mole of thiol) is consumed without production of hydrox yl radicals. In the second reaction phase excess oxygen is consumed in a hy drogen peroxide-mediated process with significant (OH)-O-. production. The reaction rate in the second phase is decreased for cysteine derivatives wit h 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 c ysteine complex. The reaction mechanism consists of two simultaneous reacti ons (superoxide-dependent and peroxide-dependent) in the first phase, and p eroxide-dependent in the second phase. The second reaction phase begins aft er 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 producti on of hydroxyl radicals.