KINETIC-STUDIES ON THE ROLE OF DIOXYGEN IN THE COPPER-CATALYZED AUTOXIDATION OF CYSTEINE

Kinetic studies on a role of dioxygen in the copper-catalyzed autoxidation of cysteine in glycylglycine-phosphate buffers have been carried out. The rate of autoxidation was obtained by measuring the consumption of cysteine. It has been revealed that cysteine is oxidized by a ''sequential mechanism''. The reaction pathway can be shown by Eqs. 1, 2, 3, and 4; cysteine is oxidized not only by copper(II) species, but also by copper(I)-O-2 adducts: Cu(II)-L + CyS(-) reversible arrow L-Cu(II)-CyS(-) (1) L-Cu(II)-CyS(-) --> Cu(I)-L + CyS. (2) Cu(I)-L + O-2 reversible arrow L-Cu(I)-O-2 (3) L-Cu(I)-O-2 + CyS(-) + 2H(+) --> Cu(II)-L + CyS.+ H2O2 (4) Here, L represents ligands including cysteine. The oxidation step (4) catalyzed by the Cu(I)-O-2 species was proposed to be rate-determining.