Oxidation Of Thiols By Copper(II)

Abstract

Thiols were oxidized by Cu(II) in a reaction that had two phases. There was an immediate reaction between thiol and Cu(II) with loss of thiol, measured by its reaction with 5,5′-dithio-bis(2-nitrobenzoic acid), and formation of Cu(I), measured by its reaction with bathocuproinedisulfonic acid. With equal molar concentrations of thiols and Cu(II), only the first phase was observed and thiols were oxidized in less than 15 sec. When the concentration of Cu(II) was limiting (200 μM thiol and 10 to 160 μM Cu(II)), the second phase of the reaction was observed; this phase was much slower for glutathione (GSH) than it was for cysteine and accounts for previous reports that GSH is oxidized by Cu(II) at a slower rate than cysteine. It is proposed that Cu(II) reacts with thiols to form Cu(II)-thiol complexes, Cu(II) is reduced to Cu(I), and the thiols are oxidized to the corresponding radicals. Two of the radicals react to form a disulfide and Cu(I) reacts with a second thiol to form a Cu(I)-thiol complex in which the Cu(I) is oxidized to Cu(II) by oxygen. The rate of reoxidation of Cu(I) is dependent on the R group of the thiol and is the rate limiting step of the reaction.