Amino acids and peptides. X. The reduction of cystine and related disulphides by cuprous copper in the presence of ascorbic acid

Abstract

Cystine, and penicillamine disulphide, are quantitatively reduced by cuprous copper in the presence of ascorbic acid to give the cuprous mercaptide, according to the equation RSSR + 2Cu⁺ +2e (from ascorbate) + 2RSCu. In practice, cupric copper can be used as titrant, the overall equation then being RSSR + 2Cu⁺⁺ + 2C₆H₈O₆ → 2RSCu + 2C₆H₆0₆ +4H⁺. The reactions can be followed polarographically and cystine determined amperometrically by measuring the current due to Cu⁺, which appears on further addition of Cu⁺⁺ beyond the end-point. Protein disulphides fail to react, but thiols interfere according to RSH + Cu⁺ + RSCu + H⁺. Hence cysteine can also be determined amperometrically by titration with Cu⁺⁺ in the presence of ascorbic acid.