
%0 Journal Article
%A Swan, JM 
%T Amino acids and peptides. X. The reduction of cystine and related disulphides by cuprous copper in the presence of ascorbic acid
%J Australian Journal of Chemistry
%D 1965/03/01
%V 18
%N 3
%P 411-415
%K 
%R https://doi.org/10.1071/CH9650411
%U https://www.publish.csiro.au/paper/CH9650411 
%X 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 &#8594; 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. 
