Amino acids and peptides. VI. Studies on cystine and αα'-dimethylcystine in relation to the alkaline degradation of protein disulphides

Abstract

The synthesis and properties of α-methyl-DL-cysteine and of racemic αα?-dimethyl-cystine and its dimethyl ester are recorded. The new disulphide is very much more stable to aqueous alkali than is cystine, its dimethyl ester is quite stable to methanolic triethylamine whereas cystine dimethyl ester breaks down readily with liberation of sulphur. These results support the β-elimination hypothesis for the alkaline degradation of cystine and its derivatives, namely, that attack occurs at the amino acid α-carbon atom, with synchronous displacement of a sulphenyl thiol anion, RSS^-.