Specific Cleavage of Cystine Peptides by Cyanide

The mechanism of cyanide cleavage of peptide bonds involving the cystine amino group has been shown to involve scission of the disulfide bond to yield a sulfhydryl and a thiocyano group. In pH ranges below 8, the latter cyclizes to produce an acyliminothiazolidine moiety on the peptide chain. This product is unstable and hydrolyzes spontaneously to effect cleavage of the peptide link on the cystine amino group. Increasing the pH favors disulfide scission but decreases the rate of ring closure. Above pH 10, thiocyanate ion is eliminated from the primary reaction products. Since the disulfide scission is unsymmetrical, reoxidation of the newly formed sulfhydryl groups is necessary to convert all cysteine residues to iminothiazolidine rings at the NH2-terminal ends of peptide chains.