Specific chemical cleavage in high yield at the amino peptide bonds of cysteine and cystine residues.

A protocol is presented for the quantitative conversion of cysteine and cystine residues in proteins to residues of Scyanocysteine, using the reagent 2-nitro-S-thiocyanobenzoic acid (DEGANI, Y., AND PATCHORNIK, A. (1971) J. Org. Chem. 36, 2727). Cleavage of the amino peptide bond of the Scyanocysteine residue is obtained upon exposure to 6 M guanidinium chloride-O.1 M sodium borate, pH 9.0, at 37” for 12 hours, with concomitant formation of Z-iminothiazolidine-4-carboxylyl peptides. Application of the method to several proteins indicates that virtually complete cleavage can be obtained and that there are no significant side reactions due to exposure to alkaline pH. A mechanism for the cleavage reaction is proposed in which specific hydroxide ion catalysis is followed by concerted peptide bond cleavage and ring closure, without the intermediate formation of an acyliminothiazolidine.