1-Cyano-4-dimethylamino-pyridinium salts: new water-soluble reagents for the cyanylation of protein sulphydryl groups

1-Cyano-4-dimethylaminopyridinium perchloroate or fluoroborate rapidly react with thiols in neutral or acidic medium: in 11 min at pH 3·6, 98% of the catalytic activity of papain is inhibited; cysteine residues nos. 7 and 19 of the reduced B-chain of bovine insulin are quantitatively cyanylated at pH 3·5, then the N-peptide bonds are selectively cleaved at pH 9·5.