Mechanism of cyanogen bromide reaction with methionine in peptides and proteins: I. Formation of imidate and methyl thiocyanate

The reaction of cyanogen bromide with peptides and proteins in anhydrous formic acid has been studied. Cleavage of the side chain of methionine to yield methyl thiocyanate was quantitative for six of the seven compounds in the study. A cyclic imidate, proposed by earlier workers as an intermediate in the cleavage of the peptide chain, has now been identified. The formation of methyl thiocyanate was followed quantitatively by gas chromatography. This procedure also offers a very sensitive and specific test for methionine in biological materials.