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Cysteine is converted into S-4-pyridylmethylcysteine [Gosden, Stevenson &amp; Young (1972) J. Chem. Soc. Chem Commun. 1123-1124] by 4-pyridylmethyl chloride in aqueous propanol at pH 8.3. The derivative is stable to the conditions of total acid hydrolysis. Reduction and alkylation of bovine insulin (pH 8.3, aq. 50% propanol) gives fully S-substituted derivatives in excellent yields. The S-pyridylmethylated A- and B-chains of insulin were separated by gel filtration: each of them has good solubility properties. The pyridylmethyl group is cleaved by electrolysis in a dilute acid medium, pH 2.6, to give reduced chains. They can be recombined to give insulin in the same yield and with the same degree of biological activity as chains which had not been subjected to the protection and de-protection steps. The results indicate that pyridylmethyl satisfactorily meets requirements for partial synthesis and suggest that it warrants more general use.

4-pyridylmethyl, a thiol-protecting group suitable for the partial synthesis of proteins