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In the first paper of this series (Elliott, 1952) it was shown that N, O-acyl migration could be used to bring about cleavage of the silk fibroin molecule at some of the serine residues, but that other peptide bonds involving serine remained resistant to the reagent. It seemed possible that this resistance may have been due to steric hindrance by bulky side chains in the neighbourhood of serine residues. In view of the very high content of glycine and alanine in silk fibroin it could be assumed that they are linked frequently to serine and that it is these bonds which undergo the acyl migration reaction, whereas bonds involving serine and amino acids with larger side chains remained unaffected. In order to test this hypothesis a quantitative Cterminal end-group method of wide scope was needed. No such method existed when this work commenced, but reduction of the C-terminal residues with a metallo-hydride seemed promising. In addition it was hoped that reduction of the carboxyl groups ofpeptides would form the starting point for a stepwise degradation method as outlined below. Fromageot, Jutisz, Meyer & P6nasse (1950) had already shown that lithium aluminium hydride reduced the free carboxyl groups of proteins to primary alcohol groups and that amino alcohols were obtained on subsequent hydrolysis, but the results obtained on insulin were far from quantitative. Chibnall & Rees (1951, 1952) had used lithium borohydride to determine the distribution of amide groups in insulin. Our initial experiments consisted in the reduction of a few simple peptide derivatives by lithium aluminium hydride and lithium borohydride, in connexion with the stepwise degradation method. Attempts were then made to develop a reliable end-group method for large peptides and proteins.

A search for specific chemical methods for fission of peptide-bonds. Experiments involving reduction of C-terminal residues