On cysteine and cystine peptides. 3. Synthesis of a fragment of insulin containing the intrachain disulfide bridge.

The application of methods developed mostly in this laboratory permitted the synthesis of the fully S,Nprotected heptapeptides N-carbobenzoxy-S-trityl-L-cysteinylS-diphenylmethylL cysteinyl-LalanylglycylLvalyl-S-trityl-L-cysteinyl-L-serine methyl ester (VIII, Figure 21, N-t-butyloxycarbonyl-S-trityl-L-cysteiny1-Sdiphenylmethyl-LcysteinylL-alanylglycyl-LvalylStrityl-L-cysteinyl-L-serine methyl ester ( X , Figure 21, and N-o-nitrophenylsulfenyl-S-benzoyl-L-cysteinyl-Strityl-LcysteinylLalanylglycyl-Lvalyl-SbenzoylLcysteinyl-L-serine methyl ester ( X X I I , Figure 3). For these syntheses, a variety of N and S-protecting groups was used; this allowed selective removal of either the N-, or two of three S-protecting groups according to desired aims. Thus, selective removal of the two Strityl groups f r o m VIII and X and of the two S-benzoyl groups f r o m X X I I led to the formation of the corresponding dithiol compounds X X V , X X V I , and XXVI I . B y oxidation of these thiol compounds a disuljde bridge was established specijcally between two of the three cysteine residues incorporated in the above three heptapeptides. The corresponding oxidation products XXVIII , X X I X , and XXX are derivatives of the 6-12 sequence of the A chain of sheep insulin bearing the 6-11 intrachain bridge (Figure 1). Removal of the N-protecting groups f r o m the cyclic peptide esters X X I X and XXX aflorded the cyclic peptide ester hydrochlorides X X X I and X X X I I from which the remaining S-protecting group can be removed b y established methods. The significance of the above cyclic peptides as regards the problem of insulin synthesis is discussed, The possibility of an S+N acyl migration should be taken into account when using S-acylcysteines in peptide synthesis. It was proved that such a migration does not take place, at least in detectable extent, during the course of the synthesis of the S,N-protected heptapeptide X X I I .