Three cyclohexapeptides modelling the oxytocin ring moiety: Synthesis and circular dichroism

A series of cyclic disulfides cysteinyl-triglycyl-asparaginyl-cysteine (Ib), cysteinyl-diglycyl-glutaminyl-asparaginyl-cysteine (Ic), and cysteinyl-glycyl-isoleucyl-glutaminyl-asparaginyl-cysteine (Id) has been prepared. The effect of gradual attachment of side chains to the ring on the peptide backbone and the disulfide group conformation has been studied using circular dichroism. Introduction of side chains reduces substantially the conformational mobility of the backbone , but not enough to let any conformational β-turn type predominate (in polar solvents). Conformation of the disulfide group is essentially independent of the peptide moiety of the molecule and is influenced by specific solvation.