Linear oligopeptides. Part 406.1 Helical screw sense of peptide molecules: the pentapeptide system (Aib)4/L-Val[L-(αMe)Val] in solution

Abstract

A variety of N ^α-blocked pentapeptide esters, each containing four helicogenic, achiral α-aminoisobutyric acid residues and one chiral L-valine or C ^α-methyl-L-valine residue in the N-terminal, internal or C-terminal position of the sequence, have been synthesized by solution methods and fully characterized. The results of a solution conformational analysis, performed by using FTIR absorption and ¹H NMR techniques, favour the conclusion that all of the pentapeptides examined fold into a 3₁₀-helical structure. In addition, a CD study of the N ^α-para-bromobenzoylated peptides strongly supports the view that the prevailing screw sense of the 3₁₀-helical structure that is formed is strongly dependent upon the position in the sequence of the single chiral C ^α-tri- or C ^α-tetrasubstituted α-amino acid.