Linear oligopeptides. Part 406. Helical screw sense of peptide molecules: The pentapeptide system (Aib)(4)/L-Val[L-(alpha Me)Val] in solution

A variety of N-alpha-blocked pentapeptide esters, each containing four helicogenic, achiral alpha-aminoisobutyric acid residues and one chiral L-valine or C-alpha-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 H-1 NMR techniques, favour the conclusion that all of the pentapeptides examined fold into a 3(10)-helical structure. In addition, a CD study of the N-alpha-para-bromobenzoylated peptides strongly supports the view that the prevailing screw sense of the 3(10)-helical structure that is formed is strongly dependent upon the position in the sequence of the single chiral C-alpha-tri- or C-alpha-tetrasubstituted alpha-amino acid.