Helical Screw Sense of Homo-oligopeptides of C-alpha-methylated Alpha-amino Acids As Determined With Vibrational Circular-dichroism

Vibrational circular dichroism spectra for three homo-oligopeptides, p-BrBz-[D-(alpha Me)Phe](4,5)-OtBu and p-BrBz-(D-Iva)(5)-OtBu, are presented and interpreted to show that the C-alpha-methylated alpha-amino acids (alpha Me)Phe and Iva of the same optical configuration favor helical conformations of the opposite screw sense. The D-(alpha Me)Phe tetra- and pentapeptides are folded in a right-handed 3(10)-helix, which is consistent with previous findings regarding homo-oligomers of C alpha-methylated amino acids with gamma-branched side chains. On the other hand, the pentapeptide of D-Iva, with a linear side chain, is folded in a left-handed helical sense, in agreement with the behavior of peptides based on the D-enantiomers of protein amino acids.