Circular dichroism studies of helical oligopeptides: can 310 and α-helical conformations be chiroptically distinguished?

Abstract

Circular dichroism studies of seven helical oligopeptides containing α-aminoisobutyric acid (Aib) in methanol and trifluoroethanol (TFE) solutions are reported. Peptides ranging from 10 to 21 residues in length have been examined. In all cases distinct negative CD bands characteristic of helical peptides are obtained at 220 and 205 nm corresponding to the n-π^∗ π-π^∗ transitions, respectively. The ratio R = [θ]n-π^∗ /[ θ]π-π^∗ is < 1.0 for all peptides studied. Using crystal structure and n.m.r. results for a 10 residue 310 helical peptide and literature values for an α-helical 11-residue peptide, it is shown that both helical conformations yield R values of 0.8 in alcoholic solvents. The CD data are considered in the light of ¹H n.m.r. studies on these oligopeptides. The results suggest that 310 and α-helical conformations cannot be distinguished by CD methods.