Circular Dichroism Studies of \alpha-Aminoisobutyric Acid-Containing Peptides: Chain Length and Solvent Effects in Alternating Aib-L-Ala and Aib-L-Val Sequences

The CD spectra of the peptides $Boc-X-{(Aib-X)}_n-OMe$ (n = 1,2,3) and $Boc-{(Aib-X)}_5-OMe$, where X = L-Ala or L-Val have been examined in several solvents. The X = Ala and Val peptides behave similarly in all solvents, suggesting that the Aib residues dominate the folding preferences of these peptides. The decapeptides adopt helical conformations in methanol and trifluoroethanol, with characteristic negative CD bands at 222 and 205 nm. In the heptapeptides, similar spectra with reduced intensities are observed. Comparison with nmr studies suggest that estimates of helical content in oligopeptides by CD methods may lead to erroneous conclusions. The pentapeptides yield solvent-dependent spectra indicative of conformational perturbations. Peptide association in dioxane results in an unusual spectrum with a single negative band at 210 nm for the decapeptides. Disaggregation is induced by the addition of methanol or water to dioxane solutions. Aggregation of the heptapeptides is less pronounced in dioxane, suggesting that a critical helix length may be necessary to promote association stabilized by helix dipole-dipole interactions.