Circular dichroism of type 13 β-turn in linear tripeptides containing L-proline and D-alanine

The linear tripeptides tBoc-L-Prolyl-D-alanyl-L-leucine and tBoc-L-prolyl-D-alanyl-L-valine have been shown, from circular dichroism (CD) and infrared spectral data, to take up the 4 → 1 hydrogen bonded β-turn conformation in organic solvents. The CD spectra of these tripeptides in trifluoroethanol exhibit a positive n → π★ band around 220 nm contrary to the usual negative band observed for the type II β-turn. The observed CD spectra of the tripeptides provide the first examples of those predicted theoretically by Woody for peptides containing L,D sequences and adopting the Venkatachalam type 13 β-turn. This conformation is seen to revert to the type II β-turn when the N-terminal protecting group is acetyl or when the C-terminal residue is glycine. These data are shown to have a direct bearing on the interpretation of the CD spectra of globular proteins.