Electronic Properties and Optical Activity of Oligopeptides. III. Some cyclohexapeptides with glycine,L- and D-alanine

The wavelength range of the CD. spectra of some cyclohexapeptides containing different sequences of glycine, L - and D -alanine is extended down to 170 nm. This allows a relatively complete recording of the (n ? π*) and (π° ? π*) Cotton effects. Some striking spectral changes are observed on going from one molecule to another. The relative influence of L - and D -alanyl residues is discussed: Some spectra may be qualitatively related to each other by considering the effect of an L -residue at position q in the ring to cancel partially with the effect of a D -residue at position q ± 3. Assuming these cyclopeptides to occur in a hydrogen-bonded pleated sheet structure, certain dominant changes in the spectra are interpreted as reflecting a transition of the overall backbone conformation from one which is closer to the (optically inactive) symmetry C_i to another which is closer to the (optically active) symmetry C₂. An attempt is made to relate the influence of L - and D -substituents within hairpin bends of the pleated sheet structure to an amide sector rule.