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

Abstract

SummaryThe 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-n*) and (no-n*) 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 q1f: 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 Ci to another which is closer to the (optically active) symmetry C,. 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.