Abstract
Vibrational circular dichroism (VCD) and infrared absorption spectra in the amide A region of blocked oligopeptides containing proline have been obtained in apolar organic solvents in order to clarify the characteristic VCD for the intramolecularly hydrogen-bonded NH stretching involved in the turn structures of peptide chains as well as the intermolecularly hydrogen-bonded NH stretching in associated molecules. The γ-turn with the intramolecularly hydrogen-bonded C<sub>7</sub> conformation exhibits a characteristic positive VCD band at about 3330 cm<sup>-1</sup> in dilute solutions of Piv-Pro-NHMe and Ac-Pro-NHMe. The intramolecularly hydrogen-bonded NH stretching in the type II β-turn with C<sub>10</sub> conformation gives rise to a positive VCD band at 3345 cm<sup>-1</sup> for Piv-Pro-Gly-NHMe. The Gly NH group of Piv-Pro-Gly-OMe and Ac-Pro-Gly-OMe in dilute solutions assumes C<sub>7</sub>C<sub>5</sub> conformation stabilized by a bifurcated three-center intramolecular hydrogen bond and exhibits a positive VCD band near 3300 cm<sup>-1</sup> at a lower frequency than the intermolecularly hydrogen-bonded band. All the peptides studied give a characteristic negative-positive bisignate couplet from the high wavenumber side for the intermolecularly hydrogen-bonded NH stretching band at high concentrations.
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