The Ability of the Proline Residue to Promote Successive-Intramolecular Hydrogen Bonds in Oligopeptides

In order to investigate the ability of the Pro residue to promote helical folding in oligopeptides, the IR spectroscopic conformational study of the oligo(Leu)s containing the Pro residue was performed in dichloromethane. Their IR difference spectra suggested the occurrence of successive-intramolecular hydrogen bonds just like incipient α-helical structures formed by one, two, three, and so forth i→i–4 hydrogenbonding patterns, alluding to the ability of the Pro residue to promote helical folding in the peptides. However, the intensities of hydrogen-bonded absorption bands in the amide A region of each peptide strongly suggested that conformations of the peptides investigated were also contributed by other intramolecularly hydrogen-bonded folded structures, which had lower absorption coefficients. The initiation mechanism of successive-intramolecular hydrogen bonding in the peptides is apparently attributed to the disturbance of the β-sheet formation by the rotation of the tertiary peptide bond plane (Leu...