Infrared spectroscopic study of C₇ intramolecular hydrogen bonds in peptides

Abstract

The ir-spectra in the N-H stretching region of Piv-Pro-NHMe and Boc-Pro-NHMe have been studied in carbon tetrachloride and chloroform solutions over a wide range of concentrations. Based on the concentration dependence of the N-H stretching bands, it has been shown that the characteristic N-H stretching band due to the C₇ intramolecular hydrogen bond is around 3335 cm^-1. Intermolecular hydrogen bonding also occurs to a small extent in these peptides, giving rise to a slight concentration dependence of the N-H stretching bands. The band around 3335 cm^-1 need not necessarily be due to C₇ hydrogen bonds alone as proposed by Tsuboi et al. or to intermolecular hydrogen bonding alone as proposed by Maxfield et al.; this conclusion is supported by studies on Boc-Leu-NHMe, which undergoes only intermolecular hydrogen bonding. We have shown that Z-Aib-Aib-OMe and Z-Aib-Ala-OMe form C₇ intramolecular hydrogen bonds in addition to C₅ intramolecular hydrogen bonds. The present studies also show that all the peptides studied exist in more than one conformation in solution.