Infrared spectroscopic study of C₇ intramolecular hydrogen bonds in peptides

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 C 7 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 C 7 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 C 7 intramolecular hydrogen bonds in addition to C 5 intramolecular hydrogen bonds. The present studies also show that all the peptides studied exist in more than one conformation in solution.