Infrared Spectroscopic Study of $C_7$ 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