Infrared Absorption Study of Human Proinsulin C-Peptide Fragments in Dichloromethane

In connection with the relationship between the conformation and solubility of peptide intermediates having polar side chains, IR spectroscopic conformational analysis of human proinsulin C-peptide fragments was performed in dichloromethane. The polar amino acid residues involved in the peptide fragments are Glu(OBzl), Asp(OBzl), Gin, and Ser(Bzl). Especially, the N–H stretching absorption spectra have been explored over a wide range of concentration. Based on the concentration dependence of the N–H stretching bands, it has been shown that the characteristic N–H stretching band around 3330 cm−1 is due to the intramolecular hydrogen bond. 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. Conformational behaviors of oligopeptides having protected polar side chains of the Glu(OBzl) and Asp(OBzl) residues are just the same as those of the corresponding homooligo(Leu)s, indicating that the protected polar side chains do not disturb conformational behaviors of the oligopeptides. Conformational analysis of the peptide in dichloromethane, which contains the Pro residue and has a high potential for the formation of an α-helical structure, distinctly suggests that the Pro residue near the N-terminal in the peptide has the great ability to promote successive intramolecular hydrogen bonds, probably corresponding to the α-helical structure of the peptide.