Infrared Absorption Study of Human Proinsulin C-Peptide Fragments in the Solid State

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Abstract
In order to evaluate the propriety of the solubility prediction method for peptide intermediates having polar side chains, IR absorption spectroscopic analysis of human proinsulin C-peptide fragments was performed in the solid state. The polar amino acid residues involved in the peptide fragments are in the following: Glu(OBzl), Asp(OBzl), Gin, and Ser(Bzl). All peptide fragments except for ones containing the Pro residue exhibit a high potential for the formation of a β-sheet structure, indicating that, even when the polar side chains of the Glu, Asp, and Ser residues are protected by the Bzl group, the coil conformation parameter Pc for each amino acid residue is useful for estimating the potential for the formation of a β-sheet structure of peptide fragments smaller than critical size for development of α-helical structure in the solid state. As in the case of hydrophobic peptides, the β-sheet aggregation clearly plays an important role in reducing solubility of the peptide fragments having polar side chains. The effect of the Pro residue on the disturbance of the β-sheet structure was also observed. The ability of the short-range (polar side chain-backbone) or intraresidue interactions due to the low flexibility of the pyrrolidine ring of the Pro residue to promote helical folding in peptide fragments in the solid state was not observed for the peptide fragments corresponding to helical regions of human proinsulin.