Linear oligopeptides, 29. Infrared conformational analysis of homo‐oligopeptides in the solid state and in solution

Abstract

Abstract The IR spectra of eight series of monodisperse, chemically and optically pure, protected linear homo‐oligopeptides having the general formula Boc( L X) n OMe, where X = Ala, Nva, Val, Leu, Ile, Phe, Met, Cys(Me), and n = 2–7, were investigated by IR absorption spectroscopy in the solid state and in solution. The measurement in the solid state indicate that all pentamers and higher oligomers assume essentially a β‐form; trimers and tetramers present evidence of mixtures of β‐ and unordered structures, while in dimers the unordered structure is largely predominant. The occurrence of the unusual intermolecular β‐parallel conformation was suggested for HCl·H( L X) 7 OMe, where X = Val, Ile, and Phe. Further, the absorbance in the NH stretching region was determined as a function of concentration in deuterochloroform. This conformational analysis in solution showed that a high content of intramolecularly hydrogen‐bonded forms exist in all homo‐tetramers with the exception of those derived from the β‐branched amino acid residues, valine and isoleucine. The effect of the presence of a sulfur atom or an aromatic ring in the amino acid side chain ( S ‐methyl cysteine, methionine, and phenylalanine peptides) on the stability of the folded forms is also discussed.