Homochiral and heterochiral associations observed in crystals of ArSO2-(Aib)5-OMe

Abstract

The molecular conformations, packing structures and intermolecular interactions of homopentapeptides from achiral α-aminoisobutyric acid (Aib), ArSO₂-(Aib)₅-OMe (Ar = p-tolyl, p-bromophenyl and p-methoxyphenyl), have been investigated by single-crystal X-ray diffraction analysis. The peptides were folded in 3₁₀-helical conformations consisting of two or three consecutive ten-atom intramolecular hydrogen-bonded β-turns of type III or III′. In the packing mode, left-handed (M) and right-handed (P) 3₁₀-helical molecules formed linear network structures with head-to-tail type intermolecular hydrogen bonds. Two types of network structures consisting of homochiral sequences (⋯M⋯M⋯M⋯ or ⋯P⋯P⋯P⋯) and heterochiral sequences (⋯M⋯P⋯M⋯P⋯) were obtained depending on the functional groups or substituents of the peptides. Interestingly, peptide 1a which has a p-tolyl group crystallized differently when using a different crystallization medium.