Intramolecularly hydrogen-bonded peptide conformations. Preferred crystal-state and solution conformations of N-monochloroacetylated glycines dialkylated at the α-carbon atom

Abstract

A crystal-state structural analysis of N-monochloroacetylated glycines diethylated and di-n-propylated at the α-carbon was performed by X-ray diffraction. In both structures the α-amino acid residue shows a φ,ψ_T set of torsion angles in the region of the conformational energy map where fully extended conformations are found. The conformation about the amide bond is trans, and the substituted acetamido and carboxylic acid groups are nearly coplanar. The two structures are characterized by a three-centre, doubly intramolecular hydrogen bond (C₅,C₅ conformer) of the Cl ⋯ H(N)⋯ O (carboxylic acid) type. For both compounds these interactions survive in deuteriochloroform solution, as shown by ¹H n.m.r. and i.r. absorption analysis.