Structural characterization of N-acetylglycyl-L-prolinamide: an X-ray and thermodynamic study

Abstract

The crystal structure of N-acetylglycyl-L-prolinamide C 9H 15N 3O 3 has been determined by single-crystal X-ray analysis to an R value of 0.046. The peptide linkage between Gly and Pro residues is in a distorted trans conformation. Main-chain conformation occurs for both residues in the F region of the conventional ϕ,ψ map for peptides. The prolific residue has a “type B” geometry. The ring conformation is not single atom puckered, and can be described as C 2 C β exo  C γ endo , close to a pure twist form. The crystal packing is ruled by three intermolecular hydrogen bonds which involve all the donor groups. Wide layers of H-bonds grow parallel to the ab plane and separate along the c direction narrow apolar regions containing the proline residues at van der Waals distances. The temperatures and enthalpies of fusion and of a probable solid-solid transition, determined by differential scanning calorimetry, are discussed on the basis of the crystal features (density and hydrogenbond pattern) and in comparison with the corresponding parameters of the isomeric N-acetyl-L-prolylglycinamide.