Formula omitted]: X-ray structure, energy minimization and calorimetric determinations

Abstract

A solid-state study of N- acetyl- l-prolyl- l-leucyl-glycinamide hemihydrate (NAPLGA), C 15H 26N 4O 4·0.5H 2O has been performed using single crystal X-ray diffraction method and calorimetric determinations of the fusion thermodynamic parameters. Conformational energy map of AcProLeuGlyNH 2 molecule has also been evaluated. In the crystal, the molecular backbone is folded back between Leu and Gly residues and the conformation is stabilized by a 1 ← 4 intramolecular H-bond. In this manner a ten-membered cyclic structure with β-turn type II conformation is formed. Prolyl residue is in a slightly distorted CsC β- exo form and Leu side-chain adopts the energetically favoured t( g + t) conformation. Crystal packing is characterized by four intermolecular hydrogen bonds which involve all the donor groups. The crystallization water placed on a binary axis acts as a bridge, through two H-bonds, between two-fold related peptide molecules. All the hydrogen bonds assemble in wide layers extending parallel to the ab plane of a C2 space group. Along the c direction, adjacent layers are separated by regions characterized by loose van der Waals interactions. Potential energy calculations have been carried out using procedures as in ecepp (empirical conformational energy program for peptides) and amber programs and the most favoured conformations have been analysed in comparison with the one observed in the hydrated crystal. The observed conformation is very close to a relative minimum, whose energy is only 4 kJ mol −1 ( ecepp procedure) higher than that of the calculated absolute minimum. Thermodynamic properties concerning the fusion, when compared with those of other correlated N-acetyl peptidoamides, suggest that the intramolecular hydrogen bond is probably maintained also in NAPLGA molten, partially limiting the conformational freedom of each peptide molecule.