CRYSTAL-STRUCTURE AND THERMODYNAMICS OF PHASE-TRANSITIONS OF N-ACETYL-L-VALINAMIDE

Changes in the fundamental thermodynamic functions, such as enthalpy and entropy, during fusion and sublimation have been measured for N-acetyl-l-valinamide (l-NAVA) and compared with previous results for the same derivative of other simple amino acids (glycine, l-alanine and d-leucine). There is no discernible simple relation between the increasing molecular weight and any of the measured quantities. The fusion properties of l-NAVA are characterised by values noticeably higher than those of other homologous compounds, the sublimation properties being intermediate between those of the glycine and alanine derivatives. An accurate crystallographic analysis of l-NAVA has been carried out to support this observed behaviour. Detailed knowledge of the molecular orientations and contacts in the crystal is required because l-NAVA has the same number of H-bonds as the derivatives of glycine and alanine, although its density is very close to that of the leucine derivative, whose packing is characterised by a lower number of H-bonds. In order to optimise the intermolecular interactions and to adhere to the conformational constraints, the l-NAVA molecules are arranged in the crystal such that the hydrogen bonds are of proper length and there are good van der Waals contacts between some of the alkyl groups. The relatively high stability of l-NAVA crystals can be attributed to these factors. At the same time there are regions of small voids, which are responsible for the relatively low density. © 1990.