Solubility Equilibria and Crystallographic Characterization of the l-Threonine/l-allo-Threonine System, Part 2: Crystallographic Characterization of Solid Solutions in the Threonine Diastereomeric System

Abstract

Features of crystal structures and their thermal deformations in the l-threonine/l-allo-threonine system were investigated by means of single-crystal X-ray diffraction and temperature-resolved X-ray powder diffraction. The l-threonine/l-allo-threonine system belongs to the systems with continuous solid solutions. The comparative analysis of crystal structures of l-threonine (l-Thr), l-allo-threonine (l-aThr), and their mixed crystals was conducted based on literature and our own data. It was found that the distances between methyl groups are more sensitive to the crystal composition than the lengths of the hydrogen bonds. Statistical distribution of l-Thr and l-aThr molecules causes the occurrence of shortened and elongated distances between the methyl groups of the neighbor nonidentical molecules (l-Thr—l-aThr). Thermal deformations of crystal structures of l-Thr, l-aThr, and solid solutions containing 34 and 90% l-Thr were investigated. In the case of diastereomers, the slight negative thermal expansion along the axis a was observed. Correlations between features of crystal structure and thermal behavior of l-Thr, l-aThr, and their solid solutions were identified.