Thermal deformations of crystal structures formed in the systems of malic acid enantiomers and l-valine–l-isoleucine enantiomers

Abstract

The thermal behavior of discrete phases formed in the respective systems of malic acid enantiomers and L-enantiomers of the amino acids valine and isoleucine was studied using the temperature-resolved PXRD method. In the (S)-malic acid-(R)-malic acid system, thermal deformations in crystal structures of stable compounds (enantiomer S, racemates RSI and RSII, and non-equimolar compound S 3 R) and polymorph transformations of metastable compounds (racemate RSIII and non-equimolar compound 3S1R) were examined. In the L-valine-L-isoleucine system, thermal deformations in crystal structures of stable compounds L-Val and L-Ile and non-equimolar compound V 2 I were investigated. Thermal deformation analysis included plotting the temperature dependence of the unit cell parameters and volume, calculating thermal deformation tensors, plotting figures of thermal expansion coefficients (CTE), and estimating the extent of thermal deformation anisotropy. In all the cases studied, the maximal thermal expansion was observed in the direction of the weakest hydrogen (malic acid) or van der Waals (valine and isoleucine) intermolecular bonds, i.e. in the directions closest to that perpendicular to the dimer molecule chains (malic acid) or to molecular layers (valine and isoleucine). The strongest anisotropy of thermal deformations in monoclinic crystals was observed in the ac plane, in which the symmetrically unfixed angle β can vary.

Thermal Deformations of Crystal Structures in the L-Aspartic Acid/L-Glutamic Acid System and DL-Aspartic Acid

“…Conversely, the maximal thermal expansion occurs in the direction of the weakest interaction between the molecules, which may be related to the geometry and low concentration of relatively weak hydrogen bonds of the N-H…O type in the crystal structure of the L-glu enantiomer. The temperature dependencies of the orthorhombic cell parameters and the CTE figures of L-glu (see Figures 7a and 10 A decrease in the β angle with increase in temperature was also observed for enantiomers of two other monoclinic amino acids-namely, L-valine and L-isoleucine [9]. However, in these cases the changes in the crystal structures at elevated temperatures were not accompanied by negative thermal expansion, as revealed for L-and DL-aspartic acid.…”

“…Figure 11a,b also show the CTE figures and the regions corresponding to the positive (axis α11) and negative (axis α33) thermal expansion. A decrease in the β angle with increase in temperature was also observed for enantiomers of two other monoclinic amino acids-namely, L-valine and L-isoleucine [9]. However, in these cases the changes in the crystal structures at elevated temperatures were not accompanied by negative thermal expansion, as revealed for L-and DL-aspartic acid.…”

“…Thermal deformations in organic compounds [8,9] are considerably less studied than those in inorganic substances [10][11][12], while thermal deformations of chiral organic Thermal deformations in organic compounds [8,9] are considerably less studied than those in inorganic substances [10][11][12], while thermal deformations of chiral organic compounds are hardly investigated at all. Surprisingly enough, there are only a few reported works discussing thermal deformations of amino acids and presenting figures of their thermal expansion coefficients (CTEs).…”

Limits of Solid Solutions and Thermal Deformations in the L-Alanine–L-Serine Amino Acid System

“…In the review of B. Saha [7] and our recent work [8], it was already mentioned that the number of publications reporting thermal deformations of organic crystal structures is scarce compared to the number of related investigations on inorganic compounds. At the same time, publications on thermal deformations of amino acids or chiral substances, which play a particularly important role in living matter, are even less numerous.…”

“…Examples are the works from B. Nicolaï et al [9] 12]. We investigated the thermal deformations of crystal structures in the following systems: the components and two solid solutions formed in the L-threonine-L-allo-threonine diastereomer system [10], the components in the L-malic acid-D-malic acid system [8,11], and the components formed in the L-valine-L-isoleucine amino acid system [12].…”