DL-Norvaline

Molecular salts and cocrystals of amino acids have potential applications as molecular materials with nonlinear optical, ferroelectric, piezoelectric, and other various target physical properties. The wide choice of amino acids and coformers makes it possible to design various crystal structures. The amino acid-maleic acid system provides a perfect example of a rich variety of crystal structures with different stoichiometries, symmetries and packing motifs built from the molecular building blocks, which are either exactly the same, or differ merely by protonation or as optical isomers. The present paper reports the crystal structures of two new salts of the DL-norvaline-maleic acid system with 1:1 and 2:1 stoichiometries, namely DL-norvalinium hydrogen maleate, C5H12NO2+·C4H3O4-, (I), and DL-norvalinium hydrogen maleate-DL-norvaline, C5H12NO2+·C4H3O4-·C5H11NO2, (II). These are the first examples of molecular salts of DL-norvaline with an organic anion. The crystal structure of (I) has the same C22(12) structure-forming motif which is common for hydrogen maleates of amino acids. The structure of (II) has dimeric cations. Of special interest is that the single crystals of (I) which are originally formed on crystallization from aqueous solution transform into single crystals of (II) if stored in the mother liquor for several hours.

The amino acid DL-norvaline undergoes two solid-solid phase transitions between room temperature and -180 °C. Single-crystal X-ray diffraction studies show that the first of these transitions, taking place around -80 °C, is completely reversible with respect to crystal quality, whereas the second, taking place below -100 °C, is not due to crystal delamination. High-quality crystal structures were obtained for the higher temperature phase β (at -70 °C) and the intermediate temperature phase α (at -90 °C). They show that although side-chain disorder is present for both forms, the β-to-α phase transition induces significant side-chain rearrangements, which are accompanied by a substantial reduction in molecular volume. The observed polymorphs are compared with those found for DL-aminobutyric acid, DL-norleucine, and DL-methionine.

dl-Norvaline is a molecular crystal at room temperature and it undergoes a phase transition when cooled below 190 K. This phase transition is believed to be Martensitic, thus making it of particular interest for molecular machines. In this paper we investigate this phase transition by measuring its terahertz (THz) spectrum over a range of temperatures. Temperature-dependent THz time-domain spectroscopy (THz-TDS) measurements reveal that the transition temperature (Tβ→α) is 190 K. The influence of nucleation seeds was analyzed by determining the Tβ→α of molecular crystals with varying grain size. Grains of 5 μm or less result in a lower transition temperature (Tβ→α = 180 K) compared to larger grains of 125-250 μm (Tβ→α = 190 K). Additionally, we gain insight into the physical process of the phase transition via temperature-dependent THz-TDS spectra of doped and mixed molecular crystals. The addition of molecular dopants, which differ from dl-norvaline only at the end of the side chain which resides in the hydrophobic layers of the crystal, decreases Tβ→α. This is consistent with a solid-solid phase transition in which the unit cell shifts along this hydrophobic layer, and it leads us to believe that the phase transition in dl-norvaline is Martensitic in nature.