Solid-state phase transitions in DL-norvaline studied by single-crystal X-ray diffraction.

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.