Polymorphism of the quasiracemate d-2-aminobutyric acid:l-norvalineElectronic supplementary information (ESI) available: Movie S1, thermal polarisation microscopy video of the solid-state phase transition in a single crystal of d-2-aminobutyric acid:l-norvaline (MP4). CCDC 1550144 and 1550145. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7ce01270e

Here we report two new crystal structures of the amino acid quasiracemate d -2-aminobutyric acid: l -norvaline, as well as the fully reversible solid-state phase transition which connects the two forms. The room temperature form II is obtained by co-crystallisation through slow evaporation from an equimolar aqueous solution. Form I is obtained through cooling of form II below 150 K. The crystal structures of forms I and II have been determined using single crystal X-ray diffraction at 100 and 150 K, respectively. Both forms consist of 2D hydrogen bonded bilayers with an ld-ld hydrogen bonding pattern. The room temperature form II shows two conformations for the l -norvaline side chain in a disordered 50/50 occupancy distribution. The solid-state phase transition between the two polymorphic forms involves a displacement of the bilayers and conformational changes in all molecules. It has been further characterised using differential scanning calorimetry and thermal polarisation microscopy. These crystal structures and the single-crystal-to-single-crystal phase transition are similar to other quasiracemates and racemates of linear chain aliphatic amino acids. A new polymorphic quasiracemate d -2-aminobutyric acid: l -norvaline has been discovered and its reversible single-crystal-to-single-crystal phase transition has been studied using XRD, DSC and thermal microscopy. The low temperature form I is fully ordered, while form II shows two conformations for l -norvaline with a 50/50 occupancy.