MS18-P05 A crystallographic study of L-threonine to extremely high pressuresOur work focuses on pushing the boundaries of high-pressure crystallography on soft molecular systems. We have successfully determined crystal structures of the essential amino acid, L-threonine (C4H9NO3) to 22 GPa.
L-threonine is zwitterionic in the solid-state, comprising of an ammonium cation and a carboxylate anion. Each molecule is connected by four H-bonds, resulting in a P 212121 (Z=4) crystal structure with zig-zag chains formed along the long crystallographic axis, linked by carboxylate and hydroxyl interactions. Adjacent chains are interconnected by off-axis H-bonds between nitrile and carboxylate groups to form pleated layers. Single-crystal X-ray diffraction experiments were performed with synchrotron radiation in series using the diamond anvil cell and neon pressure transmitting medium. A reversible pressure induced phase transition going from the P 212121 (Z’=1) crystal structure to P 21 (Z’=2) was observed at 18 GPa. All H-bonds shorten resulting in conformational changes of the carboxylate, hydroxyl and methyl moieties. Structure refinements were validated against periodic-DFT (CASTEP) optimisations and prior works. Crystal lattice energies and the most energetically significant H-bond interactions for all structures were examined by the PIXEL method.
 R. O. Holanda, J. A. Lima, P. T. C. Freire, F. E. A. Melo, J. Mendes Filho and A. Polian, Journal of Molecular Structure 2015, 1092, 160-165.
 A. Gavezzotti in Bonding in Organic Molecules and Condensed Phases. The Role of Repulsions, (Ed. J. J. Novoa), The Royal Society of Chemistry, UK, 2017.
Keywords: amino acid, hydrogen-bond