Alanine at 13.6 GPa and its pressure-induced amorphisation at 15 GPa

L-Alanine is the smallest chiral amino acid, crystallising in the space group P2(1)2(1)2(1). By comparison with other amino acids its crystal structure is extremely resilient to pressure, the ambient-pressure crystalline phase persisting until 13.6 GPa. At 15.46 GPa, L-alanine undergoes a reversible transformation to an amorphous phase. Hirshfeld fingerprint analysis and PIXEL calculations show that there is no development of destabilising contacts as pressure increases, so minimisation of volume is the likely driving force for this transition. Void-space analysis suggests that interstitial voids are all but absent in alanine at 13.6 GPa, and further compression is only possible with a change of phase. With the exception of benzene, 15.46 GPa is the highest pressure for which crystal structure data have been obtained for an organic system.