High-pressure polymorphism in amino acids
Pressure up to 10 GPa is a powerful method for studying polymorphism in organic crystal structures, and this review surveys work carried out on high-pressure polymorphism in amino acids. High-pressure polymorphs have been established crystallographically for glycine, alanine, serine, cysteine and leucine. Phase transitions can be driven by the avoidance of very short intermolecular contacts or by promotion of a more stable molecular conformation. Experimental methods are also briefly surveyed, along with three methods that have proved very helpful in the analysis of high-pressure polymorphs, namely the PIXEL method for calculation of intermolecular energies, topological analysis with Voronoi–Dirichlet partitioning and Hirshfeld surfaces for gaining a graphical overview of intermolecular interactions.