zeta-Glycine: insight into the mechanism of a polymorphic phase transition

Glycine is the simplest and most polymorphic amino acid, with five phases having been structurally characterized at atmospheric or high pressure. A sixth form, the elusive zeta phase, was discovered over a decade ago as a short-lived intermediate which formed as the high-pressure "phase transformed to the gamma form on decompression. However, its structure has remained unsolved. We now report the structure of the zeta phase, which was trapped at 100 K enabling neutron powder diffraction data to be obtained. The structure was solved using the results of a crystal structure prediction procedure based on fully ab initio energy calculations combined with a genetic algorithm for searching phase space. We show that the fate of zeta-glycine depends on its thermal history: although at room temperature it transforms back to the gamma phase, warming the sample from 100 K to room temperature yielded beta-glycine, the least stable of the known ambient-pressure polymorphs.