Effect of hydrostatic pressure on the γ-polymorph of glycine -: 1.: A polymorphic transition into a new δ-form

The results of a high-resolution powder diffraction study of the effect of high hydrostatic pressure up to 8 GPa on the pure gamma-polymorph of glycine (P3(1)) are discussed. A phase transition with a jumpwise change of cell volume and cell parameters was observed. The transition starts at about 2.73 GPa and is still not complete even at 7.85 GPa. The crystal structure of the previously unknown high-pressure polymorph of glycine (delta-polymorph) could be solved and refined in the space group Pn. In this structure, glycine zwitter-ions are linked via NH...O hydrogen bonds into layers, which form double-layered bands via additional NH...O hydrogen bonds. The structure of the individual layers in the high-pressure poly morph is similar to that in the previously known alpha- (P2(1)/n) and beta- (P2(1)) forms, but the packing of the layers is essentially different. The pressure-induced polymorphic transformation in the gamma-glycine can be compared with a change in the secondary structure of a peptide, when a helix is transformed into a sheet.