Polymorphic transformations in glycine co-crystals at low temperature and high pressure: two new examples as a follow-up to a glycine-glutaric acid study

The effects of temperature and pressure on the co-crystals of glycine with DL-tartaric and phthalic acids (GTa and GPh, respectively) have been studied by X-ray diffraction and Raman spectroscopy in comparison with those on glycine-glutaric acid (GGa). On cooling, no phase transitions were observed in GTa or GPh, in contrast to the situation with GGa. On hydrostatic compression, both GTa and GPh underwent reversible phase transformations, accompanied by fracture. In the high-pressure phases, the main structural framework was preserved, and the number of crystallographically independent molecules in the unit cell increased. In GTa, dimers are squeezed together so that some hydrogen bonds get a three-centered character, and the interactions of one of the two glycine molecules change dramatically.