An Isosymmetric High-Pressure Phase Transition in α-Glycylglycine: A Combined Experimental and Theoretical Study

• DOI: 10.1021/acs.jpcb.9b07313
• Publication Date: 2019-12-03T21:57:19Z
• Authors: Samantha M. Clarke; Brad A. Steele; Matthew P. Kroonb...; Dongzhou Zhang; I-Feng W. Kuo; Elissaios Stavrou
• Keywords: Models, Molecular; X-Ray Diffraction; Glycylglycine; Molecular Conformation; Pressure; Hydrogen Bonding; Spectrum Analysis, Raman; 01 natural sciences; Density Functional Theory; Phase Transition; 0104 chemical sciences

We investigated the effects of hydrostatic pressure on α-glycylglycine (α-digly) using a combined experimental and theoretical approach. The results of powder X-ray diffraction show a change in compressibility of the axes above 6.7 GPa, but also indicate that the structure remains in the same monoclinic space group, suggesting an isosymmetric phase transition. A noticeable change in the Raman spectra between 6 and 7.5 GPa further supports the observed phase transition. First-principles-based calculations combined with the crystal structure prediction code USPEX predict a number of possible polymorphs at high pressure. An orthorhombic structure with a bent peptide backbone is the lowest enthalpy polymorph above 6.4 GPa; however, it is not consistent with experimental observations. A second monoclinic structure isosymmetric to α-digly, α'-digly, is predicted to become more stable above 11.4 GPa. The partial atomic charges in α'-digly differ from α-digly, and the molecule is bent, possibly indicating different reactivity of α'-digly. The similarity in the lattice parameters predicted from calculations and the axial changes observed experimentally support that the α'-digly phase is likely observed at high pressure. A possible explanation for the isosymmetric phase transition is discussed in terms of relaxing strained hydrogen bonding interactions. Such combined experimental and modeling efforts provide atomic-level insight into how pressure-driven conformational changes alter hydrogen-bonding networks in complicated molecular crystals.