An Isosymmetric High-Pressure Phase Transition in α-Glycylglycine: A Combined Experimental and Theoretical Study.
Samantha M ClarkeBrad A SteeleMatthew P KroonblawdDong-Zhou ZhangI-Feng W KuoElissaios StavrouPublished in: The journal of physical chemistry. B (2019)
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.
Keyphrases
- crystal structure
- molecular dynamics
- density functional theory
- molecular dynamics simulations
- high resolution
- electron microscopy
- single molecule
- computed tomography
- magnetic resonance imaging
- mass spectrometry
- quality improvement
- room temperature
- atomic force microscopy
- ionic liquid
- dual energy
- raman spectroscopy