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

Samantha M. Clarke*, Brad A. Steele, Matthew P. Kroonblawd, Dongzhou Zhang, I. Feng W. Kuo, Elissaios Stavrou

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Scopus citations

Abstract

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.

Original languageEnglish
Pages (from-to)1-10
Number of pages10
JournalJournal of Physical Chemistry B
Volume124
Issue number1
DOIs
StatePublished - 9 Jan 2020
Externally publishedYes

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