How focussing on hydrogen bonding interactions in amino acids can miss the bigger picture: a high-pressure neutron powder diffraction study of epsilon-glycine

Stephen A. Moggach, William G. Marshall, David M. Rogers, Simon Parsons

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27 Citations (Scopus)

Abstract

The crystal structures of amino acids, which are composed of molecules in their zwitterionic tautomers, are usually interpreted in terms of strong NH center dot center dot center dot O hydrogen bond formation between the ammonium and carboxylate groups supported by weaker dispersion or CH center dot center dot center dot O interactions. This view of the factors which promote thermodynamic stability in the crystalline amino acids has been re-examined in two phases of glycine, the trigonal gamma-form, which is the thermodynamically most stable form under ambient conditions, and the epsilon-form, which is generated from gamma-glycine at high pressure. A combination of Hirshfeld surface analysis, periodic DFT, PIXEL and symmetry-adapted perturbation theory calculations indicates that the conventional interpretation of intermolecular interactions in crystalline amino acids phases fails to recognise the over-whelming significance of Coulombic attraction and repulsion. There are no intermolecular interactions in either phase that can plausibly be described as dispersion-based. The interaction energies of molecules connected by so-called CH center dot center dot center dot O H-bonds are far in excess of accepted values for such interactions. Of the 14 closest intermolecular contacts in both phases, six have destabilizing interaction energies: in gamma-glycine a hydrogen bond with 'text-book' NH center dot center dot center dot O contact geometry is part of a destabilising molecule-molecule interaction. The relative stabilities of the phases are best understood not in terms of a series of stabilising atom-atom contacts, but rather as a balance between efficient filling of space in the high-pressure epsilon-phase, and more weakly repulsive electrostatic whole-molecule interactions in the gamma-phase.

Original languageEnglish
Pages (from-to)5315-5328
Number of pages14
JournalCrystEngComm
Volume17
Issue number28
DOIs
Publication statusPublished - 2015
Externally publishedYes

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