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

The crystal structures of amino acids, which are composed of molecules in their zwitterionic tautomers, are usually interpreted in terms of strong NH…O hydrogen bond formation between the ammonium and carboxylate groups, supported by weaker dispersion or CH…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 γ-form, which is the thermodynamically most stable form under ambient conditions, and the ε-form, which is generated from γ-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…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 γ-glycine a hydrogen bond with ‘text-book’ NH…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 ε-phase, and more weakly repulsive electrostatic whole-molecule interactions in the γ-phase.