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Acta Cryst. (2014). A70, C272
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It is well known that infinite head to tail chains built of zwitterions linked to each other by N-H...O hydrogen bonds are common structural motifs in crystals of amino acids. These chains coincide with directions of the smallest compressibility of a crystal structure on cooling and increasing pressure and can even remain after structural phase transitions. However one should take into account the dual nature of these chains. From the one hand zwitterions of amino acids are linked by N-H...O hydrogen bonds formed from the head, amino group, and the tail, carboxylate group. From the other hand besides hydrogen bonding there is electrostatic interactions which occur between positively charged amino group and negatively charged carboxylate group. Being guided by an idea to distinguish electrostatic contribution from the charge assisted N-H...O hydrogen bonds and to understand their role in the crystal structure distortion on increasing pressure, two crystal structures of N-methyl derivatives of the simplest amino acid glycine are considered as a case study. N-methylglycine or sarcosine has two donors for hydrogen bonding and so forms two infinite head to tail chains in the structure whereas N,N,N-trimethylglycine or betaine has no hydrogen bonds at all, but its zwitterions are lined up resembling head to tail chains. The effect of increasing hydrostatic pressure is different for two crystals. The structure of betaine compresses anisotropically, but sarcosine undergoes a phase transition accompanying crystal fragmentation and changes in N-H...O hydrogen bonds. The phase transition is kinetically controlled and strongly depends on the rate of variation of pressure. Of special interest is distortion of head to tail chains on increasing pressure comparing with those observed in polymorphic modifications of glycine.

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Acta Cryst. (2014). A70, C550
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Crystalline amino acids are considered to mimic important interactions in peptides, therefore the studies of the structure-forming factors in these systems attract much attention. N,N-dimethylglycine is an interesting model compound that was used to test the role of the N-H...O H-bonds in forming the head-to-tail chains - the main structural unit in the crystals of amino acids. It was hypothesized previously that additional side N-H...O H-bonds play an important role in forming the head-to-tail chains of amino acid zwitterions linked via N-H...O H-bonds between the charged -NH3 and -COO terminal groups. Twice methylated amino group of N,N-dimethylglycine is able to form only one N-H...O H-bond in the crystal structure, so that this hypothesis could be tested. There are two polymorphs of N,N-dimethylglycine, in which the zwitterions are packed in two different ways. In one polymorph (orthorhombic, Pbca) they form finite four member ring motifs not linked to each other via any H-bonds, but only by weak van der Waals interactions. However, in the second polymorph (monoclinic, P21/n) the zwitterions do form infinite head-to-tail chains though the N-H...O H-bond is the only one and is not assisted via any additional H-bonds. The effect of cooling on the two crystal structures was followed by single-crystal X-ray diffraction combined with polarized Raman spectroscopy of oriented single crystals, in order to compare the response of the N-H...O H-bonds to temperature variations. The crystal structure of the monoclinic polymorph compresses anisotropically on cooling, whereas that of the orthorhombic polymorph undergoes a reversible single-crystal to single-crystal phase transition at ~200 K accompanied by non-merohedral twinning, reducing the space symmetry to monoclinic (P21/b), and doubling the asymmetric unit from 2 to 4 molecules. This phase transition could not be detected by Raman spectroscopy and DSC because of the subtle related changes in intermolecular energies.
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