Effect of pressure on methylated glycine derivatives: relative roles of hydrogen bonds and steric repulsion of methyl groups

Abstract

Infinite head-to-tail chains of zwitterions present in the crystals of all amino acids are known to be preserved even after structural phase transitions. In order to understand the role of the N—H...O hydrogen bonds linking zwitterions in these chains in structural rearrangements, the crystal structures of theN-methyl derivatives of glycine (N-methylglycine, or sarcosine, with two donors for hydrogen bonding; two polymorphs ofN,N-dimethylglycine, DMG-I and DMG-II, with one donor for hydrogen bond; andN,N,N-trimethylglycine, or betaine, with no hydrogen bonds) were studied at different pressures. Methylation has not only excluded the formation of selected hydrogen bonds, but also introduced bulky mobile fragments into the structure. The effects of pressure on the systems of the series were compared with respect to distorting and switching over hydrogen bonds and inducing reorientation of the methylated fragments. Phase transitions with fragmentation of the single crystals into fine powder were observed for partially methylatedN-methyl- andN,N-dimethylglycine, whereas the structural changes in betaine were continuous with some peculiar features in the 1.4–2.9 GPa pressure range and accompanied by splitting of the crystals into several large fragments. Structural rearrangements in sarcosine and betaine were strongly dependent on the rate of pressure variation: the higher the rate of increasing pressure, the lower the pressure at which the phase transition occurred.