The crystal structure of L-serine has been determined at room-temperature at pressures between 0.3 and 4.8 GPa. Compression of this phase is facilitated through the closing-up of voids in the middle of ring motifs and compression of a long OH…OH interaction. Above 4.8 GPa the structure undergoes a phase transition to a previously uncharacterized polymorph, which we designate L-serine-II. This transition occurs via a change from a <i>gauche </i>to <i>anti</i> conformation of the OH group, and a change in the NC<i><sub>a</sub></i>CO torsion angle. In a separate high-pressure powder neutron diffraction experiment, compression of deuterated L-serine-I was found to be analogous to that observed previously in our single crystal study. On increasing pressure further to 8.1 GPa the structure undergoes another transition to a second previously unobserved phase of L-serine (L-serine-III). The crystal structure of hexagonal L-cystine has been determined at room temperature at pressures between 0.4 and 3.7 GPa; unit cell dimensions were measured up to 6.4 GPa. The structure consists of H-bonded layers which are connected on one side by the disulfide bridges within the cystine molecules, and on the other by NH….OH-bonds to other layers. Application of pressure pushes the layers closer together occurring approximately equally in the regions of the interlayer H-bonds and the disulfide bridges. The crystal structure of L-cysteine (hereafter L-cysteine-I) was compressed to 1.8 GPa, where the main effect of compression was to contract voids in the middle of ring motifs. On increasing pressure further, a transition occurs to a new phase of L-cysteine, L-cysteine-III. The phase transition is accompanied by a change in the NCCS torsion angle and small positional displacements, but with no major changes in the orientations of the molecules. L-cysteine-III was found to be stable to at least 4.2 GPa. On decompression to 1.7 GPa, another phase transition formed another previously uncharacterised phase, L-cysteine-IV. This phase is <i>not </i>observed on increasing pressure. Further decompression to ambient pressure generates L-cysteine-1. The crystal structure of <i>a</i>-glycylglycine (<i>a</i>-GLYGLY) has been determined at room temperature at pressures between 1.4 and 4.7 GPa; unit cell dimensions were measured up to 5.4 GPa. The structure can be considered to consist of layers of glycylglycine molecules which stack perpendicular to the (1 0 -1) direction. The arrangement of glycylglycine molecules within each layer resembles that of an anti-parallel β-sheet motif observed in protein structures.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657893 |
| Date | January 2006 |
| Creators | Moggach, Stephen A. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12663 |
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