Browsing by Subject "Crystallography"
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- ItemRestrictedThe mineralogy and crystallography of pyrrhotite from selected nickel and PGE ore deposits.(Society of Economic Geologists, 2010) Becker, Megan; De Villiers, Johan; Bradshaw, DeeThe nonstoichiometric sulfide pyrrhotite (Fe(1–x)S) common to many ore deposits occurs in a variety of crystallographic forms and compositions and occasionally is also intergrown with stoichiometric troilite (FeS). In this study, the mineralogy of pyrrhotite derived from several different nickel and PGE ore deposits in South Africa, Botswana, and Canada was examined in detail in terms of pyrrhotite association, crystallography, and mineral chemistry. Pyrrhotite samples were subdivided into two-phase 6C Fe11S12 pyrrhotite intergrown with 2C FeS troilite, two-phase 4C Fe7S8 pyrrhotite intergrown with 5C Fe9S10 pyrrhotite, single-phase 5C Fe9S10 pyrrhotite and single-phase 4C Fe7S8 pyrrhotite. None of the pyrrhotite samples analyzed were classified as two-phase 4C pyrrhotite intergrown with pyrite due to the scarcity of pyrite in these samples. Average solid solution Ni contents of NC pyrrhotite (0.75 ± 0.10 wt % Ni) in this study were found to be greater than in 4C pyrrhotite (0.43 ± 0.10 wt % Ni), but only when the pyrrhotite occurred as two-phase 4C pyrrhotite intergrown with NC pyrrhotite. For single-phase pyrrhotite occurrences in this study, 4C pyrrhotite was more Ni rich (up to 2 wt % Ni) than NC pyrrhotite (0.75 ± 0.19 wt % Ni). The average atomic metal/S ratios obtained for 4C Fe7S8 pyrrhotite was 0.869 ± 0.013 (n = 699), for 5C Fe9S10 pyrrhotite was 0.895 ± 0.013 (n = 316) and for 6C Fe11S12 pyrrhotite was 0.918 ± 0.017 (n = 101). The histogram comparing metal/S ratios of all the pyrrhotite samples analyzed showed a continuum of metal/S ratios, although with frequency maxima corresponding to the ideal compositions of 4C, 5C, and 6C pyrrhotite. The presence of the continuum however, was interpreted to be representative of nonstoichiometry in the pyrrhotite structure.
- ItemRestrictedTertiary interactions stabilise the C-terminal region of human glutathione transferase A1-1: a crystallographic and calorimetric study(Elsevier, 2005) Kuhnert, Diane C; Sayed, Yasien; Mosebi, Salerwe; Sayed, Muhammed; Sewell, Trevor; Dirr, Heini WThe C-terminal region in class Alpha glutathione transferase A1-1 (GSTA1-1), which forms an amphipathic a-helix (helix 9), is known to contribute to the catalytic and non-substrate ligand-binding functions of the enzyme. The region in the apo protein is proposed to be disordered which, upon ligand binding at the active-site, becomes structured and localised. Because Ile219 plays a pivotal role in the stability and localisation of the region, the role of tertiary interactions mediated by Ile219 in determining the conformation and dynamics of the C-terminal region were studied. Ligand-binding microcalorimetric and X-ray structural data were obtained to characterise ligand binding at the active-site and the associated localisation of the C-terminal region. In the crystal structure of the I219A hGSTA1-1$ S-hexylglutathione complex, the C-terminal region of one chain is mobile and not observed (unresolved electron density), whereas the corresponding region of the other chain is localised and structured as a result of crystal packing interactions. In solution, the mutant C-terminal region of both chains in the complex is mobile and delocalised resulting in a hydrated, less hydrophobic active-site and a reduction in the affinity of the protein for S-hexylglutathione. Complete dehydration of the active-site, important for maintaining the highly reactive thiolate form of glutathione, requires the binding of ligands and the subsequent localisation of the C-terminal region. Thermodynamic data demonstrate that the mobile C-terminal region in apo hGSTA1-1 is structured and does not undergo ligand-induced folding. Its close proximity to the surface of the wild-type protein is indicated by the concurrence between the observed heat capacity change of complex formation and the type and amount of surface area that becomes buried at the ligand–protein interface when the C-terminal region in the apo protein assumes the same localised structure as that observed in the wild-type complex.
- ItemOpen AccessThe crystal structure and constitution of some molecular complexes of 4:4- dinitrodiphenyl(1947) Saunder, Douglas Harold; Saunder, Douglas Harold; James, R W; Rapson, W SThe crystal structures or a number of molecular complexes formed by 4:4'-dinitrodiphenyl with various diphenyl derivatives are described. The general type of structure is undoubtedly the same for all the complexes examined and the typical arrangement may be taken as that in the complex of 4:4'-dinitrodiphenyl with 4-hydroxydiphenyl, the structure of which has been fully determined. As shown in fig. 7 the dinitrodiphenyl molecules form layers in face-centred array and lie one above the other with a spacing of about 3.7 A. The arrangement of these molecules alone is such that a set of tubular cavities, also in face-centred array, run through the structure. In the complexes these cavities are occupied by the other component molecules, the hydroxydiphenyl molecules in the case considered; which thus lie nearly normal to the planes containing the dinitrodiphenyl molecules and are seen end-on in fig. 7. In the other structures examined geometrical and symmetry conditions require that the individual molecules should be tilted in varying degrees, but the type or structure still remains essentially the same. It is shown that the ratio of the components in these complexes is determined by the length of the molecule other than dinitrodiphenyl, and that in no case is it necessary for the intermolecular distances to be shorter than those found in crystals or ordinary aromatic nitro-compounds. A bonding mechanism in terms of dipole attraction between the component molecules is shown to be consistent with all the observed data. Of interest are the periodic distortions which appear to occur in the crystal lattices of the complexes or 4:4'-dinitrodiphenyl with 4-iodo-, 4-bromo- and 4-chlorodiphenyl.