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  • Polymer and Materials Science  (1)
  • biomineralization  (1)
  • 1
    Electronic Resource
    Electronic Resource
    New York : Wiley-Blackwell
    Die Makromolekulare Chemie 187 (1986), S. 2241-2249 
    ISSN: 0025-116X
    Keywords: Chemistry ; Polymer and Materials Science
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Physics
    Notes: The crystal and molecular structure of an asymmetric diacetylene derivative, 6-(4-dimethylaminobenzoyloxy)-2,4-hexadiynyl 3,5-dinitrobenzoate (1), was determined from x-ray diffraction data. The crystals are monoclinic, P21/c with Z = 4 in a unit cell having dimensions, a = 11,116 (1) Å, b = 13,429 (2) Å, c = 14,672 (3) Å, β = 106,84 (6)°. The structure was solved by direct methods and refined by least-squares techniques to an RF of 6,2% for 1 991 independent reflections. The diacetylene backbone chains are almost perfectly parallel to the ac-plane; however, they are displaced from each other in the [010] direction. The solid-state polymerization mechanism is discussed with respect to the geometric disposition of the diacetylene chains, which in this crystal are too far apart to permit polymerization by means of 1,4-addition, consistent with the Baughman mechanistic model.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 0947-6539
    Keywords: biomineralization ; calcite ; crystal morphology ; mosaic structure ; symmetry reduction ; Chemistry ; General Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Organisms can exert a remarkable degree of control over crystal growth. One way of achieving this is by the adsorption of specialized macromolecules on specific planes of the growing crystals. With continued growth of the crystal, the macromolecules are incorporated inside the crystal bulk. Their presence does not change the crystal structure, but creates discontinuities in the perfect lattice. Here we study in detail three unusual cases of reduction in symmetry at the level of crystal domain shapes, induced by this controlled intercalation. We examined sponge spicules, which are single crystals of Mg-bearing calcite. They were specifically chosen for this study, because their morphologies do not reflect the hexagonal symmetry of calcite. Their crystal textures (coherence lengths and angular spreads) were characterized by high-resolution X-ray diffraction with well-collimated synchrotron radiation. The results are compared to analogous studies of synthetic calcite and Mg-bearing calcite. In all the selected spicules reduction in symmetry is observed in the coherence lengths among symmetry-related crystallographic directions. The reconstructed shapes of the domains of perfect structure closely match the specific spicule morphologies. The synthetic crystals show no such reduction in symmetry. Although the manner by which such exquisite control is achieved is not known, we envisage it involving a combination of oriented nucleation with either physical or stereochemically driven adsorption.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
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