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Crystals Twinned (crystal + twinned)
Selected AbstractsThe application of eigensymmetries of face forms to anomalous scattering and twinning by merohedry in X-ray diffractionACTA CRYSTALLOGRAPHICA SECTION A, Issue 3 2010H. Klapper The face form (crystal form) {hkl} which corresponds to an X-ray reflection hkl is considered. The eigensymmetry (inherent symmetry) of such a face form can be used to derive general results on the intensities of the corresponding X-ray reflections. Two cases are treated. (i) Non-centrosymmetric crystals exhibiting anomalous scattering: determination of reflections hkl for which Friedel's rule is strictly valid, i.e.I(hkl) = I() (Friedel pair, centric reflection), or violated, i.e.I(hkl) , I() (Bijvoet pair, acentric reflection). It is shown that those reflections hkl strictly obey Friedel's rule, for which the corresponding face form {hkl} is centrosymmetric. If the face form {hkl} is non-centrosymmetric, Friedel's rule is violated due to anomalous scattering. (ii) Crystals twinned by merohedry: determination of reflections hkl, the intensities of which are affected (or not affected) by the twinning. It is shown that the intensity is affected if the twin element is not a symmetry element of the eigensymmetry of the corresponding face form {hkl}. The intensity is not affected if the twin element belongs to the eigensymmetry of {hkl} (`affected' means that the intensities of the twin-related reflections are different for different twin domain states owing to differences either in geometric structure factors or in anomalous scattering or in both). A simple procedure is presented for the determination of these types of reflections from Tables 10.1.2.2 and 10.1.2.3 of International Tables for Crystallography, Vol. A [Hahn & Klapper (2002). International Tables for Crystallography, Vol. A, Part 10, edited by Th. Hahn, 5th ed. Dordrecht: Kluwer]. The application to crystal-structure determination of crystals twinned by merohedry (reciprocal space) and to X-ray diffraction topographic mapping of twin domains (direct space) is discussed. Relevant data and twinning relations for the 63 possible twin laws by merohedry in the 26 merohedral point groups are presented in Appendices A to D. [source] Diphenyldipyridinezinc(II): partial spontaneous resolution of an organometallic reagentACTA CRYSTALLOGRAPHICA SECTION C, Issue 5 2009Anders Lennartson The title compound, [Zn(C6H5)2(C5H5N)2], (I), forms conformationally chiral molecules residing on a twofold axis. The molecules are stacked along c, and these stacks are associated by edge-to-face ,,, interactions. Crystals of (I) belong to the Sohncke space group P21212 and the crystal lattice of (I) is chiral. The crystal batch that was examined consisted of a mixture of enantiomerically pure crystals and crystals twinned by inversion. [source] Structure determination of a cocaine hydrolytic antibody from a pseudomerohedrally twinned crystalACTA CRYSTALLOGRAPHICA SECTION D, Issue 12 2002Nicholas A. Larsen Few examples of pseudomerohedrally twinned macromolecular crystals have been described in the literature. This unusual phenomenon arises when a fortuitous unit-cell geometry makes it possible for twinning to occur in a space group that ordinarily does not allow twinning. Here, the crystallization, structure determination and refinement of the cocaine hydrolytic antibody 15A10 at 2.35,Å resolution are described. The crystal belongs to space group P21, with two molecules in the asymmetric unit and unit-cell parameters a = 37.5, b = 108.4, c = 111.3,Å and , fortuitously near 90°; the refined twinning fraction is , = 0.43. Interestingly, the non-crystallographic symmetry (NCS) and twin operators are nearly parallel, which appears to be a relatively frequent situation in protein crystals twinned by merohedry or pseudomerohedry. [source] | ||||||||||