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Merohedral Twin (merohedral + twin)
Selected AbstractsPseudomerohedral twinning, pseudosymmetry and complex hydrogen-bonded sheets in 2-methoxy-4-(pyrrolo[1,2- a]quinoxalin-4-yl)phenolACTA CRYSTALLOGRAPHICA SECTION C, Issue 8 2010Juan C. Castillo The title compound, C18H14N2O2, crystallizes as a nonmerohedral twin, but the structure can be satisfactorily refined as a merohedral twin, in which a monoclinic unit cell with a cell angle , close to 90° emulates a metrically orthorhombic cell. The two molecules in the asymmetric unit are very similar in structure and they are related by an approximate pseudo-screw axis. The molecules are linked into complex sheets by a combination of two O,H...N hydrogen bonds and four C,H...O hydrogen bonds. [source] K2ScSn(AsO4)3: an arsenate-containing langbeiniteACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2010William T. A. Harrison The title compound, dipotassium tri-,-arsenato-scandium(III)tin(IV), is the first arsenate-containing langbeinite to be characterized by single-crystal methods and crystallizes in the aristotype P213 cubic symmetry for this structure type in which the K+ ions and the octahedral scandium and tin cations lie on crystallographic threefold axes. The ScIII and SnIV ions show a slight segregation over the two octahedral sites, with Sc/Sn populations of 0.582,(5):0.418,(5) on one site and 0.418,(5):0.582,(5) on the other. Bond-valence-sum calculations indicate that the K+ ions are significantly underbonded in this structure and the O atoms show large anisotropic displacement parameters, as also seen in other langbeinites. The crystal studied was found to be a merohedral twin with a 0.690,(16):0.310,(16) domain ratio. [source] Crystallization and preliminary X-ray diffraction studies of a ferredoxin reductase component of carbazole 1,9a-dioxygenase from Novosphingobium sp.ACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 6 2010Carbazole 1,9a-dioxygenase (CARDO) is the initial enzyme of the carbazole-degradation pathway. The CARDO of Novosphingobium sp. KA1 consists of a terminal oxygenase, a putidaredoxin-type ferredoxin and a ferredoxin-NADH oxidoreductase (Red) and is classified as a class IIA Rieske oxygenase. Red from KA1 was crystallized at 278,K by the hanging-drop vapour-diffusion method using PEG 4000. The crystal diffracted to 1.58,Å resolution and belonged to space group P32, with unit-cell parameters a = b = 92.2, c = 78.6,Å, , = , = 90, , = 120°. Preliminary analysis of the X-ray diffraction data revealed that the asymmetric unit contained two Red monomers. The crystal appeared to be a merohedral twin, with a twin fraction of 0.32 and twin law (,h, ,k, l). [source] Multiple isomorphous replacement on merohedral twins: structure determination of deacetoxycephalosporin C synthaseACTA CRYSTALLOGRAPHICA SECTION D, Issue 12 2001Anke C. Terwisscha van Scheltinga Merohedral twinning is a packing anomaly that seriously impairs the determination of macromolecular crystal structures. Crystals of deacetoxycephalosporin C synthase (DAOCS), an enzyme involved in the expansion of the penicillin nucleus to form the core structure of the cephalosporin antibiotics, were found to be merohedrally twinned by many diagnostic criteria. Here, the structure determination of DAOCS from twinned crystals based on a combination of isomorphous replacement and the use of a multiple-wavelength diffraction data set is described. [source] | ||||||||||