Home About us Contact
|
Home About us Contact | ||
|
|
||
Twin Laws (twin + law)
Selected AbstractsPseudosymmetry, high copy number and twinning complicate the structure determination of Desulfovibrio desulfuricans (ATCC 29577) flavodoxinACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2009Megan Guelker The crystal structure of oxidized flavodoxin from Desulfovibrio desulfuricans (ATCC 29577) was determined by molecular replacement in two crystal forms, P3121 and P43, at 2.5 and 2.0,Å resolution, respectively. Structure determination in space group P3121 was challenging owing to the presence of pseudo-translational symmetry and a high copy number in the asymmetric unit (8). Initial phasing attempts in space group P3121 by molecular replacement using a poor search model (46% identity) and multi-wavelength anomalous dispersion were unsuccessful. It was necessary to solve the structure in a second crystal form, space group P43, which was characterized by almost perfect twinning, in order to obtain a suitable search model for molecular replacement. This search model with complementary approaches to molecular replacement utilizing the pseudo-translational symmetry operators determined by analysis of the native Patterson map facilitated the selection and manual placement of molecules to generate an initial solution in the P3121 crystal form. During the early stages of refinement, application of the appropriate twin law, (,h, ,k, l), was required to converge to reasonable R -factor values despite the fact that in the final analysis the data were untwinned and the twin law could subsequently be removed. The approaches used in structure determination and refinement may be applicable to other crystal structures characterized by these complicating factors. The refined model shows flexibility of the flavin mononucleotide coordinating loops indicated by the isolation of two loop conformations and provides a starting point for the elucidation of the mechanism used for protein-partner recognition. [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] Ba6Si6N10O2(CN2) , A Nitridosilicate with a NPO-Zeolite Structure Type Containing Carbodiimide IonsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2009Sandro Pagano Abstract A new precursor approach leading to NPO-zeolite analogous nitridosilicates with cavities containing carbodiimide ions is presented. The reaction of amorphous "Si(CN2)2" and barium in liquid sodium afforded Ba6Si6N10O2(CN2) as yellow crystals. The structure is a rare example of the NPO-zeolite framework type and the first nitridosilicate incorporating carbodiimide ions. The partially ordered integration of carbodiimide moieties in the channels leads to the formation of a superstructure (P, no. 174, a = 16.255(2), c = 5.4690(11) Å, Z = 3, R1 = 0.0299, 2139 data, 100 parameters) and merohedral twinning. A comprehensive structure solution is presented, taking all possible ordering variants and twin laws into account.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] The 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] The derivation of twin laws in non-merohedric twins.ACTA CRYSTALLOGRAPHICA SECTION A, Issue 5 2006Application to the analysis of hybrid twins An algorithm is presented to derive the twin laws in non-merohedric twins through the systematic search for quasi-perpendicular lattice planes/directions. The twin lattice, i.e. the sublattice common to the whole crystalline edifice built by the twinned individuals, is based on a supercell of the individual defined by a pair of quasi-perpendicular lattice elements (hkl)/[uvw]. Starting from a (real or supposed) twin element, (hkl) or [uvw], the set of quasi-perpendicular lattice elements with user-defined limits on the twin index and obliquity is explored. The degree of lattice quasi-restoration is commonly measured by the classical twin index but in some cases, especially for large supercells, this index represents only a first approximation of the degree of lattice quasi-restoration, because more than one pair (hkl)/[uvw] may exist, and more than one concurrent sublattices of nodes, based on the same twin element, may be quasi-restored, although within different obliquities. These twins, whose existence has been recently recognized, are termed hybrid twins. In hybrid twins, the degree of lattice quasi-restoration is measured by the effective twin indexnE, a generalization of the classical twin index: in the limiting case of only one (quasi-)restored sublattice, the effective twin index and the classical twin index coincide. A number of examples previously reported simply as `non-Friedelian' twins (with a twin index higher than the empirical limit of 6 established by Friedel) are analysed and reinterpreted as hybrid twins. A Fortran program is made available, which derives the possible twin laws according to this algorithm and analyses the pseudosymmetry of the concurrent sublattices defined by each pair (hkl)/[uvw]. The occurrence of hybrid concurrent components in twinning does not affect the normal procedures of dealing with diffraction patterns of twinned crystals. [source] Determination of all misorientations of tetragonal lattices with low multiplicity; connection with Mallard's rule of twinningACTA CRYSTALLOGRAPHICA SECTION A, Issue 4 2003Hans Grimmer Two congruent lattices are considered, which are misoriented in such a way that they have a fraction 1, of symmetry translations in common. Whereas for cubic lattices body or face centring does not affect the `multiplicity' or `twin index' ,, this is not generally true for tetragonal lattices. Consider a fixed misorientation and let ,P and ,I be the multiplicities for tP and tI lattices with the same axial ratio ca. Grimmer [Mater. Sci. Forum (1993), 126,128, 269,272] has given an explicit formula for ,P (depending on the misorientation and the axial ratio) and showed that ,I = ,P2, ,P or 2,P. Here stronger results on the occurrence of the three possibilities are presented. Lists of all axial ratios ca of tP and tI lattices admitting misorientations with ,, 5 are given. For each of these misorientations, the twin mirror planes and their normals are listed, so that a synopsis of all possible twin laws of tetragonal crystals by reticular merohedry with ,, 5 is obtained. It is shown that the two twin laws observed in ,-Sn can be described by reticular pseudomerohedry with ,I,=,2 and obliquity ,,=,2.6134°. [source] Symmetry relations of magnetic twin lawsACTA CRYSTALLOGRAPHICA SECTION A, Issue 6 2001J. Schlessman Symmetry relationships between two simultaneously observed domain states (domain pair) are used to determine physical properties that can distinguish between the observed domains. Here the tabulation of these symmetry relationships is extended from non-magnetic cases to magnetic cases, in terms of magnetic point groups, i.e. all possible magnetic symmetry groups and magnetic twinning groups of domain pairs are determined and tabulated. [source] | ||||||||||