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Merohedral Twinning (merohedral + twinning)
Selected AbstractsThe complex phosphate K0.92In0.46Nb0.54OPO4: a new representative of the KTiOPO4 familyACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2007Artem A. Babaryk Aliovalent K0.92In0.46Nb0.54OPO4 (KINP, potassium indium niobium oxide phosphate) features a chain structure involving corner-sharing [MO6] octahedra (M = In/Nb), which allows this compound to be recognized as a member of the KTiOPO4 (KTP) family. However, its crystallization in the polar space group P41 belonging to the 4/m Laue group generates its own subclass of KTP-related compounds. The unit cell contains 18 symmetry-independent atoms occupying general positions. The cis,cis principle of octahedral arrangement is observed for each of the separate [MO6], chains. The observed variations in the cis,cis linkage are critically important for nonlinear optical properties and distinguish the present compound from other KTP-related structures. The anionic framework adopts one-dimensional tunnels running orthogonal to the ab plane. The K+ ions are arranged in the [001] direction at a distance of c. Merohedral twinning was detected during the structure refinement. [source] MIR phasing using merohedrally twinned crystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 11 2003Anke C. Terwisscha van Scheltinga Merohedral twinning is a crystal-growth disorder that seriously hinders the determination of macromolecular crystal structures by isomorphous replacement. The strategies used in the structures solved so far are discussed. Several methods can be used to determine the extent of twinning, the twin fraction and to detwin the data. Accurate determination of the twin fraction by analysing heavy-atom refinement statistics is possible, but only influences the resulting phases slightly. It seems more crucial to restrict the variation in twin fractions between data sets, either by making the twin fractions of some data sets artificially higher or by screening crystals to obtain data with a low twin fraction. [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] 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] Overcoming merohedral twinning in crystals of bacteriorhodopsin grown in lipidic mesophaseACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2010Valentin Borshchevskiy Twinning is one of the most common crystal-growth defects in protein crystallography. There are neither efficient rational approaches for the growth of nontwinned protein crystals nor are there examples of systematic studies of the dependence of the twinning-ratio distribution on crystallization conditions. The description of the twinning phenomenon has been covered even less for membrane-protein crystals and is non-existent for crystals grown using lipidic phases (in meso). In the present work, possibilities for overcoming merohedral twinning are investigated for crystals of the membrane protein bacteriorhodopsin (bR) grown in meso. It is shown that traditional crystallization additives are not effective in the case of the in meso crystallization of bR. The twinning ratio was determined for 310 crystals grown under different crystallization conditions. A correlation of the twinning ratio with the growth rate of the crystals was observed. Slow growth indicated that crystals had a noticeable chance of avoiding twinning. Model calculations were performed in order to rationalize this observation. The calculations confirmed the experimental observation that most crystals consist of two twin domains and showed that under this condition small changes in the probability of twin-domain formation lead to dramatic changes in the number of nontwinned crystals, which explains why slow crystal growth results in a considerable number of nontwinned crystals. [source] Crystallization and preliminary X-ray crystallographic analysis of a new crystal form of hydroxylamine oxidoreductase from Nitrosomonas europaeaACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 12 2009Peder E. Cedervall Hydroxylamine oxidoreductase (HAO) from Nitrosomonas europaea is a homotrimeric protein that catalyzes the oxidation of hydroxylamine to nitrite. Each monomer, with a molecular weight of 67.1,kDa, contains seven c -type hemes and one heme P460, the porphyrin ring of which is covalently linked to a tyrosine residue from an adjacent subunit. HAO was first crystallized and structurally characterized at a resolution of 2.8,Å in 1997. The structure was solved in space group P63 and suffered from merohedral twinning. Here, a crystallization procedure is presented that yielded untwinned crystals belonging to space group P21212, which diffracted to 2.25,Å resolution and contained one trimer in the asymmetric unit. The unit-cell parameters were a = 140.7, b = 142.6, c = 107.4,Å. [source] Twinned crystals of adeno-associated virus serotype 3b prove suitable for structural studiesACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 2 2009Thomas F. Lerch Adeno-associated viruses (AAVs) are leading candidate vectors for gene-therapy applications. The AAV-3b capsid is closely related to the well characterized AAV-2 capsid (87% identity), but sequence and presumably structural differences lead to distinct cell-entry and immune-recognition properties. In an effort to understand these differences and to perhaps harness them, diffraction-quality crystals of purified infectious AAV-3b particles have been grown and several partial diffraction data sets have been recorded. The crystals displayed varying levels of merohedral twinning that in earlier times would have rendered them unsuitable for structure determination, but here is shown to be a tractable complication. [source] Expression, purification, crystallization and structure determination of two glutathione S -transferase-like proteins from Shewanella oneidensisACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 6 2008Bert Remmerie Genome analysis of Shewanella oneidensis, a Gram-negative bacterium with an unusual repertoire of respiratory and redox capabilities, revealed the presence of six glutathione S -transferase-like genes (sogst1,sogst6). Glutathione S -transferases (GSTs; EC 2.5.1.18) are found in all kingdoms of life and are involved in phase II detoxification processes by catalyzing the nucleophilic attack of reduced glutathione on diverse electrophilic substrates, thereby decreasing their reactivity. Structure,function studies of prokaryotic GST-like proteins are surprisingly underrepresented in the scientific literature when compared with eukaryotic GSTs. Here, the production and purification of recombinant SoGST3 (SO_1576) and SoGST6 (SO_4697), two of the six GST-like proteins in S. oneidensis, are reported and preliminary crystallographic studies of crystals of the recombinant enzymes are presented. SoGST3 was crystallized in two different crystal forms in the presence of GSH and DTT that diffracted to high resolution: a primitive trigonal form in space group P31 that exhibited merohedral twinning with a high twin fraction and a primitive monoclinic form in space group P21. SoGST6 yielded primitive orthorhombic crystals in space group P212121 from which diffraction data could be collected to medium resolution after application of cryo-annealing protocols. Crystal structures of both SoGST3 and SoGST6 have been determined based on marginal search models by maximum-likelihood molecular replacement as implemented in the program Phaser. [source] | ||||||||||