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Monoclinic Unit Cell (monoclinic + unit_cell)
Selected AbstractsMicrostructure Characterizations in Calcium Magnesium NiobateJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2001Hwack Joo Lee Microstructural studies on the domain boundaries in Ca(Mg1/3CNb2/3)O3 (CMN) complex perovskite compound were conducted using X-ray diffractometry and transmission electron microscopy. The 1:2 chemical ordering of B-site cations and the tilting of oxygen octahedra were involved in the CMN microstructure, as inferred from the presence of two types of domain boundaries. One type was the antiphase boundaries (APBs), which did not lie on a specific set of crystallographic planes. These boundaries were caused by the chemical 1:2 ordering of B-site cations, magnesium and niobium. The other type was the ferroelastic domain boundaries, which were parallel to a certain crystallographic plane. Therefore, CMN had the 1:2 ordered monoclinic unit cell distorted by the antiphase or in-phase tilting of oxygen octahedra. CMN had the mixed phases rather than the homogeneous phase. [source] Crystal structure and orientation behavior of transversely compressed poly(ethylene- co -1-octene) filamentsPOLYMER ENGINEERING & SCIENCE, Issue 12 2008Haifeng Shan A basic study on crystal structure and orientation behavior of transversely compressed ethylene-1-octene copolymer with different 1-octene contents was described. All polymers were first melt spun under different spinline stress and subsequently transversely compressed. For the melt-spun filaments, an orthorhombic crystal structure was found for all polymers, but a pseudo-hexagonal mesophase was also found for polymers with the highest 1-octene level (13.3 mol%). For the transversely compressed filaments, several reflection peaks from a monoclinic unit cell were found for polyethylene without octene. For those with higher octene levels, the reflection peaks from monoclinic became fainter and disappeared for the one with the highest 1-octene level. After being transversely compressed, the (110) and (200) peaks of orthorhombic crystal structures became oriented along the meridian direction, which is the fiber axis direction. The reason for this appears to be that the compression deformation of the filament induces elongation along its width direction and shrinkage along its length and thickness direction, and in this change the polymer chain orients. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers [source] Pseudomerohedral 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] Structure of a monoclinic polymorph of human carbonic anhydrase II with a doubled a axisACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2010Arthur H. Robbins The crystal structure of human carbonic anhydrase II with a doubled a axis from that of the usually observed monoclinic unit cell has been determined and refined to 1.4,Å resolution. The diffraction data with h = 2n + 1 were systematically weaker than those with h = 2n. Consequently, the scaling of the data, structure solution and refinement were challenging. The two molecules comprising the asymmetric unit are related by a noncrystallographic translation of ½ along a, but one of the molecules has two alternate positions related by a rotation of approximately 2°. This rotation axis is located near the edge of the central ,-sheet, causing a maximum distance disparity of 1.7,Å between equivalent atoms on the diametrically opposite side of the molecule. The crystal-packing contacts are similar to two sequential combined unit cells along a of the previously determined monoclinic unit cell. Abnormally high final Rcryst and Rfree values (20.2% and 23.7%, respectively) are not unusual for structures containing pseudo-translational symmetry and probably result from poor signal to noise in the weak h -odd data. [source] A case of structure determination using pseudosymmetryACTA CRYSTALLOGRAPHICA SECTION D, Issue 12 2009Sergei Radaev Here, a case is presented of an unusual structure determination which was facilitated by the use of pseudosymmetry. Group A streptococcus uses cysteine protease Mac-1 (also known as IdeS) to evade the host immune system. Native Mac-1 was crystallized in the orthorhombic space group P21212. Surprisingly, crystals of the inactive C94A mutant of Mac-1 displayed monoclinic symmetry with space group P21, despite the use of native orthorhombic Mac-1 microcrystals for seeding. Attempts to solve the structure of the C94A mutant by MAD phasing in the monoclinic space group did not produce an interpretable map. The native Patterson map of the C94A mutant showed two strong peaks along the (1 0 1) diagonal, indicating possible translational pseudosymmetry in space group P21. Interestingly, one-third of the monoclinic reflections obeyed pseudo-orthorhombic P21212 symmetry similar to that of the wild-type crystals and could be indexed and processed in this space group. The pseudo-orthorhombic and monoclinic unit cells were related by the following vector operations: am = bo,co, bm = ao and cm = ,2co,bo. The pseudo-orthorhombic subset of data produced good SAD phases, leading to structure determination with one monomer in the asymmetric unit. Subsequently, the structure of the Mac-1 mutant in the monoclinic form was determined by molecular replacement, which showed six molecules forming three translationally related dimers aligned along the (1 0 1) diagonal. Knowing the geometric relationship between the pseudo-orthorhombic and the monoclinic unit cells, all six molecules can be generated in the monoclinic unit cell directly without the use of molecular replacement. The current case provides a successful example of the use of pseudosymmetry as a powerful phase-averaging method for structure determination by anomalous diffraction techniques. In particular, a structure can be solved in a higher pseudosymmetry subcell in which an NCS operator becomes a crystallographic operator. The geometrical relationships between the subcell and parental cell can be used to generate a complete molecular representation of the parental asymmetric unit for refinement. [source] Crystallization of the pneumococcal autolysin LytC: in-house phasing using novel lanthanide complexesACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 4 2010Inmaculada Pérez-Dorado LytC, one of the major autolysins from the human pathogen Streptococcus pneumoniae, has been crystallized as needles by the hanging-drop technique using 10%(w/v) PEG 3350 as precipitant and 10,mM HEPES pH 7.5. LytC crystals were quickly soaked in mother liquor containing 2,mM of the complex Gd-HPDO3A to produce derivatized crystals (LytCGd-HPDO3A). Both native LytC and isomorphous LytCGd-HPDO3A crystals were flash-cooled in a nitrogen flow at 120,K prior to X-ray data collection using an in-house Enraf,Nonius rotating-anode generator (, = 1.5418,Å) and a MAR345 imaging-plate detector. In both cases, good-quality diffraction patterns were obtained at high resolution. LytCGd-HPDO3A crystals allowed the collection of a SAD X-ray data set to 2.6,Å resolution indexed in terms of a P21 monoclinic unit cell with parameters a = 59.37, b = 67.16, c = 78.85,Å, , = 105.69°. The anomalous Patterson map allowed the identification of one heavy-atom binding site, which was sufficient for the calculation of an interpretable anomalous map at 2.6,Å resolution. [source] A case of structure determination using pseudosymmetryACTA CRYSTALLOGRAPHICA SECTION D, Issue 12 2009Sergei Radaev Here, a case is presented of an unusual structure determination which was facilitated by the use of pseudosymmetry. Group A streptococcus uses cysteine protease Mac-1 (also known as IdeS) to evade the host immune system. Native Mac-1 was crystallized in the orthorhombic space group P21212. Surprisingly, crystals of the inactive C94A mutant of Mac-1 displayed monoclinic symmetry with space group P21, despite the use of native orthorhombic Mac-1 microcrystals for seeding. Attempts to solve the structure of the C94A mutant by MAD phasing in the monoclinic space group did not produce an interpretable map. The native Patterson map of the C94A mutant showed two strong peaks along the (1 0 1) diagonal, indicating possible translational pseudosymmetry in space group P21. Interestingly, one-third of the monoclinic reflections obeyed pseudo-orthorhombic P21212 symmetry similar to that of the wild-type crystals and could be indexed and processed in this space group. The pseudo-orthorhombic and monoclinic unit cells were related by the following vector operations: am = bo,co, bm = ao and cm = ,2co,bo. The pseudo-orthorhombic subset of data produced good SAD phases, leading to structure determination with one monomer in the asymmetric unit. Subsequently, the structure of the Mac-1 mutant in the monoclinic form was determined by molecular replacement, which showed six molecules forming three translationally related dimers aligned along the (1 0 1) diagonal. Knowing the geometric relationship between the pseudo-orthorhombic and the monoclinic unit cells, all six molecules can be generated in the monoclinic unit cell directly without the use of molecular replacement. The current case provides a successful example of the use of pseudosymmetry as a powerful phase-averaging method for structure determination by anomalous diffraction techniques. In particular, a structure can be solved in a higher pseudosymmetry subcell in which an NCS operator becomes a crystallographic operator. The geometrical relationships between the subcell and parental cell can be used to generate a complete molecular representation of the parental asymmetric unit for refinement. [source] Structure of Escherichia coli pyridoxine 5,-phosphate oxidase in a tetragonal crystal form: insights into the mechanistic pathway of the enzymeACTA CRYSTALLOGRAPHICA SECTION D, Issue 5 2005Faik N. Musayev Escherichia coli pyridoxine 5,-phosphate oxidase (ePNPOx) catalyzes the terminal step in the biosynthesis of pyridoxal 5,-phosphate (PLP) by the FMN oxidation of pyridoxine 5,-phosphate (PNP) or pyridoxamine 5,-phosphate (PMP), forming FMNH2 and H2O2. The crystal structure of ePNPOx is reported in a tetragonal unit cell at 2.6,Å resolution. The three-dimensional fold of this structure is very similar to those of the E. coli and human enzymes that crystallized in trigonal and monoclinic unit cells. However, unlike the previous structures, the tetragonal structure shows major disorder in one of the two subunit domains that has opened up both the active site and a putative tunnel. Comparison of these structures gives an insight into the mechanistic pathway of PNPOx: from the resting enzyme with no substrate bound, to the initial binding of the substrate at the active site, to the catalytic stage and to the release of the catalytic product from the active site. 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