Polar and non-polar structures of NH4TiOF3

Ammonium oxofluorotitanate, NH4TiOF3, is probably the best known precursor for the synthesis of anatase mesocrystals. Transformation of NH4TiOF3 into TiO2 through thermal decomposition, accompanied by hydrolysis, preserves some structural features of the precursor. Currently, any discussion of the mechanism of this transformation is difficult, as the exact crystal structure of the starting compound is not available and no intermediate structures are known. This article describes the outcome of single-crystal and powder X-ray diffraction studies, revealing the existence of two polymorphs of the parent NH4TiOF3 at different temperatures. A second-order phase transition from the polar Pca21 α phase (1), stable at room temperature, to the Pma2 β phase (2) above ∼433 K has been demonstrated. The direction of the pseudo-fourfold axis in NH4TiOF3 coincides with the orientation of the fourfold axis of anatase mesocrystals, consistent with a topotactical transformation.

Crystal structures of two 2,3-diethylnaphtho[2,3-g]quinoxaline-6,11-dione derivatives

Two new 5,12-disubstituted 2,3-diethylnaphtho[2,3-g]quinoxaline-6,11-dione compounds were readily synthesized from the commercial dye quinizarin. For 2,3-diethyl-5,12-dihydroxynaphtho[2,3-g]quinoxaline-6,11-dione, (II), C20H16N2O4, the molecule displays a near planar conformation and both hydroxy groups participate in intramolecular O—H...O(carbonyl) hydrogen bonds. In the crystal, π–π ring interactions [minimum ring centroid separation = 3.5493 (9) Å] form stacks of co-planar molecules down the c axis, while only minor intermolecular C—H...O interactions are present. In contrast, in 2,3-diethyl-5,12-bis(piperidin-1-yl)naphtho[2,3-g]quinoxaline-6,11-dione, (IV), C30H34N4O2, which contains two independent, but similar, molecules in the asymmetric unit, the polycyclic cores have a significant twist, with dihedral angles of 29.79 (6) and 29.31 (7)° between the terminal rings and only minor intermolecular C—H...O hydrogen-bonding interactions are present. Electron density associated with additional solvent molecules disordered about a fourfold axis was accounted for using the SQUEEZE procedure in PLATON [Spek (2015). Acta Cryst. C71, 9–18].

Crystallization and preliminary crystallographic analysis of human aquaporin 1 at a resolution of 3.28 Å

Aquaporin water channels (AQPs) are found in almost every organism from humans to bacteria. In humans, 13 classes of AQPs control water and glycerol homeostasis. Knockout studies have suggested that modulating the activity of AQPs could be beneficial for the treatment of several pathologies. In particular, aquaporin 1 is a key factor in cell migration and angiogenesis, and constitutes a possible target for anticancer compounds and also for the treatment of glaucoma. Here, a preliminary crystallographic analysis at 3.28 Å resolution of crystals of human aquaporin 1 (hAQP1) obtained from protein expressed in Sf9 insect cells is reported. The crystals belonged to the tetragonal space groupI422, with unit-cell parametersa=b= 89.28,c= 174.9 Å, and contained one monomer per asymmetric unit. The hAQP1 biological tetramer is generatedviathe crystallographic fourfold axis. This work extends previous electron crystallographic studies that used material extracted from human red blood cells, in which the resolution was limited to approximately 3.8 Å. It will inform efforts to improve lattice contacts and the diffraction limit for the future structure-based discovery of specific hAQP1 inhibitors.

Reconstructive phase transition in (NH4)3TiF7accompanied by the ordering of TiF6octahedra

An unusual phase transitionP4/mnc→ Pa\bar 3 has been detected after cooling the (NH4)3TiF7compound. Some TiF6octahedra, which are disordered in the room-temperature tetragonal structure, become ordered in the low-temperature cubic phase due to the disappearance of the fourfold axis. Other TiF6octahedra undergo large rotations resulting in huge displacements of the F atoms by 1.5–1.8 Å that implies a reconstructive phase transition. It was supposed that phasesP4/mbmand Pm\bar 3m could be a high-temperature phase and a parent phase, respectively, in (NH4)3TiF7. Therefore, the sequence of phase transitions can be written as Pm\bar 3m →P4/mbm→P4/mnc→ Pa\bar 3. The interrelation between (NH4)3TiF7, (NH4)3GeF7and (NH4)3PbF7is found, which allows us to suppose phase transitions in relative compounds.

The crystal structure of parkinsonite, nominally Pb7MoO9Cl2: a naturally occurring Aurivillius phase

The crystal structure of the sheet oxychloride mineral parkinsonite, nominally Pb7MoO9Cl2, has been determined for synthetic and natural crystals of analysed compositions, (Pb7.28Mo0.72) O8.96Cl1.96 and (Pb7.23Mo0.40V0.37)O8.90Cl1.82, respectively. Parkinsonite is tetragonal, space group I4/mmm. Unit-cell parameters for synthetic and natural crystals are: asynthetic = 3.9773(4) Å, csynthetic = 22.718(4) Å, Vsynthetic = 359.38(5) Å3, and anatural = 3.9570(3) Å, cnatural = 22.634(5) Å, Vnatural = 354.40(5) Å3. Final agreement indices (R1, wR2) for refinements of the two crystals are 0.024, 0.067 (synthetic) and 0.036, 0.078 (natural). Although a superlattice has been identified by electron diffraction for crystals of both samples (Welch et al., 1996), only the substructure could be determined by X-ray diffraction. This X-ray invisibility of the superstructure has also been observed for the closely related sheet oxychlorides asisite and schwartzembergite, for both of which superstructure motifs have been identified by electron diffraction. The Pb(1) site of both parkinsonite crystals is fully occupied by Pb. Refinement of the Pb content of the Pb(2) site for the synthetic and natural crystals gives occupancies of 0.85(1) and 0.70(1) respectively, corresponding to 3.40 and 2.80 Pb(2) a.p.f.u. respectively. The substituent cation Mo (synthetic crystal) and [Mo+V] (natural crystal) was located at a distance of 0.5 Å from Pb(2), being displaced along the fourfold axis. The reduced occupancy of Pb(2) is due to substitution by Mo or [Mo+V]. No evidence for separate Mo and V sites in the substructure of natural parkinsonite was found. Refined occupancies of the Cl site are 0.84(4) and 0.91(5) for the synthetic and natural crystals, respectively, and are consistent with the 9:1 superstructure component identified by electron diffraction.

Cryoelectron Microscopy Structures of Rotavirus NSP2-NSP5 and NSP2-RNA Complexes: Implications for Genome Replication

ABSTRACT The replication and packaging of the rotavirus genome, comprising 11 segments of double-stranded RNA, take place in specialized compartments called viroplasms, which are formed during infection and involve a coordinated interplay of multiple components. Two rotavirus nonstructural proteins, NSP2 (with nucleoside triphosphatase, single-stranded RNA [ssRNA] binding and helix-destabilizing activities) and NSP5, are essential in these events. Previous structural analysis of NSP2 showed that it is an octamer in crystals, obeying 4-2-2 crystal symmetry, with a large 35-Å central hole along the fourfold axis and deep grooves at one of the twofold axes. To ascertain that the solution structure of NSP2 is the same as that in the crystals and investigate how NSP2 interacts with NSP5 and RNA, we carried out single-particle cryoelectron microscopy (cryo-EM) analysis of NSP2 alone and in complexes with NSP5 and ssRNA at subnanometer resolution. Because full-length NSP5 caused severe aggregation upon mixing with NSP2, the deletion construct NSP566-188 was used in these studies. Our studies show that the solution structure of NSP2 is same as the crystallographic octamer and that both NSP566-188 and ssRNA bind to the grooves in the octamer, which are lined by positively charged residues. The fitting of the NSP2 crystal structure to cryo-EM reconstructions of the complexes indicates that, in contrast to the binding of NSP566-188, the binding of RNA induces noticeable conformational changes in the NSP2 octamer. Consistent with the observation that both NSP5 and RNA share the same binding site on the NSP2 octamer, filter binding assays showed that NSP5 competes with ssRNA binding, indicating that one of the functions of NSP5 is to regulate NSP2-RNA interactions during genome replication.

Structure of strontium hydroxide octahydrate, Sr(OH)2·8H2O, at 20, 100 and 200 K from neutron diffraction

The crystal structure of Sr(OH)2·8H2O has been determined at 20, 100 and 200 K from neutron diffraction data. The structure consists of double layers of H2O and OH− ions separated by Sr2+ along the c axis. The Sr2+ ions are eight-coordinated by water O atoms in a square antiprism configuration. Each H2O molecule is engaged in three hydrogen bonds. The OH− ions form chains of acceptor and donor bonds along the fourfold axis with O atoms engaged in four bonds with H2O molecules, such that both non-equivalent O atoms have square-pyramidal environments of five H atoms and the overall bonding configurations of distorted octahedra.

Calculation of the EPR Parameters and the Local Structure for Fe+ on the Zn2+ Site of ZnSiP2

The zero-field splitting D, g factors gII and g⊥ and the local structure near Fe+ on the Zn2+ site of ZnSiP2 are calculated from high-order perturbation formulas of the EPR parameters for a 3d7 ion in tetragonally distorted tetrahedra based on the cluster approach. According to these studies, we find that the impurity-ligand bonding angle αloc related to the fourfold axis is about 58.05◦ in the studied Fe+ impurity center, which is larger than the metal-ligand bonding angle αh(≈ 56.65◦) in pure ZnSiP2. The EPR parameters based on the above angle αloc agree well with the observed values. The errors of the results are analyzed

Symmetry classification of the layered perovskite-derived A n B n X 3n+2 structures

The effects of combinations of octahedral tilts on the symmetries of layered A n B n X 3n+2 structures were considered. Two complementary approaches were used to deduce the symmetries. The space groups associated with different tilt systems were determined for structures with different layer thicknesses (n values) and for structures with different layer stacking arrangements. For the most symmetrical tilts about the orthorhombic axes of the A n B n X 3n+2 structure, maximal group/subgroup relations were established. Comparison of these results with experimental data available in the literature suggests that the symmetries of most observed A n B n X 3n+2 compounds are fully determined by the tilt systems adopted by rigid BX 6 octahedra. The most common tilt system observed at room temperature is a combination of an in-phase tilt about an orthorhombic axis parallel to the pseudo-fourfold axis of the octahedron with a tilt about an orthorhombic axis perpendicular to the layers.

Crystallization and preliminary X-ray studies of flavocetin-A, a platelet glycoprotein Ib-binding protein from the habu snake venom

Flavocetin-A (FL-A) is a platelet glycoprotein Ib-binding protein, a high molecular mass oligomer (149 kDa) of C-type lectin-like subunits α and β isolated from the habu snake venom. Purified FL-A crystallized in the tetragonal space group I4 with unit-cell dimensions a = b = 121.0, c = 63.2 Å. The crystals diffract to at least 2.4 Å resolution. The structure has been solved by molecular replacement using the crystal structure of factors IX/X-binding protein (PDB code 1ixx) as a search model. The asymmetric unit contains one heterodimer, showing that FL-A is a novel tetradimer (αβ)4 composed of four heterodimers related by a crystallographic fourfold axis.