Full-matrix Least-squares Refinement Research Articles

Ultrahigh-resolution study onPyrococcus abyssirubredoxin. I. 0.69 Å X-ray structure of mutant W4L/R5S

The crystal structure of Pyrococcus abyssi rubredoxin mutant W4L/R5S was solved by direct methods. The model of the air-oxidized protein was refined by partially restrained full-matrix least-squares refinement against intensity data to 0.69 A resolution. This first ultrahigh-resolution structure of a rubredoxin provides very detailed and precise information about the Fe(SCys)(4) centre and its environment, the peptide-backbone stereochemistry, H atoms and hydrogen bonds, static and dynamic disorder, the solvent structure and the electron-density distribution. P. abyssi rubredoxin W4L/R5S is the first of a series of mutants studied by atomic and ultrahigh-resolution X-ray crystallography which are expected to contribute to the understanding of structure-function relationships in iron-sulfur proteins.

Preparation and crystal structural characterization of Cu(II) complex with piperidine-4-carboxylic acid

Copper(II) complex with piperidine-4-carboxylic acid (HPipe-4), Cu(HPipe-4)4(ClO4)2 (I) was prepared and characterized by IR, X-ray diffraction, and thermal analysis. The crystal is orthorhombic with space group Ccca, Z = 8, a =17.415(3) A, b =17.399(2) A, c = 21.426(3) A. The copper atom has a square surrounding and is coordinated by four carboxylato oxygen atoms in monodentate coordination mode. Full-matrix least-squares refinements gave a final R value of 0.0723 for 22169 reflections. The complex consists of two perpendicularly arranged molecules differing slightly in conformations and shows a three-dimensional polymeric structure formed by hydrogen bonds.

Crystal structure and vibrational spectroscopy of the new acidic diphosphate (NH4)2Zn(H2P2O7)2 · 2 H2O

Abstract The hydrated ammonium hemizinc dihydrogen diphosphate (NH4)2Zn(H2P2O7)2 · 2 H2O was synthesized. It crystallizes in the triclinic system, space group P-1, Z = 1, with the following unit-cell-parameters: a = 7.0026(2) Å, b = 7.3297(2) Å, c = 7.7885(3) Å, α = 81.229(1)°, β = 71.064(1)°, γ = 88.172(1)°, V = 373.64(2) Å3. The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F 2 gave a final R 1 index of = 0.0447 (wR 2 = 0.1336), utilizing 2055 observed reflections with I > 2σ(I). The crystal packing consists in a three-dimensional network made by layers parallel to ab plane of ZnO6 octahedra sharing four vertices with H2P2O7 double tetrahedra: (NH4)+ cations fill up the cavities between tetrahedra and octahedra; strong hydrogen bonds stabilize the framework. The dihydrogendiphosphate anion (H2P2O7)2- shows bent eclipsed conformation and the Zn2+ ion lies on inversion centre. Raman and infrared spectra of (NH4)2Zn(H2P2O7)2 · 2 H2O have been collected and interpreted using factor group analysis. No coincidences observed between most of Raman and IR spectral bands confirmed the centrosymmetric structure of the title compound; the vibrational spectra point to a bent POP bridge angle.

Crystal Structure of 4,7,13,16,21,24-Hexaoxa-1,10-diazoniabicyclo[8.8.8]hexacosane Semihydrate Oxonium Tribromide

The crystal structure of 4,7,13,16,21,24-hexaoxa-1,10-diazoniabicyclo-[8.8.8]hexacosane semihydrate oxonium tribromide, [H2(2.2.2-Crypt)·0.55H2O]2+·H3O+·3Br− (I) was determined by XRD analysis. The triclinic structure of I (space group P\(\bar 1\), a = 10.026 A, b = 11.292 A, c = 13.115 A, α = 78.37°, β = 72.11°, γ = 77.50°, Z = 2) was solved by direct methods; full-matrix least-squares refinement in an anisotropic approximation converged to R = 0.055 for all 4057 independent reflections collected (CAD-4 automatic diffractometer, λCuKα).

Pb(UO 2)(V 2O 7), a novel lead uranyl divanadate

A new lead uranyl divanadate, PbUO 2(V 2O 7), has been synthesized by high temperature solid-state reaction and its crystal structure was solved by direct methods using single-crystal X-ray diffraction data. It crystallizes in the monoclinic system with space group P2 1/ n and following cell parameters: a=6.9212(9) Å, b=9.6523(13) Å, c=11.7881(16) Å, β=91.74(1)°, V=787.01(2) Å 3, Z=4, ρ mes=5.82(3), ρ cal=5.83(1) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.029 and w R 2=0.064 for 2136 independent reflections with I > 2 σ ( I ) collected with a Bruker AXS diffractometer (Mo Kα radiation). The crystal structure of PbUO 2(V 2O 7) consists of a tri-dimensional framework resulting from the association of V 2O 7 divanadate units formed by two VO 4 tetrahedra sharing corner and UO 7 uranyl pentagonal bipyramids and creating one-dimensional elliptic channels occupied by the Pb 2+ ions. In PbUO 2(V 2O 7), infinite ribbons of four pentagons wide are formed which can be deduced from the sheets with Uranophane type anion-topology that occurs, for example, in the uranyl divanadate (UO 2) 2(V 2O 7), by replacement of half-U atoms of the edge-shared UO 7 pentagonal bipyramids by Pb atoms. Infrared spectroscopy was investigated at room temperature in the frequency range 400–4000 cm −1, showing some characteristic bands of uranyl ion and of VO 4 tetrahedra.

Growth and Structural Characterization of Needlelike Metastable Crystals in the Nd−Ba−Cu−(Al)−O System

An unconventional technique was used to grow crystals in the Nd−Ba−Cu−O system. A Nd4Ba2Cu2O10 (Nd-422) bar was supported on two NdBa2Cu3Ox (Nd-123) pellets in an alumina crucible and was heated in a horizontal furnace at a higher temperature than the peritectic decomposition point of Nd-422. Brown Al-doped Nd−Ba−Cu−O needlelike crystals belonging to a new phase were grown directly on the crucible. The structure of the new phase, whose composition is Nd6Ba9.78Sr0.22Cu2Al6O30, has been determined by single-crystal X-ray diffraction. The space group is P63mc; the unit cell parameters are a = 11.526(2) A, c = 6.963(1) A, and Z = 1. A full matrix least-squares refinement yielded R(F) = 0.0405 for 3131 independent reflections. This new phase is a metastable polymorphic modification of Nd-422 solid solution. The preferential directional growth of the needles lies along the c-axis. Many needles are affected by twinning by merohedry, with the twinning plane parallel to the (001) crystallographic plane.

A novel open-framework with non-crossing channels in the uranyl vanadates A(UO2)4(VO4)3 (A=Li, Na)

A new sodium uranyl vanadate Na(UO2)4(VO4)3 has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the tetragonal symmetry with space group I41/amd and following cell parameters: a=7.2267(4) Å and c=34.079(4) Å, V=1779.8(2) Å3, Z=4 with ρmes=5.36(3) g/cm3 and ρcal=5.40(2) g/cm3. A full-matrix least-squares refinement on the basis of F2 yielded R1=0.028 and wR2=0.056 for 52 parameters with 474 independent reflections with I⩾2σ(I) collected on a BRUKER AXS diffractometer with MoKα radiation and a CCD detector. The crystal structure is characterized by ∞2[(UO2)2(VO4)] sheets parallel to (001) formed by corner-shared UO6 distorted octahedra and V(2)O4 tetrahedra, connected by V(1)O4 tetrahedra to ∞1[UO5]4− chains of edge-shared UO7 pentagonal bipyramids alternately parallel to the a- and b-axis. The resulting three-dimensional framework creates mono-dimensional channels running down the a- and b-axis formed by face-shared oxygen octahedra half occupied by Na. The powder of Li analog compound Li(UO2)4(VO4)3 has been synthesized by solid-state reaction. The two compounds exhibit high mobility of the alkaline ions within the two-dimensional network of non-intersecting channels.

A novel open-framework with non-crossing channels in the uranyl vanadates A(UO 2) 4(VO 4) 3 ( A=Li, Na)

A new sodium uranyl vanadate Na(UO 2) 4(VO 4) 3 has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the tetragonal symmetry with space group I4 1/ amd and following cell parameters: a=7.2267(4) Å and c=34.079(4) Å, V=1779.8(2) Å 3, Z=4 with ρ mes=5.36(3) g/cm 3 and ρ cal=5.40(2) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.028 and w R 2=0.056 for 52 parameters with 474 independent reflections with I⩾2 σ( I) collected on a BRUKER AXS diffractometer with Mo Kα radiation and a CCD detector. The crystal structure is characterized by ∞ 2[(UO 2) 2(VO 4)] sheets parallel to (001) formed by corner-shared UO 6 distorted octahedra and V(2)O 4 tetrahedra, connected by V(1)O 4 tetrahedra to ∞ 1[UO 5] 4− chains of edge-shared UO 7 pentagonal bipyramids alternately parallel to the a- and b-axis. The resulting three-dimensional framework creates mono-dimensional channels running down the a- and b-axis formed by face-shared oxygen octahedra half occupied by Na. The powder of Li analog compound Li(UO 2) 4(VO 4) 3 has been synthesized by solid-state reaction. The two compounds exhibit high mobility of the alkaline ions within the two-dimensional network of non-intersecting channels.

Synthesis, crystal structure and electrical characterization of Cs 4[(UO 2) 2(V 2O 7)O 2], a uranyl divanadate with chains of corner-sharing uranyl square bipyramids

A new cesium uranyl vanadate Cs 4[(UO 2) 2(V 2O 7)O 2] has been synthesized by solid-state reaction and its structure determined from single-crystal X-ray diffraction data. It crystallizes in the orthorhombic symmetry with space group Pmmn and following cell parameters: a=8.4828(15) Å, b=13.426(2) Å and c=7.1366(13) Å, V=812.8(3) Å 3, Z=2 with ρ mes=5.39(2) g/cm 3 and ρ cal=5.38(1) g/cm 3. A full-matrix least-squares refinement on the basis of F 2 yielded R 1=0.027 and w R 2=0.066 for 62 parameters with 636 independent reflections with I⩾2 σ( I) collected on a BRUKER AXS diffractometer with Mo Kα radiation and a charge-coupled device detector. The crystal structure is characterized by ∞ 2[(UO 2) 2(V 2O 7)O 2] 4− corrugated layers parallel to (001). The layers are built up from distorted (UO 2)O 4 octahedra and divanadate V 2O 7 units resulting from two VO 4 tetrahedra sharing corner. The distorted uranyl octahedra (UO 2)O 4 are linked by corners to form infinite ∞ 1[UO 5] 4− chains parallel to the a-axis. These chains are linked together by symmetrical divanadate units sharing two corners with each chain, the two last corners being oriented towards the same interlayer. The cohesion of the structure is assured by interlayer Cs + ions. Their mobility within the interlayer space gives rise to a cationic conductivity with an important jump between 635°C and 680°C. Cs 4[(UO 2) 2(V 2O 7)O 2] is readily decomposed by water at 60°C to form the Cs-carnotite analog Cs 2(UO 2) 2(V 2O 8) compound.

Crystal Structure of Tetrafluoro(1,10-phenanthroline-N,N)silicon Acetonitrile Semisolvate

The crystal structure of tetrafluoro(1,10-phenanthroline-N,N')silicon acetonitrile semisolvate, [SiF4(phen)]·1/2CH3CN (I), was studied by X-ray diffraction analysis. The structure of I is tetragonal (space group I41/a; a=9.184(1), c=29.700(5) A, Z=8); direct methods, full-matrix least-squares refinement in an anisotropic approximation; R=0.039 for all 1820 independent reflections collected (CAD-4 automatic diffractometer, MoKα). The geometrical parameters of the [SiF4(phen)] molecule (bond lengths and angles, etc.) were determined with a high accuracy.

Synthesis, X-ray crystal structure and vibrational spectroscopy of the acidic pyrophosphate KMg 0.5H 2P 2O 7·H 2O

The hydrated potassium hemimagnesium dihydrogen pyrophosphate KMg 0.5H 2P 2O 7·H 2O was synthesized. It crystallizes in the triclinic system, space group P 1 ̄ (n. 2), Z=2, with the following unit-cell parameters: a=6.8565(2) Å, b=7.3621(3) Å, c=7.6202(3) Å, α=81.044(2)°, β=72.248(2)°, γ=83.314(3)°, V=360.90(2) Å 3. The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F 2 gave a final R index of =0.0368 (w R=0.0975), utilizing 1446 observed reflections with I>2 σ( I). The crystal packing consists in a three-dimensional network made by layers parallel to ab plane of PO 4 double tetrahedra and MgO 6 octahedra, linked by hydrogen bonds, while K atoms form complex coordination within cavities between tetrahedra and octahedra. The dihydro-pyrophosphate anion (H 2P 2O 7) 2− shows bent eclipsed conformation and the Mg 2+ ion lies on inversion center. No coincidences observed between most of infrared and Raman spectral bands confirmed the centrosymmetric structure of the title compound; the vibrational spectra point to a bent POP bridge angle.

Synchrotron radiation in atomic-resolution studies of protein structure

The paper discusses the high resolution and quality aspects of synchrotron radiation protein crystallography using examples from our own laboratory. The examples include two inhibitors of trypsin, one (CMTI) derived from plants, and one (BPTI) of mammalian origin. The structures were refined at 1 Å (CMTI) and 0.86 Å (BPTI) resolution to final R-factors of 0.114 and 0.103, respectively. In both cases it was possible to use anisotropic displacement parameters and to relax or remove some of the stereochemical restraints that are necessary in lower-resolution refinements. The results indicate that some of the standards are not exactly correct. For instance, the peptide group shows large deviations from planarity, and the N–Cα–C angle has a wide spread. Compared to lower-resolution refinements, more residues are modeled in alternative conformations. From the final full-matrix least-squares refinements, the estimated standard errors in bond lengths are about 0.005–0.01 Å (BPTI) and 0.01–0.03 Å (CMTI). These numbers demonstrate the tremendous improvement in quality when resolution is extended from 1.0 to 0.86 Å. In line with this is the presence of clear difference electron density peaks corresponding to H atoms, including those in water molecules, in the BPTI structure, but not in the CMTI structure. Some of them are seen to form C–H…O and N–H…π H bonds.

Crystal Structure of [tris(1,10-Phenanthroline)copper(II)(2+) bis{3,3 -commo-bis-[h5-1,2-Dicarba-(3)-cobalt(III)-closo-dodecaborate]}(1-)-diacetonitrile, [CuII(1,10-C12H8N2)3]2+{CoIII[h5-(3)-1,2-C2B9H11]2}2- 2CH3CN

A new compound containing the Co(III) dicarbollyl cluster anion, [CuPhen3][Co(C2B9H11)2]2·2CH3CN, where Phen is 1,10-phenanthroline, was synthesized and investigated by X-ray crystallography. Crystal data: C48H74B36N8Co2Cu, M=1333.71, monoclinic, space group C2/c, unit cell parameters a=32.069(4), b=11.317(2), c=20.963(4) A, β=118.10(1)°, V=6711.2(1.9) A3, Z=4, dcalc=1.320 g/cm3, T=294 K, F(000)=2724, μ=0.846 mm-1. The structure was solved by direct and Fourier methods. The full-matrix least-squares refinement was performed anisotropically (isotropically for hydrogens) to R1=0.0281, wR2=0.0581 for 3610 Ihkl>2 σI of 5172 Ihkl collected (CAD-4 Enraf-Nonius diffractometer, λMoKα, graphite monochromator, ω scan mode). The structure consists of [CuPhen3]2+ cations, [Co(C2B9H11)2]- anions, and CH3CN acetonitrile molecules. The central Cu atom in the cation lies on the twofold crystallographic axis and is surrounded by a distorted flattened octahedron of six nitrogen atoms of the three Phen bidentate ligands. For the Cu(II) atom of the cation, the environment is (2+4) with two short axial Cu-N bonds and four long equatorial Cu-N bonds, for which the mean values are 2.076(2) and 2.147(2) A, respectively. The anion has a sandwich structure, which is typical in commo-metallocarboranes, with two CoC2B9 icosahedra sharing a vertex occupied by the Co atom. In the anion, the -C2- groups have an arrangement corresponding to the quasi-gosh-configuration.

Absence of the Nitrone Form among the Refined Crystal Structures of 1-Methoxy- and 1-(2-Chloro-ethyl)oxy-2-hydroximino-3-methyl-4-nitro-3-phospholene-1-oxides

Crystal structure refinement has been performed by XRD for 1-methoxy- and 1-(2-chloroethyl)oxy-2-hydroximino-3-methyl-4-nitro-3-phospholene-1-oxides (I) C6H9N2O5P, Z=8, space group P21/c, and (II) C7H10ClN2O5P, Z=4, space group P21/c. Disordering of independent molecules I and I' in the crystal structure of I and of molecule II in the crystal structure of II has been found. Full-matrix least-squares refinement of I and II was performed anisotropically (isotropically for H) to R=0.052 (I) and 0.034 (II) for all 4377 (I) and 2533 (II) independent reflections (CAD-4 automatic diffractometer, λMoKα). The authors of the first XRD analysis of crystals I and II made the wrong conclusion that compounds I and II exist in crystal in NH-nitrone form; i.e., that they contain an O←N(H)=C nitrone isomer group instead of an H−O−N=C oxime group. As shown in this study, compounds I and II exist in ordinary oxime form. The authors of the first XRD analysis failed to notice the disordering of molecules I, I', and II, and they erroneously took the second, low-occupied oxygen sites of the disordered oxime groups for the H atoms at N atoms.

Layered niobium titanium oxychloride cluster compounds with six-member ring opening channels

New niobium oxychloride cluster compounds corresponding to the general formula, A 5Ti 8Nb 18Cl 53O 12 ( A=K, In), have been prepared in sealed quartz tubes from a mixture containing NbCl 5, Nb 2O 5, Nb, Ti, and KCl or In by solid-state reactions at 750°C. Their structure was determined using single-crystal X-ray diffraction; Crystal data: trigonal, P 3 ̄ ml (No. 164) (for K: a=16.8303(11) Å, c=9.0510(8) Å, V=2220.3(3) Å 3 and Z=1; for In: a=16.889(2) Å, c=9.0684(2) Å, V=2240.0(6) Å 3 and Z=1). The full-matrix least-squares refinement of all data against F 2 converged to R 1=0.044, w R 2=0.088 for K, and R 1=0.050, w R 2=0.140 for In. The structure consists of octahedral (Nb 6C1 8 iO 4 i)C1 6 a cluster units that share four outer chloride ligands with four adjacent clusters to form a two-dimensional framework that generates six- and three-member rings similar to those found in hexagonal tungsten bronze. Additional linkages within the same layer are provided by Ti 3Cl 7O 6 trimers. Adjacent layers are stacked in registry with each other leading to the formation of six-member ring openings channels parallel to the [001] direction in which a disordered [ A 5(Ti 2Cl 9)] 2+ species are located. Magnetic susceptibility studies show paramagnetic behavior with a magnetic moment of 3.74 μ B per formula unit.

Synthesis and crystal structure of α and β-Rb 6U 5V 2O 23, a new layered compound

A new alkali uranyl vanadate, Rb 6U 5V 2O 23 has been synthesized by solid-state reactions. According to synthesis method, two allotropic varieties have been found and their structures determined from single-crystal X-ray diffraction data. Both compounds crystallize in the monoclinic system with space groups C2/ c and P2 1/ n for α and β varieties, respectively. α-Rb 6U 5V 2O 23 unit-cell parameters are a=24.887(8) Å, b=7.099(2) Å, c=14.376(4) Å, β=103.92(1)°, V=2465.2(6) Å 3, Z=4, ρ mes=5.86(2) g cm −3, ρ cal=5.85(1) g cm −3 and for β-Rb 6U 5V 2O 23 a=7.1635(9) Å, b=14.079(2) Å, c=24.965(4) Å, β=90.23(1)°, V=2517.8(6) Å 3, Z=4, ρ mes=5.79(2) g cm −3, ρ cal=5.73(1) g cm −3. A full-matrix least-squares refinement yielded, respectively, R=0.032 and 0.041 for 1307 and 2276 independent reflections with I>2 σ collected on a Smart diffractometer (Mo Kα radiation). Both structures are characterized by [(UO 2) 5(VO 4) 2O 5] 6− layers which are flat and parallel to the (10 1 ̄ ) plane in α variety, corrugated and parallel to the (010) plane in the β variety. The layers are built up from VO 4 tetrahedra and UO 7 pentagonal bipyramids. In β variety, UO 6 distorted octahedra can also be considered. The UO 7 pentagonal bipyramids are associated by sharing opposite equatorial edges to form zig-zag infinite chains (UO 5) ∞ parallel to the b-axis in α variety and to the a-axis in β variety, respectively. These chains are linked together on one side by VO 4 tetrahedra and on other side in α variety by UO 7 pentagonal bipyramids sharing corner and opposite edges and in β variety by UO 6 octahedra sharing opposite edges. Both structures differ by several characteristics. In α variety, the layers are flat according to the rows of VO 4 tetrahedra parallel to the b-axis whose apical oxygen atoms are alternatively pointing on the both sides in each row whereas in β variety, they are all pointing in the same side in a row parallel to a and in opposite side in the next, yielding to corrugated layers.

Synthesis and crystal structure of two new uranyl oxychloro-vanadate layered compounds: M7(UO 2) 8(VO 4) 2O 8Cl with M=Rb, Cs

Two new alkali uranyl oxychloro vanadates M 7(UO 2) 8(VO 4) 2O 8Cl with M=Rb, Cs, have been synthesized by solid-state reactions and their structures determined from single-crystal X-ray diffraction data. They crystallize in the orthorhombic system with space groups Pmcn and Pmmn, respectively. The a and b unit cell parameters are almost identical in both compounds while the c parameter in the Rb compound is doubled: Rb— a=21.427(5) Å, b=11.814(3) Å, c=14.203(3) Å, V=3595.1(1) Å 3, Z=4, ρ mes=5.93(2) g/cm 3, ρ cal=5.82(1) g/cm 3; Cs— a=21.458(3) Å, b=11.773(2) Å, c=7.495(1) Å, V=1893.6(5) Å 3, Z=2, ρ mes=6.09(2) g/cm 3, ρ cal=6.11(1) g/cm 3. A full-matrix least-squares refinement yielded R 1=0.0221, w R 2=0.0562 for 2675 independent reflections and R 1=0.0386, w R 2=0.1042 for 2446 independent reflections, for the Rb and Cs compounds, respectively. Data were collected with Mo( Kα) radiation and a charge coupled device (CCD) detector of a Bruker diffractometer. Both structures are characterized by [(UO 2) 8(VO 4) 2O 8Cl] n 7 n− layers parallel to the (001) plane. The layers are built up from VO 4 tetrahedra, UO 7 and UO 6Cl pentagonal bipyramids, and UO 6 distorded octahedra. The UO 7 and UO 6Cl pentagonal bipyramids are associated by sharing opposite equatorial edges to form infinite chains (UO 5–UO 4Cl–UO 5) n parallel to the a axis. These chains are linked together by VO 4 tetrahedra, UO 6 octahedra, UO 7 corner sharing and UO 6Cl, Cl sharing. Both structures differ simply by the symmetry of the layers. The unit cell contains one centrosymmetric layer in the Cs compound, whereas in the two-layer unit cell of the Rb compound, two non-centrosymmetric consecutive layers are related by an inversion center. The layers appear to be held together by the alkali ions. The mobility of the M + ions within the interlayer space in M 7(UO 2) 8(VO 4) 2O 8Cl and carnotite analog compounds is compared.

A2VNb 6Cl 18 ( A=Rb, In, Tl): synthesis, crystal structure and magnetic properties of a new series of quaternary niobium chloride cluster compounds

A new series of quaternary niobium chloride cluster compounds corresponding to the general formula, A 2VNb 6Cl 18 ( A=Rb, Tl or In), has been prepared in sealed quartz tubes from a mixture containing NbCl 5, Nb, VCl 3 and RbCl, or In or Tl metal by solid state reactions at 750°C. The structure of Tl 2VNb 6Cl 18 was determined using single-crystal X-ray diffraction: Crystal data: rhombohedral, R 3 ̄ (No. 148), a=9.1122(17), c=25.178(7) Å, V=1810.5(7) Å 3 and Z=3. The full-matrix least-squares refinement against F 2 converged to R 1=0.0515, w R 2=0.1104 (all data). The structure consists of discrete octahedral cluster units, [Nb 6Cl i 12Cl a 6] 4− linked by V 2+ and A + cations, located in a 6-coordinated octahedral and 12-coordinated anticubeoctahedral chloride environment, respectively. The intra-cluster bond lengths indicate 16 valence electrons per cluster. Magnetic susceptibility studies show paramagnetic behavior with a magnetic moment of 3.37 μ B per formula unit. Electrical resistivity measurements indicate a semiconducting behavior.

Preparation of Low‐Dimensional Cluster Materials: Synthesis, Structure, and Properties of A2Ti2Nb6Cl14O5 (A: K, Rb, Cs), a Series of One‐Dimensional Titanium Niobium Oxychlorides.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The refined atomic structure of carbonic anhydrase II at 1.05 A resolution: implications of chemical rescue of proton transfer.

Using synchrotron radiation and a CCD detector, X-ray data have been collected at 100 K for the His64Ala mutant of human carbonic anhydrase II complexed with 4-methylimidazole (4-MI) to a maximal 1.05 A resolution, allowing full anisotropic least-squares refinement. The refined model has a conventional R factor of 15.7% for all reflections. The C(alpha) coordinates of the model presented here have an r.m.s. deviation of 0.10 A relative to the previously determined structure at 1.6 A resolution. Several amino-acid residues (six of the 255 observed) have been identified with multiple rotamer side-chain conformations. C, N and O atoms can be differentiated with selective electron-density map contouring. The estimated standard deviations for all main-chain non-H atom bond lengths and angles are 0.013 and 0.030 A, respectively, based on unrestrained full-matrix least-squares refinement. This structure gives detailed information about the tetrahedrally arranged zinc ion coordinated by three histidine N atoms (His94 N(epsilon 2), His96 N(epsilon2) and His119 N(delta1)) and a water/hydroxide, the multiple binding sites of the proton chemical rescue molecule 4-MI and the solvent networks linking the zinc-bound water/hydroxide and 4-MI molecules. This structure presents the highest resolution structure of a carbonic anhydrase isozyme so far determined and adds to the understanding of proton-transfer processes.