Conformation of the flexible bent helix of Leu1-zervamicin in crystal C and a possible gating action for ion passage

The membrane channel-forming polypeptide, Leu(1)-zervamicin, Ac-Leu-Ile-Gln-Iva-Ile(5)-Thr-Aib-Leu-Aib-Hyp(10) -Gln-Aib-Hyp-Aib-Pro(15)-Phol (Aib: alpha-aminoisobutyric acid; Iva: isovaline; Hyp: 4-hydroxyproline; Phol: phenylalininol) has been analyzed by x-ray diffraction in a third crystal form. Although the bent helix is quite similar to the conformations found in crystals A and B, the amount of bending is more severe with a bending angle approximate to 47 degrees, The water channel formed by the convex polar faces of neighboring helices is larger at the mouth than in crystals A and B, and the water sites have become disordered. The channel is interrupted in the middle by a hydrogen bond between the OH of Hyp(10) and the NH2 of the Gln(11) of a neighboring molecule. The side chain of Gln(11) is wrapped around the helix backbone in an unusual fashion in order that it can augment the polar side of the helix. In the present crystal C there appears to be an additional conformation for the Gln(11) side chain (with approximate to 20% occupancy) that opens the channel for possible ion passage. Structure parameters for C85H140N18O22.xH(2)O.C2H5OH are space group P2(1)2(1)2(1), a = 10.337 (2) Angstrom, b = 28.387 (7) Angstrom, c = 39.864 (11) Angstrom, Z = 4, agreement factor R = 12.99% for 3250 data observed > 3 sigma(F), resolution = 1.2 Angstrom. (C) 1994 John Wiley & Sons, Inc.

Effects of zirconia phase on the synthesis of higher alcohols over zirconia and modified zirconia

The catalytic performances of ZrO2-based catalysts were evaluated for the synthesis of higher alcohols from synthesis gas. The crystal phase structures were characterized by X-ray diffraction (XRD) and UV Raman. The results indicated that ZrO2 and Pd modified ZrO2 catalysts were effective catalysts in the synthesis of ethanol or isobutanol, and their selectivities basically depended on the crystal phase of ZrO2 surface. The ZrO2 with surface tetragonal crystal phase exhibited a high activity to form ethanol, while the ZrO2 with surface monoclinic crystal phase exhibited a high activity to form isobutanol. Temperature-programmed desorption (TPD) experiment indicated that the high activity of isobutanol formation from synthesis gas over monoclinic zirconia was due probably to the strong Lewis acidity of Zr4+ cations and the strong Lewis basicity of O2- anions of coordinative unsaturated Zr4+-O2- pairs on the surface of monoclinic ZrO2. (C) 2003 Elsevier B.V. All rights reserved.

Crystal structure and its transition for poly(aryl ether ketone ketone)s .1.

Poly(aryl ether ketone ketone)s (PEKK) was a high-performance engineering plastics, By means of Wide Angle X-ray Diffraction (WAXD) and Differential Scanning Calorimetry (DSC) methods, PEKK samples crystallized in solvent induction, from glass state and from melting state were studied, Crystal forms I and II for PEKK were found, The formation of crystal form II was dependent on thermal history and solvent induction, and this form II had melting point 10 degrees C or so lower than that of form I crystallized from glass state, All PEKK samples had low melting peaks which were relevant to the polarization of PEKK molecular chain, while they had nothing to do with thermal history, The heat of fusion for PEKK low melting peaks accounted for,percentage of 2 to 10 or so of the whole heat of fusion, And PEKK has its equilibrium melting point of 409 degrees C.

SYNTHESES OF BIS(TERT-BUTYLCYCLOPENTADIENYL)LANTHANOID CHLORIDE COMPLEXES AND CRYSTAL-STRUCTURES OF BIS(TERT-BUTYLCYCLOPENTADIENYL)CHLORO(BISTETRAHYDROFURAN)-PRASEODYMIUM AND BIS(TERT-BUTYLCYCLOPENTADIENYL)-CHLOROTETRAHYDROFURANYTTERBIUM

Reaction of lanthanoid trichloride with two equivalents of sodium t-butylcyclopentadienide in tetrahydrofuran affords bis(t-butylcyclopentadienyl)lanthanoid chloride complexes (t-BuCp)2LnCl. nTHF (Ln = Pr, Nd, n = 2; Ln = Gd, Yb, n = 1). The compound (t-BuCp)2PrCl.2THF (1) crystallizes from THF in monoclinic space group P2(1)/c with unit cell dimensions a = 15.080(3), b = 8.855(2), c = 21.196(5) angstrom, beta = 110.34(2)degrees, V = 2653.9 angstrom-3 and D(calcd) = 1.41 g/cm3 for Z = 4. The central metal Pr is coordinated to two t-BuCp ring centroids, one chlorine atom and two THF forming a distorted trigonal bipyramid. The crystal of (t-BuCp)2YbCl.THF (2) belongs to the monoclinic crystal system, space group P2(1)/n with a = 7.726(1), b = 12.554(2), c = 23.200(6) angstrom, beta = 97.77(2)degrees, V = 2229.56 angstrom-3, D(calcd) = 1.50 g/cm3 and Z = 4. The t-BuCp ring centroids, the chlorine atom and the oxygen atom of the THF describe a distorted tetrahedron around the central ion of ytterbium.

Growth, optical characterization, and laser operation of a stoichiometric crystal KYb(WO4)(2)

We present our recent achievements in the growing and optical characterization of KYb(WO4)2 (hereafter KYbW) crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies (absorption and emission measurements at room and low temperature) have been carried out in the spectral region near 1 µm characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with 41% slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort (ps/fs) pulse laser systems.

Effect of pressure-assisted heat treatment on photoluminescence emission of α-Bi2O3 needles

Materials with high photoluminescence (PL) intensity can potentially be used in optical and electronic devices. Although the PL properties of bismuth(III) oxide with a monoclinic crystal structure (α-Bi2O3) have been explored in the past few years, methods of increasing PL emission intensity and information relating PL emission to structural defects are scarce. This research evaluated the effect of a pressure-assisted heat treatment (PAHT) on the PL properties of α-Bi2O3 with a needlelike morphology, which was synthesized via a microwave-assisted hydrothermal (MAH) method. PAHT caused an angular increase between the [BiO6]-[BiO6] clusters of α-Bi2O3, resulting in a significant increase in the PL emission intensity. The Raman and XPS spectra also showed that the α-Bi2O3 PL emissions in the low-energy region (below ∼2.1 eV) are attributed to oxygen vacancies that form defect donor states. The experimental results are in good agreement with first-principles total-energy calculations that were carried out within periodic density functional theory (DFT).

Crystal structure of the sodium-proton antiporter NhaA dimer and new mechanistic insights

Sodium-proton antiporters rapidly exchange protons and sodium ions across the membrane to regulate intracellular pH, cell volume, and sodium concentration. How ion binding and release is coupled to the conformational changes associated with transport is not clear. Here, we report a crystal form of the prototypical sodium-proton antiporter NhaA from Escherichia coli in which the protein is seen as a dimer. In this new structure, we observe a salt bridge between an essential aspartic acid (Asp163) and a conserved lysine (Lys300). An equivalent salt bridge is present in the homologous transporter NapA, but not in the only other known crystal structure of NhaA, which provides the foundation of most existing structural models of electrogenic sodium-proton antiport. Molecular dynamics simulations show that the stability of the salt bridge is weakened by sodium ions binding to Asp164 and the neighboring Asp163. This suggests that the transport mechanism involves Asp163 switching between forming a salt bridge with Lys300 and interacting with the sodium ion. pKa calculations suggest that Asp163 is highly unlikely to be protonated when involved in the salt bridge. As it has been previously suggested that Asp163 is one of the two residues through which proton transport occurs, these results have clear implications to the current mechanistic models of sodium-proton antiport in NhaA.

Crystal structure of the HLA-Cw3 allotype-specific killer cell inhibitory receptor KIR2DL2

Killer cell inhibitory receptors (KIR) protect class I HLAs expressing target cells from natural killer (NK) cell-mediated lysis. To understand the molecular basis of this receptor-ligand recognition, we have crystallized the extracellular ligand-binding domains of KIR2DL2, a member of the Ig superfamily receptors that recognize HLA-Cw1, 3, 7, and 8 allotypes. The structure was determined in two different crystal forms, an orthorhombic P212121 and a trigonal P3221 space group, to resolutions of 3.0 and 2.9 Å, respectively. The overall fold of this structure, like KIR2DL1, exhibits K-type Ig topology with cis-proline residues in both domains that define β-strand switching, which sets KIR apart from the C2-type hematopoietic growth hormone receptor fold. The hinge angle of KIR2DL2 is approximately 80°, 14° larger than that observed in KIR2DL1 despite the existence of conserved hydrophobic residues near the hinge region. There is also a 5° difference in the observed hinge angles in two crystal forms of 2DL2, suggesting that the interdomain hinge angle is not fixed. The putative ligand-binding site is formed by residues from several variable loops with charge distribution apparently complementary to that of HLA-C. The packing of the receptors in the orthorhombic crystal form offers an intriguing model for receptor aggregation on the cell surface.

Crystal structures of eukaryotic translation initiation factor 5A from Methanococcus jannaschii at 1.8 Å resolution

Eukaryotic translation initiation factor 5A (eIF-5A) is a ubiquitous protein found in all eukaryotic cells. The protein is closely associated with cell proliferation in the G1–S stage of the cell cycle. Recent findings show that the eIF-5A proteins are highly expressed in tumor cells and act as a cofactor of the Rev protein in HIV-1-infected cells. The mature eIF is the only protein known to have the unusual amino acid hypusine, a post-translationally modified lysine. The crystal structure of eIF-5A from Methanococcus jannaschii (MJ eIF-5A) has been determined at 1.9 Å and 1.8 Å resolution in two crystal forms by using the multiple isomorphous replacement method and the multiwavelength anomalous diffraction method for the first crystal form and the molecular replacement method for the second crystal form. The structure consists of two folding domains, one of which is similar to the oligonucleotide-binding domain found in the prokaryotic cold shock protein and the translation initiation factor IF1 despite the absence of any significant sequence similarities. The 12 highly conserved amino acid residues found among eIF-5As include the hypusine site and form a long protruding loop at one end of the elongated molecule.

The molecular structure of matioliite – NaMgAl5(PO4)4(OH)6·2(H2O) : a pegmatite mineral from Minas Gerais, Brazil

Detailed spectroscopic and chemical investigation of matioliite, including infrared and Raman spectroscopy, scanning electron microscopy and electron probe microanalysis has been carried out on homogeneous samples from the Gentil pegmatite, Mendes Pimentel, Minas Gerais, Brazil. The chemical composition is (wt.%): FeO 2.20, CaO 0.05, Na2O 1.28, MnO 0.06, Al2O3 39.82, P2O5 42.7, MgO 4.68, F 0.02 and H2O 9.19; total 100.00. The mineral crystallize in the monoclinic crystal system, C2/c space group, with a = 25.075(1) Å, b = 5.0470(3) Å, c = 13.4370(7) Å, β = 110.97(3)°, V = 1587.9(4) Å3, Z = 4. Raman spectroscopy coupled with infrared spectroscopy supports the concept of phosphate, hydrogen phosphate and dihydrogen phosphate units in the structure of matioliite. Infrared and Raman bands attributed to water and hydroxyl stretching modes are identified. Vibrational spectroscopy adds useful information to the molecular structure of matioliite.

A comparative study of different porous amorphous silica minerals supported TiO2 catalysts

Three porous amorphous silica minerals, including diatomite, opal and porous precipitated SiO2wereadopted to prepare supported TiO2catalysts by hydrolysis–deposition method. The prepared compoundmaterials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fouriertransform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and X-ray photo-electron spectroscopy (XPS). Through morphology and physical chemistry properties of the resultingTiO2/amorphous SiO2catalysts, it was proposed that the nature of silica supports could affect the particlesize and the crystal form of TiO2and then further influence the photocatalytic property of TiO2/amorphousSiO2catalysts. The catalytic properties of these porous amorphous silica supported photocatalysts(TiO2/SiO2) were investigated by UV-assisted degradation of Rhodamine B (RhB). Compared with pureTiO2(P25) and the other two TiO2/amorphous SiO2catalysts, TiO2/diatomite photocatalyst exhibits bet-ter catalytic performance at different calcined temperatures, the decoloration rate of which can be upto over 85% even at a relatively low calcined temperature. The TiO2/diatomite photocatalyst possessesmixed-phase TiO2with relatively smaller particles size, which might be responsible for higher photo-catalytic activity. Moreover, the stable and much inerter porous microstructure of diatomite could beanother key factor in improving its activity.

Vibrational spectroscopy of the copper (II) disodium sulphate dihydrate mineral kröhnkite Na2Cu(SO4)2 · 2H2O

A natural single-crystal specimen of the kröhnkite from Chuquicamata, Chile, with the general formula Na2Cu(SO4)2 · 2H2O, was investigated by Raman and infrared spectroscopy. The mineral kröhnkite is found in many parts of the world's arid areas. Kröhnkite crystallizes in the monoclinic crystal system with point group 2/m and space group P21/c. It is an uncommon secondary mineral formed in the oxidized zone of copper deposits, typically in very arid climates. The Raman spectrum of kröhnkite dominated by a very sharp intense band at 992 cm−1 is assigned to the ν1 symmetric stretching mode and Raman bands at 1046, 1049, 1138, 1164, and 1177 cm−1 are assigned to the ν3 antisymmetric stretching vibrations. The infrared spectrum shows an intense band at 992 cm−1. The Raman bands at 569, 582, 612, 634, 642, 655, and 660 cm−1 are assigned to the ν4 bending modes. Three Raman bands observed at 429, 445, and 463 cm−1 are attributed to the ν2 bending modes. The observation that three or four bands are seen in the ν4 region of kröhnkite is attributed to the reduction of symmetry to C2v or less.

Raman spectra of ferro-electric crystals Part IV. Lithium hydrazinium sulphate (LiN2H5SO4)

The Raman spectrum of lithium hydrazinium sulphate has been recorded both in the single crystal form and in aqueous solutions. The crystal exhibits thirty-eight Raman lines having the frequency shifts 52, 70, 104, 146, 174, 220, 260, 302, 350, 454, 470, 610, 630, 715, 977, 1094, 1115, 1132, 1177, 1191, 1260, 1444, 1493, 1577, 1630, 1670, 2205, 2484, 2553, 2655, 2734, 2848, 2894, 2939, 3028, 3132, 3290 and 3330 cm.−1 The aqueous solution gave rise to six Raman lines at 452, 980, 1050–1200, 1260, 1425 and 1570 cm.−1 apart from a maximum at 180 cm.−1 in the ‘wing’ accompanying the Rayleigh line. The observed Raman lines have been assigned as arising from the vibrations of the SO4 ion, N2H5+ ion, Li-O4 group, hydrogen bond and the lattice. The influence of the hydrogen bond on the N-H stretching vibrations has been pointed out. The various features of the observed spectrum strongly support the hypothesis that the NH3 group in the crystal is rotating around the N-N axis at room temperature.

Raman spectrum of diglycine barium chloride monohydrate

The Raman spectrum of diglycine barium chloride monohydrate in the single crystal form has been recorded using λ 2536·5 excitation. 43 Raman lines (9 lattice and 34 internal) have been recorded. Satisfactory assignments have been given for most of the observed Raman lines. It is concluded from a comparison of the Raman spectrum of this compound with those of glycine and of other addition compounds of glycine, that the glycine unit exists in the zwitterion form in the structure of diglycine barium chloride monohydrate.