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At least 1,351 records · Page 75
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Synthesis, crystal structure and magnetic properties of K Ln Se 2 ( Ln = La, Ce, Pr, Nd) structures: A family of 2D triangular lattice frustrated magnets

Here, we report the detail synthesis, single crystal structure characterization and magnetic properties of KLnSe 2 series obtained via solid state molten flux growth method. The crystal structures were characterized using single crystal x-ray diffraction. The KLnSe 2 (Ln = La, Ce, Pr and Nd) series crystallizes in the trigonal crystal system with the space group of R-3m (No. 166). The overall structure contains two-dimensional (2D) layers made from edged shared LnSe 6 -octahedra. Ln 3+ ions form a perfect triangular magnetic lattice which propagates along ab-plane. These triangular lattices are separated by the K + ions. The magnetic properties confirm that, Ce, Pr and Nd do not show any long-range ordering down to 0.4 K indicating frustrated magnetism in the KLnSe 2 series.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2013

Structure and magnetic properties of a new anion-deficient perovskite Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} with crystallographic shear structure

Graphical abstract: - Highlights: • Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} was obtained by solid state synthesis. • Its structure was refined from combination of XPD and TEM. • It is a new member of the perovskite-related homologous series A{sub n}B{sub n}O{sub 3n−2} with n = 5. • Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} is antiferromagnetically ordered below T{sub N} ≈350 K. - Abstract: Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13}, a new n = 5 member of the oxygen-deficient perovskite-based A{sub n}B{sub n}O{sub 3n−2} homologous series, was synthesized using a solid-state method. The crystal structure of Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} was investigated by a combination of synchrotron X-ray powder diffraction, electron diffraction, high-angle annular dark-field scanning transmission electron microscopy and Mössbauer spectroscopy. At 900 K, it crystallizes in the Ammm space group with the unit cell parameters a = 5.8459(1) Å, b = 4.0426(1) Å, and c = 27.3435(1) Å. In the Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} structure, quasi-two-dimensional perovskite blocks are periodically interleaved with ½[1 1 0] (1{sup ¯}01){sub p} crystallographic shear (CS) planes. At the CS planes, the corner-sharing FeO{sub 6} octahedra are transformed into chains of edge-sharing FeO{sub 5} distorted tetragonal pyramids. B-positions of the perovskite blocks between the CS planes are jointly occupied by Fe{sup 3+} and Sc{sup 3+}. The chains of the FeO{sub 5} pyramids and (Fe,Sc)O{sub 6} octahedra delimit six-sided tunnels that are occupied by double columns of cations with a lone electron pair (Pb{sup 2+}). The remaining A-cations (Bi{sup 3+}, Ba{sup 2+}) occupy positions in the perovskite block. According to the magnetic susceptibility measurements, Pb{sub 2}Ba{sub 2}BiFe{sub 4}ScO{sub 13} is antiferromagnetically ordered below T{sub N} ≈350 K.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Structural Basis for Substrate Placement by an Archaeal Box C/D Ribonucleoprotein Particle

Box C/D small nucleolar and Cajal body ribonucleoprotein particles (sno/scaRNPs) direct site-specific 2'-O-methylation of ribosomal and spliceosomal RNAs and are critical for gene expression. Here we report crystal structures of an archaeal box C/D RNP containing three core proteins (fibrillarin, Nop56/58, and L7Ae) and a half-mer box C/D guide RNA paired with a substrate RNA. The structure reveals a guide-substrate RNA duplex orientation imposed by a composite protein surface and the conserved GAEK motif of Nop56/58. Molecular modeling supports a dual C/D RNP structure that closely mimics that recently visualized by electron microscopy. The substrate-bound dual RNP model predicts an asymmetric protein distribution between the RNP that binds and methylates the substrate RNA. The predicted asymmetric nature of the holoenzyme is consistent with previous biochemical data on RNP assembly and provides a simple solution for accommodating base-pairing between the C/D guide RNA and large ribosomal and spliceosomal substrate RNAs.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2012

Structural Insights to How Mammalian Capping Enzyme Reads the CTD Code

Physical interaction between the phosphorylated RNA polymerase II carboxyl-terminal domain (CTD) and cellular capping enzymes is required for efficient formation of the 5' mRNA cap, the first modification of nascent mRNA. Here, we report the crystal structure of the RNA guanylyltransferase component of mammalian capping enzyme (Mce) bound to a CTD phosphopeptide. The CTD adopts an extended {beta}-like conformation that docks Tyr1 and Ser5-PO{sub 4} onto the Mce nucleotidyltransferase domain. Structure-guided mutational analysis verified that the Mce-CTD interface is a tunable determinant of CTD binding and stimulation of guanylyltransferase activity, and of Mce function in vivo. The location and composition of the CTD binding site on mammalian capping enzyme is distinct from that of a yeast capping enzyme that recognizes the same CTD primary structure. Thus, capping enzymes from different taxa have evolved different strategies to read the CTD code.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2016

Investigating the Surface Structure of γ-Al 2 O 3 Supported WO X Catalysts by High Field 27 Al MAS NMR and Electronic Structure Calculations

The metal-support interaction in γ-Al2O3 supported WOX catalysts is investigated by a combination of high field quantitative single pulse (SP) 27Al MAS NMR spectroscopy, 2D MQMAS, 1H-27Al CP/MAS, and electronic structure calculations. NMR allows the observation of at least seven different Al sites, including a pentahedral Al site, three different tetrahedral Al sites, and three octahedral Al sites. It is found that the penta-coordinated Al (AlP) site density decreases monotonically with an increased WOX loading while the octahedral Al (AlO) site density increases concurrently. This suggests that the Alp sites are the preferred surface anchoring positions for the WOX species. Importantly, the AlP site isotropic chemical shift observed for the unsupported γ-Al2O3 at about 38 ppm migrates into the octahedral region with a new isotropic chemical shift value appearing near 7 ppm when the Alp site is anchored by WOX species. Density functional theory (DFT) computational modeling of the NMR parameters on proposed cluster models is carried out to accurately interpret the dramatic chemical shift changes from which the detailed anchoring mechanisms are obtained. It is found that tungsten dimers and monomers are the preferred supported surface species on γ-Al2O3, wherein one monomeric and several dimeric structures are identified as the most likely surface anchoring structures.

27Al MAS NMR↗
DOE OSTI2012

Structure and Electrochemistry of Two-Electron Redox Couples in Lithium Metal Fluorophosphates Based on the Tavorite Structure

An electrochemical and structural study of the two-electron redox couple comprising the tavorite-type series of fluorophosphates Li{sub 1{+-}x}VPO{sub 4}F (x = 0, 1) shows that both intercalation of LiVPO{sub 4}F with Li (to give Li{sub 2}VPO{sub 4}F) and deintercalation (to give VPO{sub 4}F) proceed by a two-phase mechanism. Structural models for each of the three phases were determined by Rietveld refinements of combined neutron and X-ray diffraction data of the isolated pure phase materials. LiVPO{sub 4}F crystallizes in the triclinic space group P1{sup -} and is isostructural to many known fluorophosphates whereas both Li{sub 2}VPO{sub 4}F and VPO{sub 4}F crystallize in the monoclinic space group C2/c, although they have very closely related structures to the parent. Solid-state {sup 6,7}Li NMR studies of Li{sub 2}VPO{sub 4}F reveal the two lithium sites are clearly distinguishable, with more than 100 ppm separation between the resonances. 2D exchange NMR is used to demonstrate the time scale of ion dynamics between the two sites.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2012

Differential Pair Distribution Function Study of the Structure of Arsenate Adsorbed on Nanocrystalline [gamma]-Alumina

Structural information is important for understanding surface adsorption mechanisms of contaminants on metal (hydr)oxides. In this work, a novel technique was employed to study the interfacial structure of arsenate oxyanions adsorbed on {gamma}-alumina nanoparticles, namely, differential pair distribution function (d-PDF) analysis of synchrotron X-ray total scattering. The d-PDF is the difference of properly normalized PDFs obtained for samples with and without arsenate adsorbed, otherwise identically prepared. The real space pattern contains information on atomic pair correlations between adsorbed arsenate and the atoms on {gamma}-alumina surface (Al, O, etc.). PDF results on the arsenate adsorption sample on {gamma}-alumina prepared at 1 mM As concentration and pH 5 revealed two peaks at 1.66 {angstrom} and 3.09 {angstrom}, corresponding to As-O and As-Al atomic pair correlations. This observation is consistent with those measured by extended X-ray absorption fine structure (EXAFS) spectroscopy, which suggests a first shell of As-O at 1.69 {+-} 0.01 {angstrom} with a coordination number of 4 and a second shell of As-Al at 3.13 {+-} 0.04 {angstrom} with a coordination number of 2. These results are in agreement with a bidentate binuclear coordination environment to the octahedral Al of {gamma}-alumina as predicted by density functional theory (DFT) calculation.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2012

Role of support-nanoalloy interactions in the atomic-scale structural and chemical ordering for tuning catalytic sites

The understanding of the atomic-scale structural and chemical ordering in supported nanosized alloy particles is fundamental for achieving active catalysts by design. This report shows how such knowledge can be obtained by a combination of techniques including x-ray photoelectron spectroscopy and synchrotron radiation based x-ray fine structure absorption spectroscopy and high-energy x-ray diffraction coupled to atomic pair distribution function analysis, and how the support-nanoalloy interaction influences the catalytic activity of a ternary nanoalloy (platinum-nickel-cobalt) particles on three different supports: carbon, silica and titania. The reaction of carbon monoxide with oxygen is employed as a probe of the catalytic activity. This ternary composition, in combination with the different support materials, is demonstrated to be capable of fine-tuning the catalytic activity and stability. The support-nanoalloy interaction is shown to influence structural and chemical ordering in the nanoparticles, leading to support-tunable active sites on the nanoalloys for oxygen activation in the catalytic oxidation of carbon monoxide. A nickel/cobalt-tuned catalytic site on the surface of nanoalloy was revealed for oxygen activation, which differs from the traditional oxygen-activation sites known for oxide-supported noble metal catalysts. The discovery of such support-nanoalloy interaction enabled oxygen-activation sites introduces a very promising strategy for designing active catalysts in heterogeneous catalysis.

activation↗
DOE OSTI2012

Structure-Based Design of Potent and Ligand-Efficient Inhibitors of CTX-M Class A [beta]-Lactamase

The emergence of CTX-M class A extended-spectrum {beta}-lactamases poses a serious health threat to the public. We have applied structure-based design to improve the potency of a novel noncovalent tetrazole-containing CTX-M inhibitor (K{sub i} = 21 {mu}M) more than 200-fold via structural modifications targeting two binding hot spots, a hydrophobic shelf formed by Pro167 and a polar site anchored by Asp240. Functional groups contacting each binding hot spot independently in initial designs were later combined to produce analogues with submicromolar potencies, including 6-trifluoromethyl-3H-benzoimidazole-4-carboxylic acid [3-(1H-tetrazol-5-yl)-phenyl]-amide, which had a K{sub i} value of 89 nM and reduced the MIC of cefotaxime by 64-fold in CTX-M-9 expressing Escherichia coli. The in vitro potency gains were accompanied by improvements in ligand efficiency (from 0.30 to 0.39) and LipE (from 1.37 to 3.86). These new analogues represent the first nM-affinity noncovalent inhibitors of a class A {beta}-lactamase. Their complex crystal structures provide valuable information about ligand binding for future inhibitor design.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2012

Structure and functional interaction of the extracellular domain of human GABA[subscript B] receptor GBR2

Inhibitory neurotransmission is mediated primarily by GABA. The metabotropic GABA{sub B} receptor is a G protein-coupled receptor central to mammalian brain function. Malfunction of GABA{sub B} receptor has been implicated in several neurological disorders. GABA{sub B} receptor functions as a heterodimeric assembly of GBR1 and GBR2 subunits, where GBR1 is responsible for ligand-binding and GBR2 is responsible for G protein coupling. Here we demonstrate that the GBR2 ectodomain directly interacts with the GBR1 ectodomain to increase agonist affinity by selectively stabilizing the agonist-bound conformation of GBR1. We present the crystal structure of the GBR2 ectodomain, which reveals a polar heterodimeric interface. We also identify specific heterodimer contacts from both subunits, and GBR1 residues involved in ligand recognition. Lastly, our structural and functional data indicate that the GBR2 ectodomain adopts a constitutively open conformation, suggesting a structural asymmetry in the active state of GABA{sub B} receptor that is unique to the GABAergic system.

59 BASIC BIOLOGICAL SCIENCES↗