Kooperativer Bibliotheksverbund

Berlin Brandenburg

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  • Journal Of The American Chemical Society
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  • 1
    Language: English
    In: Journal of the American Chemical Society, 02 March 2011, Vol.133(8), pp.2626-31
    Description: The proximities of specific subgroups of nearest-neighbor chains in glassy polymers are revealed by distance-dependent (13)C-(13)C dipolar couplings and spin diffusion. The measurement of such proximities is practical even with natural-abundance levels of (13)C using a 2D version of centerband-only detection of exchange (CODEX). Two-dimensional CODEX is a relaxation-compensated experiment that avoids the problems associated with variations in T(1)(C)'s due to dynamic site heterogeneity in the glass. Isotropic chemical shifts are encoded in the t(1) preparation times before and after mixing, and variations in T(2)'s are compensated by an S(0) reference (no mixing). Data acquisition involves acquisition of an S(0) reference signal on alternate scans, and the active control of power amplifiers, to achieve stability and accuracy over long accumulation times. The model system to calibrate spin diffusion is the polymer itself. For a mixing time of 200 ms, only (13)C-(13)C pairs separated by one or two bonds (2.5 Å) show cross peaks, which therefore identify reference intrachain proximities. For a mixing time of 1200 ms, 5 Å interchain proximities appear. The resulting cross peaks are used in a simple and direct way to compare nonrandom chain packing for two commercial polycarbonates with decidedly different mechanical properties.
    Keywords: Polycarboxylate Cement -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: Journal of the American Chemical Society, 14 December 2011, Vol.133(49), pp.19638-41
    Description: Noble-metal-free nickel-iron alloy nanoparticles exhibit excellent catalytic performance for the complete decomposition of hydrous hydrazine, for which the NiFe nanocatalyst, with equimolar compositions of Ni and Fe, shows 100% hydrogen selectivity in basic solution (0.5 M NaOH) at 343 K. The development of low-cost and high-performance catalysts may encourage the effective application of hydrous hydrazine as a promising hydrogen storage material.
    Keywords: Decomposition (Chemistry) -- Analysis ; Hydrazines (Class of compounds) -- Chemical Properties ; Metal Catalysts -- Chemical Properties ; Metal Catalysts -- Usage ; Nickel Alloys -- Chemical Properties;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 3
    Language: English
    In: Journal of the American Chemical Society, August 17, 2011, Vol.133(32), p.12458-12461
    Description: Biomimetic proton transfer catalysis with a new chiral organic catalyst is performed to realize an unprecedented enantioselective and general olefin isomerization. The proposed method is able to transform a broad range of mono- and disubstituted [beta],[gamma]-unsaturated butenolides into the corresponding chiral [alpha],[beta]-unsaturated butenolides in high enantioselectivity and yield in the presence of as low as 0.5 mol % catalyst.
    Keywords: Alkenes -- Chemical Properties ; Alkenes -- Structure ; Catalysis -- Analysis ; Heterocyclic Compounds -- Chemical Properties ; Isomerization -- Analysis
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Journal of the American Chemical Society, July 10, 2013, Vol.135(27), p.10149-10154
    Description: The monoclinic [Sr.sub.1-x][Na.sub.x][SiO.sub.3-0.5x] oxide ion are superior electrolytes. The use of low-cost [Na.sup.+] rather than [K.sup.+] as the dopant in monoclinic SrSi[O.sub.3] (C12/C1) provides a larger solid solution range and good oxide ion conductivity.
    Keywords: Chemical Kinetics -- Analysis ; Fuel Cells -- Innovations ; Fuel Cells -- Research ; Strontium Compounds -- Chemical Properties ; Strontium Compounds -- Thermal Properties ; Surface Energy -- Measurement
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Journal of the American Chemical Society, 27 May 2015, Vol.137(20), pp.6464-7
    Description: The presence of antimony, as a dopant in the colloidal growth reaction for CuIn(1-x)Ga(x)S2 (CIGS) nanocrystals, causes end-to-end fusion of nanorod pairs into nanodumbbells at high yield. The influence of the dopant on shape is indirect; antimony catalyzes the incorporation of gallium, which is found in high concentration at the junction between the fused nanorods.
    Keywords: Copper Compounds – Chemical Properties ; Catalysis – Research ; Gallium – Chemical Properties;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 6
    Language: English
    In: Journal of the American Chemical Society, 29 May 2013, Vol.135(21), pp.7835-8
    Description: The formation of Cu2SnSe3 tetrapod nanocrystals is reported using a hot injection colloidal synthesis. The ternary copper chalcogenide nanocrystals nucleate with a cubic core with four short wurzite arms.
    Keywords: Copper Compounds -- Chemical Properties ; Copper Compounds -- Thermal Properties ; Crystal Structure -- Analysis ; Nucleation (Physics) -- Analysis ; Surface Energy -- Measurement;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 7
    Language: English
    In: Journal of the American Chemical Society, 28 March 2012, Vol.134(12), pp.5610-7
    Description: Adenosine triphosphate (ATP)-capped silver nanoparticles (ATP-Ag NPs) were synthesized by reduction of AgNO(3) with borohydride in water with ATP as a capping ligand. The NPs obtained were characterized using transmission electron microscopy (TEM), UV-vis absorption spectroscopy, X-ray diffraction, and energy-dispersive X-ray analysis. A typical preparation produced ATP-Ag NPs with diameters of 4.5 ± 1.1 nm containing ~2800 Ag atoms and capped with 250 ATP capping ligands. The negatively charged ATP caps allow NP incorporation into layer-by-layer (LbL) films with poly(diallyldimethylammonium) chloride at thiol-modified Au electrode surfaces. Cyclic voltammetry in a single-layer LbL film of NPs showed a chemically reversible oxidation of Ag NPs to silver halide NPs in aqueous halide solutions and to Ag(2)O NPs in aqueous hydroxide solutions. TEM confirmed that this takes place via a redox-driven solid-state phase transformation. The charge for these nontopotactic phase transformations corresponded to a one-electron redox process per Ag atom in the NP, indicating complete oxidation and reduction of all Ag atoms in each NP during the electrochemical phase transformation.
    Keywords: Phase Transition ; Adenosine Triphosphate -- Chemistry ; Metal Nanoparticles -- Chemistry ; Silver -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 8
    Language: English
    In: Journal of the American Chemical Society, 15 February 2012, Vol.134(6), pp.2910-3
    Description: The quaternary copper chalcogenide Cu(2)ZnSnS(4) is an important emerging material for the development of low-cost and sustainable solar cells. Here we report a facile solution synthesis of stoichiometric Cu(2)ZnSnS(4) in size-controlled nanorod form (11 nm × 35 nm). The monodisperse nanorods have a band gap of 1.43 eV and can be assembled into perpendicularly aligned arrays by controlled evaporation from solution.
    Keywords: Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 9
    Language: English
    In: Journal of the American Chemical Society, 01 June 2011, Vol.133(21), pp.8059-61
    Description: Standard molecular mechanics (MM) force fields predict a nearly linear decrease in hydration free energy with each successive addition of a methyl group to ammonia or acetamide, whereas a nonadditive relationship is observed experimentally. In contrast, the non-additive hydration behavior is reproduced directly using a quantum mechanics (QM)/MM-based free-energy perturbation (FEP) method wherein the solute partial atomic charges are updated at every window. Decomposing the free energies into electrostatic and van der Waals contributions and comparing the results with the corresponding free energies obtained using a conventional FEP method and a QM/MM method wherein the charges are not updated suggests that inaccuracies in the electrostatic free energies are the primary reason for the inability of the conventional FEP method to predict the experimental findings. The QM/MM-based FEP method was subsequently used to evaluate inhibitors of the diabetes drug target fructose-1,6-bisphosphatase adenosine 5'-monophosphate and 6-methylamino purine riboside 5'-monophosphate. The predicted relative binding free energy was consistent with the experimental findings, whereas the relative binding free energy predicted using the conventional FEP method differed from the experimental finding by an amount consistent with the overestimated relative solvation free energies calculated for alkylamines. Accordingly, the QM/MM-based FEP method offers potential advantages over conventional FEP methods, including greater accuracy and reduced user input. Moreover, since drug candidates often contain either functionality that is inadequately treated by MM (e.g., simple alkylamines and alkylamides) or new molecular scaffolds that require time-consuming development of MM parameters, these advantages could enable future automation of FEP calculations as well as greatly increase the use and impact of FEP calculations in drug discovery.
    Keywords: Acetamides -- Chemistry ; Diabetes Mellitus, Type 2 -- Drug Therapy ; Enzyme Inhibitors -- Therapeutic Use ; Fructose-Bisphosphatase -- Antagonists & Inhibitors ; Methylamines -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 10
    Language: English
    In: Journal of the American Chemical Society, 12 February 2014, Vol.136(6), pp.2575-82
    Description: Enzyme catalysis has been studied extensively, but the role of enzyme dynamics in the catalyzed chemical conversion is still an enigma. The enzyme dihydrofolate reductase (DHFR) is often used as a model system to assess a network of coupled motions across the protein that may affect the catalyzed chemical transformation. Molecular dynamics simulations, quantum mechanical/molecular mechanical studies, and bioinformatics studies have suggested the presence of a "global dynamic network" of residues in DHFR. Earlier studies of two DHFR distal mutants, G121V and M42W, indicated that these residues affect the chemical step synergistically. While this finding was in accordance with the concept of a network of functional motions across the protein, two residues do not constitute a network. To better define the extent and limits of the proposed network, the current work studied two remote residues predicted to be part of the same network: W133 and F125. The effect of mutations in these residues on the nature of the chemical step was examined via measurements of the temperature-dependence of the intrinsic kinetic isotope effects (KIEs) and other kinetic parameters, and double mutants were used to tie the findings to G121 and M42. The findings indicate that residue F125, which was implicated by both calculations and bioinformatic methods, is a part of the same global dynamic network as G121 and M42, while W133, implicated only by bioinformatics, is not. These findings extend our understanding of the proposed network and the relations between functional and genomic couplings. Delineating that network illuminates the need to consider remote residues and protein structural dynamics in the rational design of drugs and of biomimetic catalysts.
    Keywords: Tetrahydrofolate Dehydrogenase -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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