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81.
Dilatometrical indication can be employed to the neutralizing reactions of polybasic acids and is demonstrated by the following acid-base couples: H2SO4/NaOH, H3PO4/NaOH and H5JO6/NaOH. With amounts between 0.5 and 4 mMol several, frequently all, equivalence points can be determined with a deviation of less than 0.2%. The determination of the changes of molar volume of the reactions depending on the concentrations allow conclusions as to the course of the reactions. 相似文献
82.
Jelan Kuhn Kenji Yajima Toshihiro Tomita Joachim Gross Freek Kapteijn 《Journal of membrane science》2008
The all silica DDR membrane turns out to be well suited to separate water from organic solvents under pervaporation conditions, despite its hydrophobic character. All-silica zeolites are chemically and hydrothermally more stable than aluminum containing ones and are therefore preferred for membrane applications, including for dehydration, even though these type of membranes are hydrophobic. Permeation of water, ethanol and methanol through an all-silica DDR membrane has been measured at temperatures ranging from 344 to 398 K. The hydrophobic membrane shows high water fluxes (up to 20 kg m−2 h−1). The pure water permeance is insensitive to temperature and is well described assuming weak adsorption. Excellent performance in dewatering ethanol (N=2 kg m−2 h−1and αw=1500 at 373 K and xw=0.18) is observed and the membrane is also able to selectively remove water from methanol (N=5 kg m−2 h−1 and αw=9). Water could also be removed from methanol/ethanol/water (αwater/EtOH=1500, αMeOH/EtOH=70 at 373 K) mixtures, even at water feed concentrations below 1.5 mol%. 相似文献
83.
Homogeneous hydrogenation of barbituric acid derivatives with parahydrogen yields a substantial increase of the (1)H NMR signals of the reaction products. These physiologically relevant compounds were hydrogenated at both ambient and elevated temperatures and pressures using a standard cationic rhodium catalyst. The resulting nonthermal nuclear spin polarization (hyperpolarization) is limited by the spin-lattice relaxation time T(1) of the corresponding nuclei in the products, being shorter than the time constant of the hydrogenation. The signal-to-noise ratio of the NMR spectra could be further increased upon signal averaging the antiphase PHIP signals of 25 successive scans following 30 degrees pulse experiments and a delay of 10 s. 相似文献
84.
Dzubiella J 《Journal of the American Chemical Society》2008,130(42):14000-14007
The structure of a single alanine-based Ace-AEAAAKEAAAKA-Nme peptide in explicit aqueous electrolyte solutions (NaCl, KCl, NaI, and KF) at large salt concentrations (3-4 M) is investigated using approximately 1 mus molecular dynamics (MD) computer simulations. The peptide displays 71% alpha-helical structure without salt and destabilizes with the addition of NaCl in agreement with experiments of a somewhat longer version. It is mainly stabilized by direct and indirect (" i + 4")EK salt bridges between the Lys and Glu side chains and a concomitant backbone shielding mechanism. NaI is found to be a stronger denaturant than NaCl, while the potassium salts hardly show influence. Investigation of the molecular structures reveals that consistent with recent experiments Na (+) has a much stronger affinity to side chain carboxylates and backbone carbonyls than K (+), thereby weakening salt bridges and secondary structure hydrogen bonds. At the same time, the large I (-) has a considerable affinity to the nonpolar alanine in line with recent observations of a large propensity of I (-) to adsorb to simple hydrophobes, and thereby "assists" Na (+) in its destabilizing action. In the denatured states of the peptide, novel long-lived (10-20 ns) "loop" configurations are observed in which single Na (+) ions and water molecules are hydrogen-bonded to multiple backbone carbonyls. In an attempt to analyze the denaturation behavior within the preferential interaction formalism, we find indeed that for the strongest denaturant, NaI, the protein is least hydrated. Additionally, a possible indication for protein denaturation might be a preferential solvation of the peptide backbone by the destabilizing cosolute (sodium). The mechanisms found in this work may be of general importance to understand salt effects on protein secondary structure stability. 相似文献
85.
Silvia Richter Stepan Kyrsta Jochen Schneider David Hajas Joachim Mayer 《Mikrochimica acta》2006,155(1-2):257-262
Fibre reinforced NiAl offers new possibilities for the development of high strength structural materials of low density applicable
in gas turbines at high operating temperatures. The properties of composite materials are strongly influenced by the strength
of the fibre–matrix interface. In addition, if fibre and matrix differ in their thermal expansion coefficients, a well controlled
interface reaction at high temperature changes is demanded. Therefore, two layers consisting of BN and Hf were embedded between
a sapphire fibre and NiAl and heated at 1350 °C to find a compromise between adhesion and ductility. The control and characterization
of the reaction zone is essential for the development of these new materials. Especially, the characterization of the fibre-coating
interface is a challenge. The different hardness of fibre and coating makes it nearly impossible to use a conventional cross-section
preparation. Further, the small dimension of the reaction zone requires the use of analytical techniques providing high lateral
resolution. In order to accomplish these requirements, a newly developed technique FIB (Focused Ion Beam)-EPMA (Electron Probe
Microanalysis) was combined with XRD (X-ray diffraction). XRD was performed for the identification of the phases. The reaction
zone was exposed by a special FIB preparation technique and examined by surface-sensitive EPMA. This allowed to determine
the spatial distribution of the different phases. 相似文献
86.
Hochrein MB Reich C Krause B Rädler JO Nickel B 《Langmuir : the ACS journal of surfaces and colloids》2006,22(2):538-545
Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer. 相似文献
87.
A series of synthetic [2Fe-2S] complexes with terminal thiophenolate ligands and tethered ether or thioether moieties has been prepared and investigated in order to provide models for the potential interaction of additional donor atoms with the Fe atoms in biological [2Fe-2S] clusters. X-ray crystal structures have been determined for six new complexes that feature appended Et (1(C)), OMe (1(O)), or SMe (1(S)) groups, or with a methylene group (2(C) ), an ether-O (2(O)), or an thioether-S (2(S)) linking two aryl groups. The latter two systems provide a constrained chelate arrangement that induces secondary bonding interactions with the ether-O and thioether-S, which is confirmed by density functional theory (DFT) calculations that also reveal significant spin density on those fifth donor atoms. Structural consequences of the secondary bonding interactions are analyzed in detail, and effects on the spectroscopic and electronic properties are probed by UV-vis, M?ssbauer, and (1)H NMR spectroscopy, as well by SQUID measurements and cyclic voltammetry. The potential relevance of the findings for biological [2Fe-2S] sites is considered. 相似文献
88.
Liuqing Yang Marco Hill Dr. Meitian Wang Dr. Santosh Panjikar Dr. Joachim Stöckigt Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(28):5211-5213
Cutting carbons : The three‐dimensional structure of polyneuridine aldehyde esterase (PNAE) gives insight into the enzymatic mechanism of the biosynthesis of C9‐ from C10‐monoterpenoid indole alkaloids (see scheme). PNAE is a very substrate‐specific serine esterase. It harbors the catalytic triad S87‐D216‐H244, and is a new member of the α/β‐fold hydrolase superfamily. Its novel function leads to the diversification of alkaloid structures.
89.
Daniel Göbke Yuriy Romanyshyn Sébastien Guimond Jacobus Marinus Sturm Dr. Helmut Kuhlenbeck Dr. Jens Döbler Dr. Ulrike Reinhardt Maria Veronica Ganduglia‐Pirovano Dr. Joachim Sauer Prof. Dr. Hans‐Joachim Freund Prof. Dr. 《Angewandte Chemie (International ed. in English)》2009,48(20):3695-3698
Hydroxy‐mediated methoxy formation or stabilization is probably an important process in many methanol adsorption systems. Hydrogen atoms originating from the scission of the methanol O? H bond react with the substrate and form water. This process may result 1) in the production of additional surface defects as reactive centers for methoxy formation and 2) in the stabilization of methoxy groups by suppression of methanol formation.
90.
Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations 下载免费PDF全文
Dr. Joachim Reimer Prof. Dr. Frédéric Vogel Prof. Dr. Matthew Steele‐MacInnis 《Chemphyschem》2016,17(10):1446-1453
Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular‐scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na?SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K?SO4 show significantly less clustering under equivalent conditions. In mixed systems (NaxK2?xSO4), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure‐breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K+ and SO42?, compared with the analogous Na+?SO42? interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na?K?SO4. Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium–sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting‐in in ternary hydrothermal solutions of Na?K?SO4 reflects the opposing, but complementary, natures of Na?SO4 versus K?SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system. 相似文献