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排序方式: 共有643条查询结果,搜索用时 15 毫秒
561.
Kathrin Heidbrink 《Nachrichten aus der Chemie》1999,47(8):1049-1051
562.
We present a Jacobi–Davidson like correction formula for left and right eigenvector approximations for non-Hermitian nonlinear eigenvalue problems. It exploits techniques from singularity theory for characterizing singular points of nonlinear equations. Unlike standard nonlinear Jacobi-Davidson, the correction formula does not contain derivative information and works with orthogonal projectors only. Moreover, the basic method is modified in that the new eigenvalue approximation is taken as a nonlinear Rayleigh functional obtained as root of a certain scalar nonlinear equation the existence of which – as well as a first order perturbation expansion – is shown. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
563.
The threshold energy Et = 10.06 eV (0.002 eV standard deviation) is determined for photoelectron emission by liquid water and is correlated with Et = 8.45 eV for OH? (aq). Free energy changes and standard reduction potentials are calculated for both emission processes. Reorganization free energies are correlated to solvation free energies for H2O+(aq) and OH?(aq). 相似文献
564.
Kathrin Schwarzmeier Maria Knauer Natalia P. Ivleva Reinhard Niessner Christoph Haisch 《Analytical and bioanalytical chemistry》2013,405(16):5387-5392
Bacterial contamination of indoor air is a serious threat to human health. Pathogenic germs can be transferred from the liquid to the aerosol phase, for instance, when water is sprayed in the air, such as in shower rooms, air conditioners, or fountains. Existing analytical methods for biological indoor air-quality assessment and contamination monitoring are mostly time consuming as they generally require a cultivation step. The need for a rapid, sensitive, and selective detection method for bioaerosols is evident. Our approach is based on the combination of a commercial wet particle sampler (Coriolis μ, Bertin Technologies, France) and a label-free microarray readout based on surface-enhanced Raman scattering (SERS) for detection, which was established in our laboratories. Heat-inactivated Escherichia coli bacteria were used as test microorganisms. An E. coli suspension was sprayed into the chamber by a jet air nebulizer. The resulting bioaerosol was dried, neutralized, and then collected by a Coriolis μ sampler. The bacteria collected were detected by a recently developed microarray readout system, based on label-free SERS detection. A special data evaluation procedure was applied in order to fully exploit the selectivity of the detection scheme, resulting in a detection limit of 144 particles per cubic centimeter. 相似文献
565.
Höbartner C Rieder R Kreutz C Puffer B Lang K Polonskaia A Serganov A Micura R 《Journal of the American Chemical Society》2005,127(34):12035-12045
The derivatization of nucleic acids with selenium is a new and highly promising approach to facilitate their three-dimensional structure determination by X-ray crystallography. Here, we report a comprehensive study on the chemical and enzymatic syntheses of RNAs containing 2'-methylseleno (2'-Se-methyl) nucleoside labels. Our approach includes the first synthesis of an appropriate purine nucleoside phosphoramidite building block. Most importantly, a substantially changed RNA solid-phase synthesis cycle, comprising treatment with threo-1,4-dimercapto-2,3-butanediol (DTT) after the oxidation step, is required for a reliable strand elongation. This novel operation allows for the chemical syntheses of multiple Se-labeled RNAs in sizes that can typically be achieved only for nonmodified RNAs. In combination with enzymatic ligation, biologically important RNA targets become accessible for crystallography. Exemplarily, this has been demonstrated for the Diels-Alder ribozyme and the add adenine riboswitch sequences. We point out that the approach documented here has been the chemical basis for the very recent structure determination of the Diels-Alder ribozyme which represents the first novel RNA fold that has been solved via its Se-derivatives. 相似文献
566.
Bicreosol, labelled in different ring positions (2.2′ or 3.3′ or 6.6′) with14C, was oxygenated in 0.25m-NaOH at 70°. A number of low molecular fragments was isolated and their specific activities were determined. It was possible to show, that most of the isolated acetone, formed by ring splitting, originates from ring-C-atoms 4 and 5 and the methyl group; the largest part of the isolated oxalic acid (formed in relatively small amounts) derives from ring-C-atoms 2 and 3, which are mainly split off as CO2. 相似文献
567.
The catalytic mechanism of limonene epoxide hydrolase (LEH) was investigated theoretically using the density functional theory method B3LYP. LEH is part of a novel limonene degradation pathway found in Rhodococcus erythropolis DCL14, where it catalyzes the hydrolysis of limonene-1,2-epoxide to give limonene-1,2-diol. The recent crystal structure of LEH was used to build a model of the LEH active site composed of five amino acids and a crystallographically observed water molecule. With this model, hydrolysis of different substrates was investigated. It is concluded that LEH employs a concerted general acid/general base-catalyzed reaction mechanism involving protonation of the substrate by Asp101, nucleophilic attack by water on the epoxide, and abstraction of a proton from water by Asp132. Furthermore, we provide an explanation for the experimentally observed regioselective hydrolysis of the four stereoisomers of limonene-1,2-epoxide. 相似文献
568.
Cerda BA Horn DM Breuker K McLafferty FW 《Journal of the American Chemical Society》2002,124(31):9287-9291
Although a poly(ethylene/propylene glycol) (PEG/PPG) copolymer mixture is far too complex (approximately 150 oligomeric formulas) for conventional purification, oligomer ion compositions of <1% abundance can be separated by Fourier transform mass spectrometry and dissociated into sequence-specific fragment ions. Using collisionally activated dissociation (CAD) or other conventional energetic methods, we found that misleading rearrangements are common; however, these are negligible with electron capture dissociation (ECD), consistent with its nonergodic mechanism. Despite the lack of reference compounds, ECD of five oligomers ranging from PEG(1)PPG(18) to PEG(9)PPG(15) shows that approximately 80% of their isomers have all PEG units at one end, while CAD gave lower values because of an approximately 21% rearrangement loss of internal monomer units. In contrast to the indicated triblock "PEG/PPG/PEG" sample designation of this commercial surfactant, all of these oligomers are found to consist primarily of diblock PEG/PPG structures, so that their termini differ significantly in hydrophobicity, as expected for a surfactant. 相似文献
569.
Threshold energies (6.1 <Et ? 8.6 eV) are determined for photoelectron emission by 16 inorganic cations in aqueous solution Et values are correlated with gas-phase ionization potentials, solvation and reorganization free energies, standard reduction potentials and ligand field stabilization energies (five transition metals). Dielectric saturation is shown to drastically lower threshold energies. 相似文献
570.
Ottinger F Kroslakova I Hametner K Reusser E Nesper R Günther D 《Analytical and bioanalytical chemistry》2005,383(3):489-499
Single crystals of new nitridosilicates and nitridoaluminosilicates with excellent R values in X-ray investigations were analysed quantitatively using 30 to 60 μm single-spot LA-ICP-MS. Significant discrepancies
between expected and measured chemical composition could not be explained by the crystallographic data. High spatial resolution
analysis using electron probe microanalysis (EPMA, 10 μm) leads to the discovery of inhomogeneities in the crystalline material.
The application of standard single-spot LA-ICP-MS with a spatial resolution of 30 to 60 μm is not suitable for the analysis
of these crystals as the existing inhomogeneities dominate and alter the determined concentrations. However, owing to the
better detection capabilities, a scanning LA-ICP-MS procedure enables a more representative analysis of single crystals of
Ca5Si2Al2N8 than single-spot LA-ICP-MS as a result of a larger sampling volume. It is highly likely that these impurities consist of
amorphous, vitreous phases as powder diffraction X-ray data indicates the existence of a significant fraction of an X-ray
amorphous material besides crystalline silicates. These microdomains contain less aluminium, silicon and calcium or are nearly
free of aluminium, which explains the detected discrepancies in the chemical composition. 相似文献