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排序方式: 共有388条查询结果,搜索用时 156 毫秒
101.
102.
Libor Mat
jka Oksana Dukh Ji
í Brus William J. Simonsick Jr Bohumil Meissner 《Journal of Non》2000,270(1-3):34-47
The sol–gel polymerization of 3-glycidyloxypropyltrimethoxysilane (GTMS) was followed by size exclusion chromatography and 29Si NMR. Extensive non-random cyclization under formation of polyhedral cycles – cubic cages – predominates at the beginning of the reaction. Structure growth of polysilsesquioxanes proceeds by combining the incompletely condensed cage frameworks. The extent of the cage formation increases with dilution and the amount of water and depends appreciably on a catalyst. The cage fraction was isolated from a reaction mixture using preparative size exclusion chromatography and identified by 29Si NMR spectroscopy and electrospray ionization mass spectrometry (ESI-MS). High content of polyhedral cages prevents gelation of the trifunctional GTMS monomer. Reaction of pendant epoxy groups is much slower; however, at a late reaction stage the epoxy hydrolysis can be significant. Under some conditions, like base catalysis, polysilsesquioxane clusters are crosslinked by intermolecular condensation of SiOH with hydrolyzed epoxy groups and the system gels. The cage with epoxide functionalities may serve as a rigid precursor of crosslinking. 相似文献
103.
Václav Matěj Bierhanzl Martina Riesová Lukáš Taraba Radomír Čabala Gabriela Seydlová 《Analytical and bioanalytical chemistry》2015,407(23):7215-7220
A new, fast, selective, and reliable capillary electrophoresis method has been developed for analysis of selected phosphoesters (phosphoserine, phosphoethanolamine, phosphoglycerol) and phosphate. The method is based on separation of specific phosphate containing headgroups (phosphoesters) which are cleaved from the glycerol skeleton of a phospholipid by a regioselective enzyme (phospholipase C). Analysis of intact phospholipids with the same polar headgroup but different fatty acids shows that fatty acid composition has a high impact on separation of phospholipids, so analysis of separated polar headgroups, which avoids this influence, represents a much more suitable approach for phospholipid class research. Optimization of method parameters results in running buffers of relatively narrow pH interval (pH about 10) where all phosphoesters are separated. Further method validation has shown that direct UV detection has a sufficient detection limit for all analytes to perform suitable analyses of cell membrane lipids. The optimized method was tested on the lysate of cell membrane of Bacillus subtilis, where all analytes were determined. 相似文献
104.
Najim N Bathich Y Zain MM Hamzah AS Shaameri Z 《Molecules (Basel, Switzerland)》2010,15(12):9340-9353
The aim of this study was to investigate the in vitro cellular activity of novel spiroisoxazoline type compounds against normal and cancer cell lines from lung tissue (Hs888Lu), neuron-phenotypic cells (SH-SY5Y), neuroblastoma (SH-SY5Y), human histiocytic lymphoma (U937), lung cancer (A549), and leukaemia (HL-60). Our bioassay program revealed that the spiroisoxazoline type compounds show cytotoxicity only in lymphoma cell lines, which is in contrast with the pyrrolidine precursor of these spiroisoxazoline compounds, where significant cytotoxicity is seen in all normal and cancer cell lines. These data suggest a tumour-specific mechanism of action. In addition these data also show that spiroisoxazoline compounds are non-toxic in the human neuronphenotypic neuroblastoma SH-SY5Y cell line, and furthermore that they might protect cells from neurodegenerative disease. 相似文献
105.
Gabriela Broncová Sergey Anikin Ulrich Lange Pavel Matějka Martin Krondak Vladimír Král Vladimir M. Mirsky 《Journal of Solid State Electrochemistry》2010,14(6):1035-1044
The structure and the electrochemical and spectral properties of two conductive electrochemically polymerized substituted
bipyrroles 4,4′-methoxy-2,2′-bipyrrole and 4,4′-buthoxy-2,2′-bipyrrole were studied and compared. The polymers were characterized
by cyclic voltammetry, FT-Raman spectroscopy, scanning electron microscopy, and in situ conductivity measurements at different
pH and redox state. 相似文献
106.
Miroslava Trchová Pavel Matějka Andrea Kalendová Jaroslav Stejskal 《Polymer Degradation and Stability》2006,91(1):114-121
Polyaniline base has been exposed to various temperatures between 100 °C and 1000 °C for 2 h in air. The mass loss has increased with increasing temperature. FTIR and Raman spectroscopies show the gradual destruction of the PANI structure, the possible formation of intermediate oxime and nitrile groups, and the final conversion to graphitic material. The elemental analysis confirmed the dehydrogenation while the content of nitrogen was nearly constant even after treatment at 800 °C. The conductivity of PANI base, 10−8 S cm−1, increased to ∼10−4 S cm−1 after treatment at 1000 °C; most of the products, however, were non-conducting. Another series of experiments involved the polyaniline base heated at 500 °C for 1-8 h. The studies were performed in connection with the potential flame-retardant application of polyaniline. 相似文献
107.
[reaction: see text] A new iron(III) halide-promoted aza-Prins cyclization between gamma,delta-unsaturated tosylamines and aldehydes provides six-membered azacycles in good to excellent yields. The process is based on the consecutive generation of gamma-unsaturated-iminium ion and further nucleophilic attack by the unsaturated carbon-carbon bond. Homoallyl tosylamine leads to trans-2-alkyl-4-halo-1-tosylpiperidine as the major isomer. In addition, the alkyne aza-Prins cyclization between homopropargyl tosylamine and aldehydes gives 2-alkyl-4-halo-1-tosyl-1,2,5,6-tetrahydropyridines as the only cyclic products. 相似文献
108.
Matěj Stehlík 《Graphs and Combinatorics》2006,22(3):421-426
A hypergraph is τ-critical if τ(−{E})<τ() for every edge E ∈ , where τ() denotes the transversal number of . We show that if is a connected τ-critical hypergraph, then −{E} can be partitioned into τ()−1 stars of size at least two, for every edge E ∈ . An immediate corollary is that a connected τ-critical hypergraph has at least 2τ()−1 edges. This extends, in a very natural way, a classical theorem of Gallai on colour-critical graphs, and is equivalent
to a theorem of Füredi on t-stable hypergraphs. We deduce a lower bound on the size of τ-critical hypergraphs of minimum degree at least two. 相似文献
109.
Marek Omelka Matías Salibián-Barrera 《Annals of the Institute of Statistical Mathematics》2010,62(5):897-927
In this paper we find verifiable regularity conditions to ensure that S-estimators of scale and regression and MM-estimators
of regression are uniformly consistent and uniformly asymptotically normally distributed over contamination neighbourhoods.
Moreover, we show how to calculate the size of these neighbourhoods. In particular, we find that, for MM-estimators computed
with Tukey’s family of bisquare score functions, there is a trade-off between the size of these neighbourhoods and both the
breakdown point of the S-estimators and the leverage of the contamination that is allowed in the neighbourhood. These results
extend previous work of Salibian-Barrera and Zamar for location-scale to the linear regression model. 相似文献
110.
Kazem Naghavi Elias Saion Khadijeh Rezaee Wan Mahmood Mat Yunus 《Radiation Physics and Chemistry》2010,79(12):1203-1208
Colloidal silver nanoparticles were synthesized by γ-irradiation-induced reduction method of an aqueous solution containing silver nitrate as a precursor in various concentrations between 7.40×10?4 and 1.84×10?3 M, polyvinyl pyrrolidone for capping colloidal nanoparticles, isopropanol as radical scavenger of hydroxyl radicals and deionised water as a solvent. The irradiations were carried out in a 60Co γ source chamber at doses up to 70 kGy. The optical absorption spectra were measured using UV–vis spectrophotometer and used to study the particle distribution and electronic structure of silver nanoparticles. As the radiation dose increases from 10 to 70 kGy, the absorption intensity increases with increasing dose. The absorption peak λmax blue shifted from 410 to 403 nm correspond to the increase of absorption conduction electron energy from 3.02 to 3.08 eV, indicating the particle size decreases with increasing dose. The particle size was determined by photon cross correlation spectroscopy and the results showed that the particle diameter decreases exponentially with the increase of dose. The transmission electron microscopy images were taken at doses of 20 and 60 kGy and the results confirmed that as the dose increases the diameter of colloidal silver nanoparticle decreases and the particle distribution increases. 相似文献