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Cyclodextrin glucanotransferase, produced by Bacillus megaterium, was characterized, and the biochemical properties of the purified enzyme were determined. The substrate specificity of the
enzyme was tested with different α-1,4-glucans. Cyclodextrin glucanotransferase displayed maximum activity in the case of
soluble starch, with a K
m value of 3.4 g/L. The optimal pH and temperature values for the cyclization reaction were 7.2 and 60 °C, respectively. The
enzyme was stable at pH 6.0–10.5 and 30 °C. The enzyme activity was activated by Sr2+, Mg2+, Co2+, Mn2+, and Cu2+, and it was inhibited by Zn2+and Ag+. The molecular mass of cyclodextrin glucanotransferase was established to be 73,400 Da by sodium dodecyl sulfate–polyacrylamide
gel electrophoresis, 68,200 Da by gel chromatography, and 75,000 Da by mass spectrometry. The monomer form of the enzyme was
confirmed by the analysis of the N-terminal amino acid sequence. Cyclodextrin glucanotransferase formed all three types of
cyclodextrins, but the predominant product was β-cyclodextrin. 相似文献
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Hadjieva B Ilieva S Cheshmedzhieva D Galabov B 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(7):1321-1326
Experimental IR spectroscopic data for the N-H stretching mode frequencies for several types of tri-substituted ureas containing benzyl and/or phenyl substituents as well as theoretical results from B3LYP/6-31G(d,p) computations on selected compounds provide sufficient evidence to determine the conformational state of these molecules. Two types of N-H bands may be found the spectra: (a) A type band due to a classical trans conformation (trans I) of the CONH structure; (b) B type band arising from an alternative trans form (trans II), in which the N-H band is involved in a hydrogen bond like interaction with the aromatic ring at the neighbouring nitrogen atom (benzyl or phenyl substituents). The N-H band of trans ICONH structure is observed at frequencies higher than 3460 cm-1, the actual position depending on weather the non-substituted N-H group is linked to aryl or alkyl substituents. The N-H band of the trans II rotameric structure is observed at 3430-3420 cm-1. 相似文献
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Daniela Nedeltcheva Vanya Kurteva Boriana Damyanova Simeon Popov 《Rapid communications in mass spectrometry : RCM》2009,23(11):1724-1734
Gas‐phase tautomerism in 1‐phenylazonaphthalene‐4‐ol (1) was studied by using electron ionization (EI) mass spectrometry on the basis of the fragmentations of the model enol and keto tautomers, where the movable proton is replaced by a methyl group. These fixed tautomers were obtained as an easy separable mixture by simple methylation of the cheap and easily accessible diazene (1). It was found that their EI mass spectral fragmentations are in full congruence with the already published theoretical predictions. The relative energies required for bond cleavage in 1 and its fixed tautomers were estimated by stepwise increasing of the electron energy of the ion source of the mass spectrometer. A simple equation for the approximate estimation of the molar fractions of the individual tautomers was suggested. It was shown that the enol form is dominant in the temperature range of 200–300°C. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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N-K electron energy-loss near-edge structures for TiN/VN layers: an ab initio and experimental study
Lazar P Redinger J Strobl J Podloucky R Rashkova B Dehm G Kothleitner G Sturm S Kutschej K Mitterer C Scheu C 《Analytical and bioanalytical chemistry》2008,390(6):1447-1453
We study N-K-edge electron energy-loss near-edge structures for well-defined TiN/VN bilayers grown on a MgO(100) substrate
by both calculations and experiments. The structural relaxations and the electronic structure of TiN/VN multilayers are calculated
using the Vienna Ab Initio Simulation Package computer code, which uses density functional theory to describe the electronic
interaction. The effects of the core hole created in the excitation process are included in the calculations. For VN, off-stoichiometric
effects due to nitrogen vacancies are modelled. The partial density of states (PDOS) for the N-K edge of atoms in the vicinity
of the TiN/MgO interface revealed that two new peaks appear between 7 and 9 eV instead of a broad shoulder typical for the
bulk. For the VN/TiN interface, the PDOS is modified only slightly, owing to similar bonding on both sides of the interface,
and is thus very similar to the respective bulk spectra. An experimental spectrum taken at the VN/TiN interface is, however,
well described by an average of the simulated spectra for VN and TiN bulk (interface). Such a finding is characteristic of
an intermixed interface.
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New insights into structural alteration of enamel apatite induced by citric acid and sodium fluoride solutions 总被引:1,自引:0,他引:1
Wang X Klocke A Mihailova B Tosheva L Bismayer U 《The journal of physical chemistry. B》2008,112(29):8840-8848
Attenuated total reflectance infrared spectroscopy and complementary scanning electron microscopy were applied to analyze the surface structure of enamel apatite exposed to citric acid and to investigate the protective potential of fluorine-containing reagents against citric acid-induced erosion. Enamel and, for comparison, geological hydroxylapatite samples were treated with aqueous solutions of citric acid and sodium fluoride of different concentrations, ranging from 0.01 to 0.5 mol/L for citric acid solutions and from 0.5 to 2.0% for fluoride solutions. The two solutions were applied either simultaneously or consecutively. The citric acid-induced structural modification of apatite increases with the increase in the citric acid concentration and the number of treatments. The application of sodium fluoride alone does not suppress the atomic level changes in apatite exposed to acidic agents. The addition of sodium fluoride to citric acid solutions leads to formation of surface CaF2 and considerably reduces the changes in the apatite P-O-Ca framework. However, the CaF2 globules deposited on the enamel surface seem to be insufficient to prevent the alteration of the apatite structure upon further exposure to acidic agents. No evidence for fluorine-induced recovery of the apatite structure was found. 相似文献
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Dr. Gergana Koleva Prof. Boris Galabov Prof. Boriana Hadjieva Prof.Dr. Henry F. Schaefer III Prof.Dr. Paul von R. Schleyer 《Angewandte Chemie (International ed. in English)》2015,54(47):14123-14127
Experimental evidence is reported for the first intermediate in the classic SEAr reaction of benzene nitration with mixed acid. The UV/Vis spectroscopic investigation of the reaction showed an intense absorption at 320 nm (appearing as a band shoulder) arising from a reaction intermediate. Our theoretical modeling shows that the interaction between the two principal reactants with solvent (H2SO4) molecules significantly affects the structure of the initial complex. In this complex, a larger distance between the aromatic ring and nitronium ion precludes the possibility for electronic charge transfer from the benzene π‐system to the electrophile. The computational modeling of the potential energy surface reveals that the reaction favors a stepwise mechanism with intermediate formation of π‐ and σ‐ (arenium ion) complexes. 相似文献