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991.
Hu D Croutxé-Barghorn C Feuillade M Carré C 《The journal of physical chemistry. B》2005,109(32):15214-15220
Hybrid sol-gel materials have been prepared by hydrolytic polycondensation of an alkoxysilane. The sol-gel process of methacryloxypropyltrimethoxysilane (MAPTMS) has been followed by fluorescence spectroscopy with 2-naphthol as a probe. The experimental results showed that this photoprobe was dramatically sensitive to the microenvironment polarity. Spectroscopic studies revealed fluctuations of the maximum emission intensity and wavelength as a function of time. These fluctuations were attributed to the amphiphilic behavior of the hydrolyzed precursor. The maximum emission wavelength of the probe corresponding to its protonated form was higher than in pure water. All the results suggest that the presence of water molecules, tightly bonded to the polar head of the silanols, increased locally the sol polarity and induced a red-shifted emission. Fluorescence spectroscopy emphasized the reversibility of monomeric silanol aggregates and the changes in hydroxy group number of the silica network during the sol maturation. The behavior of this system upon shaking confirmed this statement. 相似文献
992.
van Dongen BE Schouten S Damsté JS 《Rapid communications in mass spectrometry : RCM》2001,15(7):496-500
Monosaccharides were derivatized using methylboronic acid and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), and the delta13C values of these derivatives measured by gas chromatography/combustion/isotope-ratio-monitoring mass spectrometry to determine the original 13C-content of the monosaccharides. Comparison with the measured off-line delta13 values of the monosaccharides shows that no fractionation in 13C takes place during derivatization. The methylboronic derivatization method has proven to be a new method for natural abundance isotopic analysis of intact monosaccharides (arabinose, xylose, fucose, fructose and glucose). The method is rapid, does not involve isotopic fractionation during derivatization, and gives more precise delta13C values than other methods reported. The method was successfully applied to determine the delta13C value of glucose of the freshwater alga Scenedesmus communis. 相似文献
993.
Avian eggshell matrix proteins were studied by two analytical approaches. Peptide mapping was done by trypsin and pepsin followed by collagenase cleavage; analyses were carried out by capillary electrophoresis and reversed-phase high-performance liquid chromatography (HPLC). Comparison of peptide maps obtained by both methods revealed a complex mixture of peptides in the insoluble layers of the eggshell; it was concluded that there are at least three different insoluble protein/peptide layers in the avian eggshell (cuticle, palisade, and mammillary layer). Partial characterization of peptides in each layer was made by HPLC-mass spectrometry analysis. There is an evidence that the eggshell insoluble proteins contain species susceptible to collagenase cleavage, however, the sequences split by this enzyme probably are not those typical for the main triple-helical core of collagenous proteins. It is proposed that the action of collagenase upon eggshell proteins is caused by the side effect of collagenase described previously with synthetic peptides. Some of the proteins present are probably glycosylated. Fatty acid content in the insoluble eggshell layers (after decalcification) was in the range of 2-4% (which reflected both lipid and lipoproteins bound fatty acids). Porphyrin pigments are dominant in the cuticle layer. 相似文献
994.
Molecular mechanics (MM) calculations have been performed on the title compounds. For the MM minimum energy conformation obtained by conformational analysis, molecular orbital (MO) calculations (MNDO and AM1) have also been performed. The geometries obtained have been compared with the experimental ones extracted from the Cambridge Structural Database (CSD). A qualitative structure-activity relationship has been pointed out based on the electrostatic potentials calculated at different positions on the electronic surface. 相似文献
995.
Jayaraman Chandrasekhar Eluvathingal D. Jemmis Paul von Ragué Schleyer 《Tetrahedron letters》1979,20(39):3707-3710
The 3,5-dehydrophenyl cation (I), which may represent the structure of C6H3 ions observed mass spectroscopically, illustrates : two different aromatic systems in orthogonal planes. 相似文献
996.
Barluenga J Fernández MA Aznar F Valdés C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(8):2276-2283
A novel approach for the synthesis of the important indole ring is described. Indoles are obtained from o-bromoanilines and alkenyl halides in a Pd-catalyzed cascade process that involves an alkenyl amination followed by an intramolecular Heck reaction. The overall process represents the first example of the participation of alkenyl amination reactions in Pd-catalyzed cascade reactions. Initially, the relative reactivity of aryl and alkenyl bromides and chlorides towards Pd-catalyzed amination was investigated. Competition experiments were carried out in the presence of primary and secondary amines, and these revealed the reactivity order alkenyl bromides > aryl bromides > alkenyl chlorides > aryl chlorides, as well as very high chemoselectivity; the more reactive halide was always favored. Thereafter, optimized reaction conditions for the sequential alkenyl amination/Heck cyclization to give indoles were investigated with the model reaction of o-bromoaniline with alpha-bromostyrene. An extensive screening of ligands, bases, and reaction conditions revealed that the [Pd2(dba)3]/DavePhos, NaOtBu, toluene combination at 100 degrees C were the optimized reaction conditions to carry out the cascade process (dba=dibenzylideneacetone, DavePhos=2-dicyclohexylphosphino-2'-N,N-dimethylaminobiphenyl). The reaction proceeds with aryl, alkyl, and functionalized substitutents in both starting reactants. The cyclization was also studied with N-substituted o-bromoanilines (which would give rise to N-substituted indoles); however, in this case, indole formation occurred only with 1-substituted-2-bromoalkenes. Finally, the application of this methodology to o-chloroanilines required further optimization. Although the catalyst based on DavePhos failed to promote the cascade process, a catalytic combination based on [Pd2(dba)3]/X-Phos promoted the formation of the indole ring also from the less reactive chloroanilines. 相似文献
997.
Crespo A Martí MA Estrin DA Roitberg AE 《Journal of the American Chemical Society》2005,127(19):6940-6941
A novel technique for computing free energy profiles in enzymatic reactions using the multiple steering molecular dynamics approach in the context of an efficient QM-MM density functional scheme is presented. The conversion reaction of chorismate to prephenate catalyzed by the Bacillus subtilis enzyme chorismate mutase has been chosen as an illustrative example. 相似文献
998.
Multicomponent Ni-base alloys exhibit good mechanical properties even at elevated temperatures and they are widely used for
industrial production of exertion-resistive parts of engines. These properties are mainly determined by the coexistence of
a disordered γ matrix with a face centred cubic lattice and cuboidal domains of its ordered γ′ structure. Therefore it is
useful to study phase equilibria in Ni-base systems, namely in the regions involving both mentioned phases. One of the conclusions
of our recent work on Ni–Al–Cr–W system was a necessity of modification of selected thermodynamic parameters of the ternary
Ni–Al–W subsystem in order to achieve a better agreement of our experimental observations with theoretical modelling. This
involves new measurements of the microstructure of selected samples of the Ni–Al–W system at 900°C and the comparison of the
results with existing literature data in order to confirm our conclusions on higher order system investigated before. It is
a first step on the way to an assessment of the Ni–Al–W system, which has not been done before. 相似文献
999.
G. Wytze Meindersma Lara M. Galán Sánchez Antje R. Hansmeier André B. de Haan 《Monatshefte für Chemie / Chemical Monthly》2007,138(11):1125-1136
Summary. Ionic liquids offer tremendous opportunities to intensify reactions and separations in process technologies by tuning their
physical and chemical properties. Several ionic liquids are suitable for the separation of aromatic and aliphatic hydrocarbons.
CO2 absorption behavior was influenced by the functionalized chains appended to the room temperature ionic liquid (RTIL) cation.
Ionic liquids seem able to combine the chemical features of amine solutions with the characteristic advantages of the physical
solvents used for CO2 absorption. 相似文献
1000.
Emilia Iglesias-Martínez Isabel Brandariz Francisco Penedo 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):573-576
The UV-vis absorption and the fluorescence emission spectra of novocaine were analysed in aqueous cyclodextrin (CD) solutions.
Either the absorbance read at λmax 290 nm or the fluorescence emission intensity at λems 346 nm increase in the presence of both α- and β-CD due to the formation of 1:1 inclusion complexes. The lower polarity of
the CD-cavity sensed by the included drug enhances the emitted fluorescence; in fact, the same effect was observed in aqueous
mixtures of acetonitrile, dioxane, or dimethylsulfoxide. The inclusion complex formation between the monocation of novocaine
and CDs diminishes the electrical conductance of aqueous solutions of novocaine hydrochloride (NoHCl). Both the nitrosation
reaction in aqueous acid medium and the ester hydrolysis in alkaline medium are retarded in the presence of CDs. The strongest
effect was observed with β-CD as a consequence of the higher stability inclusion complex. 相似文献