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251.
Liquid chromatography (LC) was coupled on-line to a homogeneous continuous-flow protease assay using fluorescence resonance energy transfer (FRET) as a readout for the screening of inhibitors of an enzyme (e.g., Subtilisin Carlsberg). The inhibitors aprotinin (a protein of approximately 6500 g/mol) and 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF, 240 g/mol) were mixed with other, non-active compounds and separated on a size-exclusion chromatography column. After the separation, the analytes were eluted to the postcolumn reactor unit where the enzyme solution and subsequently the FRET peptide substrate were added; by measuring the fluorescence intensity the degree of inhibition was monitored on-line. As expected, only the two inhibitors caused a change in the FRET response. Detection limits for aprotinin were 5.8 microM in the flow injection analysis (FIA) mode and 12 microM in the on-line LC mode. System validation was performed by determining IC50 values for aprotinin for the FIA mode (19 microM) and the on-line mode (22 microM). These IC50 values were in line with the value determined in batch experiments (25 microM). With this system, chemical information (i.e., chromatographic retention time) and biological information (i.e., enzyme inhibition) can be combined to characterize mixtures. 相似文献
252.
We have developed a model calculation for the electrical field gradient correlation function on a probe atom in the liquid, cEFG(t)=20(0)V20(t)>. In this model, symmetry of the liquid is introduced explicitly and the distribution function for therelative coordinate ri(t) between the probe atom and particle i is calculated using Smoluchowski's diffusion equation with a mean force potential Φ(r)=kBT In g(r). The results for cEFG(t) can be characterized by two correlation times, , the shorter one being responsible for the small values of RQ in pure liquid metals, the longer one producing the increase of RQ in alloys. Also good agreement is found with recent results for cefg(t) from molecular dynamics studies. 相似文献
253.
Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt3H?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR4X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)4Br to Pd(ac)2 in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)4]2PdCl2Br2 with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×103 catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;103 catalytic cycles. 相似文献
254.
Reduction of NO by Propene Over Pt, Pd and Rh-Based ZSM-5 Under Lean-Burn Conditions 总被引:1,自引:0,他引:1
Javier Pérez-Ramírez Jorge M. García-Cortés María José Illán-Gómez Freek Kapteijn Jacob A. Moulijn Concepción Salinas-Martínez de Lecea 《Reaction Kinetics and Catalysis Letters》2000,69(2):385-392
Selective catalytic reduction of NO by propene has been investigated over noble metal (Pt, Pd, Rh)-based ZSM-5 catalysts. These samples were tested in a gas mixture system in the presence of excess oxygen, simulating lean-burn exhaust gases. The sequence in activity for NO reduction was Pt > Rh Pd. Regarding the selectivity of the reaction to N2, an opposite trend was observed: Rh > Pd Pt. The catalytic systems have presented stable operation under isothermal conditions during time-on-stream experiments. 相似文献
255.
de Rijke E de Kanter F Ariese F Brinkman UA Gooijer C 《Journal of separation science》2004,27(13):1061-1070
Previous studies revealed that the main isoflavones in extracts of leaves of T. pratense L. are biochanin A and formononetin, their 7-O-glucosides, and two glucoside malonate isomers of each of them. Since LC-MS(/MS) did not provide sufficient information to distinguish the glucoside malonate isomers, in the present paper LC-NMR as well as off-line two-dimensional NMR were used to obtain further structural information. Matrix solid-phase dispersion (MSPD) was applied to obtain sufficiently high analyte concentrations to perform LC-NMR. Stop-flow reversed-phase LC-NMR was performed using a gradient of deuterated water and deuterated acetonitrile. Offline COSY and NOESY experiments were carried out to determine the positions of the glucose moiety on the flavonoid aglycone, and of the malonate moiety on the glucose. Based on the fragmentation patterns in MS/MS and the NMR spectra, the two formononetin glucoside malonate isomers were identified as 7-O-beta-D-glucoside 6"-O-malonate and 7-O-beta-D-glucoside 4"-O-malonate; i.e. they only differ in the substitution position of the malonate group on the glucoside ring. The biochanin A glucoside malonate isomers, however, have quite different structures. The main and later eluting isomer is biochanin A 7-O-beta-D-glucoside 6"-O-malonate, and the minor and earlier eluting isomer is 5-hydroxy-7-methoxyisoflavone 4'-O-beta-D-glucoside 4"-O-malonate: the positions of the methoxy group and the glucoside 6"-O-malonate group on the flavonoid skeleton are interchanged. 相似文献
256.
Carbon insertion or extrusion mechanisms transforming one fullerene to another are presented as patch replacements on the fullerene surface. A systematic catalogue is constructed for the topologically distinct local insertion/extrusion transformations of fullerenes, classified by patch boundary and pentagon content. All pairs of patches with the same boundary but different numbers of atoms, i.e., growth patches, containing up to five pentagons, with an upper limit for the boundary length that depends on the number of pentagons, are listed. New transformations and infinite series of transformations are identified. 相似文献
257.
258.
Alber T Bächler J Bartke J Bialkowska H Bloomer MA Bock R Braithwaite WJ Brinkmann D Brockmann R Buncic P Chan P Cramer PB Cramer JG Derado I Eckardt V Eschke J Favuzzi C Ferenc D Fleischmann B Foka P Freund P Fuchs M Gazdzicki M Gladysz E Harris JW Hoffman M Jacobs P Kabana S Kadija K Kosiec J Kowalski M Kühmichel A Lahanas M Lee JY Ljubicic A Margetis S Mitchell JT Morse RJ Nappi E Odyniec G Paic G Panagiotou AD Petridis A Piper A Posa F Poskanzer AM Pühlhofer F Rauch W Renfordt R Retyk W 《Physical review letters》1995,74(8):1303-1306
259.
T. Alber H. Appelsh?user J. B?chler J. Bartke H. Bia?kowska M. A. Bloomer R. Bock W. J. Braithwaite D. Brinkmann R. Brockmann et al. 《Zeitschrift fur Physik C Particles and Fields》1994,64(2):195-207
Multiplicities and spectra of strange particles (
andK
– produced in central32S+S,32S+Ag and32S+Au collisions at 200 GeV per nucleon are presented and compared with data on strange particle production in protonnucleus and nucleon-nucleon interactions. It is shown that strangeness production in32S+Ag collisions is enhanced by a factor of two, similar to that found previously in central32S+S collisions. 相似文献
260.
de Klerk JS Szemik-Hojniak A Ariese F Gooijer C 《The journal of physical chemistry. A》2007,111(26):5828-5832
This article describes the exceptional photophysics of 2-butylamino-6-methyl-4-nitropyridine N-oxide (2B6M). It is known from the literature that this compound may undergo excited-state intra- or intermolecular proton-transfer reactions. In nonpolar solvents, 2B6M exhibits an unusual fluorescence behavior: there is a substantial difference between the relative band intensities of the excitation and absorption spectra. Furthermore, in emission two bands are observed, and their relative intensities depend on the excitation wavelength, thus violating the Kasha-Vavilov rule. It is the objective of this research to interpret these results. For this purpose, steady-state fluorescence excitation and emission spectra in the liquid state were recorded and quantum yields were determined for the two types of emission. In addition, absorption spectra were measured at room temperature and under low-temperature conditions. Finally, fluorescence lifetimes of the emitting species were determined using the time-correlated single photon counting technique. The results suggest that in the liquid state only one (monomeric) ground state species dominates, which can emit via two different pathways (from the normal and the tautomeric excited state). The excitation spectra point at two different internal proton-transfer processes, one starting at the S1 state and one starting at the S2 state. On the basis of the measured lifetimes and fluorescence quantum yields, a kinetic scheme was completed that can quantitatively explain the observations. 相似文献