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201.
The field of selenium speciation has been studied for decades and the growing interest in this field seems never to reach a plateau. Although powerful techniques based on mass spectrometry are nowadays used for selenium determination/speciation, few laboratories can support the high cost of such techniques. The hyphenation of chromatography to atomic absorption or atomic fluorescence spectrometry (AAS or AFS) is still a reliable and low-cost alternative for routine laboratories. In this work we present the most important parameters dealing with selenium speciation along with the latest trends in this subject, namely in the items related with sample treatment and hyphenation techniques with AAS and AFS detection. 相似文献
202.
Ude S Fernandez de la Mora J Alexander JN Saucy DA 《Journal of colloid and interface science》2006,293(2):384-393
A technique for generating charged aerosols of polystyrene (pSty) with narrow size distributions has been developed. It is based on electrospraying commercial narrow mass standards of pSty dissolved in l-methyl-2-pyrrolidone (NMP) seeded with the newly synthesized salt dimethyl ammonium formate. This salt imparts a much larger electrical conductivity than previously known NMP electrolytes, leading to higher quality sprays with greatly reduced attachment of impurities. Controlling the solute concentration enables forming polystyrene particles containing from one up to more than ten single polystyrene molecules, whereby 4 mass standards with molecular weights from 9200 up to 96,000 g/mol yield particles covering densely the diameter range from 3 to 11 nm. Combined mobility and mass measurement with a differential mobility analyzer and a mass spectrometer in tandem are carried out with a pSty sample 9200 amu in molecular weight. They fix directly the mass versus mobility relation near 9200 amu, and indirectly for the other standards and their clusters. The apparent particle density resulting from mobility versus mass data agrees with the bulk density of the polymer, indicating that the particles are dense and spherical. Although these standards have been studied only in gaseous suspension, their injection in liquids such as water where pSty is insoluble should keep them spherical. 相似文献
203.
Dementin S Belle V Bertrand P Guigliarelli B Adryanczyk-Perrier G De Lacey AL Fernandez VM Rousset M Léger C 《Journal of the American Chemical Society》2006,128(15):5209-5218
In NiFe hydrogenases, electrons are transferred from the active site to the redox partner via a chain of three Iron-Sulfur clusters, and the surface-exposed [4Fe4S] cluster has an unusual His(Cys)3 ligation. When this Histidine (H184 in Desulfovibrio fructosovorans) is changed into a cysteine or a glycine, a distal cubane is still assembled but the oxidative activity of the mutants is only 1.5 and 3% of that of the WT, respectively. We compared the activities of the WT and engineered enzymes for H2 oxidation, H+ reduction and H/D exchange, under various conditions: (i) either with the enzyme directly adsorbed onto an electrode or using soluble redox partners, and (ii) in the presence of exogenous ligands whose binding to the exposed Fe of H184G was expected to modulate the properties of the distal cluster. Protein film voltammetry proved particularly useful to unravel the effects of the mutations on inter and intramolecular electron transfer (ET). We demonstrate that changing the coordination of the distal cluster has no effect on cluster assembly, protein stability, active-site chemistry and proton transfer; however, it slows down the first-order rates of ET to and from the cluster. All-sulfur coordination is actually detrimental to ET, and intramolecular (uphill) ET is rate determining in the glycine variant. This demonstrates that although [4Fe4S] clusters are robust chemical constructs, the direct protein ligands play an essential role in imparting their ability to transfer electrons. 相似文献
204.
de la Fuente JM Alcántara D Eaton P Crespo P Rojas TC Fernandez A Hernando A Penadés S 《The journal of physical chemistry. B》2006,110(26):13021-13028
The preparation, characterization and the magnetic properties of gold and gold-iron oxide glyconanoparticles (GNPs) are described. Glyconanoparticles were prepared in a single step procedure in the presence of aqueous solution of thiol functionalized neoglycoconjugates and either gold salts or both gold and iron salts. Neoglycoconjugates of lactose and maltose disaccharides with different linkers were used. Iron-free gold or gold-iron oxide GNPs with controlled gold-iron ratios were obtained. The average core-size diameters are in the range of 1.5-2.5 nm. The GNPs are fully characterized by (1)H NMR spectrometry, transmission electron microscopy (TEM), and UV-vis and X-ray absorption (XAS) spectroscopies. Inductive plasma-atomic emission spectrometry (ICP) and elemental analysis gave the average number of neoglycoconjugates per cluster. The magnetic properties were measured in a SQUID magnetometer. The most remarkable results was the observation of a permanent magnetism up to room temperature in the iron-free gold GNPs, that was not present in the corresponding gold-iron oxide GNPs. 相似文献
205.
[reaction: see text] A strategy based on the control of the electron-donating capabilities of the coordinating groups of the ligand has been applied in the catalytic asymmetric addition of organometallic reagents to ketoesters. Mandelamides having deprotonated alcohol and carboxyamido groups catalyzed the addition of dimethylzinc to alpha-ketoesters with good yields and ee (up to 90%). 相似文献
206.
207.
Marleen de Veij Peter Vandenabeele Krystyn Alter Hall Facundo M. Fernandez Michael D. Green Nicholas J. White Arjen M. Dondorp Paul N. Newton Luc Moens 《Journal of Raman spectroscopy : JRS》2007,38(2):181-187
During the last decade there has been an apparent increase in the prevalence of counterfeit medicines in developing as well as developed countries. The pivotal antimalarial artesunate has been counterfeited on a large scale in SE Asia. In this work, the possibilities of Raman spectroscopy are explored as a fast and reliable screening method for the detection of counterfeit artesunate tablets. In this study, 50 ‘artesunate tablets’, purchased in SE Asia, were examined. This spectroscopic method was able to distinguish between genuine and counterfeit artesunate and to identify the composition of the counterfeit tablets. These contained no detectable levels of artesunate, but consisted mostly of starch, calcite (CaCO3), and paracetamol (4‐acetamidophenol). In one particular case an admixture of rutile (TiO2) and artesunate was detected. The results of the investigation by Raman spectroscopy were in agreement with those of colorimetric tests and of liquid chromatography‐mass spectrometry on the artesunate. Moreover, principal components analysis (PCA) was combined with hierarchical cluster analysis to establish an automated approach for the discrimination between different groups of counterfeits and genuine artesunate tablets. These results demonstrate that Raman spectroscopy combined with multivariate analysis is a promising and reliable methodology for the fast characterization of genuine and counterfeit artesunate antimalarial tablets. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
208.
Blay G Fernandez I Marco-Aleixandre A Pedro JR 《The Journal of organic chemistry》2006,71(17):6674-6677
The (S,S)-mandelamide III catalyzes the additions of both aryl- and alkylalkynylzinc reagents to heteroaromatic aldehydes with good yields and enantioselectivities up to 92%. This catalyst is easily prepared in a one-step procedure, and both enantiomers are available. Unlike most other described methods, using this catalyst does not require the addition of Ti(O(i)Pr)4. 相似文献
209.
In this paper we study Chaplygin’s Reducibility Theorem and extend its applicability to nonholonomic systems with symmetry
described by the Hamilton-Poincaré-d’Alembert equations in arbitrary degrees of freedom. As special cases we extract the extension
of the Theorem to nonholonomic Chaplygin systems with nonabelian symmetry groups as well as Euler-Poincaré-Suslov systems
in arbitrary degrees of freedom. In the latter case, we also extend the Hamiltonization Theorem to nonholonomic systems which
do not possess an invariant measure. Lastly, we extend previous work on conditionally variational systems using the results
above. We illustrate the results through various examples of well-known nonholonomic systems. 相似文献
210.
Bistable space–time discrete systems commonly possess a large variety of stable stationary solutions with periodic profile. In this context, it is natural to ask about the fate of trajectories composed of interfaces between steady configurations with periodic pattern and in particular, to study their propagation as traveling fronts. Here, we investigate such fronts in piecewise affine bistable recursions on the one-dimensional lattice. By introducing a definition inspired by symbolic dynamics, we prove the existence of front solutions and uniqueness of their velocity, upon the existence of their ground patterns. Moreover, the velocity dependence on parameters and the co-existence of several fronts with distinct ground patterns are also described. Finally, robustness of the results to small $C^1$ -perturbations of the piecewise affine map is argued by mean continuation arguments. 相似文献