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51.
A new sensitive kinetic method has been developed for the determination of trace amounts of I- over the range of 50-300 ng cm(-3). The method was based on the inhibitory effect of iodide on the oxidation of Victoria Blue 4-R by KBrO3. The reaction is monitored spectrophotometrically at 596.3 nm. Method development includes optimization of reagent concentration and temperature. The kinetic parameters of the reaction were reported and a rate equation was suggested. The effects of certain foreign ions upon the reaction rate were determined for the assessment of the selectivity of the method. The proposed method was successfully applied to the determination of iodide in real samples. The new developed method was found to have fairly good selectivity, sensitivity, simplicity and rapidity. 相似文献
52.
Violeta D. Mitic Jovana Pavlovic Jelena Nikolic Strahinja R. Simonovic Vesna P. Jovanovic Stankov 《Analytical letters》2018,51(6):908-920
Currant fruit extracts were characterized by (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity, 2,2-azinobis-3 ethyl benzothiazoline-6-sulfonic acid cation decolorization activity, total reducing power, cupric ion reducing antioxidant power, and ferric ion reducing antioxidant power (FRAP) assays to evaluate their antioxidant activity. All five antioxidant assays revealed the highest antioxidant activity to be present for the black currants. The highest concentrations of phenolics were present in the black currants (1690?±?10?mg gallic acid equivalents (GAE)/100?g fresh weight), while the lowest value was obtained in the white currants (579?±?5?mg GAE/100?g fresh weight). The correlation between the total phenolic content and antioxidant activity assays was evaluated using regression analysis. A significant positive correlation was obtained between the total phenols and the cupric ion reducing antioxidant power (r?=?0.97, p?0.05), followed by that between the total phenols and FRAP (r?=?0.93, p?0.05). Hierarchical cluster analysis separated these selected fruit species into two statistically significant clusters with black currants as a distinctive cluster. 相似文献
53.
Jan Mrázek Vlastimil Matejec Ivan Kasik Milos Hayer Daniela Berková 《Journal of Sol-Gel Science and Technology》2004,31(1-3):175-178
This paper presents results on using the sol-gel process for the preparation of microstructure fibers (MSFs). In this preparation the sol-gel method is used for the formation of bonds between silica tubes and silica rods in the input stacks for MSF drawing and for filling-in the interstices between these parts. Gels applied on the stacks were prepared from sols composed of tetraethoxysilane, phosphorous oxychloride, boron tribromide, ethanol, HCl and water. Pastes prepared from the sols and solid fabricated through heat treatment of the sols at 1000°C were also used. The sols were applied on the stacks by the dip-coating method and the pastes were applied manually. The formed xerogels were heat-treated at temperatures of up to 1000°C. The MSFs of the grapefruit type were drawn from the stacks at a temperature of about 1900°C. Waveguiding properties of the fibers were examined under their excitation by an inclined collimated beam. It has been found that the sol-gel method is suitable particularly for filling-in the interstices at the end of the stack, which enables pressure control inside holes of the stack during MSF drawing. 相似文献
54.
Meijer L Thunnissen AM White AW Garnier M Nikolic M Tsai LH Walter J Cleverley KE Salinas PC Wu YZ Biernat J Mandelkow EM Kim SH Pettit GR 《Chemistry & biology》2000,7(1):51-63
BACKGROUND: Over 2000 protein kinases regulate cellular functions. Screening for inhibitors of some of these kinases has already yielded some potent and selective compounds with promising potential for the treatment of human diseases. RESULTS: The marine sponge constituent hymenialdisine is a potent inhibitor of cyclin-dependent kinases, glycogen synthase kinase-3beta and casein kinase 1. Hymenialdisine competes with ATP for binding to these kinases. A CDK2-hymenialdisine complex crystal structure shows that three hydrogen bonds link hymenialdisine to the Glu81 and Leu83 residues of CDK2, as observed with other inhibitors. Hymenialdisine inhibits CDK5/p35 in vivo as demonstrated by the lack of phosphorylation/down-regulation of Pak1 kinase in E18 rat cortical neurons, and also inhibits GSK-3 in vivo as shown by the inhibition of MAP-1B phosphorylation. Hymenialdisine also blocks the in vivo phosphorylation of the microtubule-binding protein tau at sites that are hyperphosphorylated by GSK-3 and CDK5/p35 in Alzheimer's disease (cross-reacting with Alzheimer's-specific AT100 antibodies). CONCLUSIONS: The natural product hymenialdisine is a new kinase inhibitor with promising potential applications for treating neurodegenerative disorders. 相似文献
55.
The kinetics and mechanism of transformation reaction of S-[1-(4- methoxyphenyl)pyrrolidin-2-one-3-yl]-N-methyl-isothiuronium bromide into 2-methylimino-5-[2-(4-methoxyphenylamino) ethyl)]thiazolidin-4-one have been studied in aqueous solutions of amine buffers (pH 8.1-11.5) and sodium hydroxide solutions (0.005-0.5 mol l-1) at 25 degrees C and at I = 1 mol l-1 at pseudo-first-order reaction conditions. The kinetics observed shows that the transformation reaction is subject to general base, general acid, and hydroxide-ion catalyses. The rate-limiting step of transformation is the splitting-off a proton from the tetrahedral intermediate In. The value of pKa for S-[1-(4-methoxyphenyl)- pyrrolidin-2-one-3-yl]-N-methylisothiuronium bromide has been determined from the kinetic data (pKa = 8.75 +/- 0.10) and by potentiometric titration (pKa = 8.90 +/- 0.05). With increasing pKa value of the acid buffer component, the value of Br?nsted coefficient beta gradually decreases from about 0.7 to almost zero. The value of pKa approximately 10 for the intermediate to base-catalysed transformation has been found from this dependence. In the N-methylpyrrolidine and triethylamine buffers, the rate-limiting step of transformation is changed into ring opening of In-, and the general-base-catalysed reaction changes into a specific-base-catalysed one. 相似文献
56.
Darko P. A?anin Sne?ana Rajkovi? Dora Molnar-Gabor Milos? I. Djuran 《Monatshefte für Chemie / Chemical Monthly》2004,13(3):1445-1453
Hydrolytic reactions between various palladium(II) complexes of the type cis-[Pd(L)(H2O)2]2+ in which L is ethylenediamine (en), 1,2-propylenediamine (1,2-pn), isobutylenediamine (ibn), 1,2-diaminocyclohexane (1,2-dach), N-methylethylenediamine (Meen), N,N,N,N-tetramethylethylenediamine (Me4en), S-methyl L-cysteine (MeS-L-HCys), L-methionine (L-HMet), and 2,5-dithiahexane (dth) and dipeptide N-acetylated L-methionylglycine (MeCOMet-Gly) were studied by 1H NMR spectroscopy. The reactions were carried out in the pH range 2.0–2.5 and at 50°C. In all these reactions, palladium(II) complex bound to a methionine residue effects the regioselective cleavage of the amide bond involving the carboxylic group of methionine. We found that the rate of hydrolysis and mechanism of this reaction are strongly dependent from the nature of the chelate ligand L in palladium(II) complexes of the type cis-[Pd(L)(H2O)2]2+. 相似文献
57.
Milos Hudlicky 《Journal of fluorine chemistry》1984,25(3):353-361
Treatment of - and -α-bromo-α′-fluorosuccinic acids with aqueous alkali results in the elimination of hydrogen fluoride in preference to hydrogen bromide and gives almost exclusively bromofumaric acid. The elimination of hydrogen fluoride from the two above-mentioned acids is faster by 1–2 orders of magnitude than the elimination of hydrogen fluoride from any fluorinated succinic acids. Kinetic data based on 1H NMR measurements are presented and possible mechanisms and stereochemistry involved are discussed. 相似文献
58.
We proposed EPR spectroscopy using spin-trap DEPMPO as a novel method for the detection of a hydrogen atom (*H) produced by chemical and biological systems. In complex EPR spectra of DEPMPO adducts in biological systems, spectral lines of unknown origin have been observed. We have assumed (Baci?, G.; Mojovi?, M. Ann. N. Y. Acad. Sci. 2005, 1048, 230-243) that those lines represent the spectrum of a hydrogen atom (*H) adduct i.e., DEPMPO/H. An electrochemical system known to produce only *H radicals was used here in order to obtain a separate spectrum of the DEPMPO/H adduct. An acquired spectrum as well as a computer spectral simulation of the DEPMPO/H adduct showed considerable resemblance with additional lines in the EPR spectra of DEPMPO adducts in biological systems-plant plasma membranes and cell walls. This shows that such a radical is produced by plants as well as that DEPMPO is suitable for detection in both electrochemical and biological systems. 相似文献
59.
Unmodified silica gel size-exclusion columns were used in an on-line combination with light-scattering detection for a size characterization of dextrans. The influence of electrostatic interactions on analyte retention was briefly investigated. Size-exclusion chromatography was also used for evaluation of the fluorescence labeling procedure for dextrans with 8-aminonaphthalene-1,3,6-trisulfonic acid. The derivatives obtained through this procedure were used for electrophoretic measurements with laser-induced fluorescence detection. A comparison between the size-exclusion data and capillary electrophoresis indicates that the effectiveness of fluorescent labeling decreases with molecular mass of the dextran analytes. 相似文献
60.
[formula: see text] A protocol for achieving stereoselective aldol reactions with cyclic ketones is presented. In terms of yield, the process is particularly effective when a quaternary center at the alpha-carbon of the beta-hydroxy ketone product is created. The stereochemical outcome, anti or syn, is achieved by the Lewis acid-mediated ring expansion of stereochemically homogeneous epoxides in a reaction related to the pinacol rearrangement. 相似文献