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81.
82.
Herbert Hommer Gabriel Cuevas Barbara Gordillo 《Phosphorus, sulfur, and silicon and the related elements》2013,188(10):2421-2437
By following a previously reported method,1 the synthesis of r-2-alkoxy-cis-4-cis-5-dimethyl-1,3,2-λ3-dioxaphospholanes ligands (1 and 3) was carried out. The purpose of this work is the kinetic study of the inversion barrier at phosphorus for 1 and 3 and the comparison with the already informed dioxaphospholane 2. The kinetic measurements of the thermal isomerization cis-to-trans were performed by 31P NMR spectroscopy, observing a first order kinetics for both compounds. The energy of activation (Ea) for the epimerization of compounds cis-1 and cis-3 was calculated to be 16.0 ± 0.6 and 11.8 ± 0.8 kcal/mol, respectively. The values of the thermodynamic parameters of the transition state (Δ H≠, Δ S≠, Δ G≠) suggest that the inversion at phosphorus not only depends on the spatial requirements of the alkoxy substituent but also on entropic effects. The thermodynamic parameters Δ H°, Δ S°, and Δ G° were also evaluated and they show that the cis isomers are preferred from enthalpic point of view, but entropic effects dominate the equilibrium trans ? cis leading to the entropically favored trans isomers. Furthermore, the results are supported by density functional theory calculations of 1–4 at the B3LYP/6-31G** level. 相似文献
83.
Zaira Domínguez Javier Hernández Lorena Silva-Gutiérrez Magali Salas-Reyes Mario Sánchez Gabriel Merino 《Phosphorus, sulfur, and silicon and the related elements》2013,188(4):772-784
The effect of electron-withdrawing (EW) and electron-releasing (ER) substituents on the 31P NMR chemical shifts and the structural parameters of a series of tris-(p-X-aryl)selenophosphates is reported in this article. Similarly to O-aryl phosphates and O-aryl thiophosphates, EW groups attached to aromatic rings induce a shielding effect on the 31P NMR signal. After a detailed experimental and theoretical analysis, we confirmed that the selenium atom is the main part responsible for the charge density transfer toward phosphorus through a back-bonding effect. The obtained 1JP-Se values for the complete series agree with this observation. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. 相似文献
84.
Abstract The distribution of the alkoxide groups obtained in the metallation of starch in dimethyl sulfoxide solution by alkali metal naphthalenes was studied. The starch alkoxide was reacted with methyl iodide, and the methylated starch was hydrolyzed and analyzed for glucose and O-methyl glucose derivatives. The metallation reaction was found to be random, as seen from the fact that at low alkoxide concentration (D.S = 0.6), 2,3,6-tri-O-methyl glucose was formed, while at relatively high alkoxide concentration (D.S. = 1.5) unreacted glucose was still present. At low alkoxide concentration (D.S. ? 0.6) there was, to a certain extent, preferential metallation at the C2 hydroxyl groups, and to a lesser extent at the C6 hydroxyl groups, as seen from the relative molar ratios of about 10:4:1 of the 2-, 6-, and 3-O-methyl glucose derivatives obtained, respectively. An increase in the metallation at the C3 hydroxyl occurred with increasing alkoxide concentration. The distribution of the methoxyl groups with the three alkali metals potassium, sodium, and lithium was similar; there were differences only in the ease of metallation of the starch by the various alkali metal naphthalenes and in the efficiency of the coupling reaction with methyl iodide. 相似文献
85.
The signatures of nanosolvation on the one‐ and two‐dimensional (1D and 2D) IR spectra of a proton‐transfer mode in a hydrogen‐bonded complex dissolved in polar solvent molecule nanoclusters of varying size are elucidated by using mixed quantum–classical molecular dynamics simulations. For this particular system, increasing the number of solvent molecules successively from N=7 to N=9 initiates the transition of the system from a cluster state to a bulk‐like state. Both the 1D and 2D IR spectra reflect this transition through pronounced changes in their peak intensities and numbers, but the time‐resolved 2D IR spectra also manifest spectral features that uniquely identify the onset of the cluster‐to‐bulk transition. In particular, it is observed that in the 1D IR spectra, the relative intensities of the peaks change such that the number of peaks decreases from three to two as the size of the cluster increases from N=7 to N=9. In the 2D IR spectra, off‐diagonal peaks are observed in the N=7 and N=8 cases at zero waiting time, but not in the N=9 case. It is known that there are no off‐diagonal peaks in the 2D IR spectrum of the bulk version of this system at zero waiting time, so the disappearance of these peaks is a unique signature of the onset of bulk‐like behavior. Through an examination of the trajectories of various properties of the complex and solvent, it is possible to relate the emergence of these off‐diagonal peaks to an interplay between the vibrations of the complex and the solvent polarization dynamics. 相似文献
86.
Tomáš Větrovský Petr Baldrian Jiří Gabriel 《Applied biochemistry and biotechnology》2013,169(1):100-109
Production of the lignocellulose-degrading enzymes endo-1,4-β-glucanase, 1,4-β-glucosidase, cellobiohydrolase, endo-1,4-β-xylanase, 1,4-β-xylosidase, Mn peroxidase, and laccase was characterized in a common wood-rotting fungus Fomes fomentarius, a species able to efficiently decompose dead wood, and compared to the production in eight other fungal species. The main aim of this study was to characterize the 1,4-β-glucosidase produced by F. fomentarius that was produced in high quantities in liquid stationary culture (25.9 U?ml?1), at least threefold compared to other saprotrophic basidiomycetes, such as Rhodocollybia butyracea, Hypholoma fasciculare, Irpex lacteus, Fomitopsis pinicola, Pleurotus ostreatus, Piptoporus betulinus, and Gymnopus sp. (between 0.7 and 7.9 U?ml?1). The 1,4-β-glucosidase enzyme was purified to electrophoretic homogeneity by both anion-exchange and size-exclusion chromatography. A single 1,4-β-glucosidase was found to have an apparent molecular mass of 58 kDa and a pI of 6.7. The enzyme exhibited high thermotolerance with an optimum temperature of 60 °C. Maximal activity was found in the pH range of 4.5–5.0, and K M and V max values were 62 μM and 15.8 μmol?min?1?l?1, respectively, when p-nitrophenylglucoside was used as a substrate. The enzyme was competitively inhibited by glucose with a K i of 3.37 mM. The enzyme also acted on p-nitrophenylxyloside, p-nitrophenylcellobioside, p-nitrophenylgalactoside, and p-nitrophenylmannoside with optimal pH values of 6.0, 3.5, 5.0, and 4.0–6.0, respectively. The combination of relatively low molecular mass and low K M value make the 1,4-β-glucosidase a promising enzyme for biotechnological applications. 相似文献
87.
Gabriel G.A. de Carvalho Mário A. Feres Jr José R. Ferreira Valerie H. Kennedy 《International journal of environmental analytical chemistry》2013,93(9):686-696
A simple and reliable method for Hg determination in fish samples has been developed. Lyophilised fish tissue samples were extracted in a 25% (w/v) tetramethylammonium hydroxide (TMAH) solution; the extracts were then analysed by FI-CVAFS. This method can be used to determine total and inorganic Hg, using the same FI manifold. For total Hg determination, a 0.1% (w/v) KMnO4 solution was added to the FI manifold at the sample zone, followed by the addition of a 0.5% (w/v) SnCl2 solution, whereas inorganic Hg was determined by adding a 0.1% (w/v) L-cysteine solution followed by a 1.0% (w/v) SnCl2 solution to the FI system. The organic fraction was determined as the difference between total and inorganic Hg. Sample preparation, reagent consumption and parameters that can influence the FI-CVAFS performance were also evaluated. The limit of detection for this method is 3.7 ng g?1 for total Hg and 4.3 ng g?1 for inorganic Hg. The relative standard deviation for a 1.0 µg L?1 CH3Hg standard solution (n = 20) was 1.1%, and 1.3% for a 1.0 µg L–1 Hg2+ standard solution (n = 20). Accuracy was assessed by the analysis of Certified Reference Material (dogfish: DORM-2, NRCC). Recoveries of 99.1% for total Hg and 93.9% inorganic Hg were obtained. Mercury losses were not observed when sample solutions were re-analysed after a seven day period of storage at 4°C. 相似文献
88.
89.
90.
Separation of natural antioxidants using PDMS electrophoresis microchips coupled with amperometric detection and reverse polarity
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Bruno Gabriel Lucca Susan Marie Lunte Wendell Karlos Tomazelli Coltro Valdir Souza Ferreira 《Electrophoresis》2014,35(23):3363-3370
This report describes the use of PDMS ME coupled with amperometric detection for rapid separation of ascorbic, gallic , ferulic, p‐coumaric acids using reverse polarity. ME devices were fabricated in PDMS by soft lithography and detection was accomplished using an integrated carbon fiber working electrode aligned in the end‐channel configuration. Separation and detection parameters were investigated and the best conditions were obtained using a run buffer consisting of 5 mM phosphate buffer (pH 6.9) and a detection voltage of 1.0 V versus Ag/AgCl reference electrode. All compounds were separated within 70 s using gated injection mode with baseline resolution and separation efficiencies between 1200 and 9000 plates. Calibration curves exhibited good linearity and the LODs achieved ranged from 1.7 to 9.7 μM. The precision for migration time and peak height provided maximum values of 4% for the intrachip studies. Lastly, the analytical method was successfully applied for the analysis of ascorbic and gallic acids in commercial beverage samples. The results achieved using ME coupled with amperometric detection were in good agreement with the values provided by the supplier. Based on the data reported here, the proposed method shows suitability to be applied for the routine analysis of beverage samples. 相似文献