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排序方式: 共有288条查询结果,搜索用时 31 毫秒
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Y. A. Coutinho J. A. Martins Simões C. M. Porto 《Zeitschrift fur Physik C Particles and Fields》1997,73(4):705-709
In this paper we investigate the effects of heavy Majorana neutrinos in the reaction e + e ? → W + W ?. We consider neutrino masses in the 1–10 TeV region. We show that at LEP II and NLC energies it is possible to use this processes to verify indirect evidence of heavy neutral particles with mixing angles of the order sin2 α = 0.01. We discuss the unitarity restrictions that can be obtained for vector singlet and fermion-mirror-fermion models. 相似文献
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Alpeshkumar K. Malde Santosh A. Khedkar Evans C. Coutinho 《Journal of Physical Organic Chemistry》2007,20(2):151-160
Modification of peptides to produce peptidomimetics is of great interest, with the aim of designing potent, selective, and metabolically stable analogs having certain conformational properties. Organoboranes have been reported in the literature with a wide range of therapeutic applications. One of the therapeutically important class of molecules is amine‐carboxyboranes derived from amino acids by replacement of the Cα atom of an amino acid/peptide by boron. The conformational preferences of these peptides, with respect to backbone ω, ?, and ψ torsion angles, have been investigated by theoretical calculations. The amide bond in these molecules has the same geometry in the ground and transition states as the natural peptides. However, the boron isosteres of glycine and alanine peptides are less structured than their natural derivatives, but are distinguished by structures with a positive value for the ? angle, which is normally disfavored for natural peptides. This property could be used to build peptides with a geometry not usually seen in natural peptides. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
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J. H. de Araújo J. B. M. da Cunha A. Vasquez L. Amaral J. T. Moro F. C. Montenegro S. M. Rezende M. D. Coutinho Filho 《Hyperfine Interactions》1991,67(1-4):507-511
57Fe Mössbauer effect measurements in the diluted Ising antiferromagnet Fe x Zn1?x F2 withx=0.25 andx=0.57 at temperatures between 4.2 and 55 K, are reported. DC suceptibility measurements show a spin-glass (SG) phase at low temperatures forx≤0.31. Our Mössbauer spectra show a phase transition to a SG state with antiferromagnetic order (AFSG) forx=0.25 and only antiferromagnetic order forx=0.57. 相似文献
5.
N. S. Oliveira J. Dorgan J. A. P. Coutinho A. Ferreira J. L. Daridon I. M. Marrucho 《Journal of Polymer Science.Polymer Physics》2007,45(5):616-625
The sorption of carbon dioxide in glassy Poly(lactic acid) (PLA) films was studied by quartz crystal microbalance (QCM) at high pressures. Two thermal treatments, melted and quenched, were performed in PLA with two different L:D contents, 80:20 and 98:2, films and compared with a third thermal protocol, annealed, and used in a previous work. The results obtained show that for pressures higher than 2 MPa, the carbon dioxide solubility is larger in PLA 80:20 than in PLA 98:2, indicating that the L:D plays a dominant role on this property. The thermal treatments only affect the gas solubility in PLA 98:2. Sorption isotherms at temperatures 303, 313, and 323 K, below the glass transition temperature of the polymer, and pressures up to 5 MPa were measured and analyzed with three different models, the dual‐mode sorption model, the Flory–Huggins equation, and a modified dual‐mode sorption model where the Henry's law term was substituted by the Flory–Huggins equation. This last model performs especially well for CO2 in PLA 80:20, due to the convex upward curvature of the solubility isotherms for that system. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 616–625, 2007 相似文献
6.
Silva L Coutinho A Fedorov A Prieto M 《Journal of photochemistry and photobiology. B, Biology》2003,72(1-3):17-26
Nystatin is a polyene antibiotic frequently applied in the treatment of topical fungal infections. In this work, a 7-nitrobenz-2-oxa-1,3-diazole (NBD) hexanoyl amide derivative of nystatin was synthesized and its detailed photophysical characterization is presented. The average conformation of the labelled antibiotic in tetrahydrofuran, ethanol and methanol was determined by intramolecular (tetraene to NBD) fluorescence resonance energy transfer measurements. At variance with the literature [Can. J. Chem. 63 (1985) 77-85], it was concluded that there is no need to invoke a solvent-dependent conformational equilibrium between extended and closed conformers of the antibiotic, because the mean tetraene-to-NBD separating distance was found to remain constant (approximately 18 A) in all the solvents studied. In addition, the large solvent dependence of the fluorescence anisotropy observed for the non-derivatized nystatin, was rationalized on the basis of the prolate ellipsoidal geometry of the molecule. It was concluded that the rod shaped and amphipathic antibiotic remains monomeric in different solvents within the concentration range studied (2-20 microM). 相似文献
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A fast system for steel analysis is described. Immediately after collection, the solidified sample is polished and placed on a small electrochemical dissolution unit. The dissolution step is accomplished in a few seconds, and the dissolved material is passed directly “on line” to a flow-injection manifold. The feasibility of the approach is demonstrated by determining soluble aluminium in steels (0.01–0.13% w/w) spectrophotometrically. Dissolution conditions such as current density, electrolysis duration, composition and flow rate of the electrolyte carrier stream are described. The proposed system is very stable and the consumption of reagents is low; 20–40 samples per hour can be handled. The results compare well with the values for reference standard steels. 相似文献
10.
André Coutinho Vieira Mariane Gonçalves Santos 《International journal of environmental analytical chemistry》2017,97(1):29-41
Triazole fungicides are pesticides widely employed in the cultivation of fruits, vegetables and grains. However, their ability to change the steroid hormone biosynthesis may result in endocrine complications for mammals, as well as changes in cholesterol and triglyceride levels and hepatotoxicity. The analysis of the triazole fungicides in superficial waters is important in order to monitor the risk for the biota. However, the use of efficient extraction procedures has been necessary in order to concentrate these pesticides before the analysis. In-disk solid-phase extraction (SPE) can be highlighted as a potential pre-concentration technique, mainly because the possibility to extract the analytes from a large sample volume, increasing the method detectability. Carbon nanotubes (CNTs) have been often used as solid extraction phase due to their high sorption capacity, surface area and internal volume, as well as mechanical, chemical and thermal stability. In this paper, we proposed the preparation of a new SPE disk impregnated with CNTs for the extraction of triazole fungicides from environmental water samples. The disks were obtained by acid corrosion of a cellulose membrane followed by its impregnation with CNTs. The developed method was validated for the analysis of triadimenol, tebuconazole and epoxiconazole, according to international validation protocols. The limits of quantification obtained for triadimenol, tebuconazole and epoxiconazole were 0.1, 0.1 and 0.05 µg L?1, respectively. The linearity ranged from 0.05 to 10.00 µg L?1 for epoxiconazole and from 0.1 to 10.00 µg L?1 for triadimenol and tebuconazole, with correlation coefficients higher than 0.999 for all of them. The precisions, expressed as relative standard deviation, were lower than 12%. The accuracies were within ?12.07% to 17.7% (expressed as relative error). 相似文献