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161.
A multiply charged ion source based on E.C.R. heating is described and abundances for xenon ions up to Xe13+ are given. 相似文献
162.
For a hypergraphH, we denote by
- τ(H) the minimumk such that some set ofk vertices meets all the edges,
- ν(H) the maximumk such that somek edges are pairwise disjoint, and
- λ(H) the maximumk≥2 such that the incidence matrix ofH has as a submatrix the transpose of the incidence matrix of the complete graphK k .
163.
164.
P. D. Seymour 《Combinatorica》1996,16(2):223-231
LetG be an eulerian digraph; let (G) be the maximum number of pairwise edge-disjoint directed circuits ofG, and (G) the smallest size of a set of edges that meets all directed circuits ofG. Borobia, Nutov and Penn showed that (G) need not be equal to (G). We show that (G)=(G) provided thatG has a linkless embedding in 3-space, or equivalently, if no minor ofG can be converted toK
6 by –Y andY– operations. 相似文献
165.
Seymour Zigman John Reddan Joanne B. Schultz Thurma McDaniel 《Photochemistry and photobiology》1996,63(6):818-824
Part one of this study shows that exposure of purified beef liver catalase in buffered solutions to BL lamps that provide a mixture of 99% UVA and 1% UVB (to be labeled UVA) alters its chemistry and enzymatic activity. Thus, its spectral absorbance lost detail, it aggregated and exhibited a lower isoelectric point and its enzymatic activity was substantially reduced. These photochemically induced changes were increased by irradiation in phosphate buffer or in physiological medium (minimal essential medium) containing riboflavin and tryptophan. Neither α-tocopherol nor de-feroxamine were protective against these UVA-induced changes in pure catalase. We further investigated the effect of UVA radiation on the activity of catalase in cultured lens epithelial cells and the protective effects of antioxidants. Cultured lens epithelial cells of rabbits and squirrels were exposed to near-UV radiation with representation in the UVA region of 99% and 1% UVB. Catalase assays were done on ho-mogenate supernatants of cells kept dark or UV exposed. In some instances, cells were cultured in medium containing a-tocopherol or deferoxamine prior to UV radiation. Comparisons were made between UV-exposed lens cell catalase activity when exposure was done with or without the antioxidants. The UVA radiation was strongly inhibitory to both rabbit and squirrel lens epithelial cell catalase activities. The range of fluxes of near UV radiation was compatible with that which could reach the lens from the sunlit environment. Catalase inactivation was lessened in cells preincubated with a-tocopherol and deferoxamine. This suggests that both singlet oxygen and hydroxyl radical formation may be involved in near-UV damage to lens epithelial cell catalase. Such inhibition of catalase by near-UV would enhance H2O2 toxicity and stimulate SH oxidation so as to damage the lens. 相似文献
166.
The dominant primary reactions of propionitrile under electron-impact effect loss of HCN and loss of H·. Deuterium labeling shows that the hydrogen atom lost as HCN comes chiefly from C-2, but that lost as a free atom comes chiefly from C-3. Both reactions are probably preceded by a 1,2 hydrogen migration to yield an allylic-like molecular ion, \documentclass{article}\pagestyle{empty}\begin{document}$ \left[{{\rm CH}_{\rm 3}{\rm C}^+ {\rm HCH = N}^{\rm .} \leftrightarrow {\rm CH}_{\rm 3} \mathop {\rm C}\limits^{\rm .} {\rm HCH = N}} \right]^+ $\end{document}. 相似文献
167.
A. A. Geller A. G. Yereshchenko Zh. A. Zgibneva M. V. Polovnikova 《Journal of polymer science. Part A, Polymer chemistry》1974,12(10):2327-2335
The kinetic and thermodynamic specific features of the process of heterogeneous alkaline saponification of the copolymer of acrylonitrile (92.2%), methyl acrylate (6.3%), and sodium monoithaconate (1.5%) were investigated. Chemical and infrared spectroscopic methods reveal that the saponification of the nitrile groups of copolymer of acrylonitrile proceeds faster to yield amide groups and then slower to yield carboxylate groups. The order of both stages of the process with respect to the accumulation of the respective groups was evaluated. The dependence of the rate of saponification on the packing density of the structural units was demonstrated. With increasing conversion, the fiber structure becomes looser, as demonstrated by density and sorption investigations. If acrylonitrile copolymer is treated with alkali, the carboxylate groups in the polymer substrate accumulate as a result of saponification of the ester groups of the copolymer, while the nitrile groups are predominantly transformed into the amide groups. If the degree of conversion (with respect to the CN groups) is higher than 62.7%, the copolymer abruptly suffers a marked change in its mechanical properties and gradually dissolves. The products of partial saponification of copolymer were investigated by thermomechanical methods and DTA. The polycyclization of the polymer substrate was found to proceed at a temperatures which decreases as the content of the amide groups increases. 相似文献
168.
Chemistry of singlet oxygen. XVIII. Rates of reaction and quenching of alpha-tocopherol and singlet oxygen 总被引:4,自引:0,他引:4
Abstract —α-Tocopherol scavenges singlet oxygen (produced by methylene blue photosensitization in methanol) by a combination of chemical reaction (4.6 times 107M-1s-1) and quenching (6.2 times 108M-1 s-1). The total rate of scavenging (6.7 times 108 M-1s-1) makes it an effective protective agent against photooxidation mediated by singlet oxygen. 相似文献
169.
Electrospray ionization of mixtures of isomeric and isobaric amino acids was investigated with the goal of distinguishing and quantifying the components. Isomeric amino acids leucine and isoleucine were readily distinguished and quantified in 90 : 10 to 10 : 90 binary mixtures using two-stage (MS(2)) and three-stage (MS(3)) tandem mass spectrometric dissociations of ternary Cu(2+)-2, 2'-bipyridyl (bpy) complexes, [Cu(AA - H)bpy](+). The complexes self-assembled in solution upon mixing the components and provided a convenient means of efficient derivatization that increased the efficiency of amino acid ionization by electrospray and shifted the mass of the analytes to a region which was free of solvent interferences. Low-energy dissociations of [Cu(AA - H)bpy](+) complexes in a quadrupole ion trap were achieved at >90% conversions and >80% trapping efficiencies for the MS(2) and MS(3) precursor and fragment ions. Isobaric amino acids glutamine and lysine were also distinguished through MS(2) and MS(3) of their ternary complexes with Cu(2+) and bpy. ESI of [Cu(Gln - H)bpy](+) was enhanced in the presence of [Cu(Lys - H)bpy](+), which resulted in non-linear response at low Lys concentrations. 相似文献
170.
Electrochemical detection of aniline: an electrochemically initiated reaction pathway 总被引:1,自引:0,他引:1
The electrochemically initiated reaction of aromatic amines with catechol has been examined as a facile route to the electrochemical determination of the former. The effects of the different structural isomers of sulfanilic acid on the analytical system has been studied and appraised. The report illustrates how by varying oxidation times of the catechol species or by increasing the number of repetitive voltammetric scans from one to four the analytical parameters (linear range and limit of detection) can be improved. The results provide a linear detection range of 2-40 muM for aniline with a corresponding limit of detection of 1.4 muM. The efficacy of the approach has been shown by a 99.5% (RSD=5.5%, N=5) recovery of 2.47 muM spiked aniline in river water. 相似文献