排序方式: 共有60条查询结果,搜索用时 203 毫秒
41.
Eiserbeck C Nelson RK Grice K Curiale J Reddy CM Raiteri P 《Journal of chromatography. A》2011,1218(32):5549-5553
18α(H)-, 18β(H)-oleanane and lupane are angiosperm-derived biomarkers that are used as age indicators for the Late Cretaceous onwards when the first proliferation of angiosperms occurred. In addition, the 18α(H)-/18β(H)-oleanane ratio is employed as a thermal maturity parameter of crude oil. However, evidence has shown that accurate quantification of these compounds has been impeded by inadequate chromatographic separation by traditional one-dimensional gas chromatography. In this study, we present the separation of 18α(H)-, 18β(H)-oleanane and lupane with comprehensive two-dimensional gas chromatography (GC×GC). Furthermore, it was observed that 18β(H)-oleanane elutes earlier than 18α(H)-oleanane in second dimension (polarity) which we attribute to steric hindrance effects. Two GC conditions have been developed in order to achieve baseline separation of the triterpenoids of interest in complex mixtures such as sediment extracts and crude oils. 相似文献
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Lawrence CR Tafoya C William Shuttleworth Yuchio Yanagawa Kunihiko Obata Michael C Wilson 《BMC neuroscience》2008,9(1):105
Background
The soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, comprised of SNAP-25, syntaxin 1A, and VAMP-2, has been shown to be responsible for action potential (AP)-dependent, calcium-triggered release of several neurotransmitters. However, this basic fusogenic protein complex may be further specialized to suit the requirements for different neurotransmitter systems, as exemplified by neurons and neuroendocrine cells. In this study, we investigate the effects of SNAP-25 ablation on spontaneous neuronal activity and the expression of functionally distinct isoforms of this t-SNARE in GABAergic and glutamatergic neurons of the adult brain. 相似文献44.
James P. Kutney Peter Grice Keith O. Pascoe Krystyna Piotrowska Steven J. Rettig Jerzy Szykula James Trotter 《Helvetica chimica acta》1982,65(7):2345-2354
The thujone derived enone 1 is converted in one step to the pentacyclic compound 4 . The structure of 4 was determined by X-ray diffraction analysis. The acid-catalyzed cyclopropane-ring opening of 4 and of the model compound 9 are described. 相似文献
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Yi-Brunozzi HY Brabazon DM Lener D Le Grice SF Marino JP 《Journal of the American Chemical Society》2005,127(47):16344-16345
To probe structural features of a polypurine tract (PPT) that mediate its specific recognition and processing, a model 20 bp RNA/DNA hybrid duplex, which includes the full PPT sequence of the Saccharomyces cerevisiae LTR-retrotransposon Ty3, has been investigated using solution NMR spectroscopy. While homonuclear NOESY and DQF-COSY analyses indicate that this PPT-containing RNA/DNA hybrid adopts an overall A-form-like helical geometry, an unexpected sugar pucker switch has been detected for the ribose at position +1, relative to the cleavage site, on the RNA strand. A model of the conformational changes induced by the A- to B-type sugar pucker switch shows a significant change in the backbone trajectory of the RNA strand, which alters the presentation of backbone phosphate and 2' hydroxyl groups 3' of this residue. This observation implies that part of the mechanism governing RNase H fidelity may be through distortion of the RNA/DNA helix one base ahead of the scissile bond. 相似文献
47.
Dariush Habibollahzadeh Jane S. Murray M. Edward Grice Peter Politzer 《International journal of quantum chemistry》1993,45(1):15-20
The X–NO2 rotational energy barriers of nitromethane, nitroethylene, nitrobenzene, and a group of nitramines have been computed using a local density functional (LDF ) procedure, using ab initio Hartree–Fock (HF )-optimized structures of the ground and rotational transition states. The results have been discussed in relation to HF and some correlated ab initio values and the available experimental data. Our LDF barriers are overall quite reasonable, in generally satisfactory agreement with the experimental and correlated ab initio results. © 1993 John Wiley & Sons, Inc. 相似文献
48.
Density functional analysis of a decomposition of 4-nitro-1,2,3-triazole through the evolution of N2
Peter Politzer M. Edward Grice Jorge M. Seminario 《International journal of quantum chemistry》1997,61(3):389-392
A density functional computational study shows that 4-nitro-1,2,3-triazole, which is highly impact sensitive, can decompose through ring opening and subsequent N2 evolution, with the net release of 12 kcal/mol. An input of 52 kcal/mol is required to initiate the process. © 1997 John Wiley & Sons, Inc. 相似文献
49.
Reactive scattering of O atoms with I2 molecules has been studied at an initial translational energy E = 43 kJ mol-1 using a supersonic beam of O atoms seeded in He and E = 18 kJ mol-1 using O atoms seeded in Ne. Velocity distributions of OI product were measured by cross-correlation time-of-flight analysis. Full contour maps of the differential reaction cross section were obtained which show predominantly rebound scattering at both initial translational energies. Scattering in the forward direction has a product translational energy distribution similar to that predicted for a long-lived collision complex but scattering in the backward direction has a much higher product translational energy. The greater predominance of rebound scattering observed for O + I2 compared with O + Br2 may be attributed to the greater exoergicity of the O + I2 reaction. However, comparison with the O + IBr reaction which exhibits a long-lived collision complex mechanism indicates that the predominance of backward scattering for O + I2 also arises from diminished forward scattering from larger impact parameter collisions. 相似文献
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Angular distribution measurements of KX reactive scattering of a potassium dimer K2 beam by I2 and by a series of halomethane molecules are reported. The K2 + I2 reactive scattering is similar to that previously observed for K2 + Br2. The predominant reaction path yields K + KI + I with the K and KI product recoiling in the forward direction. However, the forward peak of the KI differential cross section is lower than that for K from K2 + I2 and is broader than that observed for KBr from K2 + Br2. This is attributed to slow dissociation of the I 2 - ion formed in the electron jump mechanism previously proposed for K2 + Br2. In the halomethane reactions, both alkali atoms of the K2 dimer become bound alkali halide molecules in all reactive collisions, despite the direct dynamics of the corresponding supersonic K atom reactions. Thus, these reactions provide compelling evidence for a second electron jump mechanism, previously proposed for the reactions of K2 dimers with polyhalide molecules. The differential cross sections for the K2 dimer plus halomethane reactions indicate an osculating collision complex with a lifetime at least comparable to its rotational period, perhaps much longer. This reaction complex is identified with the doubly ionic state formed by the second electron jump transition. 相似文献