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41.
Effects of high-pH environments on a stationary phase prepared by gamma-radiation immobilization of poly(methyloctylsiloxane) on titanium-grafted silica were investigated by HPLC testing with standard sample mixtures. The HPLC parameters indicate good stationary phase stability to 10000 column volumes each of mobile phases with pH of 7, 9 and 12. At pH 13, the efficiency decreases slowly, although reasonably good separations are still possible until increasing flow resistance no longer allows easy passage of the mobile phase. 相似文献
42.
A procedure is presented for the rapid evaluation of HPLC stationary phase stability at pH 8.4 or 10.1 using a temperature of 60 degrees C. Mobile phase (MeOH-0.1 mol l(-1) aqueous NaHCO3, 50:50, v/v) is continuously passed through the column with periodic injections of a test solution until the several chromatographic parameters of the resulting chromatograms are degraded. The tests were applied to several commercial and laboratory-made stationary phases. After degradation two of these phases, one commercial and one laboratory-made, were examined by elemental analysis and scanning electron microscopy to elucidate the degradation process. 相似文献
43.
Ahmad Waleh Jack R. Collins Gilda H. Loew M. C. Zerner 《International journal of quantum chemistry》1986,29(5):1575-1589
Restricted open-shell ground state properties and electronic spectra of two closely related low-spin, ferric, 6-coordinate, model cytochrome P450 complexes, one with a methyl mercaptide and the other a mercaptan as the second axial ligands, have been calculated with a newly modified, semiempirical INDO-SCF-CI method. The sensitivity of the calculated spectra to protonation of the sixth axial ligand, and the ability of the method to predict characteristic spectral features for the complexes investigated, are determined. Assignment of transitions, including xy- and z-polarized transitions, are made and compared with experimental observations where available. In particular, the origin of the anomalous split Soret spectrum observed in low-spin ferric complexes with mercaptide but not a mercaptan is investigated. Finally, a two part hypothesis is presented which provides a general explanation for the origin of both the observed split Soret and the red-shifted normal Soret in various ferrous and ferric P450 complexes in terms of the ground state orbital characters and simple symmetry considerations. 相似文献
44.
Wang YN Topol IA Collins JR Burt SK 《Journal of the American Chemical Society》2003,125(43):13265-13273
Phosphate hydrolysis by GTPases plays an important role as a molecular switch in signal transduction and as an initiator of many other biological processes. Despite the centrality of this ubiquitous reaction, the mechanism is still poorly understood. As a first step to understand the mechanisms of this process, the nonenzymatic hydrolysis of mono-phosphate and tri-phosphate esters were systematically studied in gas phase and aqueous solution using hybrid density functional methods. The dielectric effect of the environment on the energetics of these processes was also explored. Theoretical results show that for mono-phosphate ester, the dissociative pathway is much more favorable than the associative pathway. However, the reaction barriers for the dissociative and associative pathways of tri-phosphate hydrolysis are very close in aqueous solution, though the dissociative pathway is more favorable in the gas phase. High dielectric solvents, such as water, significantly lower the activation barrier of the associative pathway due to the greater solvation energy of the associative transition states than that of the reactant complex. By contrast, the barrier of the dissociative pathway, with respect to the gas phase, is less sensitive to the surrounding dielectric. In the associative hydrolysis pathway of the tri-phosphate ester, negative charge is transferred from the gamma-phosphate to beta-phosphate through the bridging ester oxygen and results in Pgamma-O bond dissociation. No analogous charge transfer was observed in the dissociative pathway, where Pgamma-O bond dissociation resulted from proton transfer from the gamma-phosphate to the bridge oxygen. Finally, the active participation of local water molecules can significantly lower the activation energy of the dissociative pathway for both mono-phosphate and tri-phosphate. 相似文献
45.
C. B. Collins 《General Relativity and Gravitation》1991,23(3):321-334
A particular class of solutions of Einstein's field equations, in which the source is a perfect fluid and the geometry admits a two-parameter Abelian isometry group with spacelike orbits, is examined for the possible admission of higher symmetries. It is shown that in certain cases the geometry is spatially homogeneous, being either of the orthogonal Bianchi type I or of the tilted Bianchi-Behr type VIUh withh = –4, and that no other cases of higher symmetry (in the sense of isometry groups) are possible. The global behaviour of the Bianchi-Behr type VIUh, models is studied, and conformal diagrams are given. This investigation is extended to the case when additional homothetic vectors are admitted, and a parallel study of the global behaviour, complete with conformal diagrams, is provided. A brief argument also shows that, within the class considered, whenever the fluid flow-lines possess identical thermal histories, the geometry is necessarily spatially homogeneous. 相似文献
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