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The complete Boolean homomorphisms from the category algebra C(X) of a complete matrix space X to the category algebra C(Y) of a Baire topological space Y are characterized as those σ-homomorphisms which are induced by continuous maps from dense G8-subsets of Y into X. This result is used to deduce a series of related results in topology and measure theory (some of which are well-known). Finally a similar result for the complete Boolean homomorphisms from the category algebra C(X) of a compact Hausdorff space X tothe category algebra C(Y) of a Baire topological space Y is proved.  相似文献   
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Activation energies Eα and free enthalpies of activation ΔG? were determined by NMR for the ring inversions of nineteen 1.3-dioxanes, 1.3-dithianes and 1.3-oxathianes and their methyl-substituted derivatives. The rate constant k of the chair to chair-inversion of these rings depend on the number and positions of the substituents. In substituted 1.3-dioxanes k is lower than in 1.3-dioxane itself if two geminal CH3-groups are situated in position 5. However, k becomes higher when the geminal CH3-groups are in positions 2 or 4. The rate constants are particularly high for 4.4.6.6-tetramethyl-1.3-dioxane. A similar dependence of k on substituents has been observed in the oxathianes, while with 1.3-dithianes only a small influence is noticed. The effect of the methyl substituents in positions 2 or 4 in 1.3-dioxanes can be explained by assuming that the chair conformation is deformed by 1.3-interactions. In 1.3-dithianes such 1.3-interactions are expected to be smaller because of the larger C? S bond length. For 1.3-oxathianes the dependence of k on the substituents is more difficult to understand since these molecules are not symmetrical.  相似文献   
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The first direct assignment of highly dynamic enantiofacial discrimination acting on a single heterocyclic substrate has been achieved by a combination of experimental and theoretical CD spectroscopy. The interaction of chirally modified hosts based on triphenylene ketals with appropriate prochiral guests can lead to the preferential formation of one diastereomeric host-guest complex. This reversible stereoselective binding transmits the chiral information from remote chiral groups in the host to the strongly absorbing triphenylene chromophore, which gives rise to self-induced CD. This effect was exploited for the determination of the enantiofacial recognition in various host-guest systems. Inversion of the steric demand either of the chiral substituents at the host or of the prochiral guest leads to almost complete inversion of the resulting CD spectra. For the assignment of the absolute stereochemistry of the complexes, a combined molecular dynamics/quantum-chemical approach was successfully employed. Despite the size and the highly dynamic character of the supramolecular systems, fundamental properties of the systems and details of the spectra were simulated accurately, providing access to fast and reliable assignment of the enantiofacial preference. The results are highly consistent with available X-ray data.  相似文献   
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Exposure to UVB irradiation is a major risk factor for the development of skin cancer. Therefore, it is important to identify agents that can offer protection against UVB-caused damage. Photocarcinogenesis is caused largely by mutations at sites of incorrectly repaired DNA photoproducts, of which the most common are the cyclobutane pyrimidine dimers (CPDs). In this study, we demonstrated that 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] protects primary human keratinocytes against the induction of CPDs by UVB. This protection required pharmacologic doses 1,25(OH)2D3 and an incubation period of at least 8 h before irradiation. Furthermore, we provided arguments indicating that the anti-proliferative capacity of 1,25(OH)2D3 underlies its protective effect against UVB-induced DNA damage. Finally, we showed that 19-nor-14-epi-23-yne-1,25(OH)2D3 (TX 522) and 19-nor-14,20-bisepi-23-yne-1,25(OH)2D3 (TX 527), two low-calcemic analogues of 1,25(OH)2D3, were even 100 times more potent than the parent molecule in inhibiting UVB-caused DNA damage. These molecules are therefore promising candidates for the chemoprevention of UVB-induced skin cancer.  相似文献   
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