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排序方式: 共有3344条查询结果,搜索用时 31 毫秒
41.
Bhattarai KM Davis AP Perry JJ Walter CJ Menzer S Williams DJ 《The Journal of organic chemistry》1997,62(24):8463-8473
The macrocyclic "cholaphanes" 3a-c were synthesized from the inexpensive steroid cholic acid. Like earlier relatives they feature substantial cavities with inward-directed hydroxyl groups, suitable for binding polar molecules such as carbohydrates in nonpolar media. New features are the externally directed alkyl chains, promoting solubility in organic solvents, and (in the case of 3b/c) reduced conformational freedom resulting from truncation of the steroidal side-chain. In particular, modeling shows that the smallest macrocycle 3c possesses very little flexibility, preferring an open conformation which is also revealed in the X-ray crystal structure of its pentahydrate. NMR studies indicated that all three cholaphanes form 1:1 complexes with octyl beta-D-glucoside in CDCl(3), with K(a) = 600-1560 M(-)(1). Cholaphanes 3b/c proved able to extract methyl beta-D-glucoside from aqueous solutions into CHCl(3). The transport of methyl beta-D-glucoside across a chloroform barrier was also demonstrated for 3c. 相似文献
42.
Klaus van Benthem Stephan Krämer Wilfried Sigle Manfred Rühle 《Mikrochimica acta》2002,138(3-4):181-193
An understanding of the correlation between microstructures and properties of materials require the characterization of the
material on many different length scales. Often the properties depend primarily on the atomistics of defects, such as dislocations
and interfaces. The different techniques of transmission electron microscopy allow the characterization of the structure and
of the chemical composition of materials with high spatial resolution to the atomic level: high resolution transmission electron
microscopy allows the determination of the position of the columns of atoms (ions) with high accuracy. The accuracy which
can be achieved in these measurements depends not only on the instrumentation but also on the quality of the transmitted specimen
and on the scattering power of the atoms (ions) present in the analyzed column.
The chemical composition can be revealed from investigations by analytical microscopy which includes energy dispersive X-ray
spectroscopy, mainly quantitatively applied for heavy elements, and electron energy-loss spectroscopy. Furthermore, the energy-loss
near-edge structure of EELS data results in information on the local band structure of unoccupied states of the excited atoms
and, therefore, on bonding. A quantitative evaluation of convergent beam electron diffraction results in information on the
electron charge density distribution of the bulk (defect-free) material.
The different techniques are described and applied to different problems in materials science. It will be shown that nearly
atomic resolution can be achieved in high resolution electron microscopy and in analytical electron microscopy. Recent developments
in electron microscopy instrumentation will result in atomic resolution in the foreseeable future. 相似文献
43.
Probing the water coordination of protein-targeted MRI contrast agents by pulsed ENDOR spectroscopy.
Stephan G Zech Wei-Chuan Sun Vincent Jacques Peter Caravan Andrei V Astashkin Arnold M Raitsimring 《Chemphyschem》2005,6(12):2570-2577
A novel methodology based on electron-nuclear double resonance (ENDOR) spectroscopy is used for the direct determination of the water coordination number (q) of gadolinium-based magnetic resonance imaging (MRI) contrast agents. Proton ENDOR spectra can be obtained at approximately physiological concentrations for metal complexes in frozen aqueous solutions either in the presence or absence of protein targets. It is shown that, depending on the structure of the co-ligand, the water hydration number of a complex in aqueous solution can be significantly different to when the complex is noncovalently bound to a protein. From the ENDOR spectra of the exchangeable protons, precise information on the metal-proton distance can be derived as well. These essential parameters directly correlate with the efficacy of MRI contrast agents and should therefore aid the development of novel, highly efficient compounds targeted to various proteins. 相似文献
44.
Vitamin-B12-Catalyzed C, C-Bond Formation: Synthesis of Jasmonates via Sequential Radical Reaction The Cbl-catalyzed electroreduction of 3-(2′-bromo-1′-ethoxyethoxy)cyclopenten ( 1a ) in presence of 1-cyanovinyl-acetate ( 8 ) gave, in a sequential radical reaction (5-exo-trig-cyclization of 1a followed by addition to 8 ), 1-cyano-2-(2′-ethoxy-hexahydro-2′H-cyclopenta[b] furan-4′-yl)ethyl acetate ( 10a ). This intermediate was transformed to methyl jasmonate ( 7 ; four steps) and epituberolide ( 9 ; three steps) in 20 and 31% yield, respectively, from cyclopent-2-en-l-ol. 相似文献
45.
Stephan Wehner 《Archive for Mathematical Logic》1995,34(3):183-196
To date the problem of finding a general characterization of injective enumerability of recursively enumerable (r.e) classes of r.e. sets has proved intractable. This paper investigates the problem for r.e. classes of cofinite sets. We state a suitable criterion for r.e. classesC such that there is a boundn with |-A|n for allAC. On the other hand an example is constructed which shows that Lachlan's condition (F) does not imply injective enumerability for r.e. classes of cofinite sets. We also look at a certain embeddability property and show that it is equivalent with injective enumerability for certain classes of cofinite sets. At the end we present a reformulation of property (F).Thank you for technical support, Wolfgang Eppler, for intellectual support, Alistair Lachlan, and for proof-reading, Martin Kummer. Thanks also to the anonymous referee 相似文献
46.
47.
Alves GA Amato S Anjos JC Appel JA Astorga J Bracker SB Cremaldi LM Darling CL Dixon RL Errede D Fenker HC Gay C Green DR Halling AM Jedicke R Karchin PE Kwan S Leuking LH Mantsch PM de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Passmore D Rafatian A dos Reis AC Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wallace A Wu Z 《Physical review D: Particles and fields》1994,49(9):R4317-R4320
48.
The present paper studies the properties of Hubbard-like models in high spatial dimensionsD. In a first par the limit of infinite dimension and its main features-i.e.i) the mapping onto a generalized atomic model with an additional auxiliary field andii) the validity of the local approximation for the self-energy-are worked out in a systematic (1/D)-expansion. Since the hopping matrix elements have to be properly scaled with the dimensionD, the (1/D)-expansion is also an expansion in the hopping amplitude. Thus for small hopping theD-limit may serve as a proper approximation for finite-dimensional systems. The second part of the paper adopts the hybridisation-perturbation theory of the single impurity Anderson model in order to construct a perturbation theory for the auxiliary field of the generalized atom which can also be interpreted as an expansion in the hopping amplitude. The non-crossing approximation (NCA) is used to study the antiferromagnetic phase transtion of theD-Hubbard model in the case of half filling: the critical temperature, the antiferromagnetic order parameter and the free energy of the lattice system are calculated. The NCA-results are in quite good agreement with recent results from the imaginary-time discretisation method. 相似文献
49.
Alves GA Amato S Anjos JC Appel JA Astorga J Bracker SB Cremaldi LM Dagenhart WD Darling CL Dixon RL Errede D Fenker HC Gay C Green DR Jedicke R Karchin PE Kennedy C Kwan S Lueking LH de Mello Neto JR Metheny J Milburn RH de Miranda JM da Motta Filho H Napier A Passmore D Rafatian A dos Reis AC Ross WR Santoro AF Sheaff M Souza MH Spalding WJ Stoughton C Streetman ME Summers DJ Takach SF Wallace A Wu Z 《Physical review letters》1996,77(12):2388-2391
50.