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991.
Larrosa I Da Silva MI Gómez PM Hannen P Ko E Lenger SR Linke SR White AJ Wilton D Barrett AG 《Journal of the American Chemical Society》2006,128(43):14042-14043
The first total synthesis of (+)-clavilactone B, a potent antifungal agent and novel tyrosine kinase inhibitor, is described. The absolute configuration of clavilactones has been unambiguously established by using Sharpless asymmetric epoxidation to generate the enantiomerically pure substrate. The strategy highlights the use of a powerful and convergent three-component benzyne coupling with a methylallyl Grignard and a chiral epoxy-aldehyde to generate two C-C bonds and install the carbon skeleton of clavilactone. Oxidative lactonization, ten-membered ring construction by ring closing metathesis, and oxidation gave clavilactone B. 相似文献
992.
The hydration structures and dynamics of phenol in aqueous solution at infinite dilution are investigated using molecular-dynamics simulation technique. The simulations are performed at several temperatures along the coexistence curve of water up to the critical point, and above the critical point with density fixed at 0.3 g/cm3. The hydration structures of phenol are characterized using the radial, cylindrical, and spatial distribution functions. In particular, full spatial maps of local atomic (solvent) density around a solute molecule are presented. It is demonstrated that in addition to normal H bonds with hydroxyl group of phenol, water forms pi-type complexes with the center of the benzene ring, in which H2O molecules act as H-bond donor. At ambient conditions phenol is solvated by 38 water molecules, which make up a large hydrophobic cavity, and forms on average 2.39 H bonds (1.55 of which are due to the hydroxyl group-water interactions and 0.84 are due to the pi complex) with its hydration shell. As temperature increases, the hydration structure of phenol undergoes significant changes. The disappearance of the pi-type H bonding is observed near the critical point. Self-diffusion coefficients of water and phenol are also calculated. Dramatic increase in the diffusivity of phenol in aqueous solution is observed near the critical point of simple point-charge-extended water and is related to the changes in water structure at these conditions. 相似文献
993.
Lyudmila F. Sharanda Igor V. Babich Igor V. Plyuto Jerzy Stoch 《Applied Surface Science》2006,252(24):8549-8556
Hybrid carbon-coated alumina supports have been synthesised using 4,4′-methylenebis(phenylisocyanate) as carbon precursor. The adsorption of 4,4′-methylenebis(phenylisocyanate) on the alumina support is irreversible, the resulting organic moiety can undergo pyrolysis under elevated temperature with the formation of carbon coating on the alumina support. Carbon loading in the synthesised materials and thus a degree of coverage of the alumina surface with carbon layer can be increased by repetition of 4,4′-methylenebis(phenylisocyanate) adsorption-pyrolysis cycles. The carbon coating does not substantially influence the pore structure of the initial alumina support. Upon increasing the carbon loading, the carbon coating becomes more uniform with respect to carbon localisation both on the internal and the external surface of the alumina support. The carbon coating on an alumina support can be discriminated from carbonaceous deposits due to a difference in the steady-state surface charging of the samples. Moreover, carbonaceous surface species which associated with CO, CO and OCO groups in carbon coating can also be identified. 相似文献
994.
Igor Nikolaev 《Proceedings of the American Mathematical Society》2006,134(4):973-981
The ``noncommutative geometry' of complex algebraic curves is studied. As a first step, we clarify a morphism between elliptic curves, or complex tori, and -algebras , or noncommutative tori. The main result says that under the morphism, isomorphic elliptic curves map to the Morita equivalent noncommutative tori. Our approach is based on the rigidity of the length spectra of Riemann surfaces.
995.
We show that if α > 1 is any fixed integer, then for a sufficiently large x > 1, the nth Fibonacci number Fn is a base α pseudopfime only for at most (4 + o(1))π(x) of posifive integers n x. The same result holds for Mersenne numbers 2n — 1 and for one more general class of Lucas sequences. A slight modification of our method also leads to similar results for polynomial sequences f(n), where f ∊ [X]. Finally, we use a different technique to get a much sharper upper bound on the counting fimction of the positive integers n such that φ(n) is a base α pseudoprime, where φ is the Euler function. 相似文献
996.
Copolymers of N-acryloyl-m-aminophenylboronic acid (NAAPBA) with acryamide (AA), N,N-dimethylacrylamide (DMAA), and N-isopropylacrylamide (NIPAM) were found to adsorb on cross-linked agarose gel (Sepharose CL-6B) in the pH range from 7.5-9.2, due to specific boronate-sugar interactions. The molar percentages of phenylboronic acid (PBA) groups in the boronate-containing copolymers (BCCs), as estimated by 1H NMR spectroscopy, were 13, 10, and 16%, respectively, whereas the apparent ionization constants, the pKa values, of the copolymers were similar and equal to 9.0 +/- 0.2 at 20 degrees C. The copolymers adsorption capacities were in the range of 15-30 mg x ml(-1) gel (14-36 micromol pendant PBA ml(-1) gel) at pH 9.2 and decreased with decreasing pH value. The interaction of monomeric NAAPBA with Sepharose CL-6B was characterized by an equilibrium association constant of 53 +/- 17 M(-1), the chromatographic capacity factor k' = 1.8, and a total content of binding sites of 27 +/- 10 micromol x ml(-1) gel at pH 9.2. The weak reversible binding of monomeric NAAPBA and almost irreversible binding of NAAPBA copolymers to the gel at pH 9.2 suggested a multivalent character of the copolymer adsorption. At pH 7.5, the maximal adsorption capacity was displayed by the AA-NAAPBA copolymer (15 mg x ml(-1) gel). All the BCCs could be completely desorbed from the gel by 0.1 M fructose in aqueous buffered media with pH values from 7.5-9.2. The strong adsorption of AA-NAAPBA on agarose gel probably relates to the conformation of the copolymer in aqueous solution and provides opportunities for biomedical applications of the copolymer under physiological conditions. Multivalent, weak-affinity adsorption of BCCs to the agarose gel seems to be a tentative model for the copolymers' binding to oligo- and polysaccharides of cell membranes and mucosal surfaces. 相似文献
997.
998.
Clark M Guarnieri F Shkurko I Wiseman J 《Journal of chemical information and modeling》2006,46(1):231-242
A new application of the grand canonical thermodynamics ensemble to compute ligand-protein binding is described. The described method is sufficiently rapid that it is practical to compute ligand-protein binding free energies for a large number of poses over the entire protein surface, thus identifying multiple putative ligand binding sites. In addition, the method computes binding free energies for a large number of poses. The method is demonstrated by the simulation of two protein-ligand systems, thermolysin and T4 lysozyme, for which there is extensive thermodynamic and crystallographic data for the binding of small, rigid ligands. These low-molecular-weight ligands correspond to the molecular fragments used in computational fragment-based drug design. The simulations correctly identified the experimental binding poses and rank ordered the affinities of ligands in each of these systems. 相似文献
999.
Cravetchi IV Taschuk MT Tsui YY Fedosejevs R 《Analytical and bioanalytical chemistry》2006,385(2):287-294
The analytical performance of femtosecond laser-induced breakdown spectroscopy (LIBS) for elemental microanalysis of aluminium
alloys and for mapping precipitate distribution on the sample surface has been studied in detail. A Ti–sapphire laser system
producing pulses of 130 fs at 800 nm was used to generate the laser-induced plasma. Multi-element microanalysis of commercially
available aluminium alloys was performed in air at atmospheric pressure. Crater characteristics such as diameter and crater
morphology were characterized by optical and scanning-electron microscopy. Scaling of plasma emission and limit of detection
as a function of laser pulse energy was also investigated. Current experimental results are presented and are compared with
previous nanosecond microLIBS measurements. 相似文献
1000.
Regulation of mesenchymal stem cell adhesion and orientation in 3D collagen scaffold by electrical stimulus 总被引:3,自引:0,他引:3
Cell adhesion and orientation are important for both natural and engineered tissues to fully achieve physiologic functions. Based on diverse cellular responses induced by electrical stimulus on 2D substrate, we applied non-invasive electrical stimulus to regulate cell adhesion and orientation of bone marrow-derived mesenchymal stem cells (MSCs) and fibroblasts in a reconstituted 3D collagen-based scaffold. While fibroblasts were induced to reorient perpendicularly in response to direct current electrical stimulus, rat MSCs showed only slight changes in cell reorientation. Multiphoton microscopy revealed that rat MSCs exhibited much stronger 3D adhesion, which appears to resist cell reorientation. Only in response to a large electrical stimulus (e.g., 10 V/cm), collagen fibers around rat MSCs became disconnected and loosely reorganized. In contrast, the collagen fibers surrounding the fibroblasts were entangled in a random network and became preferentially aligned in the direction of the electrical stimulus. When incubated with integrin antibodies, both fibroblasts and rat MSCs failed to respond to electrical stimulus, providing evidence that integrin-dependent molecular mechanisms are involved in 3D cell adhesion and orientation. Elucidation of physical regulation of 3D cell adhesion and orientation may offer a novel approach in controlling cell growth and differentiation and could be useful for stem cell-based therapeutic application and engineering tissue constructs. 相似文献