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661.
We discuss the application of random walks to generating a random basis of a totally unimodular matrix and to solving a linear program with such a constraint matrix. We also derive polynomial upper bounds on the combinatorial diameter of an associated polyhedron.Supported by NATO grant RG0088/89.Corresponding author. Supported by NSF grants CCR-8900112, CCR-9024935 and NATO grant RG0088/89.  相似文献   
662.
Ion-exchange experiments were carried out for the removal of137Cs, and90Sr/90Y radioisotopes by different cationic forms of amorphous zirconium phosphate. The effect of temperature and concentration on the percent uptake and distribution coefficients (K d ) values (milli-equivalents of ion per gram of exchanger/milli-equivalents of ion per ml of solution) were determined. Experiments were made in simulated pond water at pH 11.4.  相似文献   
663.
(R)-tuberculostearic acid (2) was synthesized in seven steps from (S)-citronellol (5). The carbon chain of 2 was assembled by copper-catalyzed cross coupling of (S)-citronellol tosylate (6) and hexylmagnesium bromide; subsequent ozonolysis and reaction with 6-benzyloxyhexylmagnesium bromide furnished alcohol 10. Functional group manipulation afforded (R)-2 in 49% overall yield from 5. DCC coupling of (R)-2 with 3-O-benzyl-1-O-palmitoyl-sn-glycerol (16), followed by hydrogenolytic removal of the benzyl group and treatment with benzyl bis(diisopropyl)phosphoramidite, afforded phosphoramidite 20. Tetrazole-mediated coupling of 20 with PIM1 head group 21 gave 22, and subsequent debenzylation afforded phosphatidylinositol mono-mannoside, PIM1 (23). Similarly, coupling of 20 and 24 and removal of the benzyl protecting groups gave PIM2 (1c). Both 23 and 1c have a clearly defined acylation pattern, which was confirmed by mass spectrometry, with sn-1 palmitoyl and sn-2 tuberculostearoyl groups on the glycerol moiety. Both 23 and 1c were shown to modulate the release of the pro-inflammatory cytokine, IL-12, in a dendritic cell assay.  相似文献   
664.
A series of tripodal imidazole frameworks (TIFs) are reported based on a tripodal, cavity-containing tris(imidazole) derivative. In the case of [Co(3)Cl(6)(1)(2)]·n(solvent) (TIF-1) which possesses a doubly interpenetrated framework structure, the material exhibits rigid, permanent porosity and selectively absorbs CO(2). The non-interpenetrated [Co(1)(2)(H(2)O)(2)]Cl(2)·4H(2)O (TIF-2) also absorbs gases and vapours fully reversibly exhibiting a reversible phase change in the process and considerable conditioning and hysteresis. The very highly hydrated [Co(1)(2)]Cl(2)·22H(2)O (TIF-3) irreversibly dehydrates to the layered structure [Co(1)(2)]Cl(2)·H(2)O (TIF-4). A nickel analogue [Ni(1)(2)]Cl(2)·22H(2)O (TIF-5) closely related to TIF-3 is also reported along with two isostructural, non-porous materials [MCl(2)(1)] (M = Mn, TIF-6; M = Cd, TIF-7) based on d(5) and d(10) Mn(II) and Cd(II). Some of the materials may be prepared by mechanochemical as well as solution based methods. We liken TIF-1 to a gas cylinder, TIF-2 to a sponge and TIF-3 to a fragile soda can that is crushed on emptying to give TIF-4.  相似文献   
665.
A highly stereocontrolled synthesis of (+)‐chamuvarinin has been completed in 1.5 % overall yield over 20 steps. The key fragment coupling reactions were the addition of alkyne 8 to aldehyde 7 (under Felkin–Anh control), followed by the two step activation/cyclization to close the C20–C23 2,5‐cis‐substituted tetrahydrofuran ring and a Julia–Kocienski olefination at C8–C9 to introduce the terminal butenolide. The inherent flexibility of our coupling strategy led to a streamlined synthesis with 17 steps in the longest sequence (2.2 % overall yield), in which the key bond couplings are reversed. In addition, a series of structural analogues of chamuvarinin have been prepared and screened for activity against HeLa cancer cell lines and both the bloodstream and insect forms of Trypanosoma brucei, the parasitic agent responsible for African sleeping sickness.  相似文献   
666.
We recently demonstrated that ultraviolet laser desorption ionization orthogonal time-of-flight mass spectrometry (UV-LDI o-TOF MS) could be used for the matrix-free analysis of cuticular lipids (unsaturated aliphatic and oxygen-containing hydrocarbons and triacylglycerides) directly from individual Drosophila melanogaster fruit flies (Yew, J. Y.; Dreisewerd, K.; Luftmann, H.; Pohlentz, G.; Kravitz, E. A., Curr. Biol. 2009, 19, 1245–1254). In this report, we show that the cuticular hydrocarbon, fatty acid, and triglyceride profiles of other insects and spiders can also be directly analyzed from intact body parts. Mandibular pheromones from the jaw of a queen honey bee are provided as one example. In addition, we describe analytical features and examine mechanisms underlying the methodology. Molecular ions of lipids can be generated by direct UV-LDI when non-endogenous compounds are applied to insect wings or other body parts. Current sensitivity limits are in the 10 pmol range. We show also the dependence of ion signal intensity on collisional cooling gas pressure in the ion source, laser wavelength (varied between 280–380 nm and set to 2.94 μm for infrared LDI), and laser pulse energy.  相似文献   
667.
We discuss experimentally and theoretically neutron production from the laser driven explosion of gas clusters prepared near the liquid-gas critical point. We let deuterated methane that was prepared very close to its critical temperature and pressure expand through a conical nozzle to create clusters, and then irradiated those clusters with a high intensity pulse from the Texas Petawatt Laser. After ionization, the clusters explode producing energetic ions, some of which fuse with resultant neutron emission. We show that the critical fluctuations present in the nozzle before the expansion influence the dynamics of neutron production. Neutron production near the critical point follows a power law, which is a signature of a second order phase transition and it is consistent with the Fisher model. This result might be relevant for energy production from fusion reactions.  相似文献   
668.
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