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At least 343 records · Page 19Linked to original sources
PubMed2006

Symbiotic reagent activation: Oppenauer oxidation of magnesium alkoxides by silylglyoxylates triggers second-stage aldolization.

The treatment of silylglyoxylates with magnesium alkoxides at ambient temperature results in symbiotic Oppenauer oxidation of the alkoxide and Meerwein-Ponndorf-Verley reduction of the silylglyoxylate. The reduced silylglyoxylate undergoes subsequent [1,2]-Brook rearrangement and aldol reaction with the carbonyl oxidation product. The magnesium alkoxide may be accessed via deprotonation of primary or secondary alcohols with EtMgBr, via addition of Grignard reagents to aldehydes, or via CuI-catalyzed alkylation of epoxides. For aliphatic primary alkoxides, moderate levels of anti diastereoselection are observed. A crossover experiment reveals that dissociation of the nascent aldehyde from the magnesium center is faster than [1,2]-Brook rearrangement and aldolization.

Aldehydes↗
PubMed1990

Retinobenzoic acids. 5. Retinoidal activities of compounds having a trimethylsilyl or trimethylgermyl group(s) in human promyelocytic leukemia cells HL-60.

The retinoidal activities of trimethylsilyl or trimethylgermyl-containing retinobenzoic acids are discussed on the basis of differentiation-inducing activity on human promyelocytic leukemia cells HL-60. Compounds with a trimethylsilyl or trimethylgermyl group at the meta position of the generic formula 2 have more potent activities than the corresponding retinobenzoic acids with a m-tert-butyl group. Compounds having two m-trimethylsilyl or -trimethylgermyl groups also have strong activities, and (E)-4-[3-[3,5-bis(trimethylsilyl)phenyl]-3-oxo-1-propenyl]benzoic acid (22, Ch55S) and (E)-4-[3-[3,5-bis(trimethylgermyl)phenyl]-3-oxo-1- propenyl]benzoic acid (35, Ch55G) are more active than retinoic acid by 1 order of magnitude. However, in the para-substituted chalcone derivatives, the replacement of a tert-butyl group (49, Ch40) with a trimethylsilyl (27, Ch40S) or a trimethylgermyl (30, Ch40G) group caused the disappearance of the activity.

Antineoplastic Agents↗
PubMed2004

On the mechanism of nanopore filling of SAPO-5 molecular sieve by nitrogen molecules.

SAPO-5 molecular sieve was synthesized according to patent literature and characterized with X-ray diffraction, electron microscopy (SEM, TEM), and solid state magic angle spinning NMR spectroscopy. The material of particles in the micrometer region was found to consist of ca. 20 nm microcrystallites packed in mostly parallel orientation to ca. 200 nm sized agglomerates. The nitrogen adsorption isotherm was measured at 77.6 K over ca. 7 decades of pressure up to pore saturation. The course of the isotherm is interpreted to consist of filling of the nanopores (diameter, 0.73 nm) up to 2N(2)/unit cell, subsequent multilayer adsorption on the outer surface of the agglomerates, and, finally, pore condensation in the interparticle adsorption space. The nanopore adsorption can be quantitatively reproduced with the statistical mechanical model of a quasi one-dimensional lattice gas taking intermolecular interactions into account. The evaluated energy parameters are of physically reasonable magnitude and agree with literature data. The multilayer part of the adsorption isotherm can be well represented by the Brunauer-Emmett-Teller model yielding a specific outer space area (63 m(2) g(-)(1)) which is consistent with estimated geometrical and pore size analysis data.

Adsorption↗
PubMed2003

X-ray tomographic imaging of Ti/SiC composites.

In this paper, high-resolution tomographic synchrotron X-ray imaging is applied to study the occurrence and evolution of damage in Ti-6Al-4V/SCS6 SiC fibre composite materials. Three composite morphologies of increasing complexity have been studied, namely single fibre, single-ply and multi-ply composites. The single fibre composite was strained to full fibre fragmentation and the progressive introduction of damage monitored. For the single-ply composite, damage was introduced deliberately by laser drilling to establish the effect of damaged fibres on their neighbours, whereas for the multi-ply composite the morphology of a fibre bridging fatigue crack was studied. In addition to traditional mode I fibre fractures, subsequent fibre wedge cracks were observed presumably nucleating from damage introduced into the fibre surface by the first fracture event. In addition to these crack morphologies, spiral defects were observed for the single ply during failure. Finally, for the multi-ply composite, the matrix crack front showed a number of characteristic features, including advancement in fibre-free regions, crack bifurcation near fibres and different crack plane heights either side of a fibre.

Alloys↗
PubMed1986

Adhesion of the clay minerals montmorillonite, kaolinite, and attapulgite reduces respiration of Histoplasma capsulatum.

The respiration of three phenotypes of Histoplasma capsulatum, the causal agent of histoplasmosis in humans, was markedly reduced by low concentrations of montmorillonite but was reduced less by even higher concentrations of kaolinite or attapulgite (palygorskite). The reduction in respiration followed a pattern that suggested saturation-type kinetics: an initial sharp reduction that occurred with low concentrations of clay (0.01 to 0.5% [wt/vol]), followed by a more gradual reduction with higher concentrations (1 to 8%). Increases in viscosity (which could impair the movement of O2) caused by the clays were not responsible for the reduction in respiration, and the clays did not interfere with the availability of nutrients. Scanning electron microscopy after extensive washing showed that the clay particles were tightly bound to the hyphae, suggesting that the clays reduced the rate of respiration of H. capsulatum by adhering to the mycelial surface and, thereby, interfered with the movement of nutrients, metabolites, and gases across the mycelial wall.

Adhesiveness↗
PubMed2006

Self-assembly of well-defined polyacrylamide-polystyrene copolymer on fibrillar clays via ultrasonic-assisted surface-initiated atom transfer radical polymerization.

Well-defined polyacrylamide-polystyrene copolymers were grafted from the fibrillar clay, attapulgite, by a four-step self-assembly process: (i) the gamma-aminopropyltriethoxyl silane was self-assembled onto the surfaces of the attapulgite; (ii) the surface amino groups were amidated with bromoacetylbromide; (iii) the bromo-acetamide modified attapulgite was used as macro-initiator for the surface-initiated atom transfer radical polymerization of styrene with the catalyst of the complex of 1,10-phenanthroline and Cu(I)Br; (iv) the polystyrene grafted attapulgite was then used as macroinitiator for the polymerization of acrylamide. The two steps of the surface-initiated atom transfer radical polymerizations were all conducted under ultrasonic irradiation at room temperature. The product, polyacrylamide-polystyrene copolymers grafted attapulgite, had been characterized with elemental analysis, Fourier transform infrared spectroscopy, Thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy, X-ray diffractometry, and transmission electron microscopy.

Acrylic Resins↗
PubMed1990

Modulation of lung-associated natural killer activity by resident and activated alveolar macrophages.

Alveolar macrophages (AM) freshly obtained by bronchoalveolar lavage suppressed significantly, in a dose-dependent fashion, lung interstitial lymphocytes cytotoxicity against the NK-sensitive target cells, YAC-1. Kinetic experiments revealed that AM-mediated suppression of NK activity was seen following short-term incubation of AM with lymphocytes (4 h) and was unchanged after a 24 h co-culture period. Freshly obtained lung lymphocytes and lymphocytes incubated for 24 h were similarly inhibited by AM. In addition, incubation of AM for 24 h did not abrogate their suppressive effect on lung NK activity. Interestingly, AM-conditioned media, also caused a significant inhibition of lung NK activity. Furthermore, in vitro activation of AM with lipopolysaccharide (LPS, 5 micrograms/ml) and muramyl dipeptide (MDP, 20 micrograms/ml) significantly enhanced the inhibitory effect of AM on lung NK activity. Similarly, in vivo activation of AM locally by intratracheal instillation of attapulgite, an inflammatory agent, resulted in greater AM-mediated down regulation. Taken together, these data indicate that lung NK activity is modulated by locally derived factors and suggest that pharmacologic manipulation of AM may play a determining role in the activation of lung NK activity by biological response modifiers (BRM).

Animals↗
PubMed2005

[Applications of moissanite anvil cell for Raman spectroscopy under high-temperature and high-pressure].

This paper introduces the structure and the feature of moissanite anvil cell, which is composed of moissanite anvil, supporting and creating pressure system, heater system and metal gasket. Because of its high hardness, high heat conductance, low thermal expansion, good thermal stability and low price, moissanite is a good material of anvil for high-temperature and high-pressure experimental studies. With this cell, the Raman spectrum of sodium carbonate solution, sodium sulfate solution and distilled water has been in situ measured under high-temperature and high-pressure. With increasing pressure, it can be observed that the 1066 cm(-1) Raman modes of sodium carbonate solution and the 982 cm(-1) Raman modes of sodium sulfate solution shift to high wave numbers obviously. With increasing temperature, the frequency at maximum intensity shifts to high wave numbers, the full width at half-maximum intensity decreases in the Raman spectrum of water.

Carbon Compounds, Inorganic↗
PubMed1986

A primer on organosilicon chemistry.

Organosilicon chemistry is easily understood because a few generalizations allow us to compare it with the chemistry of carbon, hydrogen and the metals. A silyl group is used in organic synthesis as a metallic substitute for a hydrogen atom, metallic enough in character to control both the site of attack by an electrophile and the position of the final double bond. The silyl group can be thought of as a kind of super-proton. Organosilicon biochemistry is beginning to exist because of the possibility of incorporating silyl groups into drugs, and because the secondary metabolism of silicon-containing compounds is being studied.

Chemical Phenomena↗
PubMed1993

Examples of evolution of microstructure in ceramics and composites.

In this article we describe a number of studies involving the direct observation of microstructural evolution. In general these investigations were carried out to establish the mechanistic paths involved. The materials studied range from fibers being evaluated for use in high-temperature ceramic composites to energetic materials used as propellants. In particular we discuss the room temperature imaging of materials difficult to image by conventional means and the use of the chamber atmosphere to influence microstructural evolution. Imaging of hydroxyapatite formed by chemical means is briefly described as an example of a difficult microstructure. Microstructural evolution during calcium aluminate cement hydration relies on the chamber atmosphere to control moisture loss from the hydrating specimens. In some instances microstructural evolution with heating occurred independently of the chamber atmosphere. Grain growth in PZT films formed by sol-gel processes depends strongly on temperature but does not appear to depend on the chamber atmosphere. This is also the case for the combustion of nitroamine propellants in that their combustion does not depend on access to an external source of oxygen. In other studies, the chamber atmosphere played an indirect role in determining microstructure. However, the mechanistic path driving microstructural evolution in copper-based inks used as conductive paths on electronic substrates is atmosphere dependent. These inks are formulated from copper powder, glass, and an organic binder, and the interaction of the binder with an oxidizing atmosphere allows it to be burned out before significant interaction occurs between the copper powder and the glass. Finally, the microstructural variations during the oxidation of structural composites at high temperature were used to allow assessments of their likely failure mechanisms.

Animals↗
PubMed1994

Synthesis and characterization of silica-based aliphatic ion exchangers.

The chemical literature describing preparation of aliphatic ion exchangers is limited, and although such phases are available commercially, the synthetic schemes are proprietary. The course of our research required the preparation of silica-based aliphatic cation and anion exchangers for which the desired base silica and/or ligand properties were not commercially available. We developed synthetic schemes to prepare silica-based aliphatic sulfonic acid, carboxylic acid and quaternary ammonium ion-exchange phases with active exchange capacities of 0.2-0.9 mumol/m2. Multiple techniques were used to characterize the intermediate and final phases produced in the syntheses, including Fourier transform diffuse reflectance infrared spectroscopy, spot tests, elemental analysis, acid-base titration and elution analysis.

Anion Exchange Resins↗
PubMed2006

Recognition-directed orthogonal self-assembly of polymers and nanoparticles on patterned surfaces.

We demonstrate the patterning of silica substrates with thymine (Thy-PS) and positively charged N-methylpyridinium (PVMP) polymers using photolithography and the subsequent orthogonal modification of these surfaces using diaminopyridine-functionalized polystyrene (DAP-PS) and carboxylate-derivatized CdSe/ZnS core-shell nanoparticles (COO-NP) through diamidopyridine-thymine three-point hydrogen bonding and pyridinium-carboxylate electrostatic interactions, respectively. This two-component orthogonal surface modification was accomplished in a self-sorting, single-step fashion, providing a versatile tool for the rapid and efficient creation of complex materials.

Cadmium Compounds↗
PubMed1991

Comparison of Kaopectate with barium for negative and positive enteric contrast at MR imaging.

In an evaluation of safe, inexpensive, orally administered contrast agents for use at magnetic resonance imaging, different concentrations of the clay agents kaolin-pectate and attapulgite were compared in an in vitro phantom by performing spin-echo imaging with variable echo times to estimate T2 relaxation time. Additional phantoms containing from 0% to 100% attapulgite or barium preparations were compared by using spin-echo and spoiled-gradient-echo techniques to estimate T1, T2, and T2* relaxation times. In vivo spin-echo and gradient-echo images were obtained of 10 healthy volunteers and of five patients with pancreatic disease after oral administration of 12-16 oz of attapulgite preparation. T1 and T2 relaxation times obtained with use of attapulgite were slightly lower than those obtained with kaolin-pectate at both 50% and 25% concentrations. Compared with barium, attapulgite had shorter T1, T2, and T2* relaxation times, and concentration-dependent relaxivity was greater by nearly an order of magnitude. All subjects were able to drink the attapulgite, and the only complaints were of bad taste and mildly loose stools.

Barium Sulfate↗
PubMed1998

Properties of maxillofacial silicone elastomers reinforced with silica powder.

Compounds of a silicone elastomer suitable for preparing maxillofacial prostheses, reinforced with various amounts of silica powder, have been studied for their mechanical response and wetting properties in terms of contact angle. Tensile strength and elongation at break showed an increase with increasing silica volume fraction up to 35%, whereas the Young modulus displayed small dependence on the silica content and the resistance to tear increased continuously with filler volume fraction (Vf). The wetting properties assessed via the contact angle, seemed to degrade with increasing silica Vf, but a dependence on the elastomer network density has also been recorded.

Elasticity↗