The Constituents of Cynanchum Taiwanianum

The isolation and identification of eleven crystalline components from the aerial part of Cynanchum taiwanianum Yamazaki (Asclepiadaceae) are described. Their structures were determined on the basis of spectral evidence and chemical transformation. Besides caffeic acid, β-amyrin, and methyl phaeophorbide a, the isolated flavonoid components are classified into two groups, i.e. kaempferol derivatives (kaempferol, astragalin, afzelin, trifolin) and quercetin derivatives (quercetin, isoquercitrin, quercitrin, hyperin).     

DNA-PROTEIN CROSSLINKING IN NORMAL HUMAN SKIN FIBROBLASTS EXPOSED TO SOLAR ULTRAVIOLET WAVELENGTHS

Three normal human skin fibroblast cell lines were exposed to the simulated solar UV radiation produced by a fluorescent sunlamp under conditions in which the wavelength components shorter than either 295, 305 or 315 nm were excluded. The level of DNA-protein crosslinks (DPC) was then measured in those cells using the alkaline elution technique either immediately after irradiation or following a 24 h incubation. In each case, cells were exposed to fluences that induce similar levels of DPC. For cells exposed to 10 kJ m(-2) of sunlamp UV > 295 nm, the level of DPC exhibited a 2-5-fold increase following incubation. In contrast, 40-100% of the DPC were removed upon incubation of cells irradiated with either 100 kJ m(-2) of sunlamp UV > 305 nm or 150 kJ m(-2) of sunlamp UV > 315 nm. A major difference between the effects induced by these wavelength regions is that, in addition to DPC, a very high level of pyrimidine dimers is also produced by sunlamp UV > 295 nm, whereas much lower dimer yields result from treatment with either sunlamp UV > 305 nm or sunlamp UV > 315 nm. A potential role for type II DNA topoisomerase in the formation of these DPC resulting from either the change in conformational structure caused by the presence of a high level of dimers or an involvement of this enzyme in dimer excision repair is discussed.     

Oxidative Addition vs. Ligand-Exchange: Fe(II)-Mixed-Phenylchalcogenolate Complexes fac-[PPN][Fe(CO)3(TePh)n(SePh)3-N] (n = 1, 2)

Diphenyldichalcogenides (PhE)₂ (E = Te, Se) react with Fe(0)-phenylchalcogenolate [PPN] [PhEFe(CO)₄] to yield the products of oxidative addition, Fe(II)-mixed-phenylchalcogenolate fac- [PPN][Fe(CO)₃(TePh)_n(ScPh)_3-n] (n = 1, 2). Reactions of [PPN][REFe(CO)₄] (E=Se, R=Me; E=S, R=Et) and diphenyldichalcogenides yielded ligand-exchange products [PPN][PhEFe(CO)₄] (E=Te, Se, S). The compounds [Fe(CO)₃(TePh)(ScPh)₂]^? (l) and [Fe(CO)₃(TePh)₂ (2) crystallize in the isomorphous monoclinic space group C_2/e, with a = 32.035(8), b = 11.708(6), c = 28.909(6) Å, Z = 8, R = 0.048, and R_w = 0.044 (1); with a = 32.089(5), b= 11.745(2), c = 28.990(8) Å, Z = 8, R = 0.048, and R_w = 0.048 (2). The complexes 1 and 2 crystallize as discrete cations of PPN⁺ and anions of [Fe(CO)₃(TcPh)_u(ScPh)_3-n] (n=1, 2), and one half solvent molecule THF. The geometry around Fe(II) is a distorted octahedron with three carbonyl groups and three phenylchalcogenolate ligands occupying facial positions.     

Kinetic and mechanistic studies of the copolymerization of bis-4-substituted-1,2,4-triazoline-3,5-diones with styrenes

Highly reactive 4-substituted-1,2,4-triazoline-3,5-diones (TDs) have been studied extensively as dienophiles, but little work has been done on their role as enophiles and particularly on their use as propagating species in polymerization studies. The copolymerization between bis-4-substituted-1,2,4-triazoline-3,5-diones (bis-TDs) and styrene has been reported. The purpose of the present work was to synthesize new copolymers derived from a variety of substituted styrenes and bis-TDs and to study the mechanism and kinetics of this novel polymerization. Three bis-TDs were prepared: 3,3′-dimethyl-4,4′-bis[3,5-dioxo-1,2,4-triazoline-4-yl] biphenyl (8), t-1,4-bis[3,5-dioxo-1,2,4-triazoline-4-yl] methyl cyclohexane (9), and 4,4′-bis[3,5-dioxo-1,2,4-triazoline-4-yl] phenyl ether (10). Their structures were fully established by spectroscopic studies, elemental analyses, and indirectly, their quantitative ene reactions with 2,3-dimethyl-2-butene. Copolymerization between bis-TDs and substituted styrenes was carried out in dimethylformamide (DMF), tetrahydrofuran (THF), or dichloroethane (DCE). Polymers formed were characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, differential scanning calorimetry (DSC), gel permeation chromatography (GPC), and viscometry. Molecular weights of polymers range from 5000 to 16,000 in most cases. They were stable up to 250°C and higher. Polymers derived from bis-TDs and p-t-butylstyrene, α-methylstyrene, p-nitrostyrene, and p-acetoxystyrene contained only Diels-Alder-ene (DAe) repeating units, whereas those derived from styrene, p-chlorostyrene, p-bromostyrene, p-methylstyrene, p-methoxystyrene, and 4-vinylbiphenyl contained both DAe and double Diels-Alder (dDA) repeating units. A kinetic study of the copolymerization of 4,4′-bis-(3,5-dioxo-1,2,4-triazoline-4-yl) phenyl ether with α-methylstyrene, p-t-butylstyrene, styrene, p-chlorostyrene, and p-nitrostyrene in DCE was carried out; the copolymerization rate constants were 60.9, 49.8, 8.4, 5.5, and 0.8 (1 mol^?1s), respectively.     

Solid-phase synthesis of imidazo[1,2-a]pyridine using sodium benzenesulfinate as a traceless linker

[formula: see text] The preparation of the first library of imidazo[1,2-a]pyridine derivatives on a solid support is described. A sulfone linker strategy was applied in the synthesis. Key steps involved in the solid-phase synthetic procedure include (i) alpha-haloketone resin formation by sulfinate-->sulfone alkylation, (ii) imidazo[1,2-a]pyridine ring formation by treatment with 2-aminopyridine, (iii) sulfone anion alkylation, and (iv) traceless product release by oxidation-elimination. A library of 12 imidazo[1,2-a]pyridines was synthesized.     

Comparison between the pervaporation and vapor permeation performances of polycarbonate membranes

The pervaporation and vapor permeation performance of symmetrical and asymmetrical polycarbonate membranes, prepared via a dry-phase inversion and wet-phase inversion methods, respectively, were studied by measuring the permeation rate and separation factor. It was found that the polymer concentration effect on the pervaporation performance for the symmetrical polycarbonate membrane was lower than that for the asymmetrical polycarbonate membrane. Compared with pervaporation, vapor permeation has a significantly increased separation factor with a decreased permeation rate for the symmetrical polycarbonate membrane. Water molecules preferentially dissolve into the symmetrical polycarbonate membrane and diffuse easily through the membrane.     

Transient electrophoresis of a charged porous particle

The starting electrophoretic motion of a porous, uniformly charged, spherical particle, which models a solvent-permeable and ion-penetrable polyelectrolyte coil or floc of nanoparticles, in an arbitrary electrolyte solution due to the sudden application of an electric field is studied for the first time. The unsteady Stokes/Brinkman equations with the electric force term governing the fluid velocity fields are solved by means of the Laplace transform. An analytical formula for the electrophoretic mobility of the porous sphere is obtained as a function of the dimensionless parameters , , , and , where a is the radius of the particle, κ is the Debye screening parameter, λ is the reciprocal of the square root of the fluid permeability in the particle, ρ_p and ρ are the mass densities of the particle and fluid, respectively, ν is the kinematic viscosity of the fluid, and t is the time. The electrophoretic mobility normalized by its steady-state value increases monotonically with increases in and , but decreases monotonically with an increase in , keeping the other parameters unchanged. In general, a porous particle with a high fluid permeability trails behind an identical porous particle with a lower permeability and a corresponding hard particle in the growth of the normalized electrophoretic mobility The normalized electrophoretic acceleration of the porous sphere decreases monotonically with an increase in the time and increases with an increase in from zero at .     

Probing stereoselective inhibition of the acyl binding site of cholesterol esterase with four diastereomers of 2'-<Emphasis Type="Italic">N</Emphasis>-α-methylbenzylcarbamyl-1, 1'-bi-2-naphthol

Background  

Recently there has been increased interest in pancreatic cholesterol esterase due to correlation between enzymatic activity in vivo and absorption of dietary cholesterol. Cholesterol esterase plays a role in digestive lipid absorption in the upper intestinal tract, though its role in cholesterol absorption in particular is controversial. Serine lipases, acetylcholinesterase, butyrylcholinesterase, and cholesterol esterase belong to a large family of proteins called the α/β-hydrolase fold, and they share the same catalytic machinery as serine proteases in that they have an active site serine residue which, with a histidine and an aspartic or glutamic acid, forms a catalytic triad. The aim of this work is to study the stereoselectivity of the acyl chain binding site of the enzyme for four diastereomers of an inhibitor.     

Aggregation-driven growth of size-tunable organic nanoparticles using electronically altered conjugated polymers

A novel class of monodisperse conjugated polymer nanoparticles have been readily prepared by the facile reprecipitation of poly{1,3-bis[2-(3-alkylthienyl)]azulene} or poly{1,3-bis[2-(3-alkoxythienyl)]azulene}. The multicomponent poly(bithiophene-alt-azulene) macromolecules were efficiently self-assembled into a wide range of size-tunable nanoparticles from a few tens to five hundred nanometers via the hydrophobic and pi-stacking effects in the mixed chloroform/methanol solutions. Electronically altered polymer structures with different alkyl or alkoxy substitutes exhibited variable self-assembling behaviors to precisely tune the size and the optical/electronic properties of nanoparticles. A strong size dependence of continuous bathochromic absorption and significant enhanced emission were observed with the increase of particle size. A linear relationship between the absorption or fluorescence intensities and the particle size was demonstrated as well, and this is very useful to probe the intermolecular interactions and the size evolutions of conjugated polymer nanoparticles. After the size-dependent optical and electronic properties are created, they can be further optimized to improve the performance of materials prior to the use in novel organic nanodevices in a cost-effective way.