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991.
A high-performance capillary electrophoretic (CE) method with electrochemical detection (ED) has been developed for determination of the pharmacologically active flavonoids in Ginkgo biloba L. and phytopharmaceuticals containing its extract. Epicatechin, catechin, rutin, apigenin, luteolin, and quercetin are important flavonoids in this plant. Operated in a wall-jet configuration, a 300 micro m diameter carbon-disk electrode was used as working electrode with good response to the six analytes at +1000 mV (relative to the SCE). Under the optimum conditions, the analytes were separated within 22 min in a borax buffer (pH 9.0). Excellent linearity was obtained over two orders of magnitude and detection limits (S/N=3) ranged from 1.4 x 10(-7) to 5.0 x 10(-7) g mL(-1) for all six analytes. The method was successfully used for assay of Ginkgo biloba L. and its phytopharmaceuticals after a relatively simple extraction procedure; the results obtained were satisfactory. 相似文献
992.
The photoelectron spectrum of tetrakis (methylidene)cyclobutane ( 1 , ‘[4]radialene’) is reported. The electronic states of 1 + are assigned on the basis of model calculations and with reference to related systems. Jahn-Teller activity in the degenerate states is discussed. A failure of the simple LCBO-model for the π(eg)-orbital of 1 is noted and traced to the fact that this orbital, though having a symmetry-equivalent π*-counterpart, does not interact with it. This feature is confined to [4n]radialenes; their total π-energies are therefore higher than those of the other members. It is shown that radialenes, in principle, do not constitute a class analogous to that of the linear polyenes as inferred earlier. 相似文献
993.
The most prominent ion in the mass spectra of C6F5CH2X (X ? H, Br, CH:CH2, COCl, and CH2C6F5) is C7F5H2+, formulated as the pentafluorotropylium cation. This ion is also found, in an amount comparable to the parent ion, in the spectrum of (C6F5)2CH2. The heptafluorotropylium cation is found similarly in the spectrum of C6F5CF3. The mass spectra of (C6F5)2CHBr and [(C6H5)2CH]2 exhibit an ion C13F10H+ as the base peak, which is probably a pentafluorophenylpentafluorotropylium cation. The alcohol (C6F5)2CHOH shows loss of C6F5, followed by 2H, as a major breakdown pathway. The mode of formation, and the subsequent fragmentation, of the major ions in these spectra, are discussed. 相似文献
994.
Ionizable residues play important roles in protein structure and activity, and proton binding is a valuable reporter of electrostatic interactions in these systems. We use molecular dynamics free energy simulations (MDFE) to compute proton pKa shifts, relative to a model compound in solution, for three aspartate side chains in two proteins. Simulations with explicit solvent and with an implicit, dielectric continuum solvent are reported. The implicit solvent simulations use the generalized Born (GB) model, which provides an approximate, analytical solution to Poisson's equation. With explicit solvent, the direction of the pKa shifts is correct in all three cases with one force field (AMBER) and in two out of three cases with another (CHARMM). For two aspartates, the dielectric response to ionization is found to be linear, even though the separate protein and solvent responses can be nonlinear. For thioredoxin Asp26, nonlinearity arises from the presence of two substates that correspond to the two possible orientations of the protonated carboxylate. For this side chain, which is partly buried and has a large pKa upshift, very long simulations are needed to correctly sample several slow degrees of freedom that reorganize in response to the ionization. Thus, nearby Lys57 rotates to form a salt bridge and becomes buried, while three waters intercalate along the opposite edge of Asp26. Such strong and anisotropic reorganization is very difficult to predict with Poisson-Boltzmann methods that only consider electrostatic interactions and employ a single protein structure. In contrast, MDFE with a GB dielectric continuum solvent, used for the first time for pKa calculations, can describe protein reorganization accurately and gives encouraging agreement with experiment and with the explicit solvent simulations. 相似文献
995.
Robert Weis Reto Brun Robert Saf Werner Seebacher 《Monatshefte für Chemie / Chemical Monthly》2003,134(7):1019-1026
Summary. 4-Aminobicyclo[2.2.2]octanones which were prepared in one-pot-reactions from benzylidene acetone and dialkylammonium rhodanides
were reduced stereoselectively to their corresponding alcohols. The activities of the bicyclic compounds against causative
organisms of tropical deseases were examined. The 4-aminobicyclo[2.2.2]octan-2-ols were in general more active against Trypanosoma b. rhodesiense and Plasmodium falciparum than the corresponding keto compounds.
Corresponding author. E-mail: robert.weis@uni-graz.at
Received November 27, 2002; accepted December 2, 2002
Published online May 2, 2003 相似文献
996.
WO3 and WO3:P (5 mol% H3PO4) thin films were prepared using the sol-gel route and the electrochromic properties of the films were investigated using
in situ spectroelectrochemical methods. The measurements were performed in propylene carbonate solution with 0.1 M LiClO4 as electrolyte. During the cathodic polarization at –0.8 V a blue coloration is observed with a reversible variation between
14% and 84% of the transmittance at λ=633 nm. The kinetics for the bleaching process is faster for the WO3:P film than for the undoped WO3 film.
Electronic Publication 相似文献
997.
The structures of the new compound La(1.2)Sr(2.7)IrO(7.33) and the recently discovered La(1.2)Sr(2.7)RuO(7.33) have been solved using a combination of X-ray and neutron diffraction. Both compounds crystallize in the trigonal space group Rm and consist of isolated MO6 (M = Ru, Ir) octahedra, which are arranged in well-defined hexagonal perovskite slabs. These slabs are separated by (Sr2O(1+delta)) layers containing both O2- and (O2)2- ions. The composition can therefore be written as La(1.2)Sr(2.7)MO(7-delta)(O2)delta with delta = 0.33. Results of the magnetic susceptibility and XANES measurements show that the transition metal cations are in a pentavalent state. While in La(1.2)Sr(2.7)RuO(7.33) an antiferromagnetic interaction between the Ru5+ ions is found, La(1.2)Sr(2.7)IrO(7.33) shows a very small temperature-independent paramagnetism down to 1.8 K due to the strong spin-orbit coupling characteristic for the 5d element iridium. 相似文献
998.
Sonesson AW Callisen TH Brismar H Elofsson UM 《Langmuir : the ACS journal of surfaces and colloids》2005,21(25):11949-11956
We have analyzed surface diffusion properties of a variant of Thermomyces lanuginosa lipase (TLL) on hydrophilic silica and silica methylated with dichlorodimethylsilane (DDS) or octadecyltrichlorosilane (OTS). For this study a novel method for analysis of diffusion on solid surfaces was developed. The method is based on fluorescence recovery after photobleaching using confocal microscopy. When a rectangular area of the sample was photobleached, fluorescence recovery could be analyzed as one-dimensional diffusion, resulting in simplified mathematical expressions for fitting the data. The method was initially tested by measuring bovine serum albumin diffusion on glass, which led to a diffusion coefficient in good correspondence to earlier reports. For the analysis of TLL diffusion, ellipsometry data of TLL adsorption were used to calibrate fluorescence intensity to surface density of lipase, enabling measurements of the diffusion coefficient at different surface densities. The average diffusion coefficient was calculated in two time intervals after adsorption. Mobile fraction and diffusion coefficient were lowest on the OTS surface, when extrapolated to infinite surface dilution. Moreover, the diffusion rate decreased with time on the hydrophobic surfaces. Our observations can be explained by the surface dependence on the distribution of orientations and conformations of adsorbed TLL, where the transition from the closed to the catalytically active open and more hydrophobic structure is important. 相似文献
999.
Gaballah ST Hussein YH Anderson N Lian TT Netzel TL 《The journal of physical chemistry. A》2005,109(48):10832-10845
Femtosecond, picosecond, and nanosecond transient absorbance (TA) and picosecond emission kinetics results are presented for three 5-(1-pyrenyl)-2'-deoxyuridine nucleosides each with a different two-atom linker joining pyrenyl C-1 to uracil C-5. The linkers are respectively -NHCO-, -(CH(2))(2)-, and -C[triple bond]C- for PAdU, PEdU, and PYdU. For all three nucleoside conjugates, most conformers undergo intramolecular charge transfer (CT) from their pyrenyl (1)(pi,pi) excited states to form Py(*+)/dU(*-) CT products in ultrashort times: 相似文献
1000.
The chance to prepare sterically and inductively stabilized arsa‐ and phosphaalkenes of the type PhE=C(CF3)2 (E = As, P) by reacting phenyl‐bis(trimethylsilyl)‐arsane ( 1 ) and ‐phosphane ( 5 ), respectively, with hexafluoroacetone (HFA) was investigated. The insertion of the carbonyl function in one of the Si–E bonds was found to occur at temperatures between ?78 and 20 °C. The elimination of hexamethyldisiloxane, which in case of acylamides and ketones spontaneously follows the insertion and in case of RE(SiMe3)–CR′2(OSiMe3) at least can be initiated by solid sodium hydroxide as catalyst, turned out to be impossible for the primary products PhE(SiMe3)–C(CF3)2‐OSiMe3 [E = As ( 2 ), P ( 6 )]. 2 and 6 were characterized by analytical (C, H) and spectroscopic methods (IR, NMR, MS). 相似文献