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831.
Castro M Griffiths D Patel A Pattrick N Kitson C Ladlow M 《Organic & biomolecular chemistry》2004,2(19):2814-2820
The synthesis and associated structure-activity relationships for gene transfection of a series of spermine-derived cationic gemini surfactants incorporating diamino acid headgroups and either identical (symmetrical) or different (unsymmetrical) lipophilic tailgroups is described. Transfection activity is found to depend critically upon the structural elements present. 相似文献
832.
A simple and rapid potentiometric method for the estimation of ascorbic acid in pharmaceutical dosage forms has been developed. The method is based on treating ascorbic acid with iodine and titration of the iodide produced equivalent to ascorbic acid with silver nitrate using Copper Based Mercury Film Electrode (CBMFE) as an indicator electrode. Interference study was carried to check possible interference of usual excipients and other vitamins. The precision and accuracy of the method was assessed by the application of lack-of-fit test and other statistical methods. The results of the proposed method and British Pharmacopoeia method were compared using F and t-statistical tests of significance. 相似文献
833.
Stellmack MA Viemeister NF Byrne AJ 《The Journal of the Acoustical Society of America》2004,116(2):1149-1159
This study examined whether the level effects seen in monaural intensity discrimination (Weber's law and the "near miss") in a two-interval task are also observed in discrimination of interaural intensity differences (IIDs) in a single-interval task. Both tasks were performed for various standard levels of 4-kHz pure tones and broadband noise. The Weber functions (10 log deltaI/I versus I in dB) in the monaural and binaural conditions were parallel. For noise, the Weber functions had slopes close to zero (Weber's law) while the Weber functions for the tones had a mean slope of -0.089 (near miss). The near miss for the monaural and binaural tasks with tones was eliminated when a high-pass masker was gated with the listening intervals. The near-miss was also observed for 250- and 1000-Hz tones in the binaural task despite overall decreased sensitivity to changes in IID at 1000 Hz. The binaural thresholds showed a small (about 2-dB) advantage over monaural thresholds only in the broadband noise conditions. More important, however, is the fact that the level effects seen monaurally are also seen binaurally. This suggests that the basic mechanisms responsible for Weber's law and the near miss are common to monaural and binaural processing. 相似文献
834.
We build upon Mac Lane's definition of a tensor category to introduce the concept of a product system that takes values in a tensor groupoid . We show that the existing notions of product systems fit into our categorical framework, as do the -graphs of Kumjian and Pask. We then specialize to product systems over right-angled Artin semigroups; these are semigroups that interpolate between free semigroups and free abelian semigroups. For such a semigroup we characterize all product systems which take values in a given tensor groupoid . In particular, we obtain necessary and sufficient conditions under which a collection of -graphs form the coordinate graphs of a -graph. 相似文献
835.
Peter Wardman Mark J. Burkitt Kantilal B. Patel Andrew Lawrence Clare M. Jones Steven A. Everett Borivoj Vojnovic 《Journal of fluorescence》2002,12(1):65-68
Lucigenin (LC2+, bis-N-methylacridinium) and 2,7-dichlorofluorescin (DCFH2) are widely used as chemiluminescent or fluorescent probes for cellular oxidative stress, to reflect levels of superoxide (O2
·–) and hydrogen peroxide, respectively. We report mechanistic studies that add to the growing evidence for the unsuitability of either probe except in very well-defined circumstances. The ability for lucigenin to generate superoxide via reduction of LC2+ to LC·+ and redox cycling with oxygen depends on the reduction potential of the LC2+/LC·+ couple. Redox equilibrium between LC·+ and the redox indicator benzyl viologen is established in microseconds after generation of the radicals by pulse radiolysis and indicated E(LC2+/LC·+) –0.28 V vs. NHE. Reaction of LC·+ with O2 to generate O2
·– was also observed directly similarly, occurring in milliseconds, with a rate constant k 3 × 106
M
–1 s–1. Quinones act as redox mediators in LC·+/O2 redox cycling. Oxidation of DCFH2 to fluorescent DCF is not achieved by O2
·– or H2O2, but NO2
·) reacts rapidly: k 1 × 107
M
–1 s–1. Oxidation by H2O2 requires a catalyst: cytochrome c (released into the cytosol in apoptosis) is very effective (even 10 nM). Fluorescence reflects catalyst level as much as O2
·–) production. 相似文献
836.
Williams PA Papadakis SJ Patel AM Falvo MR Washburn S Superfine R 《Physical review letters》2002,89(25):255502
We report on the characterization of torsional oscillators which use multiwalled carbon nanotubes as the spring elements. Through atomic-force-microscope force-distance measurements we are able to apply torsional strains to the nanotubes and measure their torsional spring constants, and estimate their effective shear moduli. The data show that the nanotubes are stiffened by repeated flexing. We speculate that changes in the intershell mechanical coupling are responsible for the stiffening. 相似文献
837.
Theories in which supersymmetry is broken on another brane, which is separated from the minimal supersymmetry standard model (MSSM) matter fields in an extra dimension, are attractive because they may solve the supersymmetric flavor problem. We consider the effects in such theories of new messenger fields with standard model gauge charges and with direct couplings to the supersymmetry breaking sector. The effect on the masses of the MSSM superpartners can be dramatic. In particular, the tachyonic slepton problem of anomaly mediation and the stable slepton problem of gaugino mediation can be cured. 相似文献
838.
Triphenylarsine catalyses the formation of epoxides from carbonyl compounds and tosylhydrazone salts. This convergent synthesis gives complete trans selectivity for all aldehyde and tosylhydrazone salt coupling partners. 相似文献
839.
The formation of ReS(2) inorganic fullerene-like structures containing Re(4) parallelogram units and metal-metal bonds 总被引:2,自引:0,他引:2
Coleman KS Sloan J Hanson NA Brown G Clancy GP Terrones M Terrones H Green ML 《Journal of the American Chemical Society》2002,124(39):11580-11581
The encapsulation of ReO(x) within ReS(2) inorganic fullerene-like cages is described for the first time. The encapsulate was prepared by the sulfidization of both hand-milled and ball-milled samples of ReO(2); partial conversion of the oxide to the sulfide was achieved with the degree of sulfidization depending on the exposure to the sulfidizing agent, H(2)S. 相似文献
840.
H M Al-Hashimi A Majumdar A Gorin A Kettani E Skripkin D J Patel 《Journal of the American Chemical Society》2001,123(4):633-640
We present a new NMR procedure for determining the three-dimensional fold of C2-symmetric nucleic acid homodimers that relies on long-range orientational constraints derived from the measurement of two independent sets of residual dipolar couplings under two alignment conditions. The application is demonstrated on an (15)N/(13)C-enriched deoxyoligonucleotide sequence, d(G-G-G-T-T-C-A-G-G), shown previously to dimerize into a quadruplex in solution and form a pair of G.(C-A) triads and G-G-G-G tetrads (G-tetrad) motifs. One-bond (1)H-(15)N ((1)D(NH)) and (1)H-(13)C ((1)D(CH)) residual dipolar couplings have been measured between nuclei in the bases of these motifs using bacteriophage as an ordering medium, and under direct magnetic field alignment (800 MHz). By combining the two dipolar data sets in an order matrix analysis, the orientation of the G.(C-A) triad relative to the G-tetrad within a contiguous monomeric unit can directly be determined, even in the presence of interstrand/intrastrand NOE ambiguity. We further demonstrate that the orientation of the C2-axis of molecular symmetry in the homodimer relative to the G.(C-A) triad and G-tetrad motifs can unambiguously be determined using the two sets of independent dipolar coupling measurements. The three-dimensional fold of the homodimer determined using this procedure is very regular and in excellent agreement with a previously determined high-resolution NOE-based NMR structure, where interstrand/intrastrand NOEs were treated as ambiguous and where noncrystallographic symmetry constraints were implicitly imposed during the structure calculation. 相似文献