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991.
Heinz B Malkmus S Laimgruber S Dietrich S Schulz C Rück-Braun K Braun M Zinth W Gilch P 《Journal of the American Chemical Society》2007,129(27):8577-8584
The photochromicity of fulgimides rests on the existence of open (E) and closed ring (C) isomers. As predicted by the Woodward-Hoffmann rules both isomers can photochemically be interconverted. This interconversion has been studied by femtosecond fluorescence and transient absorption spectroscopy. For either direction (E --> C cyclization and C --> E cycloreversion) a biphasic fluorescence decay on the 0.1-1 ps time scale is observed. The longer time constants of the decays equal the formation times of the photoproducts. The time constants retrieved (0.06 and 0.4 ps for E --> C, 0.09 and 2.4 ps for C --> E) and the associated spectral signatures differ substantially. This indicates that no common excited-state pathway for the two directions exists, as one would infer from a simple Woodward-Hoffmann consideration. These findings support recent quantum dynamic calculations on the excited-state topology of fulgimides. 相似文献
992.
993.
Atesin AC Duckett SB Flaschenriem C Brennessel WW Eisenberg R 《Inorganic chemistry》2007,46(4):1196-1204
Two chiral iridium(I) (R)-BINAP complexes, IrI(CO)((R)-BINAP) (1) and [Ir(CO)2((R)-BINAP)][SbF6] (2), have been synthesized and characterized, and their reactivity with dihydrogen has been studied. Complex 1 is formed on the addition of (R)-BINAP to [Bu4N][IrI2(CO)2] in toluene, and 2 is generated by the addition of AgSbF6 to a solution of 1 in dichloromethane under CO. A structure determination of complex 2 confirms a square planar coordination geometry, while that of 1 reveals a significant tetrahedral distortion from the expected planar coordination. Additionally, the structure of 1 shows a disorder between iodide and CO ligands. The reaction of 1 with H2 proceeds under kinetic control and shows a high degree of kinetic and thermodynamic selectivity; the kinetic product is formed by H2 addition across the P-Ir-CO axis of IrI(CO)((R)-BINAP) and yields two diastereomers which then convert over time to two more stable diastereomers which correspond to oxidative addition across the P-Ir-I axis. The kinetically favored diastereomers are formed in an initial ratio of 8.6:1, corresponding to a DeltaDeltaG* of 1.27 kcal/mol. The reaction of H2 with the C2-symmetric complex 2 also leads to the formation of two diastereomers, with one favored over the other kinetically by a 9.9:1 ratio on extrapolation to t = 0. When these reactions are followed using parahydrogen NMR methods, only one of the initially formed diastereomers in each case is found to exhibit substantial parahydrogen-induced polarization in the hydride resonances at room temperature. 相似文献
994.
Several heteroleptic and homoleptic ruthenium-terpyridine complexes bearing two and four ethynylpyrenyl or ethynyltoluyl residues have been prepared from complexes carrying reactive bromo functions. Cross-coupling promoted by low-valent palladium(0) on these preformed complexes has advantageously been used to prepare the target complexes. The structure of a bis-terpyridine complex carrying four ethynylpyrenyl subunits was determined by single-crystal X-ray diffraction, showing a distorted octahedral geometry around the metal center, with the ethynylpyrenyl fragment being slightly tilted (about 5 degrees) from the terpyridine plane. The molecular packing is characterized by intermolecular pi...pi interaction within dimers. The counteranions and the solvent molecules are entrapped in well-defined channels spanning along the a-axis. The complexes are redox active with a Ru oxidation overlapping the pyrene oxidation and two well-defined ligand-centered reduction processes. Pyrene reduction is irreversible and strongly cathodic. The new multichromophoric complexes are luminescent both in solution and in rigid matrix at 77 K, with room-temperature lifetimes and quantum yields significantly larger than those of [Ru(terpy)2]2+. At room temperature, the toluyl-substituted complexes are triplet metal-to-ligand charge-transfer (3MLCT) emitters, whereas for the pyrene-grafted complexes pyrene-centered emission is observed. For the latter complexes, the energy gap, DeltaTT, between higher 3MLCT levels and lower ligand-centered (3pipi*, 3LC) levels is in the 640-730 cm(-1) range, which results in the interstate dynamics at the basis of the observed luminescent behavior. At 77 K, for the pyrene-grafted complexes, the emission reveals features that are tentatively ascribed to intraligand interactions involving the pyrene and terpyridine units. 相似文献
995.
Microgel particles containing methacrylic acid: pH-triggered swelling behaviour and potential for biomaterial application 总被引:1,自引:0,他引:1
Lally S Mackenzie P LeMaitre CL Freemont TJ Saunders BR 《Journal of colloid and interface science》2007,316(2):367-375
pH-responsive microgels are crosslinked polymer particles that swell when the pH approaches the pK(a) of the ionic monomer incorporated within the particles. In recent work from our group it was demonstrated that the mechanical properties of degenerated intervertebral discs (IVDs) could be restored to normal values by injection of poly(EA/MAA/BDDA) (ethylacrylate, methacrylic acid and butanediol diacrylate) microgel dispersions [J.M. Saunders, T. Tong, C.L. Le Maitre, T.J. Freemont, B.R. Saunders, Soft Matter 3 (2007) 486]. In this work we report the pH dependent swelling and rheological properties of poly(MMA/MAA/EGDMA) (methylmethacrylate and ethyleneglycol dimethacrylate) microgel dispersions. This system was investigated because it contains monomers that are already used as biomaterials. The poly(MMA/MAA/EGDMA) particles exhibit pH-triggered volume swelling ratios of up to ca. 250. The swelling onset for these particles occurs at pH values greater than ca. 6.0. A pK(a) for these particles of ca. 6.7 is consistent with titration and swelling data. Fluid-to-gel phase diagrams for concentrated poly(MMA/MAA/EGDMA) dispersions were determined as a function of polymer volume fraction and pH using tube-inversion measurements. The rheological properties for the gelled microgel dispersions were investigated using dynamic rheology measurements. The elastic modulus data for the poly(MMA/MAA/EGDMA) gelled dispersions were compared to data for poly(EA/MAA/BDDA) microgels. A similar pH-dependence for the elastic modulus was apparent. The maximum elastic modulus was achieved at a pH of about 7.0. The elastic modulus is an exponentially increasing function of polymer volume fraction at pH 7.0. Preliminary cell challenge experimental data are reported that indicate that gelled poly(MMA/MAA/EGDMA) microgel dispersions are biocompatible with cells from human intervertebral discs. However, the duration over which these experiments could be performed was limited by gradual redispersion of the gelled microgel dispersions. Based on the results presented it is suggested that poly(MMA/MAA/EGDMA) microgel would be a good candidate as a biomaterial for structural support of soft connective tissues. 相似文献
996.
Luscombe CK Proemmel S Huck WT Holmes AB Fukushima H 《The Journal of organic chemistry》2007,72(15):5505-5513
The rational design, synthesis, and characterization of a series of novel perfluorinated dendrons 14a,b, 25a,b, 26a,b, and 18 are described. The dendrimers were designed to have a thiol at the focal point for attachment to a gold surface to enable the fabrication of self-assembled monolayers (SAMs). Perfluorinated tails were attached to the periphery to provide solubility in supercritical carbon dioxide, and to increase the hydrophobicity and the stability of self-assembled monolayers formed. Mitsunobu reactions were utilized to provide high-yielding steps allowing large-scale production of the novel dendrimers. 相似文献
997.
A high-performance liquid chromatography (HPLC) system using a porous graphitic carbon (PGC) stationary phase interfaced with an electrospray ionization (ESI) source and a tandem mass spectrometer (MS/MS) for the analysis of cytarabine (ara-C) in mouse plasma samples has been developed in support of a pharmacodynamic study. The graphitized carbon column was adopted for the separation of ara-C and endogenous peaks from mouse plasma samples under the reversed-phase phase mode in liquid chromatography. The retention characteristics of the PGC column and the ionization efficiencies of all analytes based on the experimental factors such as the composition of mobile phases were investigated. The potential of ionization suppression resulting from the endogenous biological matrices on the PGC column during HPLC/ESI-MS/MS was investigated using post-column infusion. The concentrations of ara-C in mouse plasma obtained by using PGC-HPLC/MS/MS and ion-pairing HPLC/MS/MS were found to be in good agreement in terms of analytical accuracy. 相似文献
998.
Breuzard G El-Khoury V Millot C Manfait M Millot JM 《Journal of photochemistry and photobiology. B, Biology》2007,87(2):113-123
The binding and the diffusion of mitoxantrone (MTX) through the plasma membrane was performed by F?rster resonance energy transfer (FRET) from the membrane fluorescent donor (4Di-10ASP) to the co-localized acceptor MTX. The MTX addition to living 4Di-10ASP-tagged cells resulted in the rapid quenching of the probe emission (1s), revealing the MTX binding to the outer leaflet. Then, a slower quenching (about 90s) occurred which corresponded to the MTX flip-flop into the inner leaflet. Changes of MTX integration into the plasma membrane were described in BCRP-overexpressed cells (HCT-116R) treated with (i) the BCRP inhibitor fumitremorgin C (FTC), (ii) cyclosporin A (CSA) and (iii) benzyl alcohol (BA). Treatments with FTC or CSA showed 80% and 40% higher flip-flop of MTX from the outer to the inner leaflet of HCT-116R cells. The addition of BA clearly increased the MTX integration into both outer and inner leaflets. Confocal fluorescence microscopy displayed that FTC, CSA and BA enhanced MTX accumulation in HCT-116R. In conclusion, Fumitremorgin C and agents modulating MTX accumulation resulted in higher MTX integration in the resistant cell membrane and could disrupt the membrane cohesion. This energy transfer method appears well-adapted to describe the drug diffusion through the plasma membrane of living cells. 相似文献
999.
Lewis HD Leveridge M Strack PR Haldon CD O'neil J Kim H Madin A Hannam JC Look AT Kohl N Draetta G Harrison T Kerby JA Shearman MS Beher D 《Chemistry & biology》2007,14(2):209-219
In this report, inhibitors of the gamma-secretase enzyme have been exploited to characterize the antiproliferative relationship between target inhibition and cellular responses in Notch-dependent human T cell acute lymphoblastic leukemia (T-ALL) cell lines. Inhibition of gamma-secretase led to decreased Notch signaling, measured by endogenous NOTCH intracellular domain (NICD) formation, and was associated with decreased cell viability. Flow cytometry revealed that decreased cell viability resulted from a G(0)/G(1) cell cycle block, which correlated strongly to the induction of apoptosis. These effects associated with inhibitor treatment were rescued by exogenous expression of NICD and were not mirrored when a markedly less active enantiomer was used, demonstrating the gamma-secretase dependency and specificity of these responses. Together, these data strengthen the rationale for using gamma-secretase inhibitors therapeutically and suggest that programmed cell death may contribute to reduction of tumor burden in the clinic. 相似文献
1000.
Lepetit C Chermette H Gicquel M Heully JL Chauvin R 《The journal of physical chemistry. A》2007,111(1):136-149
Mono- and polycyclic valence isomers of carbo-[3]oxocarbon C(9)O(3) and carbo-[5]oxocarbon C(15)O(5) have been characterized on the singlet spin state potential energy surface. By contrast to their geometry, their relative stability is highly sensitive to the calculation level. The performance of LDA, GGA, meta-GGA, and hybrid functionals is compared to that of HF, post-HF, and multiconfigurational calculations. The results obtained for C(9)O(3) are compared to those obtained for hydrocarbon analogues such as &[3]pericyclyne C9(H2)3 and carbo-[3]radialene C9(CH2)3 and are analyzed on the basis of an energy decomposition scheme. The respective role of the exchange and correlation counterparts of the functional in the discrepancy of the results is discussed. 相似文献