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41.
42.
P.D. Collier T. Bryce R. Mallows P.E. Thomas D.J. Frost O. Korver C.K. Wilkins 《Tetrahedron》1973,29(1):125-142
The structures, relative configurations and precursors of seven pigments isolated from the theaflavin fraction of black tea have been established by synthesis, and by NMR and mass spectrometry. The four principal pigments are the bis-flavan-substituted 1′,2′-dihydroxy-3,4-benzotropolones theaflavin, the corresponding isomeric 3- and 3′-monogallates, and the 3,3′-digallate. Collapsed spin-spin couplings and nuclear Overhauser effects in these and a range of mono-flavan-substituted model compounds are interpreted in terms of steric hindrance to rotation of flavan groups with respect to the benzotropolone ring. As a result of these steric effects, the 3-gallate and 3,3′-digallate exist in rotameric forms which are clearly distinguished in the temperature-dependent CD spectra. 相似文献
43.
The use of organofunctional silane chemistry is a flexible and general method for immobilizing biomolecules on silicon oxide surfaces, including fabricating DNA, small-molecule, and protein microarrays. The biggest hurdle in employing dip-pen nanolithography (DPN) for extending this general approach to the nanoscopic domain is the tendency of trialkoxy- and trichlorosilanes to rapidly polymerize due to hydrolysis reactions. The control of the local water concentration between the substrate surface and the scanning AFM tip is critical, both to the physical and chemical processes involved in DPN writing and to the ability to form well-defined thin layers of reactive silanes without extensive polymerization induced disorder. We found that we could control the degree of polymerization through careful choice of the alkoxysilane used as the "ink" for DPN and through control of the relative humidity during inking and writing with the coated AFM tip. As a proof-of-principle, we demonstrate that areas patterned with an alkoxysilane on glass with DPN are functional for subsequent immobilization of fluorescently labeled streptavidin via covalent attachment of biotin. This preliminary result sets the stage for the ability to capture proteins in their fully hydrated state from buffered solution, by molecular recognition onto previously written reactive nanoscopic regions on oxidized silicon and glass. 相似文献
44.
Laser-induced excitation spectra recorded for the electric-quadrupole 3d(6)4s a6D(J)<--3d(5)4s2a6S(5/2) transitions of atomic Mn, allow assignment of the red emission features, previously observed in Mn/RG (RG=Ar, Kr and Xe) matrices with resonance 3d(5)4s4pz6P(5/2)<--3d(5)4s2 a6S(5/2) excitation, to the metastable a6D(9/2) state. Narrow excitation bands recorded for the red site in the Mn/Kr system allow identification of all five spin-orbit levels (J=1/2, 3/2, 5/2, 7/2 and 9/2) in the a6D state. The coincidence of the lowest energy excitation band and the observed 585.75 nm (17,072 cm(-1)) emission band of atomic Mn in Kr matrices, yielded a definitive assignment of this emission to a transition from the J=9/2 spin-orbit level. Temperature dependent emission scans lead to the identification of the zero phonon line for the a6D(9/2)-->a6S(5/2) transition at 585.75 nm. The identified matrix-shift of +20 cm(-1) allows an assessment of the extent of the ground state stabilization in the red (secondary) site of atomic Mn isolation in solid Kr. Emission produced with direct a6D state excitation yielded both the 585.75 and 626 nm features. The former band arises for Mn atoms occupying the red site--the latter from blue site occupancy in solid Kr. The excitation linewidths recorded for these two sites differ greatly, with the blue site yielding a broad featureless profile, in contrast to the narrow, structured features of the red site. The corresponding red site a6D(J)<-->a6S(5/2) transitions in Ar and Xe matrices are broader than in Kr--a difference considered to originate from the site sizes available in these hosts and the interatomic Mn x RG potentials. The millisecond decay times recorded for the red emission bands in the Mn/RG systems are all much shorter than the 3 s value predicted for the gas phase a6D(9/2)-->a6S(5/2) transition. This enhancement allows optical pumping of the forbidden a6D(J)<-->a6S transitions with low laser powers when atomic manganese is isolated in the solid state. However all the emission decays are complex, exhibiting triple exponential decays. This behavior may be related to the dependence of the excitation linewidths on the J value, indicating removal of the J degeneracy due to weak matrix-induced, crystal field splitting. 相似文献
45.
Colin Hester Russell G. Baughman Harvest Collier 《Journal of chemical crystallography》1996,26(10):695-699
The reaction between cadmium(II) chloride and 2,2'-bi-1H-imidazole (H2biim) in an acidic solution affords [Cd(H3biim)2Cl4] (H3biim=2-(2-1H-imidazolyl)-1H-imidazolium) in 63% yield. The compound crystallizes in the triclinic space groupP1, wherea=8.072(2),b=8.100(2),c=8.593(2) Å, =75.89(2), =62.94(2), =63.29(1)°,V=446.4(2) Å3, andZ=1. The central Cd atom exhibits an octahedral geometry composed of a Cl4N2 core. The Cd-N bond distance is 2.392(2) Å. Cd–Cl distances are 2.5919(9) and 2.671(1) Å. 相似文献
46.
KA Saeed FO Ayorinde BE Eribo M Gordon L Collier 《Rapid communications in mass spectrometry : RCM》1999,13(19):1951-1957
Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) was used for the characterization of a partially transesterified poly(beta-hydroxyalkanoate), PHA, polymer produced by the bacterial strain Alcaligenes eutrophus using saponified vegetable oils as the sole carbon sources. The transesterification was carried out separately under acidic and basic conditions to obtain PHA oligomers weighing less than 10 kDa. The intact oligomers were detected in their cationized [M + Na](+) and [M + K](+) forms by MALDI-TOFMS. A composition analysis, using the MALDI-TOF spectra, indicate that the oligomers obtained via acid catalysis were terminated with a methyl 3-hydroxybutyrate end group, and those obtained by base catalysis had a methyl crotonate (olefinic) termination. In addition to HB (hydroxy butyrate), the oligomers were found to contain a small percentage of HV (hydroxy valerate). This was independently confirmed using gas chromatography/mass spectrometry (GC/MS). In comparison, the analysis of a commercial PHA polymer, transesterified under identical conditions, only showed the presence of HB, i.e. a pure PHB homopolymer. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
47.
Enhanced para-xylene selectivity in the toluene alkylation reaction at ultralow contact time 总被引:1,自引:0,他引:1
Breen J Burch R Kulkarni M Collier P Golunski S 《Journal of the American Chemical Society》2005,127(14):5020-5021
Dramatic improvements in the para-xylene selectivity of the toluene alkylation reaction can be effected by operating the catalytic reaction at ultralow contact time. Unexpectedly, the rate of alkylation is sustained, while unwanted side reactions are suppressed. By demonstrating that contact time directly influences the fate of para-xylene, which is known to form and diffuse preferentially within the zeolite catalyst, we conclude that external mass transfer is a key parameter in controlling selectivity. Even non-optimized catalysts can be made to achieve near-perfect selectivity, without sacrificing conversion. 相似文献
48.
Using properties of convex functionals, it is shown that closed and bounded convex sets in a class of Banach spaces which
includes separable conjugate spaces are the closed convex hulls of their strongly exposed points.
This research was supported by the National Research Council of Canada, Grant A-3999. 相似文献
49.
50.
Acta physica Academiae Scientiarum Hungaricae - Im Teil I wird die allgemeinrelativistische Dirac-Gleichung mittels geeigneter Nullteiler exakt auf eine mit der Paulischen σ(a)-Algebra... 相似文献