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31.
32.
Nidetzky Bernd Griessler Richard Weinhausel Andreas Haltrich Dietmar Kulbe Klaus D. 《Applied biochemistry and biotechnology》1997,(1):159-172
Some important process properties of α-l,4-D-ghican phosphorylases isolated from the bacteriumCorynebacterium callunae and potato tubers (Solatium tuberosum) were compared. Apart from minor differences in their stability and specificity (represented by the maximum degree of maltodextrin
conversion) and a 10-fold higher affinity of the plant phosphorylase for maltodextrin (K
M of 1.3 g/L at 300 mM of orthophosphate), the performances of both enzymes in a continuous ultrafiltration membrane reactor
were almost identical. Product synthesis was carried out over a time course of 300–400 h in the presence or absence of auxiliary
pullulanase (increasing the accessibility of the glucan substrate for phosphorolytic attack up to 15–20%). The effect of varied
dilution rate and reaction temperature on the resulting productivities was quantitated, and a maximum operational temperature
of 40°C was identified. 相似文献
33.
Riehn C Matylitsky VV Jarzeba W Brutschy B Tarakeshwar P Kim KS 《Journal of the American Chemical Society》2003,125(52):16455-16462
In this paper, we report the use of femtosecond time-resolved degenerate four-wave mixing rotationally resolved spectroscopy to obtain very accurate structural information on the symmetric top cyclohexane. Apart from highlighting the versatility of this method in determining accurate structures of large and complex molecules without dipole moment, the present study also details the comparison of the experimentally determined rotational constant B(0) with that obtained from high-level ab initio calculations. The theoretical calculations, which were carried out at both the second-order M?ller-Plesset (MP2) and coupled-cluster with single, double, and perturbative triple substitutions [CCSD(T)] levels of theory, also take into account vibrational averaging effects. A detailed investigation of the vibrational averaging effects reveals that the corrections emerge from only the six highly symmetric A(1g) modes, a justification of which is provided by an analysis of these modes. 相似文献
34.
A careful deformation density study of tetrafluoroterephthalodinitrile at 98K has been made from X-ray diffraction measurements. Prominent ‘bonding density’ peaks are found at or near the mid-points of the C, C- and C, N-bonds but not for the C, F-bonds, which show only weak density. Similarly weak bonding densities for C, F-bonds are also found for 1, 1, 4, 4-tetrafluorocyclohexane. The possible significance of these results in terms of bonding theory is briefly discussed. 相似文献
35.
The binding of palladium to high-molecular-mass compounds in palladium-treated lettuce is investigated as an example for a biological matrix. The total palladium concentration in lettuce leaves is 10.3 ng/g wet weight. After homogenization, high-molecular-mass compounds (> 10 kDa) are isolated by ultrafiltration. For separation of these palladium species a combination of preparative gel permeation chromatography (GPC) and preparative isotachophoresis (ITP) is used. Palladium is determined in separated fractions by using a highly sensitive total reflection X-ray fluorescence (TXRF) method after preconcentration. After GPC separation, four main fractions of palladium species are collected, each containing palladium in ng quantities (3-10 ng). Two of these fractions are further separated by ITP, yielding at least three main peaks per GPC fraction, each containing palladium in the range of 0.3-3 ng. These palladium containing peaks are characterized by high-performance size exclusion chromatography (HPSEC) and capillary isotachophoresis (cITP) in parallel. HPSEC enables the estimation of the molecular mass of six main palladium peaks, covering a molecular mass range of 69-200 kDa. It is also shown that the estimation of molecular mass after separation is more reliable than the respective estimation directly in the first GPC run. However, cITP reveals that each of the separated peaks is still a mixture of at least five different compounds. 相似文献
36.
37.
Kammerer B Kahlich R Ufer M Laufer S Gleiter CH 《Rapid communications in mass spectrometry : RCM》2004,18(4):458-464
Phenprocoumon is a commonly used oral anticoagulant of the coumarin type, and has found extensive clinical use in the treatment of thrombophlebitis, pulmonary embolism and atrial fibrillation. In the course of a clinical study to investigate the influence of genetic polymorphisms of the CYP2C9 enzyme on phenprocoumon metabolism, we developed a new enantioselective liquid chromatography/electrospray ionisation tandem mass spectrometry (LC/MS/MS) method to quantify (R)- and (S)-phenprocoumon in human plasma. HPLC separation of the enantiomers was achieved on a Chira-Grom-2 column under isocratic conditions using a water/acetonitrile/formic acid eluent. For detection and quantification a triple-quadrupole MS system was used in the selected reaction monitoring (SRM) mode. As an internal standard the structurally homologous compound warfarin was chosen. The detector response was linear with a correlation coefficient of 0.988-0.999 for (R)-phenprocoumon and 0.989-0.999 for (S)-phenprocoumon in the investigated concentration range between 62.5 and 1000 ng/mL (per enantiomer). The limit of detection (LOD) was 12.5 ng/mL. 相似文献
38.
Zhao Y Slepkov AD Akoto CO McDonald R Hegmann FA Tykwinski RR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,11(1):321-329
Monodisperse, cross-conjugated perphenylated iso-polydiacetylene (iso-PDA) oligomers, ranging from monomer 15 to pentadecamer 25, have been synthesized by using a palladium-catalyzed cross-coupling protocol. Structural characteristics elucidated by X-ray crystallographic analysis demonstrate a non-planar backbone conformation for the oligomers due to the steric interactions between alkylidene phenyl groups. The electronic absorption spectra of the oligomers show a slight red-shift of the maximum absorption wavelength as the chain length increases from dimer 17 b to pentadecamer 25, a trend that has saturated by the stage of nonamer 22. Fluorescence spectroscopy confirms that the pendent phenyl groups present on the oligomer framework enhance emission, and the relative emission intensity consistently increases as a function of chain length n. The molecular third-order nonlinearities, gamma, for this oligomer series have been measured via differential optical Kerr effect (DOKE) detection and show a superlinear increase as a function of the oligomer chain length n. Molecular modeling and spectroscopic studies suggest that iso-PDA oligomers (n>7) adopt a coiled, helical conformation in solution. 相似文献
39.
Coordination of dinitrogen to Sellmann-type iron (II) complexes in a sulfur-dominated coordination sphere, which emulates
the environment of iron centers in the FeMo-cofactor of nitrogenase, is analyzed with respect to spin states, spin barriers,
and the effect of trans-ligands. Such detailed investigations became only recently feasible when the reliability of density functional methods, which
are the only quantum chemical methods capable of describing large transition metal complexes, could significantly be improved
for the calculation of energies for states of different spin. It is found that the actual binding energy of dinitrogen is
of sufficient magnitude for a reasonably strong fixation of N2 by Sellmann-type coordination compounds. However, potential fixation is determined by additional factors which reduce the
binding energy. One factor is the change in spin state of the N2-free metal fragment, which lowers the total energy and quenches the thermodynamic stabilization effect of the binding energy.
In addition, the metal fragment rearranges and gains even more stabilization energy for the un-coordinated state. Apart from
these thermodynamical effects, the existence of spin barriers, which must be overcome upon binding of dinitrogen, leads to
kinetical effects, which cannot be neglected. 相似文献
40.
Two protocols for functionalization of glass supports with hexaethylene glycol (HEG)-linked oligonucleotides were developed.
The first method (standard amidite protocol) made use of the 2-cyanoethyl-phosphoramidite derivative of 4,4′-dimethoxytrityl-protected
HEG. This was first coupled to the support by standard solid-phase phosphoramidite chemistry followed by extension with a
thymidylic acid icosanucleotide. Stepwise addition of the linker phosphoramidite graduated at 1% (relative to the total sites
available) perstep at 50°C resulted in an optimal yield of immobilized oligonucleotides at a density of 2.24 × 1010 strands/mm2. This observed loading maximum lies well below the theoretical maximum loading owing to nonspecific adsorption of HEG on
the glass and subsequent blocking of reactive sites. Surface loadings as high as 3.73 × 1010/mm2 and of excellent sequence quality were achieved with a reverse amidite protocol. The support was first modified into a 2-cyanoethyl-N,N-diisopropylphosphoramidite analog followed by coupling with 4,4′-dimethoxytrityl-protected HEG. This protocol is conveniently
available when using a conventional DNA synthesizer. The reverse amidite protocol allowed for control of the surface loading
at values suitable for subsequent analytical applications that make use of immobilized oligonucleotides as probes for selective
hybridization of sample nucleic acids of unknown sequence and concentration. 相似文献