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排序方式: 共有318条查询结果,搜索用时 15 毫秒
31.
Malik H Kohlen W Jamil M Rutjes FP Zwanenburg B 《Organic & biomolecular chemistry》2011,9(7):2286-2293
Strigolactones are signaling compounds in plants of increasing importance. In this paper the focus is on their activity as germinating agents for seeds of parasitic weeds. The syntheses of aromatic A-ring analogues of the germination stimulant orobanchol have been described. Starting substrate is the ABC unit of the stimulant GR24. Oxidation at the C-4 position gives a 4-oxo derivative which on subsequent reduction produces two C-4 epimeric alcohols, syn and anti in a ratio of 82 : 3. For practical access of the C-4 anti alcohol, the predominant syn epimer is inverted by a Mitsunobu procedure. The anti C-4 alcohol is then coupled with the D-ring in a one-pot two-step process involving a formylation and a reaction with bromobutenolide to give a mixture of the diastereomeric aromatic A-ring analogues of orobanchol. In contrast, the syn C-4 alcohol cannot be coupled directly with the D-ring. Protection of the C-4 syn OH is a prequisite. The best protecting function is the SEM group as deprotection after coupling with the D-ring can then readily be achieved. The structures of these new analogues have been ascertained by X-ray analyses. Both diastereomers of the C-4 syn as well as the C-4 anti orobanchol analogues have been tested as germination agents of seeds of Striga hermonthica and Orobanche ramosa. In addition, the acetates of both epimeric C-4 alcohols have been prepared and tested. Both diastereomers of the 4-oxo derivative have been prepared and bioassayed as well. The bioassays reveal that the diastereomers having the natural relative configuration are most active. The data also suggest that hydrogen bonding is not an important factor in the binding of the stimulant molecules in the receptor. 相似文献
32.
Mid-infrared spectrometry of milk for dairy metabolomics: a comparison of two sampling techniques and effect of homogenization 总被引:1,自引:0,他引:1
Milk production is a dominant factor in the metabolism of dairy cows involving a very intensive interaction with the blood circulation. As a result, the extracted milk contains valuable information on the metabolic status of the cow. On-line measurement of milk components during milking two or more times a day would promote early detection of systemic and local alterations, thus providing a great input for strategic and management decisions. The objective of this study was to investigate the potential of mid-infrared (mid-IR) spectroscopy to measure the milk composition using two different measurement modes: micro attenuated total reflection (μATR) and high throughput transmission (HTT). Partial least squares (PLS) regression was used for prediction of fat, crude protein, lactose and urea after preprocessing IR data and selecting the most informative wavenumber variables. The prediction accuracies were determined separately for raw and homogenized copies of a wide range of milk samples in order to estimate the possibility for on-line analysis of the milk. In case of fat content both measurement modes resulted in an excellent prediction for homogenized samples (R(2)>0.92) but in poor results for raw samples (R(2)<0.70). Homogenization was however not mandatory to achieve good predictions for crude protein and lactose with both μATR and HTT, and urea with μATR spectroscopy. Excellent results were obtained for prediction of crude protein, lactose and urea content (R(2)>0.99, 0.98 and 0.86 respectively) in raw and homogenized milk using μATR IR spectroscopy. These results were significantly better than those obtained by HTT IR spectroscopy. However, the prediction performance of HTT was still good for crude protein and lactose content (R(2)>0.86 and 0.78 respectively) in raw and homogenized samples. However, the detection of urea in milk with HTT spectroscopy was significantly better (R(2)=0.69 versus 0.16) after homogenization of the milk samples. Based on these observations it can be concluded that μATR approach is most suitable for rapid at line or even on-line milk composition measurement, although homogenization is crucial to achieve good prediction of the fat content. 相似文献
33.
Hauchecorne D Moiana A van der Veken BJ Herrebout WA 《Physical chemistry chemical physics : PCCP》2011,13(21):10204-10213
Using FTIR and Raman spectroscopy, the formation of halogen bonded complexes of the trifluorohalomethanes CF(3)Cl, CF(3)Br and CF(3)I with dimethyl sulfide (DMS) dissolved in liquid krypton has been investigated. For CF(3)Br and CF(3)I, evidence was found for the formation of C-XS halogen bonded 1:1 complexes. At higher concentrations of CF(3)I weak absorptions due to a 2:1 complex were also observed. Using spectra recorded at temperatures between 118 and 163 K, the complexation enthalpies for the complexes were determined to be -9.5(5) kJ mol(-1) for CF(3)Br·DMS, -17.4(1) kJ mol(-1) for CF(3)I·DMS and -30.8(16) kJ mol(-1) for (CF(3)I·)(2)DMS. The results from the cryospectroscopic study are compared with ab initio calculations at the MP2/aug-cc-pVDZ(-PP) level. Apart from vibrational modes localized in the trifluorohalomethanes and the DMS moieties, for both CF(3)Br and CF(3)I, an additional band, which we assign as the intermolecular stretching mode in the complex, was identified in the infrared and Raman spectra. 相似文献
34.
Camilla Mohrdieck Alexander Wanner Wouter Roos Alexander Roth Erich Sackmann Joachim P Spatz Eduard Arzt 《Chemphyschem》2005,6(8):1492-1498
Arrays of elastic pillars are used in biophysical experiments as sensors for traction forces. The evaluation of the forces can be complicated if they are coupled to the pillar displacements over large distances. This is the case if many of the pillars are interconnected by elastic linkages as, for example, in fiber networks that are grown on top of pillars. To calculate the traction forces in such a network, we developed a set of nonlinear inhomogeneous equations relating the forces in the linking elements to the resulting pillar deflections. We chose a homogeneous, activated two-dimensional network of cytoskeletal actin filaments to illustrate that a pillar substrate is generally not a force sensor but a force-gradient sensor. In homogeneous networks the forces acting along the filaments can be approximated by analyzing only pillar deflections in the edge zones of the substrate and by integration over the corresponding force gradients. 相似文献
35.
36.
Rados?aw Przenios?o Wouter van BeekIzabela Sosnowska 《Solid State Communications》2003,126(9):485-488
The structural phase transition in annealed CaMn7O12 has been investigated by using high resolution synchrotron radiation powder diffraction. There is a phase coexistence phenomenon: two different crystallographic phases coexist in the material between 410 and 458 K. The first one is trigonal and it has a charge ordering (CO) of the Mn3+ and Mn4+ ions, while the second one is cubic and charge delocalized (CD). The volume fraction of the CD phase increases with temperature from 22% at 418 K up to 100% at 468 K. Both phases have domains of at least 150 nm at each temperature. The annealing of CaMn7O12 relaxed a part of the strains in the lattice, but did not influence the phase coexistence phenomenon. 相似文献
37.
Herrebout WA Melikova SM Delanoye SN Rutkowski KS Shchepkin DN van der Veken BJ 《The journal of physical chemistry. A》2005,109(13):3038-3044
Mid-infrared spectra of mixed solutions in liquid xenon containing fluoroform and either ammonia or pyridine have been investigated at temperatures between 173 and 213 K. For both Lewis bases, a new band is found in the CH stretching region at a frequency approximately 5 cm(-1) higher than that of monomer fluoroform, which is assigned to a complex between fluoroform and the Lewis base. A detailed analysis of the nu1/2nu(4) Fermi resonance in the proton donor shows that the blue shifts observed for the complexes are not caused by a strengthening of the CH bond during the complexation, but are due to the changes in the Fermi resonance interactions. Information on the nu1/2nu(4) Fermi resonance was also obtained for the complexes of fluoroform with dimethyl ether and trimethyl amine. 相似文献
38.
This paper exemplifies discrete analogs obtained by transformation of the continuum equations governing porous media flow to a system of algebraic equations. To give insight in the underlying physics, the numerical examples are worked out algebraically. The volume-centered approximation turns out to be a reasonable simplification of the algebraically exact face-centered method. 相似文献
39.
Wouter Caarls M. Soledad Celej Alexander P. Demchenko Thomas M. Jovin 《Journal of fluorescence》2010,20(1):181-190
Fluorescence probes with multiparametric response based on the relative variation in the intensities of several emission bands
are of great general utility. An accurate interpretation of the system requires the determination of the number, positions
and intensities of the spectral components. We have developed a new algorithm for spectral deconvolution that is applicable
to fluorescence probes exhibiting a two-state ground-state equilibrium and a two-state excited-state reaction. Three distinct
fluorescence emission bands are resolved, with a distribution of intensities that is excitation-wavelength-dependent. The
deconvolution of the spectrum into individual components is based on their representation as asymmetric Siano-Metzler log-normal
functions. The application of the algorithm to the solvation response of a 3-hydroxychromone (3HC) derivative that exhibits
an H-bonding-dependent excited-state intramolecular proton transfer (ESIPT) reaction allowed the separation of the spectral
signatures characteristic of polarity and hydrogen bonding. This example demonstrates the ability of the method to characterize
two potentially uncorrelated parameters characterizing dye environment and interactions. 相似文献
40.
Bertine L. Stehouwer Dennis W.J. KlompMies A. Korteweg Helena M. VerkooijenPeter R. Luijten Willem P.Th.M. MaliMaurice A.A.J. van den Bosch Wouter B. Veldhuis 《Magnetic resonance imaging》2013