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51.
Peroxynitric acid (O2NOOH) nitrates L-tyrosine and related compounds at pH 2-5. During reaction with O2(15)NOOH in the probe of a 15N NMR spectrometer, the NMR signals of the nitration products of L-tyrosine, N-acetyl-L-tyrosine, 4-fluorophenol and 4-methoxyphenylacetic acid appear in emission indicating a nitration via free radicals. Nuclear polarizations are built up in radical pairs [15NO2* , PhO*]F or [15NO2* , ArH*+]F formed by diffusive encounters of 15NO2 with phenoxyl-type radicals PhO or with aromatic radical cations ArH*+. Quantitative 15N CIDNP investigations with N-acetyl-L-tyrosine and 4-fluorophenol show that the radical-dependent nitration is the only reaction pathway. During the nitration reaction, the 15N NMR signal of 15NO3- also appears in emission. This is explained by singlet-triplet transitions in radical pairs [15NO2* , 15NO3*]S generated by electron transfer between O2(15)NOOH and H15NO2 formed as a reaction intermediate. During reaction of peroxynitric acid with ascorbic acid, 15N CIDNP is again observed in the 15N NMR signal of 15NO3- showing that ascorbic acid is oxidized by free radicals. In contrast to this, O2(15)NOOH reacts with glutathione and cysteine without the appearance of 15N CIDNP, indicating a direct oxidation without participation of free radicals. 相似文献
52.
[reaction: see text] In a convenient one-pot process, easily accessed propargyl vinyl ethers and aromatic amines are effectively converted into tetra- and pentasubstituted 5-methylpyrroles which can further be transformed into 5-formylpyrroles via IBX-mediated oxidation. The cascade reaction proceeds through a silver(I)-catalyzed propargyl-Claisen rearrangement, an amine condensation, and a gold(I)-catalyzed 5-exo-dig heterocyclization. 相似文献
53.
Roland Duduchava Ralf Kirsch Sergej Rjasanow 《Journal of Mathematical Fluid Mechanics》2006,8(2):242-266
We present new estimates of the Boltzmann collision operator in weighted Lebesgue and Bessel potential spaces. The main focus
is put on hard potentials under the assumption that the angular part of the collision kernel fulfills some weighted integrability
condition. In addition, the proofs for some previously known
-estimates have been considerably shortened and carried out by elementary methods. For a class of metric spaces, the collision
integral is seen to be a continuous operator into the same space.
Furthermore, we give a new pointwise lower bound as well as asymptotic estimates for the loss term without requiring that
the entropy is finite. 相似文献
54.
55.
Andreas Kirsch 《Applicable analysis》2013,92(3-4):319-350
This paper introduces a new approach to characterize the shape ω of a scattering medium (either an acoustically soft obstacle or an inhomogeneous medium) by the far field data. In contrast to the Linear Sampling Method normality of the far field operator is not needed. Therefore, also scattering by limited far field data and absorbing media can be treated. While in the Linear Sampling Method the points in the interior of ω are characterized by the solution of an integral equation of the first kind, for our new method a constrained optimization problem has to be solved. Although this new approach is more time consuming some numerical experiments at the end of this paper show the practicability of the method. 相似文献
56.
A variety of novel 2-alkylthiopyrimidines were synthesized through simple condensation of arylidenemalononitriles with different 2-alkylthiouronium halide derivatives catalyzed by anhydrous potassium carbonate (K2CO3). The reactions have been carried out under mild conditions in i-PrOH, and the products were obtained in moderate to good yields with a simple work-up method. Subsequently, some examples of these compounds have been converted into Tacrine analogs by applying the Friedländer reaction. 相似文献
57.
Journal of Theoretical Probability - We study a multi-group version of the mean-field Ising model, also called Curie–Weiss model. It is known that, in the high-temperature regime of this... 相似文献
58.
59.
Zarei M Kirsch S Müthing J Bindila L Peter-Katalinić J 《Analytical and bioanalytical chemistry》2008,391(1):289-297
The coupling of nano high-performance liquid chromatography (nanoHPLC) with matrix-assisted laser desorption/ionization (MALDI)
mass spectrometry (MS) via an automatic spotting roboter was developed and adapted for the first time for the analysis of
complex mixtures of glycosphingolipids (GSLs). The 2,5-dihydroxybenzoic acid and 6-azo-2-thiothymine matrix systems were adjusted
to concurrently meet the requirements for reproducible and homogeneous crystal formation with the liquid chromatography (LC)
eluent under the variable LC solvent composition over the course gradient and high ionization efficiency of the GSL species,
without the need for recrystallization. Precise adjustment of the automatic spotting parameters in terms of matrix flow rate,
on-tip collection time of the matrix/LC eluent solution and the matrix spotting mode, i.e., continuous and discontinuous,
was accomplished to collect individually nanoHPLC-separated species within distinct spots and consequently recover by MALDI
MS screening all major and minor GSL species in the mixtures. The nanoHPLC/MALDI MS coupling protocol was developed and applied
to a mixture of neutral GSLs purified from human erythrocytes and a monosialoganglioside mixture expressed by the murine MDAY-D2
cell line. Additionally, on-line nanoHPLC/MALDI doping with lithium cations of individually separated neutral GSLs was introduced
to enhance data interpretation of the GSL MS pattern, while preserving the same level of information and ultimately to enhance
structural assignment of components of interest. The method is demonstrated to be highly sensitive, reaching the low femtomole
level of detection of individual GSL species and is highlighted as a versatile analytical tool for glycolipidomic studies.
Figure
Automatic LC/MALDI MS profiling of glycosphingolipids
Mostafa Zarei and Stephan Kirsch contributed equally to this work. 相似文献
60.