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101.
Duarte MF Martins F Fernandez MT Langley GJ Rodrigues P Barros MT Costa ML 《Rapid communications in mass spectrometry : RCM》2003,17(9):957-962
A series of derivatives of 2-azidoacetic acid and 2-azidoacetone were synthesized and their behaviour under electron ionization conditions was investigated. This paper reports the electron ionization fragmentation mechanisms for five aliphatic alpha-carbonyl azides, which were clarified by accurate mass measurements and B/E linked scans. The substituent influences the abundance and the nature of the ions resulting from the molecular ion fragmentation. 相似文献
102.
[reaction: see text] The directing ability of an aziridine group for the epoxidation of adjacent double bonds is demonstrated. The aziridine group is also used to effectively protect a double bond in a cycloenone system for a short synthesis of the title compound. 相似文献
103.
Barros W de Sousa PL Engelsberg M 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,165(1):175-179
Intermolecular double-quantum coherence (i-DQC) signals in liquids are usually associated with high magnetic fields. We demonstrate that, in a magnetic field of only 16mT, i-DQC imaging of water protons is feasible thanks to the nuclear magnetization enhancement produced by the Overhauser effect. i-DQC images of a phantom containing an aqueous solution of a trityl free radical, with phase encoding in the DQC evolution period or in the acquisition period, are presented. Possible applications of low field i-MQC images are proposed. 相似文献
104.
Wilkinson PN Henstock DR Browne IW Polatidis AG Augusto P Readhead AC Pearson TJ Xu W Taylor GB Vermeulen RC 《Physical review letters》2001,86(4):584-587
Using very long baseline interferometry we have searched a sample of 300 compact radio sources for examples of multiple imaging produced by gravitational lensing; no multiple images were found with separations in the angular range 1.5--50 milliarcsec. This null result allows us to place a limit on the cosmological abundance of intergalactic supermassive compact objects in the mass range approximately 10(6)M( middle dot in circle) to approximately 10(8)M( middle dot in circle); such objects cannot make up more than approximately 1% of the closure density ( 95% confidence). A uniformly distributed population of supermassive black holes forming soon after the big bang does not, therefore, contribute significantly to the dark matter content of the Universe. 相似文献
105.
Topological properties of native folds are obtained from statistical analysis of 160 low homology proteins covering the four structural classes. This is done analyzing one, two and three-vertex joint distribution of quantities related to the corresponding network of amino acid residues. Emphasis on the amino acid residue hydrophobicity leads to the definition of their center of mass as vertices in this contact network model with interactions represented by edges. The network analysis helps us to interpret experimental results such as hydrophobic scales and fraction of buried accessible surface area in terms of the network connectivity. Moreover, those networks show assortative mixing by degree. To explore the vertex-type dependent correlations, we build a network of hydrophobic and polar vertices. This procedure presents the wiring diagram of the topological structure of globular proteins leading to the following attachment probabilities between hydrophobic–hydrophobic 0.424(5), hydrophobic-polar 0.419(2) and polar–polar 0.157(3) residues. 相似文献
106.
Guilherme D. VilelaRafaela R. da Rosa Paulo H. SchneiderIvan H. Bechtold Juliana EccherAloir A. Merlo 《Tetrahedron letters》2011,52(49):6569-6572
A collection of isoxazoles derivatives has been efficiently synthesized in three steps. The oximation reaction of aldehydes followed by nitrile oxide [3+2] 1,3-dipolar cycloaddition and MnO2-oxidation reaction furnished the title compounds which were purified by simple filtration on celite®. 相似文献
107.
108.
Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in the quick assess of food components. In this work, procyanidins were extracted with methanol and acetone/water from the seeds of white and red grape varieties. A fractionation by graded methanol/chloroform precipitations allowed to obtain 26 samples that were characterised using thiolysis as pre-treatment followed by HPLC-UV and MS detection. The average degree of polymerisation (DPn) of the procyanidins in the samples ranged from 2 to 11 flavan-3-ol residues. FTIR spectroscopy within the wavenumbers region of 1800-700 cm−1 allowed to build a partial least squares (PLS1) regression model with 8 latent variables (LVs) for the estimation of the DPn, giving a RMSECV of 11.7%, with a R2 of 0.91 and a RMSEP of 2.58. The application of orthogonal projection to latent structures (O-PLS1) clarifies the interpretation of the regression model vectors. Moreover, the O-PLS procedure has removed 88% of non-correlated variations with the DPn, allowing to relate the increase of the absorbance peaks at 1203 and 1099 cm−1 with the increase of the DPn due to the higher proportion of substitutions in the aromatic ring of the polymerised procyanidin molecules. 相似文献
109.
S. Pessanha M.L. Carvalho M. Manso A. Guilherme A.F. Marques C.A. Perez 《Spectrochimica Acta Part B: Atomic Spectroscopy》2009,64(8):805
The pigment vermilion (HgS) was used to color the fore edge, tail and head of books. Dissemination and quantification of Hg present in the ink used to color books from XVIII and XIX centuries are reported. Mercury is a very toxic element for the human body, therefore it is extremely important to know whether Hg tends to disseminate throughout the paper or stays confined to the borders of the books with less danger for readers. Synchrotron X-ray microprobe was used to evaluate Hg dissemination from the border to the centre of the paper sheet. The diffusion pattern of Hg was compared with the results obtained by a portable X-ray fluorescence spectrometer and mean quantitative calculations were obtained by a stationary X-ray fluorescence system with triaxial geometry. The results showed high concentrations of Hg in the external regions, but no diffusion was observed for the inner parts of the paper. 相似文献
110.
Reactions of dimethyl acetylenedicarboxylate with 3-methylindole and 2,3-dimethylindole under boron trifluoride-diethyl ether catalysis were carried out at low temperature (0–25°). Thermally unstable cyclobutenes were isolated, mainly in apolar solvents, which isomerized to benzazepines. The other compounds isolated were Michael adducts and dihydro-2-oxocarbazoles. 相似文献