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121.
Sisu E Bosker WT Norde W Slaghek TM Timmermans JW Peter-Katalinić J Cohen-Stuart MA Zamfir AD 《Rapid communications in mass spectrometry : RCM》2006,20(2):209-218
A combined methodology for obtaining at the preparative scale and characterization by nanoelectrospray ionization (nanoESI) quadrupole time-of-flight (QTOF) mass spectrometry (MS) and tandem MS (MS/MS) of linear polysaccharides modified at the reducing end is presented. Two polydisperse maltodextrins (1000 and 3000 Da) and a high molecular weight polydisperse dextran (6000 Da) were coupled with hexamethylenediamine (HMD). The coupling products were analyzed by nanoESI-QTOF-MS in the positive ion mode and MS/MS using collision-induced dissociation (CID) at low energies. In the HMD-M1000 mixture, the polysaccharide chains containing from 2 to 8 Glc residues were detected, while in HMD-M3000 we identified a complete series of chains containing from 8 to 21 Glc moieties. The employed ESI conditions enhanced the detection of chains with up to 46 Glc residues in the HMD-D6000 sample. By optimized MS/MS, HMD-modified polysaccharides of 3, 4, 5, 12 and 46 degrees of polymerization yielded product ion spectra exhibiting the whole set of Y- and B-fragment ions. The MS structural data were obtained within a few minutes of signal acquisition, with a sample consumption situating the analysis sensitivity in the picomolar range. 相似文献
122.
Modern microfluidic devices are currently introduced in electrospray (ESI) mass spectrometry (MS), tending to substitute the classical capillary-based ESI infusion. Automated systems using the combination of robotized sample handling and chip-based ESI are significantly increasing the analysis reproducibility, precision, throughput, and efficiency. In the last couple of years our group developed the chip-based ESI-MS approach for glycomics in biomedical research and applied it for oligosaccharide, glycopeptide and ganglioside investigation. Here we report upon the optimization and application of this modern technique for the analysis of differential ganglioside expression patterns in human fetal and adult hippocampus. By this methodology, ganglioside species exhibiting high degree of heterogeneity in the ceramide motifs and biologically-relevant modifications could be identified in human hippocampus. The ultra-high reproducibility of the experiments uniquely provided by the chip-ESI approach allowed for a reliable MS-based ganglioside comparative assay. Moreover, the particular feature of chip ESI-tandem MS to provide structural information at high sensitivity was useful for detailed characterization of hippocampus-associated species. The experimental data presented in this study indicate the benefits of microfluidic/MS for determination of the topospecific brain ganglioside composition and development-related changes in their expression, which might be of high value in clinical investigation and for studies related to ganglioside-based therapy of central nervous system diseases. 相似文献
123.
Sun Z Winschel GA Borovika A Nagorny P 《Journal of the American Chemical Society》2012,134(19):8074-8077
Catalytic enantioselective and diastereoselective spiroketalizations with BINOL-derived chiral phosphoric acids are reported. The chiral catalyst can override the inherent preference for the formation of thermodynamic spiroketals, and highly selective formation of nonthermodynamic spiroketals could be achieved under the reaction conditions. 相似文献
124.
Nicoleta Simona Vedeanu Dana Alina Magdas Laura Bolojan Grigore Damian 《Chemical Papers》2012,66(6):612-616
Antioxidant status of foods, plant, or fruit products is generally characterized by means of spectroscopic methods. Methods
like HPLC, UV-VIS, or MS spectroscopy are used to understand the chemical and physical properties of different samples, and
also EPR spectroscopy seems to be a valuable tool to characterize antioxidant activity of juice beverages. In this technique,
certain antioxidants present in fruit juices interact with free radicals interrupting the chain reaction that can possibly
damage essential molecules. Recording the EPR signal decay caused by the reaction with a natural or artificial reducing agent,
it is possible to draw conclusions about the antioxidant capability of materials. IRMS is a powerful tool to distinguish between
an authentic fruit juice and a juice obtained by concentrate dilution. This technique allows also the detection of commercial
C4 cane or corn derived sugar syrups in C3 fruit juices. In the present study, four commercial fruit juices were investigated using stable isotope measurements (oxygen,
hydrogen, and carbon) and EPR measurements in order to check the correct labeling in the Romanian markets and to compare antioxidant
activity of the studied juices and the reference. It was proven that the number of paramagnetic species decreases in time
with different reaction rates and this was correlated with the antioxidant activity of the studied juices. Stable isotope
ratio measurements have demonstrated that the fruit juices studied were reconstructed from concentrates with tap water, according
to their label. 相似文献
125.
The determination of the content of stable isotopes, 18O and 2H, respectively, in juice water facilitates the distinction between authentic juices and juices made from concentrates by
redilution with tap water. At the same time, the detection of C4 cane or corn-derived sugar syrups in fruit juices which are produced from C3 fruit types is thus facilitated by the characteristic differences in 13C/12C, expressed as δ
13C (‰) values due to photosynthetic CO2 assimilation via the C3−, C4−, and crassulacean acid metabolism pathways. In this study, the quantitative determination of water added to an authentic
juice, on the basis of δ
18O, and δ
2H values, respectively, was successfully performed. Also, the δ
18O, and δ
2H of juice water and δ
13C of the whole juice in 18 samples were also determined. The results obtained provided us with the possibility of distinguishing
between authentic fruit juices and those obtained by redilution of concentrated fruit juices and the detection of C4 type added sugar. 相似文献
126.
We estimate multiplicative character sums over the integers with a fixed sum of binary digits and apply these results to study the distribution of products of such integers in residues modulo a prime p. Such products have recently appeared in some cryptographic algorithms, thus our results give some quantitative assurances of their pseudorandomness which is crucial for the security of these algorithms. 相似文献
127.
Piazza ZA Li WL Romanescu C Sergeeva AP Wang LS Boldyrev AI 《The Journal of chemical physics》2012,136(10):104310
The structures and chemical bonding of the B(21)(-) cluster have been investigated by a combined photoelectron spectroscopy and ab initio study. The photoelectron spectrum at 193 nm revealed a very high adiabatic electron binding energy of 4.38 eV for B(21)(-) and a congested spectral pattern. Extensive global minimum searches were conducted using two different methods, followed by high-level calculations of the low-lying isomers. The global minimum of B(21)(-) was found to be a quasiplanar structure with the next low-lying planar isomer only 1.9 kcal/mol higher in energy at the CCSD(T)/6-311-G* level of theory. The calculated vertical detachment energies for the two isomers were found to be in good agreement with the experimental spectrum, suggesting that they were both present experimentally and contributed to the observed spectrum. Chemical bonding analyses showed that both isomers consist of a 14-atom periphery, which is bonded by classical two-center two-electron bonds, and seven interior atoms in the planar structures. A localized two-center two-electron bond is found in the interior of the two planar isomers, in addition to delocalized multi-center σ and π bonds. The structures and the delocalized bonding of the two lowest lying isomers of B(21)(-) were found to be similar to those in the two lowest energy isomers in B(19)(-). 相似文献
128.
129.
130.
In this paper we show a lower bound for the on-line version of Heilbronn's triangle problem in d dimensions. Specifically,
we provide an incremental construction for positioning n points in the d-dimensional unit cube, for which every simplex defined
by d + 1 of these points has volume Ω(1/n(d+1)ln(d-2)-0.265 d+2.269) (for d ≥ 5). 相似文献