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41.
Ciucanu I 《Analytica chimica acta》2006,576(2):147-155
Per-O-methylation of carbohydrates is an important sample preparation step in structural analysis of complex carbohydrates, which has generated considerable interest as shown by thousands of citations in the last 10 years. This article provides a critical overview of the per-O-methylation methods applied for structural analysis of carbohydrates by mass spectrometry. The understanding of the O-methylation mechanism can help the researchers to apply the adequate O-methylation method and can generate new ideas in the effort of improving this reaction. The per-O-methylation of carbohydrates is relied upon stepwise reactions. The parameters that affect the reaction are discussed for the most important methods and are critically commented for each reaction step. The limits of each method are emphasized. The improvements of the per-O-methylation reaction are described in detail with their advantages and disadvantages and some illustrative examples are given. The methods that give complete O-methylation in non-hazardous conditions with high yields within minutes at room temperature with a very low amount of side-products are especially highlighted. 相似文献
42.
Positivity - The aim of this paper is to prove that the cosine Fourier coefficients $$a_{mn}$$ (with $$m,n\ge 1)$$ of a Popoviciu convex function of two variables are nonnegative. 相似文献
43.
A fast, efficient, general and environmentally friendly method for preparation of highly fluorescent derivatives containing the pyrrolodiazine moiety using microwave (MW) irradiation, in liquid phase, is reported. Under MW irradiation the yields are much higher, sometimes substantially (by almost double) and, the amount of solvent used is at least 5-fold less. The pyrrolopyridazine (PP) derivatives are very intense blue emitters and have high quantum yields (up to 90%) while pyrrolophthalazine (PHP) compounds are still intense blue emitters but the quantum yield is negligible. A certain influence of the substituents concerning fluorescence was found, those ones at the 7 position being crucial for fluorescence. The number of the substituents from the pyrrolo ring seems not to play an important role in regard with the fluorescence but, with an increasing number of substituents a certain hypsochromic shift in the absorption spectra was found. 相似文献
44.
Irina Popescu Dana Mihaela Suflet Irina Mihaela Pelin Marcel Ionel Popa 《Journal of Macromolecular Science: Physics》2013,52(3):590-604
Five alternating copolymers of maleic acid with N-vinylpyrrolidone, vinyl acetate, styrene, 2-vinylnaphthalene, and vinyl cyclohexane were used in order to study the influence of the comonomer on the polyelectrolyte behavior. The dissociation of the maleic acid copolymers was investigated by electrochemical methods in pure water and in the presence of supporting electrolytes. The conformational modifications during the dissociation were directly investigated by viscosimetry and fluorescence probing. The shape of the potentiometric and conductometric titration curves was influenced by the comonomer structure. In salt-free solution, only the first dissociation step was observed. The addition of univalent counterions permitted the evidencing of the second ionization step only for the copolymers with hydrophilic comonomers, but the addition of divalent counterions permitted the evidencing of the second ionization step for all five copolymers. The apparent dissociation constants increase with the increase of the comonomer hydrophobicity. A conformational transition was evidenced especially in the case of the maleic acid-2-vinylnaphthalene copolymer. 相似文献
45.
José S. Casas Eduardo E. Castellano Javier Ellena Ionel Haiduc Agustín Sánchez José Sordo 《Journal of chemical crystallography》1999,29(7):831-836
The crystal structure of the title compound, Hg[S2P(OPri)2]2, has been determined by single-crystal X-ray diffraction. The compound crystallizes in the centrosymmetric P21/c space group with a = 11.800(1), b = 8.925(2), c = 22.167(2) Å, = 94.988(7)°, and Z = 4. The same compound has been previously reported but the structure is described there in the space group C2/c. In both cases, one phosphorodithioate moiety acts as a chelating group and the other as a bridging group between neighboring mercury atoms (related to one another by the twofold screw axis), giving rise to an infinite polymer along the direction of the b axis. It turns out that in spite of the difference in space group symmetry, the structures are remarkable similar in that they are formed from infinite polymers of similar geometry. The coordination around the Hg ion shows, however, significant differences, mainly for the bridging Hg—S(4) bond length, which is 0.16 Å smaller than that previously reported. 相似文献
46.
The possible oxidative degradation mechanism occurring during the per-O-methylation of carbohydrates in dimethyl sulfoxide with methyl iodide in the presence of base is described. Evidence is presented that this process occurs only under anhydrous conditions when there is a long reaction time between the carbohydrate dissolved in dimethyl sulfoxide and methyl iodide, followed by reaction with the base. Under these specific conditions, the oxidative degradation of alditols, and cyclic carbohydrates, with and without a free hemiacetal group, is observed. The reaction between carbohydrate and methyl iodide in dimethyl sulfoxide can result in a complete oxidative degradation depending on the type of carbohydrate and the time of reaction. The oxidative degradation can be accelerated by replacing methyl iodide with dimethyl sulfate. Mass spectrometric identification of the degradation products of d-glucitol indicates simultaneous oxidation processes at all the hydroxyl groups, with site dependent rates of their reactivity. This oxidative process is not a characteristic of the methylation of carbohydrates in dimethyl sulfoxide with methyl iodide in the presence of solid sodium hydroxide and can be totally avoided by treating the carbohydrate with powdered sodium hydroxide before introduction of methyl iodide under nonanhydrous conditions, or by adding a trace of water in dimethyl sulfoxide before methyl iodide, or by using N,N-dimethylacetamide as the solvent. The degradation of the per-O-methylated carbohydrates in the liquid-liquid extraction process is also taken into account, and it is found that the degradation process can be avoid by neutralization of the base before extraction, by using benzene or tetrachloromethane as extraction solvent, and by drying the final organic extract. 相似文献
47.
Vasilichia Bejan Costel Moldoveanu Ionel I. Mangalagiu 《Ultrasonics sonochemistry》2009,16(3):312-315
A new, fast, efficient and general method for preparation of steroid analogous of anticipated biological activity under ultrasound irradiation and classical heating is reported. The reaction pathway involves two steps: quaternization of phthalazine heterocycle followed by a 3+2 dipolar cycloaddition of cycloimmonium ylides to dienophiles. Under ultrasound the reaction time decreases substantially, the yields are higher, the reaction conditions are milder. A comparative study ultrasound verses classical heating has been done. 相似文献
48.
Ionel–Dumitrel Ghiba 《Meccanica》2008,43(6):639-649
The aim of this paper is to study the asymptotic partition of the energy associated with the solution of the initial-boundary
value problem who describes the behavior of binary homogeneous micropolar mixtures of an isotropic micropolar elastic solid
with an incompressible micropolar viscous fluid. Some Lagrange-Brun identities are established. Using the Cesáro means of
various parts of total energy, the relations that describe the asymptotic behavior of mean energies are established.
The author acknowledges support from the Romanian Ministry of Education and Research through CEEX program, contract CERES-2-CEx06-11-12/25.07.2006. 相似文献
49.
50.