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991.
A theoretical model has been developed which provides analytical expressions for the elastic moduli of disordered isotropic ensembles of spheres interconnected by physical bonds. Young's and shear moduli have been derived assuming an ideal random isotropic network and the radial distribution function for disordered packings of spheres. The interparticle interactions are accounted for in terms of surface forces for the two distinct cases of perfectly rigid spheres and spheres deformable at contact. A theoretical expression is also derived in a similar way for the bulk or compressibility modulus. In this case, an atomistic approach has been followed based on the analogy with noble gas solids and colloidal crystals. Also in this case, disordered spatial distribution of the spheres is described statistically. For the case of colloidal aggregates, a total two-body mean-field interaction potential is used which includes the Born repulsion energy. This latter contribution plays an essential role in determining the compression behavior of systems of particles aggregated in the primary minimum of the potential well and, therefore, must not be neglected. Both the expression of the Young's modulus and that of the compressibility modulus derived in this work are found to be consistent with two distinct sets of experimental data which recently appeared in the literature. 相似文献
992.
Shaw R Laye RH Jones LF Low DM Talbot-Eeckelaers C Wei Q Milios CJ Teat S Helliwell M Raftery J Evangelisti M Affronte M Collison D Brechin EK McInnes EJ 《Inorganic chemistry》2007,46(12):4968-4978
We report the synthesis, by solvothermal methods, of the tetradecametallic cluster complexes [M14(L)6O6(OMe)18Cl6] (M=FeIII, CrIII) and [V14(L)6O6(OMe)18Cl6-xOx] (L=anion of 1,2,3-triazole or derivative). Crystal structure data are reported for the {M14} complexes [Fe14(C2H2N3)6O6(OMe)18Cl6], [Cr14(bta)6O6(OMe)18Cl6] (btaH=benzotriazole), [V14O6(Me2bta)6(OMe)18Cl6-xOx] [Me2btaH=5,6-Me2-benzotriazole; eight metal sites are VIII, the remainder are disordered between {VIII-Cl}2+ and {VIV=O}2+] and for the distorted [FeIII14O9(OH)(OMe)8(bta)7(MeOH)5(H2O)Cl8] structure that results from non-solvothermal synthetic methods, highlighting the importance of temperature regime in cluster synthesis. Magnetic studies reveal the {Fe14} complexes to have ground state electronic spins of S相似文献
993.
A highly selective and sensitive method for identification and quantification of intracellular metabolites involved in central carbon metabolism (including glycolysis, pentose phosphate pathway and tricarboxylic acid cycle) by means of liquid chromatography-tandem quadrupole mass spectrometry (LC-MS/MS) was developed. The volatile ion pair modifier tributylammonium acetate (TBAA) was employed in the mobile phase for simultaneously separation of 29 negatively charged compounds including sugar phosphates, nucleotides, and carboxylic acids on a common C18 reversed-phase column. Method validation results displayed that limits of detection (LODs) calculated according to DIN (German Institute for Standardization) 32645 are mostly below 60 nM, only with the exception of pyruvate and malate. The calibration curves showed excellent linearity mainly over three orders of magnitude with correlation coefficients R(2)>0.9982. This LC-MS/MS method was successfully applied to determine these metabolites in cell extracts of Escherichia coli. Most of the intracellular metabolites were found within the detection range and the relative standard deviations of the measurements were smaller than 5.65% (n=5) for a cell extract sample. 相似文献
994.
Di Antonio M Doria F Mella M Merli D Profumo A Freccero M 《The Journal of organic chemistry》2007,72(22):8354-8360
Mild activation of water-soluble naphthalene diimides (NDIs) as bisalkylating agents has been achieved by base catalysis and by chemical and electrochemical reductions. NDI activation by a single electron reduction represents a novelty in the field of activatable electrophiles. Under mild reduction, induced by S2O4(2-) in aqueous solution, the resulting NDI radical anion (NDI*-) undergoes a monomolecular fragmentation to yield a new transient species, where the NDI radical anion is tethered to a quinone methide moiety. The latter still retains electrophilic properties, reacting with amines, thiols, and ethyl vinyl ether. Owing to the NDI recognition properties, these results represent the first step toward selective and bioactivatable cross-linking agents. 相似文献
995.
Zuliani V Cocconcelli G Fantini M Ghiron C Rivara M 《The Journal of organic chemistry》2007,72(12):4551-4553
A simple and efficient approach to selectively obtain 2,4(5)-diarylimidazoles suppressing formation of 2-aroyl-4(5)-arylimidazoles is described. The yield of each of the two products strongly depends on the reaction conditions employed. This reaction provides a simple method to prepare small libraries of biologically active compounds by parallel synthesis. 相似文献
996.
Angelini G Maria PD Fontana A Pierini M Siani G 《The Journal of organic chemistry》2007,72(11):4039-4047
The keto-enol tautomerism of 2-nitrocyclohexanone (2-NCH) was studied in aqueous solution under different experimental conditions. Ketonization rate constants were measured spectrophotometrically at 25 degrees C at an ionic strength of 0.4 mol dm-3 (NaCl) in diluted hydrochloric acid, in diluted sodium hydroxide, and in several buffers by using NaHSO3 as the scavenger of the keto form. A value of pK(a)(EH) = 4.78 for the enol form was obtained from the rate-pH profile of the reaction. A value of pK(a)(KH) = 5.97 for the keto form was directly obtained from the UV-vis spectra of 2-NCH recorded at different pHs. The equilibrium constant for the keto-enol tautomerism, pK(T) = -log([enol]/[ketone]) = 1.19, was obtained by combining the two pKa values (pK(T) = pK(a)(KH) - pK(a)(EH)). A comparison of these results with the corresponding values (Keefe, J. R.; Kresge, A. J. In The Chemistry of Enols; Rappoport, Z., Ed.; Wiley & Sons: New York, 1990; pp 399-480) for cyclohexanone shows the dramatic effects of an alpha-nitro substituent on the keto-enol acidities and the tautomerization constant of alicyclic ketones. Rates and equilibria were discussed in the light of the Br?nsted equation, the principle of nonperfect synchronization, and the Marcus theory. It turns out that, on passing from nitroalkanes to nitroketones, the resonance contribution to pKa and deprotonation rate decreases, being overwhelmed by steric and inductive effects. 相似文献
997.
Interactions at the surface of organophilic-modified laponites: a multinuclear solid-state NMR study
Borsacchi S Geppi M Ricci L Ruggeri G Veracini CA 《Langmuir : the ACS journal of surfaces and colloids》2007,23(7):3953-3960
Organically modified clays are largely employed in the preparation of nanostructured materials. The structural and dynamic characterization of the clay surface appears very important in the perspective of understanding the molecular mechanisms determining the improvement of the material properties. To this aim, in this work, a synthetic clay, Laponite, was studied in its untreated hydrophilic Na+-form, after ion exchange with alkylammonium cations and after subsequent grafting reaction with an alkoxysilane. These three samples were characterized by IR, SEM, TGA, and X-ray techniques and were deeply investigated by means of a wide combination of 29Si, 13C, and 1H high- and low-resolution solid-state NMR experiments. The grafting reaction with alkoxysilanes, occurring at the clay platelet edges, resulted in a reduction of the clay inter-platelet distances, and in an increased disorder in both the arrangement of the platelets and the conformational structure of the intercalated organic cation chains, probably due to the relative twisting of adjacent platelets. 相似文献
998.
Barabas B Caglioti L Zucchi C Maioli M Gal E Micskei K Pályi G 《The journal of physical chemistry. B》2007,111(39):11506-11510
Enantiomeric excesses obtained in absolute enantioselective synthesis by chiral autocatalysis (Soai-reaction) were statistically analyzed. Two sets of parallel experiments, which were performed under chemically different conditions, are available. One group contains 37, while the other contains 84 preparative results. The former group shows some interesting tendencies but does not give conclusive statistical results. The sample of 84 parallel experiments, providing 39 R- and 45 S-excesses have shown that these data represent two distinct, non-symmetric sets with different non-Gaussian distributions. Clear S preference was found. 相似文献
999.
A theoretical density functional theory (DFT, B3LYP) investigation has been carried out on the catalytic cycle responsible for the glycosylase activity of the human DNA repair protein hOGG1: enzyme activation, cleavage of the glycosidic bond, and expulsion of the damaged base. An unprecedented large quantum mechanics (QM) model system has been used, which includes a complete oxoG molecule, the deoxyribose ring bonded to the phosphate groups, and most of the surrounding residues that simulate the protein binding pocket. It has been found that Asp268 does not play any role in Lys249 activation and that the oxoG basis acts as a coenzyme, triggering nucleophile activation by Lys249 deprotonation. An SN2 nucleophilic attack by Lys249 on the anomeric carbon then follows. This is the rate-determining step of the process with an activation barrier of 16.7 kcal mol(-1) in good agreement with the experimental value of 17.1 kcal mol(-1). The expelled oxoG plays again as an enzyme cofactor at the end of the process by activating (via proton transfer) ribose ring opening and Schiff base formation. This study suggests a recurring catalytic strategy in the enzymatic cleavage of purine nucleoside where the activation of the leaving group by protonation of the nucleoside base (via an enzymatic general acid) triggers the cleavage of the glycosidic bond. 相似文献
1000.
Molecular dynamics simulations have been used to study the dynamics of a coarse-grained model of a melt of polymer chains with associating terminal groups, confined in a narrow slit by two layers of Lennard-Jones sites. Simulations were carried out as a function of wall separation and attracting strength. We found that confinement has an important effect on the overall dynamics of the system. Strongly attracting walls can significantly modify the dynamics of the melt, giving an aggregation structure with extremely long relaxation times. A noticeable degree of anisotropy was found for the dynamics of both the individual chains and the aggregates formed by the associating terminal groups. 相似文献