Palladium-Mediated Cyclization on Carbohydrate Templates. 3. Extension of The Cyclization to the Threo Series.

ABSTRACT

The palladium(0)-catalyzed Heck-type cyclization-β-alkoxy elimination reaction leading to enantiopure bicyclic compounds in the case of erythro 2,3-unsaturated glycosides has been successfully extended to the threo stereoisomer by only changing the aglycon moiety from an ethoxy group to an aryloxy moiety.     

Extraction of Effective Cement Paste Diffusivities from X-ray Microtomography Scans

The problem of extracting effective diffusivities of cement pastes on the basis of X-ray microtomography images is considered. A general computational homogenization framework is developed and applied to a variety of cement pastes whose microstructure has been digitized to a resolution of 1 μm. At this resolution, important submicron features are not resolved. Consequently, we propose a methodology whereby the pore space is ascribed a diffusivity less than the free diffusivity. For this purpose, a simple rule that incorporates microtomography data is proposed and shown to yield satisfactory results.     

Site-dependent activity of atomic Ti catalysts in Al-based hydrogen storage materials

Doping catalytically inactive materials with dispersed atoms of an active species is a promising route toward realizing ultradilute binary catalyst systems. Beyond catalysis, strategically placed metal atoms can accelerate a wide range of solid-state reactions, particularly in hydrogen storage processes. Here we analyze the role of atomic Ti catalysts in the hydrogenation of Al-based hydrogen storage materials. We show that Ti atoms near the Al surface activate gas-phase H(2), a key step toward hydrogenation. By controlling the placement of Ti, we have found that the overall reaction, comprising H(2) dissociation and H spillover onto the Al surface, is governed by a pronounced trade-off between lowering of the H(2) dissociation barrier and trapping of the products near the active site, with a sharp maximum in the overall activity for Ti in the subsurface layer. Our findings demonstrate the importance of controlling the placement of the active species in optimizing the activity of dilute binary systems.     

Aggregation behavior and intermicellar interactions of cationic Gemini surfactants: Effects of alkyl chain,spacer lengths and temperature

The aggregation behavior of the cationic Gemini surfactants C_mH_2m+1N(CH₃)₂(CH₂)_S (CH₃)₂ N C_mH_2m+1,2Br^? with m = 12, 14 and s = 2, 4 were studied by performing surface tension, electrical conductivity, pulsed field gradient nuclear magnetic resonance (PFG-NMR), and cyclic voltammetry (CV) measurements over the temperature range 298 K to 323 K. The critical micelle concentration (CMC), surface excess (Г_max), mean molecular surface area (A_min), degree of counter ion dissociation (α), and the thermodynamic parameters of micellization were determined from the surface tension and conductance data. An enthalpy–entropy compensation effect was observed and all the plots of enthalpy–entropy compensation exhibit excellent linearity. The micellar self-diffusion coefficients (D_m) and intermicellar interaction parameters (k_d) were obtained from the PFG-NMR and CV measurements. These results are discussed in terms of the intermicellar interactions, the effects of the chain and spacer lengths on the micellar surface charge density, and the phase transition between spherical and rod geometries. The intermicellar interaction parameters were found to decrease slightly with increasing temperature for 14–4–14, which suggests that the micellar surface charge density decreases with increasing temperature. The mean values of the hydrodynamic radius, R_h, and the aggregation number, N_agg, of the Gemini surfactants’ m–4–m micelles were calculated from the micellar self-diffusion coefficient. Moreover, the N_agg values were calculated theoretically. The experimental values of N_agg increase with increases in the chain length and are in good agreement with both previous results and our theoretical results.     

Effect of some nitrogen-heterocyclic compounds on corrosion of tin, indium, and their alloys in HClO4

Abstract  

The effect of adenine, adenosine, and 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPTZ) on the electrochemical and corrosion behavior of tin, indium, and tin-indium alloys in 0.5 M HClO₄ solution at different temperatures was studied. The inhibition efficiency increases with an increase in the concentration of adenine and adenosine in the case of tin and indium. However, the effect of two mentioned compounds on the corrosion rate of the studied alloys gives an opposite effect. In the presence of TPTZ, the inhibition efficiency increases as the concentration of the inhibitor is increased in the case of tin. In the case of both indium and its investigated alloys, the maximum inhibition efficiency is obtained at the lowest concentration of TPTZ (10⁻⁶ M). The adsorption of the studied compounds is found to obey the Frumkin adsorption isotherm. The standard enthalpy \Updelta H_ads^° , \Updelta H_{\rm ads}^{^\circ } , entropy \Updelta S_ads^° , \Updelta S_{\rm ads}^{^\circ } , and free energy changes of adsorption \Updelta G_ads^° \Updelta G_{\rm ads}^{^\circ } are calculated and discussed.     

Anodic voltammetric methods for determination of the antiparasitic drug nitazoxanide in bulk form, pharmaceutical formulation, and its metabolite tizoxanide in human serum

Abstract  

The oxidative behavior of nitazoxanide was studied on a carbon paste electrode in phosphate buffer solutions using cyclic and differential-pulse voltammetry. The oxidation process was shown to be irreversible over the pH range 3.4–11.0 and was adsorption-controlled process. An analytical method was developed for the determination of nitazoxanide in phosphate buffer solution at pH 5.6 as the supporting electrolyte. The anodic peak current varied linearly with nitazoxanide concentration in the range 4.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M with a limit of detection of 7.5 × 10⁻⁸ M. Validation parameters such as sensitivity, accuracy, precision, and recovery were evaluated. The proposed method was applied to the determination of nitazoxanide in the pharmaceutical formulation. The results were compared with those obtained by a spectrophotometric method. No difference was found statistically. Based on the adsorption behavior of the metabolite tizoxanide onto the carbon paste electrode surface, a simple and sensitive differential-pulse adsorptive anodic stripping voltammetric method was developed for the determination of tizoxanide in spiked human serum without the need for extraction.     

Theoretical studies of 1,4-dihydropyridine-3,5-dicarboxamides as possible inhibitors of Mycobacterium tuberculosis enoyl reductase

Abstract  

The binding affinity of some novel 1,4-dihydropyridine-3,5-dicarboxamides to enoyl-ACP reductase (InhA) from M. tuberculosis was studied by the docking method. A molecular dynamics simulation lasting 230 ns in total was performed for 19 ligand–enzyme complexes to calculate the binding free energies of these ligands to the enzyme. All of the studied compounds stayed in the active site. An energetics analysis revealed that the van der Waals share is more important than electrostatic in binding of all ligands to the active site.     

Complexation of p-Sulphonato-calix[6]arene by Glycine, Glycyl-glycine, and Glycyl-glycyl-glycine in Aqueous Solution

The complexing ability of p-sulphonato-calix[6]arene towards glycine, glycyl-glycine, and glycyl-glycyl-glycine has been evaluated at pH?=?1.8 and 7.9 using UV?CVis spectrophotometry. At these pHs the guest molecules are in their cationic and zwitterionic forms, respectively. The results showed that the host is capable of complexing with the guests in 1:1 guest-to-host ratios. Formation constants of the systems have been determined at different temperatures (20?±?0.1 to 40?±?0.1?°C). Considering the formation constant values, the binding selectivity of the host towards the guests is in the order glycyl-glycyl-glycine?>?glycyl-glycine?>?glycine. The thermodynamic parameters have been evaluated and are interpreted in terms of the importance of the various interactions responsible for the complexation. A roughly linear relationship between ??H ^o and T??S ^o has been observed for the studied systems and is discussed.     

Resolving of Voltammetric Data for the Ni–Glycine and Cu–Glycine Complexation Systems with Reversible and Irreversible Electrochemical Response Using MCR-ALS

The Ni?Cglycine and Cu?Cglycine systems, which form successive complexes, were studied by differential pulse polarography. The Ni?Cglycine complexes were inert while the Cu?Cglycine system showed labile behavior. An irreversible electrochemical response was found for the Cu?Cglycine system. The data were analyzed both by conventional methods (hard modeling) and the multivariate curve resolution method with alternating least-squares optimization (MCR-ALS, soft modeling approach). Resolving titration data for Cu²⁺ and Ni²⁺ with glycine as the complexing ligand shows that this method is a powerful tool for the determination of stability constants of metal complexes, especially when the conventional methods involve complicated and time consuming approaches such as for systems with an irreversible reduction process. Although for labile complexes the current is not always linear with respect to the concentrations, the results obtained by the two approaches are similar, so the variation from linearity of the signals is small and can be neglected. Also, the analysis of the labile complex data by a Gaussian peak adjustment (GPA) algorithm showed that this system has a small deviation from bilinearity. Thus, the MCR method provided good estimations of the complexation parameters, and is suitable for use as a complementary tool for studying systems with successive labile or inert complexes with both reversible and irreversible responses. Differential pulse polarograms were measured at room temperature for buffer solutions with pH = 7.5 and 0.1?mol?L^?1 ionic strength.