Terbium-sensitized luminescence determination of levofloxacin in tablets and human urine and serum

A selective and sensitive luminescence method for the determination of levofloxacin is described. The method is based in the luminescence signal from a terbium(III)-levofloxacin complex, in a micellar solution of sodium dodecyl sulfate (SDS), using a chemical deoxygenation agent (Na2SO3). The method allows the determination of 8-600 ng mL-1 of levofloxacin in 10 mM SDS solution containing 0.04 M acetic acid-sodium acetate buffer (pH 6) and 7.5 mM Na2SO3 with lambda exc = 292 nm and lambda em = 546 nm. The luminescence method was applied to the determination of the levofloxacin in a Spanish commercialized pharmaceutical formulation Tavanic (Hoechst Marion Roussel). Good concordance was found between the nominal and experimental values (500 and 488 mg, respectively), with a relative standard deviation (RSD) of 0.6%. The proposed method was shown to be 100-fold more sensitive than the spectrophotometric method, and nearly 2-fold more sensitive than the fluorescence method. The method was also applied to levofloxacin determination in human serum (by external calibration method) and urine (by standard additions method), spiked at levels found after drug administration at normal clinical doses. Average recoveries found were 90.1 (RSD 1%) and 102 (RSD 1.9%), respectively.     

Photophysics and redox behavior of chiral transition metal polymers

The absorption and emission spectra, excited-state lifetimes, quantum yields, and electrochemical measurements have been obtained for a new series of chiral complexes based on three different chiral 2,2':6',2' '-terpyridine ligands, (-)-ctpy, (-)-[ctpy-x-ctpy], and (-)-[ctpy-b-ctpy], with one, two, or multiple Ru metal centers. The room-temperature absorption and emission maxima of [[((-)-ctpy)Ru]-(-)-[ctpy-b-ctpy]-[Ru((-)-ctpy)]](PF(6))(4) and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n) were shifted to lower energies and also exhibited significantly longer luminescence lifetimes when compared to [Ru((-)-ctpy)(2)](PF(6))(2), [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), and ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n). In terms of their electrochemical behavior, all of the complexes studied exhibited one Ru-centered and two ligand-centered redox waves and the [[((-)-ctpy)Ru]-(-)-[ctpy-x-ctpy]-[Ru((-)-ctpy)]](PF(6))(4), ((-)-[ctpy-x-ctpy])-[[Ru((-)-[ctpy-x-ctpy])](PF(6))(2)](n), and ((-)-[ctpy-b-ctpy])-[[Ru((-)-[ctpy-b-ctpy])](PF(6))(2)](n)() complexes were found to electrodeposit upon ligand-based reduction. The difference between the formal potentials of the Ru-centered and the first ligand-centered (least negative) waves corresponded linearly with the changes in the observed emission energies. The shifts in energy are discussed using a particle-in-a-box model, and the luminescence lifetimes are discussed in terms of the structure of the excited-state manifold.     

Direct versus resonances mediated F+OH collisions on a new 3A" potential energy surface

A theoretical study of the F(2P) + OH(2Pi) --> HF(1Sigma+) + O(3P) reactive collisions is carried out on a new global potential energy surface (PES) of the ground 3A" adiabatic electronic state. The ab initio calculations are based on multireference configuration interaction calculations, using the aug-cc-pVTZ extended basis sets of Dunning et al. A functional representation of the PES shows no nominal barrier to reaction, contrary to previous results by others. Wave packet and quasiclassical trajectory calculations have been performed for this PES to study the F + OH(v = 0,j) reactive collision. The comparison was performed at fixed and constant values of the total angular momentum from 0 to 110 and relative translational energy up to 0.8 eV. The reaction presents a dynamical barrier, essentially due to the zero-point energy for the bending vibration near the saddle point. This determines two different reaction mechanisms. At energies higher than approximately 0.125 eV the reaction is direct, while below that value it is indirect and mediated by heavy-light-heavy resonances. Such resonances, also found in the simulations of the photodetachment spectrum of the triatomic anion, manifest themselves in the quasiclassical simulations, too, where they are associated to periodic orbits.     

Anion-pi interactions in cyanuric acids: a combined crystallographic and computational study

Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography.     

Topological excitations vs intergranular phase — coherence in a HTSC Y0.5Sm0.5Ba2Cu3O7 ceramics

This paper reports a detailed study of the electrical transport properties of good quality superconducting Y_0.5Sm_0.5Ba₂Cu₃O₇ ceramic samples. Analyses of the dependence of resistance on temperature, the relations current-voltage and the magnetoresistance allows to identify:a) the mean field critical temperature (T _CO =93.73±0.01 K);b) a power law behaviorVI atT _CO . The exponent jumps abruptly from 1 to 3, at a certain temperatureT _C =93.276±0.005 K;c) an exponential inverse root-square temperature dependence of the resistivity in theT _{C–T CO} temperature range. These features, typically observed when topological excitations sets in two dimensional systems, can also be interpreted as a signature of a percolation process and a transition towards intergranular phase coherence. We analyze and discuss the relevance of both models to account for the experimental data.     

Determination of the Enthalpies of Vaporization of Cyclic Organic Peroxides by Correlation of Changes in Gas Chromatographic Retention Times

Liquid and solid cyclic peroxides derived from aliphatic ketones are explosive materials so their enthalpies of vaporization and other thermodynamic or condensed-phase properties cannot be measured directly. In this work the enthalpies of vaporization of peroxides at 298.15 K were estimated simply from gas chromatographic retention times measured at different temperatures. The technique correlates changes in the retention times of compounds whose enthalpies of vaporization are known (called the reference series), with those of the compounds of interest. If t _R′ is the adjusted retention time (retention time of each compound minus the retention time of unretained diethyl ether, used as solvent) a plot of ln t _R′ against 1/T for each compound (reference compounds and cyclic peroxides) results in a straight line (r ² > 0.99 for all compounds). The enthalpy of transfer from solution to the vapor state (Δ_sol^g H _m) can be obtained by multiplying the slope by the gas constant (R). A second plot correlates the enthalpies of transfer from solution to the vapor state (Δ_sol^g H _m), as measured by gas–liquid chromatography (GLC), with enthalpies of vaporization of reference materials (Δ_vap H _m at 298.15 K) available in the literature. C₉–C₁₅ fatty acid methyl esters and hydrocarbons were used as reference compounds. The enthalpies of vaporization of the cyclic organic peroxides were calculated from the equation of the line obtained in this second correlation, the slope of which was Δ_vap H _m (at 298.15 K)/Δ^g _sol H _m. The experiments were performed under isothermal conditions with a DB-5 capillary column, flame-ionization detection (FID), and nitrogen as carrier gas. The column temperature was varied over a range of at least 30–70 K between 403 and 473 K, with chromatograms being acquired at 10 K intervals. Enthalpies of vaporization of cyclic organic peroxides are not available in the literature, and the values given in this paper, obtained by gas chromatography, are the first to be reported.     

Determination of zinc by flow injection with fluorimetric detection in a micellar medium

A room-temperature flow injection spectrofluorimetric method is presented for the determination of Zn(II), based on the use of salicylaldehyde thiocarbohydrazone in the presence of Triton X-100 and sodium acetate-acetic acid buffer. Various physical and chemical variables affecting the reaction in the flow system were evaluated. The proposed method is very selective. The calibration graph is linear over the range 10-1000 ng/ml, with a detection limit of 5 ng/ml and a relative standard deviation at the 50 ng/ml level of 1.8 %. The method was successfully applied to the determination of Zn(II) in drinking waters and biological samples.     

Evaluation of the Use of Elicitors for the Production of Antioxidant Compounds in Liquid Cultures of Ganoderma curtisii from Costa Rica

The use of substances or conditions as elicitors can significantly increase the production of secondary metabolites. In this research, the effects of different elicitors on the production of antioxidant secondary metabolites were evaluated in a strain of Ganoderma sp. The elicitors tested were pH changes in different growth phases of the fungus (pH 3, 5.5 and 8), different concentrations of peptone as a nitrogen source (1 g/L and 10 g/L), and the addition of chemical agents to the culture medium (ethanol, growth regulators, and salts). The alkaline pH during the stationary phase and the high availability of nitrogen were effective elicitors, producing cultures with higher antioxidant activity (37.87 g/L and 43.13 g/L dry biomass) although there were no significant differences with other treatments.     

Multi-scale homogenization-based modeling of semi-crystalline polymers

This paper presents a finite-strain, multi-scale constitutive model for semi-crystalline polymers, accounting explicitly for the current state and evolution of the underlying crystallographic, lamellar and morphological texture. Specifically, a semi-crystalline polymer is modeled as a two-scale composite, assumed to be, at the larger length scale, an aggregate of randomly distributed grains that, at the smaller length scale, are made up of alternating layers of an amorphous and a crystalline phase. The model incorporates finite elasticity for the amorphous phase and crystallographic hardening for the crystalline phase. The instantaneous effective response of this composite is determined by means of multi-scale homogenization methods, consisting in the use of a “linear comparison composite” (LCC) with the same internal structure as the actual nonlinear composite, with local properties that are optimally chosen via suitably designed variational principles. The effective properties of the resulting two-scale LCC are obtained through a “sequential” homogenization procedure, involving the exact solution for the effective behavior of the lamellar grains and a self-consistent estimate for the aggregate. The latter results are also used to establish evolution laws for the appropriate internal variables in the material. The predictions of the model for the macroscopic response and texture evolution in high-density polyethylene are confronted with available experimental results and compared with those of earlier models.