Enantioseparation of basic pharmaceutical compounds by capillary electrophoresis using sulfated cyclodextrins. Application to E-6006, a novel antidepressant

In this study, a chiral capillary electrophoresis method was optimized and validated for E-6006, a thienylpyrazolylethanamine derivative (pK_a 8.9). Enantioselectivity of neutral and anionic cyclodextrins (CDs) was evaluated at acid pH (3), obtaining cathodic and anodic migration, respectively. Hydroxypropyl-β-CD, carboxymethyl-β-CD and sulfobutyl ether-β-CD led to similar and partial selectivity, whereas sulfate (S)-β-CD produced baseline separation of the enantiomers. Four types of sulfated CDs were compared considering: cavity size (, β, γ) and random substitution versus unique derivative (S-β-CD, 6-heptakis-S-β-CD). Complete peak separation was obtained in all cases, but with different affinity and binding strength. Some factors that play a role in the complex formation include: position/region/degree of substitution, size of CD cavity and proportion of derivatives in mixtures. Enantioaffinity and enantioselectivity increased with the average of sulfate groups/mol. β Cavity size complexed better, although and γ cavities did not compromise separation. 6-Heptakis-S-β-CD had less affinity and separation efficiency, attributed to its lower degree and unique position of substitution. The method was optimized with S-β-CD (Aldrich, randomly substituted, 7–11 groups/mol). With this selector, the effect of pH value (3–9) was evaluated. Around pH 7 the cross-over point with change in the direction and order of migration was observed, associated with great enantioselectivity and long migration times. Fine tuning was done by adjusting the CD concentration and the buffer counterion. Definitive conditions were: uncoated silica capillary, 10 mM S-β-CD–25 mM sodium phosphate, pH 3. Validation parameters are included.     

Supershell structure in alkali metal nanowires

Nanowires are formed by indenting and subsequently retracting two pieces of sodium metal. Their cross section gradually reduces upon retraction and the diameters can be obtained from the conductance. In previous work we have demonstrated that when one constructs a histogram of diameters from large numbers of indentation-retraction cycles such histograms show a periodic pattern of stable nanowire diameters due to shell structure in the conductance modes. Here, we report the observation of a modulation of this periodic pattern, in agreement with predictions of a supershell structure.     

Attenuation of a Stoneley wave and higher Lamb modes due to the scattering by two-dimensional irregularities of the walls of a fluid-filled borehole

Attenuation of Stoneley waves and higher Lamb modes propagating along an irregular surface of a fluid-filled borehole is investigated. This problem generalizes the problem on the attenuation of Rayleigh waves by an irregular surface of an empty borehole [10]. The technique used to evaluate the attenuation coefficient is based on the perturbation method (surface irregularity heights are considered to be small in comparison with the wavelength) and the mean field method. As a result, an expression is obtained for the partial coefficients of the eigenmode attenuation due to the scattering of eigenmodes by the irregularities of the borehole walls into the same or other eigenmodes, as well as into the bulk longitudinal and transverse waves. The frequency-dependent behavior of the partial attenuation coefficients of both Stoneley waves and higher modes is analyzed against the ratio between the irregularity correlation length and the borehole radius for different correlation functions of irregularities.     

A simulation of the mass‐transfer effects on the kinetics of solid–gas reactions

A theoretical analysis of the influence of mass‐transfer effect on the kinetic of solid–gas reactions has been carried out by assuming that the partial pressures of the gases generated in the reaction are proportional to the reaction rate. The influence of mass‐transfer phenomena on the apparent activation energy, calculated by the isoconversional methods of Friedman, and on the reaction model is discussed. In the present study, simulated nonisotherm, isotherm, and controlled rate thermal analysis (CRTA) data have been used. Master plots based on the differential forms of the kinetic equations describing solid‐state reactions have been employed by using the concept of the generalized time (θ), introduced by Ozawa; this permits the application of these master plots to the kinetic analysis of reactions whatever the type of temperature program used for recording the experimental data. It has been shown that when the simulated mass‐transfer effect is present the variable effective activation energy E remains nearly constant while the reaction model approaches zero order. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 217–222, 2008     

Locating a metro line in a historical city centre: application to Sevilla

When constructing a metro alignment under a historical city centre, it is important to generate a cost-effective path while maintaining a minimum distance between the alignment and historical buildings. This paper describes a simple methodology for generating a set of good alternative solutions. It is based on the use of Voronoi diagrams. The method was applied to data from the city of Sevilla.     

Self-assembled tin-doped ZnO nanowire and nanoplate structures grown by thermal treatment of ZnS powder

Sintering of a ZnS–SnO₂ mixture under argon flow leads to the growth of columnar nanoplate arrays as well as arrays of nanowires, nanorods and nanoplates with six-fold symmetry. The six-fold nanoplate structures correspond to a more advanced stage of growth than the nanowire structures. Cathodoluminescence (CL) in the scanning electron microscope (SEM) shows that the structures contain Sn, but the amount of this element is normally under the detection limit of X-ray energy-dispersive spectroscopy (EDS). The formation of branches in the hierarchical structures depends on the presence of Sn and on defects in the mixture powder.     

Viscosity of n-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids saturated with compressed CO2

The viscosity of imidazolium-based ionic liquids (ILs) saturated with gaseous, liquid and supercritical carbon dioxide (CO₂) was measured by a high-pressure viscometer at three different temperatures (25, 50, and 70 °C). The high-pressure viscosity of 1-ethyl-3-methylimidazolium ([EMIm]), 1-n-hexyl-3-methylimidazolium ([HMIm]), and 1-n-decyl-3-methylimidazolium ([DMIm]) cations with a common anion, bis(trifluoromethylsulfonyl)amide ([Tf₂N]), saturated with CO₂ was measured up to a maximum of 287 bar. As CO₂ pressure is increased the viscosity of the IL mixture dramatically decreases. While, the ambient pressure viscosity of 1-alkyl-3-methyl-imidazolium [Tf₂N] ILs increases significantly with increasing chain length, the viscosity of all the CO₂-saturated ILs becomes very similar at high CO₂ pressures. From previous vapor–liquid equilibrium data, the viscosity with concentration was determined and found to be the primary factor to describe the fractional viscosity reduction. Several predictive and correlative methods were investigated for the mixture viscosity given pure component properties and include arithmetic mixing rules, the Irving (Predictive Arrhenius) model, Grunberg equation, etc. The modified Grunberg model with one adjustable parameter provided an adequate fit to the data.     

Quantitative analysis of boldine alkaloid in natural extracts by cyclic voltammetry at a liquid-liquid interface and validation of the method by comparison with high performance liquid chromatography

The quantitative determination of boldine alkaloid in boldo leaf extracts by employing cyclic voltammetry, at a liquid/liquid interface as well as the validation of this methodology against the reference method, high performance liquid chromatography (HPLC), are reported in the present paper. The voltammetric analysis was performed successfully and economically using two kinds of liquid/liquid interfaces: water/1,2-dicholoroethane and water/PVC (polyvinyl chloride)-gelled 1,2-dichloroethane. Linear calibration curves in the concentration range of 1.04 × 10⁻⁵ mol L⁻¹ to 5.19 × 10⁻⁴ mol L⁻¹ were obtained with a detection limit equal to (6.1 ± 0.7) × 10⁻⁵ mol L⁻¹ and the quantitative determination of this alkaloid, in complex matrixes such as boldo leaf extracts, by the electrochemical technique proposed was found to be equal to the values obtained using the standard HPLC method. The validation analysis of this methodology against HPLC demonstrated that accuracy, linearity, limit of detection (LOD), limit of quantification (LOQ), specificity and precision are acceptable. The electroanalytical technique proposed is economical and selective, involves simple equipment and can be applied for the quantitative determination of boldine alkaloid in complex matrixes such as leaf extracts without special drug separation. Moreover, cyclic voltammetry (CV) experiments applied at the liquid/liquid interface under different experimental conditions allowed us to study the transfer mechanism of boldine, and determine a value of pK_a^w = 6.90 for protonated boldine, from the variation of voltammetric peak current with pH.