A new spectrophotometric method for determining the extraction constant of quaternary complexes

A new spectrophotometric method for determining the extraction constants of quaternary complexes is described. The method has been applied to a study of the distribution between water and toluene in the V(V)-salycylhydroxamic acid-nitrate-trioctyl methyl ammonium system, determining its conditional extraction constant in toluene for several pH values.     

On the preservation of periodic monotonicity

A 2-periodic continuous real functionf is said to beperiodically monotone if it has the following property: there exist numbert ₁t ₂t ₃t ₁+2 such thatf is nonincreasing fort ₁t ₂ and nondecreasing int ₂tt ₃. For any 2-periodic, integrable real functiong with ₀ ² |g(t|dt) we define

Field-induced uniaxial and biaxial nematic phases in the Maier–Saupe–Zwanzig (MSZ) lattice model

We report detailed statistical mechanics calculations to analyse the field-temperature phase diagram of a fully connected Maier–Saupe–Zwanzig (MSZ) lattice model for the nematic transitions in liquid-crystalline systems. If the field is applied along the uniaxial direction, with positive anisotropy, there is a first-order boundary between distinct uniaxial paranematic phases, which ends at a simple critical point. For negative values of the anisotropy, the first-order boundary becomes continuous at a tricritical point, and the transition is between field-induced uniaxial and biaxial structures. This elementary MSZ lattice model provides a way to systematically regain all of the main features of earlier phenomenological and model calculations. Due to the simplicity of the calculations, critical and tricritical points, as well as critical lines, can be located analytically, which leads to a possibility of comparisons with values for real experimental systems. Also, we formulate a generalised six-state MSZ model, which takes into account intrinsic molecular biaxiality, and perform calculations to determine the location of the critical point in the field-temperature phase diagram.     

Partial least squares multicomponent fluorimetric determination of fluoroquinolones in human urine samples

Ternary mixtures of fluoroquinolones, with a 7-piperazinyl substituted group have been simultaneously determined in human urine samples by application of a multivariate calibration partial least squares (PLS) model. The calibration set was designed with 15 urine samples containing different concentrations of the three fluoroquinolones and 16 blank urine samples. The concentration range for the fluoroquinolones were up to 25 ng ml⁻¹ for norfloxacin (NOR), 80 ng ml⁻¹ for ofloxacin (OFLO) and 300 ng ml⁻¹ for enoxacin (ENO). The method is based on the native fluorescence emission of these compounds in sodium dodecyl sulfate (SDS) medium, at pH 4.0, when exciting at 277 nm. A selection of the emission wavelength range used for the analysis was made for each component. Intraday and interday precision values were determined. Figures of merit as selectivity, sensitivity, limit of detection (LOD) and analytical sensitivity were also calculated. Using the standard addition methodology, five urine samples from five different persons, fortified with three concentration levels of the fluoroquinolones, were analyzed. The limits of detection in urine were 10.0, 0.5 and 0.8 ng ml⁻¹ for ENO, NOR and OFLO, respectively.     

Inhibition of cyclin-dependent kinases, GSK-3beta and CK1 by hymenialdisine, a marine sponge constituent

BACKGROUND: Over 2000 protein kinases regulate cellular functions. Screening for inhibitors of some of these kinases has already yielded some potent and selective compounds with promising potential for the treatment of human diseases. RESULTS: The marine sponge constituent hymenialdisine is a potent inhibitor of cyclin-dependent kinases, glycogen synthase kinase-3beta and casein kinase 1. Hymenialdisine competes with ATP for binding to these kinases. A CDK2-hymenialdisine complex crystal structure shows that three hydrogen bonds link hymenialdisine to the Glu81 and Leu83 residues of CDK2, as observed with other inhibitors. Hymenialdisine inhibits CDK5/p35 in vivo as demonstrated by the lack of phosphorylation/down-regulation of Pak1 kinase in E18 rat cortical neurons, and also inhibits GSK-3 in vivo as shown by the inhibition of MAP-1B phosphorylation. Hymenialdisine also blocks the in vivo phosphorylation of the microtubule-binding protein tau at sites that are hyperphosphorylated by GSK-3 and CDK5/p35 in Alzheimer's disease (cross-reacting with Alzheimer's-specific AT100 antibodies). CONCLUSIONS: The natural product hymenialdisine is a new kinase inhibitor with promising potential applications for treating neurodegenerative disorders.     

Phototautomerism in the lowest excited singlet state of 1-hydroxy-2-carboxyanthraquinone

The dependence of the absorption and fluorescence spectra of 1-hydroxy-2-carboxy-anthraquinone on pH and Hammett acidity have been studied. This compound exhibits phototautomerism in its uncharged and its singly-charged anionic species in aqueous media. Its ground state (pK(a)) and lowest excited singlet-state (pK(a)( *)) dissociation constants have been determined by absorptiometric and fluorimetric titrations and the assignment of the pK(a) and pK( *)(a) values to the equilibria concerned has been carefully considered.     

Determination of carbendazim, thiabendazole and fuberidazole using a net analyte signal-based method

The net analyte signal (NAS)-based method HLA/GO, modification of the original hybrid linear analysis (HLA) method, has been used to determine carbendazim, fuberidazole and thiabendazole in water samples. This approach was used after a solid-phase extraction (SPE) step, using the native fluorescence emission spectra of real samples, previously standardized by piecewise direct standardization (PDS). The results obtained show that the modification of HLA performs in a similar way that partial least-squares method (PLS-1). The NAS concept was also used to calculate multivariate analytical figures of merit such as limit of detection, selectivity, sensitivity and analytical sensitivity (γ⁻¹). With this purpose, blanks of methanol and ternary mixtures, with the target analyte at low concentration and the other two ranging according to the calibration matrix, were used, with different results. Detection limits calculated in the last way are more realistic and show the influence of the other components in the sample. Selectivity for carbendazim is higher than the corresponding values for fuberidazole and thiabendazole, whereas sensitivity, as well as the values obtained for their detection limits, are lower for carbendazim, followed by thiabendazole and fuberidazole. Results obtained by modification of HLA vary in the same way that the ones obtained by PLS-1.     

Solvent effect on the determination of sulfamethazine by room-temperature photochemically induced fluorescence

Room-temperature photochemically-induced fluorescence (RTPF) was applied to the determination of sulfamethazine (SMT) in methanol, ethanol and 2-propanol. Optimal ultraviolet irradiation times ranged between 2 and 6 min. Linear calibration graphs were obtained over a concentration range of more than one order of magnitude. The relative standard deviations were within the range 1.4-2.2%. Limits of detection were between 40 and 80 ng/ml. The method was evaluated for its applicability to the analysis of SMT in pharmaceutical formulations.