Assessment of Inorganic Fibre Burden in Biological Samples by Scanning Electron Microscopy – Energy Dispersive Spectroscopy

A method based on cloud point extraction (CPE) separation/preconcentration of trace cadmium as a prior step to its determination by graphite furnace atomic absorption spectrometry (GFAAS) has been developed. If the system temperature is higher than the cloud point temperature (CPT) of the nonionic surfactant of p-octyl polyethyleneglycolphenyether (Triton X-100), the complex of Cd²⁺ with 1-(2-pyridylazo)-2-naphthol (PAN) could be extracted into surfactant-rich phase. The chemical variables affecting CPE were evaluated and optimized. Under the optimum conditions, preconcentrating 10.0 mL of water samples permitted a limit of detection of 5.9 ng · L⁻¹ (3σ) for cadmium with an enhancement factor of 50 and a relative standard deviation of 2.1% (n = 11, c = 2.0 ng · mL⁻¹). The method was applied to the determination of cadmium in reference material and water samples with satisfactory results.     

Pharmacophore modeling as an efficient tool in the discovery of novel noncompetitive AMPA receptor antagonists

A three-dimensional pharmacophore model for the binding of noncompetitive AMPA receptor antagonists was developed in order to map common structural features of highly active compounds. This hypothesis, which consists of two hydrophobic regions, one hydrogen bond acceptor and one aromatic region, was successfully used as framework for the design of a new class of allosteric modulators containing a tetrahydroisoquinoline skeleton and for in silico screening. The promising biological results suggested that the identified molecules might be useful "lead compounds" for future drug development.     

AChE biosensor based on zinc oxide sol-gel for the detection of pesticides

Zinc oxide has been used as a matrix for immobilization of acetylcholinesterase (AChE) and detection of the pesticide paraoxon. The immobilized enzyme retained its enzymatic activity up to three months when stored in phosphate buffered saline (pH 7.4) at 4 °C. An amperometric biosensor for the detection of paraoxon was designed. The biosensor detected paraoxon in the range 0.035-1.38 ppm and can be used to detect other AChE inhibiting organophosphate pesticides.     

UVB Monochromatic Action Spectrum for the Inhibition of Photosynthetic Oxygen Production in the Green Alga Dunaliella saha

The effect of monochromatic UVB (280–315 nm) irradiation on the photosynthetic activity of the marine green alga Dunaliella salina was investigated by monitoring the rate of oxygen exchange. Samples were irradiated with narrow bands centered at different UV wavelengths (281, 290, 300, 310 and 322 nm). In a first set of experiments the samples were preirradiated keeping constant the irradiance and varying exposure time. By increasing UVB fluence, a wavelength-dependent decrease in the rate of oxygen production was observed. In a second set of experiments oxygen exchange was monitored simultaneously to UVB irradiation. The decline in the rate of oxygen production was sharper at short wavelengths and faster with increasing UVB photon flux density. The photon fluence action spectra derived from the two sets of experiments are in good agreement and indicate a decrease in the UVB-inhibiting effectiveness of two orders of magnitude from about 0.2 (mmol m-2)-1 at 281 nm to about 0.003 (mmol m-2)-1 at 322 nm. The photon flux density action spectrum derived from the second set of experiments indicates a smoother decrease from about 0.2 (pmol s-l m-2)-1 at 281 nm to about 0.05 (pmol s-l m-z)-l at 310 nm.     

Validation of the anti-factor Xa assay for the potency assessment of enoxaparin in pharmaceutical formulations

Enoxaparin is a low-molecular weight heparin used clinically for the prevention and treatment of venous and arterial thrombosis. An anti-factor Xa assay was used to evaluate the potency of the final drug preparation. Method validation investigated parameters such as the range, linearity (r2 = 0.9971), precision, accuracy, and robustness; the biological assay incorporated a chromogenic endpoint and detection at 405 nm. The method yielded good results with a quantitation limit of 0.037 IU/mL and a detection limit of 0.011 IU/mL. The results demonstrated the validity of the anti-factor Xa assay for the determination of enoxaparin.     

Spontaneous dehydration mechanism of aromatic aldehyde reactions with hydroxyl and non‐hydroxyl amines

The plot of rate constants vs. pH for the dehydration step of the reaction between furfural and 5‐nitrofurfural with hydroxylamine, N‐methylhydroxylamine, and O‐methylhydroxylamine, shows two regions corresponding to the oxonium ion‐catalyzed and spontaneous dehydration. The oxonium ion‐catalyzed dehydration region of the reaction of furfural with the above mentioned hydroxylamines exhibits general acid catalysis with excellent Brønsted correlation (Brønsted coefficients: 0.76 (r = 0.986), 0.68 (r = 0.987), and 0.67 (r = 0.993) respectively). However, the rate constants of the spontaneous dehydration of these hydroxylamines, where water is considered the general acid catalyst, exhibit a large positive deviation from the Brønsted line. This fact was not observed in the reaction of non‐hydroxyl amines with different aromatic aldehydes by other authors, thus supporting that the spontaneous dehydration steps for these reactions proceed by intramolecular catalysis. The mechanism of intramolecular catalysis might be stepwise. First, a zwitterionic intermediate is formed. It can then evolve in the second step by loss of water, or follow a concerted pathway, with the transference of a proton through a five‐membered ring (general intramolecular acid catalysis). In the case of non‐hydroxyl amines, data suggested the possibility of a mechanism of intramolecular proton transfer through one or two water molecules, from the nitrogen of the amine to the leaving hydroxide ion. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 685–692, 2002     

Multivariate Optimization and Validation of a CZE Method for the Analysis of Pridinol Mesylate and Meloxicam in Tablets

A capillary zone electrophoresis method for the simultaneous determination of pridinol mesylate (PRI) and meloxicam (MEL) employing epinastine hydrochloride and piroxicam as internal standards, was developed and optimized employing experimental design and response surface methodologies. The separation was optimally achieved in less than 2 min at 30 kV in an uncoated fused-silica capillary (41.4 cm × 75 ??m I.D.), employing an 18 mmol L^?1 sodium phosphate buffer solution (pH 5.90) at 25 °C. Samples were injected in hydrodynamic mode (50 mbar, 5 s) and the analytes were spectrophotometrically detected at 200 nm. Method robustness was demonstrated by ANOVA of determinations performed under conditions slightly different from the optimum. The method was validated regarding separation selectivity (peak purity factors > 0.99), linearity and range (PRI = 17.6?C31.4 mg L^?1; MEL = 66.5?C122.5 mg L^?1), accuracy (PRI = 100.2?C101.9%; MEL = 98.9?C100.7%) and precision. The RSD values obtained were ??1.3% for injection repeatability and ??1.9% for intra-day precision. The limits of detection (1.0 and 0.9 mg L^?1) and quantification (3.3 and 16.5 mg L^?1) of PRI and MEL, respectively, were also determined. The method was successfully applied to the determination of both drugs in three brands of tablet formulations. No statistically significant differences were observed when these results were compared with those of a RP-HPLC method.     

pKa determinations of xanthene derivates in aqueous solutions by multivariate analysis applied to UV-Vis spectrophotometric data

Xanthenes form to an important class of dyes which are widely used. Most of them present three acid-base groups: two phenolic sites and one carboxylic site. Therefore, the pKa determination and the attribution of each group to the corresponding pKa value is a very important feature. Attempts to obtain reliable pKa through the potentiometry titration and the electronic absorption spectrophotometry using the first and second orders derivative failed. Due to the close pKa values allied to strong UV-Vis spectral overlap, multivariate analysis, a powerful chemometric method, is applied in this work. The determination was performed for eosin Y, erythrosin B, and bengal rose B, and also for other synthesized derivatives such as 2-(3,6-dihydroxy-9-acridinyl) benzoic acid, 2,4,5,7-tetranitrofluorescein, eosin methyl ester, and erythrosin methyl ester in water. These last two compounds (esters) permitted to attribute the pKa of the phenolic group, which is not easily recognizable for some investigated dyes. Besides the pKa determination, the chemometry allowed for estimating the electronic spectrum of some prevalent protolytic species and the substituents effects evaluation.     

The “dimer nucleophile mechanism” for reactions with rate‐determining first step: Derivation of the whole kinetic law and further treatment of kinetic results

Overwhelming evidence has been previously reported for the existence of the so‐called “dimer nucleophile mechanism” in aromatic nucleophilic substitutions by amines in aprotic solvents, for which the most prominent feature is the fourth‐order kinetics (third order in amine) that has been observed with many different substrate–nucleophile systems, especially those in which departure of the nucleofuge is the rate‐determining step. The mechanism has been confirmed by several other features, although other alternative mechanisms were suggested to explain the fourth‐order kinetics, no one has been able to explain the other above‐mentioned features. The present paper affords additional experimental evidence and derivation of the kinetic expressions for reactions with good nucleofugues, where the first step is rate determining. The work involves studies of the reactions of 2,4‐dinitrofluorobenzene and 2,4‐dinitrochlorobenzene with aniline and with alkyldiamines in toluene. The novelty of this work lies in the selection of substrate–nucleophile systems exhibiting kinetic behavior that allows estimations of the different k's involved. The satisfactory agreement between the quotients of k's calculated from sets of data obtained under different reaction conditions hereby reported indicates that the assumptions made are correct and that the whole treatment applied to the kinetic data is justified. All together, the results fit well with the reaction scheme involving the dimer nucleophile mechanism, adding new evidence to this mechanism that it is well established in the current literature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 735–742, 2010     

An adaptive stepsize method for the chemical Langevin equation

Mathematical and computational modeling are key tools in analyzing important biological processes in cells and living organisms. In particular, stochastic models are essential to accurately describe the cellular dynamics, when the assumption of the thermodynamic limit can no longer be applied. However, stochastic models are computationally much more challenging than the traditional deterministic models. Moreover, many biochemical systems arising in applications have multiple time-scales, which lead to mathematical stiffness. In this paper we investigate the numerical solution of a stochastic continuous model of well-stirred biochemical systems, the chemical Langevin equation. The chemical Langevin equation is a stochastic differential equation with multiplicative, non-commutative noise. We propose an adaptive stepsize algorithm for approximating the solution of models of biochemical systems in the Langevin regime, with small noise, based on estimates of the local error. The underlying numerical method is the Milstein scheme. The proposed adaptive method is tested on several examples arising in applications and it is shown to have improved efficiency and accuracy compared to the existing fixed stepsize schemes.