Carbon Paste Electrode for the Potentiometric Flow Injection Analysis of Drotaverine Hydrochloride in Serum and Urine

A carbon paste electrode for drotaverine hydrochloride (DvCl) was prepared and fully characterized in terms of composition, life span, usable pH range, response time and temperature. The electrode was applied to the potentiometric determination of drotaverinium ions in pharmaceutical preparations and biological fluids in steady state and flow injection conditions (FIA). The electrode is based on a mixture of two ion exchangers, namely, drotaverinium-silicotungestate and drotaverinium-tetraphenylborate dissolved in tricresyl phosphate as pasting liquid. The modified electrode showed a near-Nernstian slope of 59.34±2mV over the concentration range of 5.0×10^–7–1.0 × 10^–2M. The electrode exhibits good selectivity for DvCl with respect to a large number of inorganic cations, organic cations, sugars and amino acids. Potentiometric titrations of DvCl with several titrants have been monitored using this modified carbon paste electrode as an end-point indicator electrode. The proposed electrode offers the advantages of simplicity, accuracy, automation feasibility and applicability to turbid and colored samples.     

X-Ray Powder Diffraction Studies of Polyhydrates of Cross-Linked Tetraisoamylammonium Polyacrylates

X-ray powder diffraction studies were performed of the polyhydrates, formed at positive temperature in swelled grains of carboxylic cation-exchange resins in the form of tetraisoamylammonium (cross-linked tetraisoamylammonium polyacrylates, differing in the degree of cross-linking: 1, 2 and 3%). It was shown that the polyhydrates exist as the crystalline phase under specific conditions, determined earlier by DTA technique. Diffraction patterns of the samples, recorded at the temperature +3 ± 1 °C, indicate that the polyhydrates crystallize in hexagonal symmetry, the unit cell parameters are close to a=12.25 Å, c=12.72 Å and do not depend significantly upon the degree of cross-linking. The suppositions were made concerning the size of hydrate crystallites, formed in the swelled grains of the studied resins.This revised version was published online in July 2005 with a corrected issue number.     

An <Emphasis Type="BoldItalic">in silico</Emphasis> approach for screening flavonoids as P-glycoprotein inhibitors based on a Bayesian-regularized neural network

Summary P-glycoprotein (P-gp), an ATP-binding cassette (ABC) transporter, functions as a biological barrier by extruding cytotoxic agents out of cells, resulting in an obstacle in chemotherapeutic treatment of cancer. In order to aid in the development of potential P-gp inhibitors, we constructed a quantitative structure–activity relationship (QSAR) model of flavonoids as P-gp inhibitors based on Bayesian-regularized neural network (BRNN). A dataset of 57 flavonoids collected from a literature binding to the C-terminal nucleotide-binding domain of mouse P-gp was compiled. The predictive ability of the model was assessed using a test set that was independent of the training set, which showed a standard error of prediction of 0.146 ± 0.006 (data scaled from 0 to 1). Meanwhile, two other mathematical tools, back-propagation neural network (BPNN) and partial least squares (PLS) were also attempted to build QSAR models. The BRNN provided slightly better results for the test set compared to BPNN, but the difference was not significant according to F-statistic at p = 0.05. The PLS failed to build a reliable model in the present study. Our study indicates that the BRNN-based in silico model has good potential in facilitating the prediction of P-gp flavonoid inhibitors and might be applied in further drug design.     

Stereoselective discrimination and quantification of arginine and N-blocked arginine enantiomers by formation and dissociation of calcium-mediated diastereomeric trimer complexes with a chiral reference compound using electrospray ionization-ion trap tandem mass spectrometry

Chiral resolution of arginine (Arg) and Arg derivatives is demonstrated using electrospray ionization-tandem mass spectrometry (ESI-MS). Calcium ion (Ca(II))-mediated trimeric clusters are generated, which incorporate the analyte of interest and an enantiomerically pure reference molecule of similar metal ion affinity. Two methods, one based on the measurement of a competitive-dissociation-based branching ratio (R(chir)) by the kinetic method (KM) and one based on the measurement of a chiral recognition ratio (CR) by a similar method, are compared. Incorporating N-blocked Arg derivatives (Z-Arg and Boc-Arg) as chiral references provides chiral resolution greater than that previously reported for Arg enantiomers. In a reciprocal manner, pure Arg enantiomers can be used as references for discriminating enantiomers of these N-blocked Arg derivatives. Condensed-phase and gas-phase Ca(II) ion affinity relative to Arg is also addressed qualitatively for other acidic, basic, and neutral amino acids. In some cases, when only one offspring ion is observed (insufficient for KM analysis), the CR method can be applied as an alternative to obtain a measurable stereoselectivity value for the system. The results of these experiments demonstrate the applicability of, and the difference between, the KM and the CR method for improved quantitative analysis of enantiomeric excess for Arg.     

Reversible energy transfer

Analysis of the transient and steady-state kinetics of reversible energy transfer shows that while the interpretation of lifetime measurements is difficult unless the donor and acceptor lifetimes are appreciably different, quantum yield measurements are relatively easy to interpret.

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Zusammenfassung Die Analyse der Kinetik der Übergangszustände und der stationären Zustände der reversiblen Energieübertragung zeigt, daß im Gegensatz zu einer schwierigen Interpretation der Messungen der Lebensdauer — es sei denn die Lebensdauer von Donor und Acceptor sind wesentlich voneinander verschieden — die Messungen der Quantumausbeute verhältnismäßig einfach zu interpretieren sind.

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Résumé L'analyse de la cinétique de l'état transitoire et de l'état stationnaire du transfert réversible d'énergie montre que, si l'interprétation des mesures de durée de vie est difficile, à moins queles durées de vie du donneur et de l'accepteur soient très différentes, il est par contre relativement facile d'interpréter les mesures de rendement quantique.

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Dedicated to the memory of Professor K. H. Hansen.     

Thermodynamic trans-Effects of the Nucleotide Base in the B12 Coenzymes

The thermodynamic effects of the nucleotide coordination on the Co-C bond strengths in the B ₁₂ coenzymes were analyzed. Methyl group transfer reactions from methylcob( III )inamides to cob( II )inamides and cob( I )inamides in neutral aqueous solution were used in equilibration experiments to determine the effect fo the intramolecular coordination of the nucleotide function on the Co-C bond dissociation energies of methylcob( III )alamin ( 4 ). In the equilibrium between 4 , cob( I )inamide ( 11 ), cob( I )alamin ( 10 ) and methylcob( III )inamide 6 (Scheme 2), 4 and 11 were found to predominate ( 4 + 11 ? 10 + 6 , equilibrium constant K_I/III≈0.004), while the equilibrium between 4 , cob( II )inamide 9 , cob( II )alamin ( 5 ), and 6 (Scheme 1) proved to be well balanced ( 4 + 9 ? 5 + 6 , equilibrium constant K_II/III=0.60). These equilibrium values indicate the nucleotide coordination to stabilize the Co–C bond in 4 both against homolysis (slight effect) and against nucleophilic heterolysis (considerable effect). They reflect a stabilization of the complete corrins 4 and 5 by the nucleotide coordination, which is also indicated for 4 and 5 by their (nucleotide) basicity. The latter information, where available for other organocobalamins, allows the analysis of the thermodynamicnucleotide trans effect there as well: e.g. in coenzyme B ₁₂ ( 1 ), the nucleotide coordination is found this way to weaken the Co–C bond towards homolysis by ca. 0.7 kcal/mol.     

Microemulsion photopolymerization of methacrylates stabilized with sodium dodecyl sulfate and poly(N-acetylethylenimine) macromonomers

Methyl methacrylate and butyl methacrylate were polymerized in oil-in-water microemulsions that were stabilized by sodium dodecyl sulphate (SDS). A poly(N-acetylethylenimine) (PNAEI) macromer was also included in the recipe, as a cosurfactant and a comonomer. Polymerizations were initiated by UV-irradiation. The average diameters of latex particles, obtained by STM, were in the range of 17-200 nm. The experimental data evidenced that the particle size was mainly dependent on the SDS/PNAEI ratio. Polymerization yields were around 75-85%. The synthesized copolymers have viscosity average molecular weights in the range of 2.1-2.4×10⁶ and glass transition temperatures of 38.0-43.5°C, lower than those obtained without using PNAEI. The investigation by means of FTIR and 1H-NMR techniques revealed that PNAEI was incorporated into the nanoparticles.     

Vergleich zwischen dem dielektrischen und dem mechanischen Relaxationsverhalten

Zusammenfassung Auf Grund theoretischer Überlegungen ist es bei der Bestimmung des Relaxationsverhaltens hochpolymerer Stoffe bei verschiedenen Beanspruchungsarten sinnvoll, zur Bestimmung der Relaxationszeiten die Maxima der Imaginärteile der elastischen Moduln und der reziproken Dielektrizitätskonstanten auszuwerten und die in Abhängigkeit von der Frequenz bei konstanter Temperatur gemessenen Dispersionskurven in die Anteile der Hauptund Nebendispersionsgebiete zu zerlegen. Durch Anwendung dieses Verfahrens auf die an Polyvinylchlorid gewonnenen Ergebnisse wird nachgewiesen, daß bei diesem Stoff die mechanisch und dielektrisch gemessenen Relaxationszeiten identisch sind und daß somit die vonThurn undWolf (1) vorgeschlagene Deutung der Unterschiede zwischen mechanischem und dielektrischem Relaxationsverhalten keine allgemeine Gültigkeit hat. Ein verbessertes Verfahren zur Bestimmung der Ultraschallgeschwindigkeit und der Ultraschalldämpfung in hochpolymeren Stoffen wird angegeben.