Three limit cycles for a first‐order exothermic chemical reaction in a continuous stirred‐tank reactor

We study a first‐order exothermic chemical reaction in a continuous stirred‐tank reactor modelled by a 3‐parameter family of vector fields in . We prove that there exist regions in which contain points that depend on parameters such that the chemical reaction has 0, 1, 2, or 3 small amplitude limit cycles that surround the origin. We conclude that this model can reach two stable small amplitude limit cycles. Finally, we show that one of these regions contains the point in the parameter space considered by Gurel and Lapidus [6] who proved numerically the existence of one stable limit cycle. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Versatile Long‐Wavelength‐Absorbing Scaffold for Eu‐Based Responsive Probes

Coumarin‐sensitized, long‐wavelength‐absorbing luminescent Eu^III‐complexes have been synthesized and characterized. The lanthanide binding site consists of a cyclen‐based chelating framework that is attached through a short linker to a 7‐hydroxycoumarin, a 7‐B(OH)₂‐coumarin, a 7‐O‐(4‐pinacolatoboronbenzyl)‐coumarin or a 7‐O‐(4‐methoxybenzyl)‐coumarin. The syntheses are straightforward, use readily available building blocks, and proceed through a small number of high‐yielding steps. The sensitivity of coumarin photophysics to the 7‐substituent enables modulation of the antenna‐absorption properties, and thus the lanthanide excitation spectrum. Reactions of the boronate‐based functionalities (cages) with H₂O₂ yielded the corresponding 7‐hydroxycoumarin species. The same species was produced with peroxynitrite in a ×10⁶–10⁷‐fold faster reaction. Both reactions resulted in the emergence of a strong ≈407 nm excitation band, with concomitant decrease of the 366 nm band of the caged probe. In aqueous solution the methoxybenzyl caged Eu‐complex was quenched by ONOO^?. We have shown that preliminary screening of simple coumarin‐based antennae through UV/Vis absorption spectroscopy is possible as the changes in absorption profile translate with good fidelity to changes in Eu^III‐excitation profile in the fully elaborated complex. Taken together, our results show that the 7‐hydroxycoumarin antenna is a viable scaffold for the construction of turn‐on and ratiometric luminescent probes.     

Determination of drug-cyclodextrin binding constants by capillary zone electrophoresis

Binding constants of the optical isomers of Deprenyl® (selegiline) and its potential metabolites with (2,6-di-O-methyl)--cyclodextrin were determined using electrophoretic mobility data gained from separations performed by capillary electrophoresis, and absorbancies obtained from spectrophotometric experiments. To calculate equilibrium constants l: l complex formation have been assumed. The comparison of the equilibrium constants calculated from different methods shows similar values in their order of magnitude. Their difference may probably be explained by the different media of the measurements. The effect of the structure of compounds on chiral discrimination were also elucidated.     

Evaluation of antioxidants in Dong quai (Angelica sinensis) and its dietary supplements

The antioxidant activity (AA), total phenolic content (TPC) and total flavonoids content (TFC) in Dong quai (DQ, Angelica sinensis) raw materials and dietary supplements (DS) containing this plant were determined using the CUPRAC, FRAP and fluorescence methods. The antioxidant activity for DQ aqueous extracts revealed by CUPRAC was (1330.45 ± 1.30) μmol Trolox equivalent (TE) per 100 g of dry mass (DM), whereas the antioxidant activity as determined by FRAP was (1813.9 ± 2.0) μmol of TE per 100 g of DM. Lower values were noted for the fluorescence method than for CUPRAC and FRAP (ranging from (35.96 ± 0.3) to (304.6 ± 1.4) μmol of TE per 100 g of DM). The highest TPC values were determined for an aqueous extract of DQ ((3330.3 ± 2.3) μmol of TE per 100 g of DM), while TFC for ethanolic extracts of DQ was ((146.50 ± 0.5) mg of quercetin equivalent (QE) per 100 g of DM). Cinnamic acid, isomers of benzoic acid and derivatives of quercetin were analysed by HPLC-PDA. The ferulic acid concentration in an ethanolic extract of DQ was (21.83 ± 0.07) mg per 100 g of DM. Of the flavonols detected, rutin exhibited the highest concentration in ethanolic extract of DQ ((3.32 ± 0.13) mg of QE per 100 g of DM). Other phytochemicals (alkaloids, saponins, flavonoids, anthraquinones, tannins, steroids, etc.) were identified by phytoscreening colour reaction. The results were analysed by principal component analysis (PCA), cluster analysis and one-way ANOVA tests.     

Investigation of the thermal and structural behaviour of diclofenac sodium-sugar ester surfactant systems

Sugar esters (SEs) have a wide range of hydrophilic-lipophilic balance (HLB) values (1–16) and hence can be applied as surfactants or as solubility or penetration enhancers. They can be used for hot melt technology and solvent method which are frequently applied techniques to preparation of solid dispersions. In this study drug-SE products were prepared by physical mixing, melt technology and solvent methods. The products were investigated by DSC, X-ray powder diffraction and dissolution tests. Diclofenac sodium (DS) as model drug and two SEs, P1670 (HLB=16) and S970 (HLB=9) were used for the preparation of the products. DSC curves revealed considerable melting range and enthalpy decreases for the DS-SE products. The dissolved drug molecules broke down the structures of the SEs but were not built into the crystalline phase of the carrier. The melt technology led to a solid dispersion while in the case of the solvent methods the DS was in molecularly dispersed form which resulted in faster dissolution. The drug release was influenced by the structures resulting from the various treatments, by the HLB and by the gel-forming behaviour of the SEs.     

Colloidal stability of pluronic F68-coated PLGA nanoparticles: a variety of stabilisation mechanisms

Poloxamers are a family of polypropylene oxide (PPO) and polyethylene oxide (PEO) tri-block copolymers that are usually employed in the micro- and nanoparticulate engineering for drug delivery systems. The aim of this work is to study the electrophoretic mobility (mu(e)) and colloidal stability of complexes formed by adsorbing a poloxamer (Pluronic F68) onto poly(d,l-lactic-co-glycolic acid) (PLGA) nanoparticles. A variety of stabilisation mechanisms have been observed for the Pluronic-coated PLGA nanoparticles, where DLVO interactions, solvent-polymer segment interactions and hydration forces play different roles as a function of the adsorbed amount of Pluronic. In addition, the mu(e) and stability data of these complexes have been compared to those obtained previously using a PLGA-Pluronic F68 blend formulation. As both the mu(e) and the stability data are identical between the two systems, a phase separation of both components in the PLGA-Pluronic blend formulation is suggested, being the PLGA located in the core of the particles and the Pluronic in an adsorbed shell.     

P-Mannich Reaction on Wool Fibre for Activation in Dyeing

Abstract

Chemical modification of wool by means of dimethyl phoaphitc and D-glucose results in an increase in dye pickup, while colour and light fastness properties did not decrease, in fact some features bccpme more favourable. Our invesigations showed that increase in fibre diameter occurred. which could rise the rate of dye penetration. Amino acid analysis of the treated wool sample h w e d a decrease in lysine and arginim content, which allows of supposing covalent bond formation betweecl dimcthyl phosphite, D-glucose and the amino and guanidino groups of wool.     

Theoretical study on the structure and stability of some unusual boron-nitrogen helices

Ab initio self-consistent field molecular orbital and density functional theory calculations have been performed on a series of helical structures comprised of boron-nitrogen analogues of extended helicenes, with helically arranged N fused benzene rings, and alternating N benzene units fused to N - 1 cyclobutadiene rings as reference structures. The electronic structure and stability of boron-nitrogen analogues of angular [N]helicenes, [N]phenylenes (N = 5, 6, 7, 12), and [N]methylenylnaphthalenes (N = 6) were investigated at the HF/6-31G(d) and the B3LYP/6-31G(d) levels of theory. The presence of an even number N of rings in the boron-nitrogen [N]helicenes leads to the possibility of angular isomers. Electron density contours were calculated in order to interpret the existing bonding patterns. These structures may provide supramolecular building blocks and macromolecular "springs" with unusual electronic properties.