On the long‐time stability of the Crank–Nicolson scheme for the 2D Navier–Stokes equations

In this article we study the stability for all positive time of the Crank–Nicolson scheme for the two‐dimensional Navier–Stokes equations. More precisely, we consider the Crank–Nicolson time discretization together with a general spatial discretization, and with the aid of the discrete Gronwall lemma and of the discrete uniform Gronwall lemma we prove that the numerical scheme is stable, provided a CFL‐type condition is satisfied. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007     

Direct zonal liquid chromatographic method for the kinetic study of actinomycin-DNA binding

The binding of an anticancer drug (actinomycin D or ACTD) to double-stranded DNA (dsDNA) was studied by means of high-performance liquid chromatography (HPLC). ACTD is an antitumor antibiotic containing one chromophore group and two pentapeptidic lactone cycles that binds dsDNA. Incubations of ACTD with DNA were performed at physiological pH. The complexed and free ligand concentrations of the mixture were quantified at 440 nm from their separation on a size-exclusion chromatographic (SEC) column using the same buffer for the elution and the sample incubation. The DNA and the ACTD-DNA complexes were eluted at the column exclusion volume while the ligand was retained on the support. An apparent binding curve was obtained by plotting the amount emerging at the exclusion column volume against that eluted at free ACTD retention volume. A dissociating effect was evidenced and the binding parameters were significantly different from those obtained at equilibrium by visible absorbance titration. The equilibrium binding parameters determined by absorption spectroscopy were used as starting data in the numerical simulations of the chromatographic process. The results showed a strong dependency of the apparent binding parameters on the reaction kinetics. Finally the comparison of the apparent binding curve obtained from the HPLC experiments and from the numerical simulations permitted an evaluation of the dissociation rate constant (kd = 0.004 s(-1)).     

Photophysical Properties of Some Flavones Probes in Homogeneous Media

Photophysical properties of five hydroxyflavones (HF) (some typical models of flavonols), (3 - HF, 6 - HF, 7-HF, 3, 6 - diHF and 3, 7 - diHF) were studied in homogeneous media by means of UV–vis and steady-state and time resolved fluorescence spectroscopies. Their absorption and fluorescence characteristics based on the flavonols structure are presented and discussed. It was found that the fluorescence of the flavonols depends on the nature of the solvent and on their molecular structure, especially on the position and the number of the -OH groups of the substituted phenyl ring. Attention is paid to the number of the -OH groups that influence the excited-state intramolecular proton transfer (ESIPT) process. The fluorescence quantum yield and the lifetime of the flavonols in heterogeneous media have been also determined. The results are discussed with relevance to the flavonols as sensitive fluorescence probe and to their microenvironments in the systems of biological interest and especially in a typical protein environment.     

Development of a nanocomposite system and its application in biosensors construction

The present work describes the development of a nanocomposite system and its application in construction of a new amperometric biosensor applied in the determination of total polyphenolic content from propolis extracts. The nanocomposite system was based on covalent immobilization of laccase on functionalized indium tin oxide nanoparticles and it was morphologically and structural characterized. The casting of the developed nanocomposite system on the surface of a screen-printed electrode was used for biosensor fabrication. The analytical performance characteristics of the settled biosensor were determined for rosmarinic acid, caffeic acid and catechol (as laccase specific substrate). The linearity was obtained in the range of 1.06×10^?6 ? 1.50×10^?5 mol L^?1 for rosmarinic acid, 1.90×10^?7 ? 2.80×10^?6 mol L^?1 for caffeic acid and 1.66×10^?6 ? 7.00×10^?6 mol L^?1 for catechol. A good sensitivity of amperometric biosensor 141.15 nA µmol^?1 L^?1 and fair detection limit 7.08×10^?8 mol L^?1 were obtained for caffeic acid. The results obtained for polyphenolic content of propolis extracts were compared with the chromatographic data obtained by liquid-chromatography with diode array detection.      

Enolate Alkylations with Vinylphosphonates

A remarkable feature of diethyl-1,3-butadiene-phosphonate, unlike the dienephosphonium salts,^2,3 is that it undergoes a simple Michael addition with enolates of aldehydes and ketones without the loss of the phosphorus group.⁴ Whether such a difference in reactivity exists between the two-carbon reagents, vinyltriphenylphosphonium bromide 1 ⁵ and diethyl vinylphosphonate 2 ⁶ is the subject of this investigation.     

Screening the thermal stability of carotenoids-α lactalbumin complex by spectroscopic and molecular modeling approach

Sea buckthorn is a natural food ingredient rich in carotenoids, tocopherols, sterols, flavonoids, lipids, vitamins, tannins and minerals. In this study, we investigated the themostability of the complex formed between α-lactalbumin (α-LA) with carotenoids from sea buckthorn berries extract (CSB) in the temperature range of 25°C to 100°C. The heat induced conformational changes of the α-LA-CSB complex were studied by using fluorescence and molecular modeling techniques. Phase diagram indicated the presence of more than one structurally distinct species as an indicator that temperature influenced the conformation of α-LA. Intrinsic fluorescence studies revealed that carotenoids trapped into the core of α-LA do not bind in locations close to tryptophan (Trp) residues. The synchronous spectra indicated that the interaction between α-LA and CSB had no apparent influence on the local conformation of Trp and tyrosine (Tyr) microenvironments within protein structure. Quenching studies with acrylamide showed that Trp residues had the highest exposure at 80°C, being least accessible to quencher at 60°C. In agreement with the fluorescence spectroscopy observations, the in silico analysis at single molecules level indicated a significant increase of 46.42 Å² and 80.07 Å² of the total accessible surface area of Trp and Tyr residues, respectively with the temperature increase from 25°C to 90°C. Concerning the thermodynamic properties of the α-LA-β-carotene model, the molecular modeling results indicate that the thermal treatment is not favorable for preserving the stability of the complex.     

Covariate selection for semiparametric hazard function regression models

We study a flexible class of nonproportional hazard function regression models in which the influence of the covariates splits into the sum of a parametric part and a time-dependent nonparametric part. We develop a method of covariate selection for the parametric part by adjusting for the implicit fitting of the nonparametric part. Asymptotic consistency of the proposed covariate selection method is established, leading to asymptotically normal estimators of both parametric and nonparametric parts of the model in the presence of covariate selection. The approach is applied to a real data set and a simulation study is presented.     

Enolate alkylations with diethylbutadiene phosphonat-I

Enolates of various cyclic ketones undergo Michael addition to 1-diethyl-phosphinyl-1,3-butadiene to yield ketophosphonates.