Modified electrodes based on lipidic cubic phases

The lipidic cubic phase can be characterized as a curved bilayer forming a three-dimensional, crystallographical, well-ordered structure that is interwoven by aqueous channels. It provides a stable, well-organized environment in which diffusion of both water-soluble and lipid-soluble compounds can take place. Cubic phases based on monoacylglycerols form readily and attract our interest due to their ability to incorporate and stabilize proteins. Their lyotropic and thermotropic phase behaviour has been thoroughly investigated. At hydration over 20%, lipidic cubic phases Ia3d and Pn3m are formed. The latter is stable in the presence of excess water, which is important when the cubic phase is considered as an electrode-modifying material. Due to high viscosity, the cubic phases can be simply smeared over solid substrates such as electrodes and used to host enzymes and synthetic catalysts, leading to new types of catalytically active modified electrodes as shown for the determination of cholesterol, CO(2), or oxygen. The efficiency of transport of small hydrophilic molecules within the film can be determined by voltametry using two types of electrodes: a normal-size electrode working in the linear diffusion regime, and an ultramicroelectrode working under spherical diffusion conditions. This allows determining both the concentration and diffusion coefficient of the electrochemically active probe in the cubic phase. The monoolein-based cubic phase matrices are useful for immobilizing enzymes on the electrode surface (e.g., laccases from Trametes sp. and Rhus vernicifera were employed for monitoring dioxygen). The electronic contact between the electrode and the enzyme was maintained using suitable electroactive probes.     

Micropatterning chemical oscillations: waves, autofocusing, and symmetry breaking

Arrays of chemical oscillators are micropatterned by Wet Stamping. The technique is used to demonstrate that chemical waves can be initiated and controlled in oscillatory systems and that such waves can give rise to phenomena not observed in excitable media. Interoscillator coupling and synchronization, kinetic autofocusing, and twist-symmetry breaking are a consequence of the dependence of the oscillation phase on the local concentrations of reagents and on systems' geometry. Conditions under which a generic oscillatory system would exhibit such behaviors are determined.     

Do proficiency tests always verify laboratories’ performance? The case of FAPAS PT 0270

In this paper, the case of FAPAS PT 0270 “Doramectin and Oxfendazole in Sheep Liver” is discussed. During evaluation of the data received from participants (determination of total, oxidised oxfendazole residue and calculation of the sum of oxfendazole and oxfendazole sulfone residues), significant differences were observed between the results obtained by use of two analytical approaches. This phenomenon can be explained by the route of oxfendazole metabolism, which results in the presence of fenbendazole in the sample. This was not predicted by the provider; consequently, not all the necessary tests on the material were conducted. Due to the high uncertainty of the z-scores in this test, the results of the PT cannot be used for purposes of evaluation, and the benefits of participation in PT 0270 are questionable.     

Studies of surface properties of Na- and La-montmorillonites

This paper presents possible applications of thermal analysis, sorptometry and porosimetry to study physico-chemical properties of Na- and La-montmorillonite samples, especially for determination of total surface heterogeneity. The quasi-isothermal thermogravimetric (Q-TG) mass loss and its first derivative (Q-DTG) curves with respect to temperature and time obtained during programmed liquid thermodesorption under quasi-isothermal conditions have been used to study adsorbed layers and heterogeneous properties of the Na- and La-montmorillonites. Calculations of the desorption energy distribution functions by analytical procedure using mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption under quasi-isothermal conditions of polar and non-polar liquid vapours preadsorbed on a material surface are presented. Parameters relating to porosity of samples were determined by sorptometry, mercury porosimetry and atomic force microscopy (AFM). From nitrogen sorption isotherms from sorptometry and porosimetry methods, the fractal dimensions of montmorillonites have been calculated. Moreover, a new approach is proposed to calculate fractal dimensions of materials obtained from Q-TG curve; this is compared with values obtained by the above methods. The total heterogeneous properties (energy distribution function and pore-size distribution functions) of samples studied were estimated. The radius and pore volume of the tested samples calculated on the basis of thermogravimetry, sorptometry and porosimetry techniques were compared and good correlations obtained.     

Anisotropic dipole polarizability of transition metal atoms: Sc(2D), Ti(3F,3P), V(4F,4P,6D), Ni(3F) and ions: Sc2+(2D), Ti2+(3F,3P)

Dipole polarizability tensor components and quadrupole moments of transition-metal atoms Sc, Ti, V, Ni, and Cu and ions Sc2+ and Ti2+ are computed using finite field complete active space self-consistent field and multireference configuration interaction ab initio methods. Perpendicular components of the dipole polarizability tensor are calculated from equations involving only parallel components of the polarizability tensor and its average value. Mean polarizability and polarizability anisotropy decrease in the Sc-Ni series. Relativistic effects are accounted for with the Douglas-Kroll Hamiltonian. The consequences of the anisotropic properties of these atoms to their interactions with spherically symmetric rare gases are also discussed.     

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.     

An Efficient Synthesis of <Emphasis Type="Italic">O</Emphasis>-Methyl Protected Emodin Aldehyde and Emodin Nitrile

Summary. An efficient synthesis of tri-O-methylemodin aldehyde was achieved via bromination of tri-O-methylemodin utilizing N-bromosuccinimide yielding the monobromo and dibromo derivatives. Sommelet reaction of the monobromomethyl derivative as well as hydrolysis of the dibromomethyl analog with aqueous silver nitrate afforded the protected aldehyde in good yield. Accordingly, both bromo derivatives can be used even when they are obtained as a mixture of the bromination reaction, which could not be controlled easily to yield the bromo products selectively. From the aldehyde the tri-O-methylemodin nitrile was prepared in a one-pot reaction using hydroxylamine-O-sulfonic acid.Received February 18, 2003; accepted March 3, 2003 Published online June 2, 2003     

Exchange interactions in copper(II) complexes with tridentate Schiff bases: EPR and magnetic studies

The influence of the nature of ligands upon the magnetic and EPR properties was investigated for the series of polynuclear Cu(II) complexes with tridentate Schiff bases. A similar tendency of changes in the |J| and D parameters was found for binuclear Cu(II) complexes with ligands derived from condensation of 3-amino-1-propanol with 2-hydroxyaldehydes and acetylacetone as well as for Cu(II) complexes with condensation products of o-aminophenol and β-diketones. The Cu(II) complex with Schiff base of 1-amino-2-propanol and acetylacetone exhibits ferromagnetic behaviour suggesting tetrameric structure.     

Effects of solute-solvent proton exchange on polypeptide chain dynamics: a constant-pH molecular dynamics study

A method for performing implicit-solvent molecular dynamics simulations at constant pH was applied to a pentapeptide acetyl-Ala-Asp-Ala-Lys-Ala-amide at pH 4. As a reference, molecular dynamics simulations were done for the same peptide with two variants of its fixed protonation patterns expected to dominate at pH 4, i.e., with a protonated and a deprotonated side chain of the Asp residue and the protonated Lys residue in both cases. The dynamic trajectories of the peptide were used to discuss the problem of the significance of the solute-solvent proton exchange phenomena for the dynamics and structural distributions of the polypeptide chain. The Asp-Lys distance was used as a probe of the overall molecular structure of the investigated pentapeptide. To characterize the dynamics, distributions of the "waiting" times for a transition from a "short" distance conformation to a "long" distance conformation were constructed, based on the generated molecular dynamics trajectories. We show that the relaxation time for the transitions, derived from the constant-pH simulations, is very close to the relaxation time characterizing a permanently protonated molecule, although the average protonation probability of the short-distance conformation is close to zero. However, the distribution of the Asp-Lys distances obtained from constant-pH simulations cannot be reproduced as a linear combination of the distributions resulting from the simulations with fixed protonation states.