Ligand binding properties of cobalamins

The main goal of the present density functional theory calculations is a comparative study of NO, O₂, ${{\rm NO}_{2}^{-}}$ , and H₂O binding to different forms of cob(II)alamins and cob(III)alamins. The comparison of binding energies of small ligands enables one to draw conclusions regarding the stability of the studied derivatives of cobalamins as well as to define the preferred form of cobalamin for each ligand. Ligands such as NO and O₂ favor cob(II)alamins, while H₂O and ${{\rm NO}_{2}^{-}}$ cob(III)alamins. The obtained results are confronted with available experimental data. Finally, our findings allow one to divide the studied small ligands into two groups: NO and O₂ for which the coordination to cobalamins significantly weakens their internal bonds, and ${{\rm NO}_{2}^{-}}$ and H₂O for which the effect is not observed.     

Ion flux through membrane channels--an enhanced algorithm for the Poisson-Nernst-Planck model

A novel algorithmic scheme for numerical solution of the 3D Poisson-Nernst-Planck model is proposed. The algorithmic improvements are universal and independent of the detailed physical model. They include three major steps: an adjustable gradient-based step value, an adjustable relaxation coefficient, and an optimized segmentation of the modeled space. The enhanced algorithm significantly accelerates the speed of computation and reduces the computational demands. The theoretical model was tested on a regular artificial channel and validated on a real protein channel-alpha-hemolysin, proving its efficiency.     

Ultraselective and Sensitive Determination of Cr(VI) in the Presence of a High Excess of Cr(III) in Natural Waters with a Complicated Matrix

In the present work, the catalytic adsorptive stripping voltammetric procedure for trace determination of Cr(VI) in the presence of a high excess of Cr(III) in natural waters with a complicated matrix has been developed. The influence from potentially interfering substances such as organic matter was successfully eliminated by exploiting adsorptive properties of polymeric resin. The detection limit estimated from 3 times the standard deviation for a low Cr(VI) concentration in the simultaneous presence of a 2×10³ fold excess of Cr(III), 50 mg L^?1 surfactants, 50 mg L^?1 humic substances and the accumulation time of 30 s was about 5.3×10^?10 mol L^?1.     

Physical Stability and the Droplet Distribution of Rice Oil–in–Water Emulsion

The objective of the current study was to evaluate long-term stability of emulsions with rice oil by assessing their physical properties. For this purpose, six emulsions were prepared, their stability was examined empirically, and the most correctly formulated emulsion composition was determined using a computer simulation. Variable parameters (oil and thickener content) were indicated with optimization software based on Kleeman's method. Synthesized emulsions were studied by numerous techniques involving determination of particle size and distribution of emulsion, optical microscopy, viscosity, and novelty analysis—Turbiscan test.

The emulsion containing 50 g of oil and 1.2 g of thickener had the highest stability. Empirically determined parameters proved to be consistent with the results obtained using the computer software. The computer simulation showed that the most stable emulsion should contain from 35.93 to 50 g of oil and 0.94 to 1.19 g of thickener. The computer software based on Kleeman's method proved to be useful for fast optimization of the composition and providing parameters of stable emulsion systems. Forming emulsions based on rice oil is a chance to introduce a new, interesting representative of functional food as well as a cosmetic product.     

Complexes of atmospheric alpha-dicarbonyls with water: FTIR matrix isolation and theoretical study

The complexes of glyoxal (Gly), methylglyoxal (MGly), and diacetyl (DAc) with water have been studied using Fourier transform infrared (FTIR) matrix isolation spectroscopy and MP2 calculations with 6-311++G(2d,2p) basis set. The analysis of the experimental spectra of the Gly(MGly,DAc)/H2O/Ar matrixes indicates formation of one Gly...H2O complex, three MGly...H2O complexes, and two DAc...H2O ones. All the complexes are stabilized by the O-H...O(C) hydrogen bond between the water molecule and carbonyl oxygen as evidenced by the strong perturbation of the O-H, C=O stretching vibrations. The blue shift of the CH stretching vibration in the Gly...H2O complex and in two MGly...H2O ones suggests that these complexes are additionally stabilized by the improper C-H...O(H2) hydrogen bonding. The theoretical calculations confirm the experimental findings. They evidence the stability of three hydrogen-bonded Gly...H2O and DAc...H2O complexes and six MGly...H2O ones stabilized by the O-H...O(C) hydrogen bond. The calculated vibrational frequencies and geometrical parameters indicate that one DAc..H2O complexes, two Gly...H2O, and three MGly...H2O ones are additionally stabilized by the improper hydrogen bonding between the C-H group and water oxygen. The comparison of the theoretical frequencies with the experimental ones allowed us to attribute the calculated structures to the complexes present in the matrixes.     

Wettability and topography of phospholipid DPPC multilayers deposited by spin-coating on glass, silicon, and mica slides

The surface free energy of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) layers deposited on glass, silicon, or mica by the spin-coating method was estimated. For this purpose, the advancing and receding contact angles of water, formamide, and diiodomethane were measured, and then two concepts of the interfacial interactions were applied. In the contact angle hysteresis approach, the apparent total surface free energy is calculated from the advancing and receding contact angles of the probe liquids, and in the Lifshitz-van der Waals/acid-base approach, the total surface free energy is calculated from previously determined components of the energy, that is, the apolar Lifshitz-van der Waals and the polar electron-donor and electron-acceptor, which are calculated from the advancing contact angles of the probe liquids alone. Comparison of the results obtained using these two approaches provided more information about changes in the hydrophobic/hydrophilic character of the DPPC layers and, simultaneously, a verification of the approaches. Moreover, the roughness and topography of the investigated layers were also examined by atomic force microscopy measurements. The hydrophilic character of the DPPC layers decreased if up to 0.5 mg of DPPC/mL was used to deposit on the substrates by the spin-coating method. Then it increased and leveled off if up to 2-2.5 mg of DPPC/mL was used. The changes in the energy were correlated with the changes in topography of the surfaces.     

Selective voltammetric determination of chromium (VI) with DTPA and nitrate

A voltammetric determination of Cr(VI) in a flow system is described based on the selective accumulation of the reduction product of Cr(VI) on an HMDE, its complexation with DTPA and subsequent reduction of the complex in presence of nitrate. The calibration graphs were linear up to 100 and 5 nmol/L for deposition times 120 and 600 s, respectively. The relative standard deviation was 2.8% (n = 5) for Cr(VI) concentrations of 5 × 10^–8 mol/L. The detection limits (3 σ) for Cr(VI) were 1.0 and 0.12 nmol/L at deposition times of 120 and 600 s, respectively. Typical interferences derived from real water samples are discussed. The method has been applied for the determination of Cr(VI) in spiked natural water samples. Received: 18 June 1998 / Revised: 28 September 1998 / Accepted: 5 October 1998     

Stereodefined dinucleoside (3′,5′)-propionamidophosphonates and β-cyanoethylphosphonates and their incorporation into modified oligonucleotides

Base-catalyzed stereospecific anti-Markovnikov addition of dinucleoside (3′,5′)-H-phosphonates to the activated alkenes acrylamide and acrylonitrile resulting in the synthesis of P-chiral diastereomerically pure dinucleoside (3′,5′)-alkylphosphonates is reported.     

Application of a renewable silver based mercury film electrode to the determination of Cr(VI) in soil samples

The design and experimental results of the application of a renewable mercury film silver based electrode to the determination of Cr(VI) in soil samples are presented. The main feature of this procedure is that it can be used in field measurements. The procedure is based on the extraction of total Cr(VI) exploiting the complexation property of diethylenetriaminepentaacetic acid (DTPA) followed by electrochemical reduction of Cr(VI) to Cr(III) with the formation of Cr(III)-H₂DTPA complex adsorbed on mercury film electrode. The voltammetric signal is caused by reduction of this complex. The validation of the proposed procedure was made by Cr(VI) determination in the certified reference material “Chromium VI in soil”. The protocol for Cr(VI) determination has also been applied to the analysis of Rendoll soil samples with satisfying precision.