Ascorbic acid as a matrix modifier for determination of tin in concentrated boric acid solutions by electrothermal atomic-absorption spectrometry

It has been found that the atomic-absorption signal for tin is reduced in the presence of 5 micro1 of 0.05-0.30M boric acid at STPF-conditions. It has been proposed that the reason for the boron interferences is the formation of SnB(g) at the atomization stage. In the presence of palladium chloride the interferences from 0.2M boric acid are reduced by a factor of 1.3. The interferences are reduced most effectively when the sample is atomized from a polycrystalline graphite platform or in the presence of ascorbic acid. The interference of up to 0.2M boric acid can be suppressed and the area of the tin signal doubled. It is proposed that the observed phenomenon is connected with the bonding of boron as non-volatile B(4)C. Ascorbic acid is the most effective matrix modifier for the determination of different trace elements in boron compounds.     

Minimizing the effect of organic matrices in the analysis of tin-containing extracts by electrothermal atomic absorption spectrometry: Determination of Tin in rocks

The effect of the nature of the extracted complex and of the organic solvent on the sensitivity of the extraction/atomic absorption determination of tin in a graphite-furnace atomizer is investigated. It is recommended that extracts of tin as its chloride complex, or chelates with N-benzoyl-N-phenylhydroxylamine or 8-quinolinol, in organic solvents which do not contain chlorine should be used. The depressive effect of the organic matrix can be significantly decreased by using ascorbic acid as matrix modifier, by atomization from a graphite platform, and by using an atomizer coated with tungsten carbide. The procedure developed is applied to the determination of tin in rocks. For tin concentrations of 0.1–10 μg g^?1, the relative standard deviation does not exceed 10%.     

Factors influencing the free oxygen content in an electrothermal atomizer

A considerable decrease in the partial pressure of free oxygen in the gas phase within a graphite atomizer during atomization with the use of ascorbic acid as a matrix modifier was proved by using the method suggested by L'vov and Ryabchuk. A similar change in the gas-phase composition was observed in graphite tubes treated with compounds of refractory metals. A mechanism for this oxygen binding is suggested. It is shown that the sensitivity of the determination of tin is improved by decreasing the free oxygen content of the gas phase.     

Solvent extraction-electrothermal atomic-absorption analysis

The review discusses the problems of rational combination of solvent-extraction separation and preconcentration of trace elements with their determination by electrothermal atomic-absorption spectrometry.     

Development of the force field parameters for phosphoimidazole and phosphohistidine

Phosphorylation of histidine-containing proteins is a key step in the mechanism of many phosphate transfer enzymes (kinases, phosphatases) and is the first stage in a wide variety of signal transduction cascades in bacteria, yeast, higher plants, and mammals. Studies of structural and dynamical aspects of such enzymes in the phosphorylated intermediate states are important for understanding the intimate molecular mechanisms of their functioning. Such information may be obtained via molecular dynamics and/or docking simulations, but in this case appropriate force field parameters for phosphohistidine should be explicitly defined. In the present article we describe development of the GROMOS96 force field parameters for phosphoimidazole molecule--a realistic model of the phosphohistidine side chain. The parameterization is based on the results of ab initio quantum chemical calculations with subsequent refinement and testing using molecular mechanics and molecular dynamics simulations. The set of force constants and equilibrium geometry is employed to derive force field for the phosphohistidine moiety. Resulting parameters and topology are incorporated into the molecular modeling package GROMACS and used in molecular dynamics simulations of a phosphohistidine-containing protein in explicit solvent.     

Implicit two-phase solvation model as a tool to assess conformation and energetics of proteins in membrane-mimetic media

 A heterogeneous implicit membrane-mimetic model is applied to simulations of membrane proteins. The model employs atomic solvation parameters for gas–water and gas–cyclohexane transfer. It is used to analyze structure, energetics, and orientation with respect to the bilayer of two polypeptides with different modes of membrane binding – hydrophobic segment of human glycophorin A (GpA) and cytotoxin II from Naja naja oxiana snake venom (CTX). The native state of GpA represents a transmembrane (TM) α helix, while CTX is a water-soluble protein, which is able to interact with the cell membrane. The conformational space of the polypeptides was explored in Monte Carlo simulations. The results show that the most stable conformers of GpA represent a TM α helix. They are additionally stabilized by an applied TM voltage. The results also show that CTX inserts with its three loops, does not cross the hydrophobic layer, and stays partially immersed in the membrane. This agrees well with the experimental data, thus confirming the validity of the solvation model. Received: 13 June 2000 / Accepted: 15 September 2000 / Published online: 19 January 2001     

Structure of Supramers Formed by the Amphiphile Biotin-CMG-DOPE

The synthetic function-spacer-lipid (FSL) amphiphile biotin-CMG-DOPE is widely used for delicate ligation of living cells with biotin residues under physiological conditions. Since this molecule has an “apolar-polar-hydrophobic” gemini structure, the supramolecular organization is expected to differ significantly from the classical micelle. Its organization is investigated with experimental methods and molecular dynamics simulations (MDS). Although the linear length of a single biotin-CMG-DOPE molecule is 9.5 nm, the size of the dominant supramer globule is only 14.6 nm. Investigations found that while the DOPE tails form a hydrophobic core, the polar CMG spacer folds back upon itself and predominantly places the biotin reside inside the globule or planar layer. MDS demonstrates that <10 % of biotin residues on the highly water dispersible globules and only 1 % of biotin residues in layer coatings are in an linear conformation and exposing biotin into the aqueous medium. This explains why in biotin-CMG-DOPE apolar biotin residues both in water dispersible globules and coatings on solid surfaces are still capable of interacting with streptavidin.     

Multistate organization of transmembrane helical protein dimers governed by the host membrane

Association of transmembrane (TM) helices taking place in the cell membrane has an important contribution to the biological function of bitopic proteins, among which receptor tyrosine kinases represent a typical example and a potent target for medical applications. Since this process depends on a complex interplay of different factors (primary structures of TM domains and juxtamembrane regions, composition and phase of the local membrane environment, etc.), it is still far from being fully understood. Here, we present a computational modeling framework, which we have applied to systematically analyze dimerization of 18 TM helical homo- and heterodimers of different bitopic proteins, including the family of epidermal growth factor receptors (ErbBs). For this purpose, we have developed a novel surface-based modeling approach, which not only is able to predict particular conformations of TM dimers in good agreement with experiment but also provides screening of their conformational heterogeneity. Using all-atom molecular dynamics simulations of several of the predicted dimers in different model membranes, we have elucidated a putative role of the environment in selection of particular conformations. Simulation results clearly show that each particular bilayer preferentially stabilizes one of possible dimer conformations, and that the energy gain depends on the interplay between structural properties of the protein and the membrane. Moreover, the character of protein-driven perturbations of the bilayer is reflected in the contribution of a particular membrane to the free energy gain. We have found that the approximated dimerization strength for ErbBs family can be related to their oncogenic ability.