Two-state irreversible thermal denaturation of anionic peanut (Arachis hypogaea L.) peroxidase

Detailed differential scanning calorimetry (DSC), steady-state tryptophan fluorescence and far-UV circular dichroism (CD) studies, together with enzymatic assays, were carried out to monitor the thermal stability of anionic peanut peroxidase (aPrx) at pH 3.0. The spectral parameters were seen to be good complements to the highly sensitive but integral method of DSC. Thus, changes in far-UV CD corresponded to changes in the overall secondary structure of the enzyme, while changes in intrinsic tryptophan fluorescence emission corresponded to changes in the tertiary structure of the enzyme. The results, supported with data concerning changes in enzymatic activity with temperature, show that thermally induced transitions for aPrx are irreversible and strongly dependent upon the scan rate, suggesting that denaturation is under kinetic control. It is shown that the process of aPrx denaturation can be interpreted with sufficient accuracy in terms of the simple kinetic scheme, , where k is a first-order kinetic constant that changes with temperature, as given by the Arrhenius equation; N is the native state, and D is the denatured state. On the basis of this model, the parameters of the Arrhenius equation were calculated.     

Shot noise sets the limit of quantification in electrochemical measurements

Detection of single molecules, particles, and rapid redox events is a challenge of electrochemical investigations and requires either an amplification strategy or significant averaging for the electrochemical current to exceed the noise level. We consider the minimum number of electrons required to reach the limit of quantification in these electrochemical measurements. A survey of the literature indicates that the state-of-the-art limit in current detection for different types of measurements (e.g. voltammetry, single-molecule redox cycling, ion channel recordings of single molecules, metal nanoparticle collision, and phase nucleation) is independent of the nature of the measurement and increases linearly with reciprocal response time, Δt^?1, over ～5 orders of magnitude (from ～10 to ～10⁶ s^?1). We demonstrate that the practical limit of quantification requires cumulative measurement of ～2100 electrons during Δt and is determined by statistics of counting electrons, that is, the shot noise in the current.     

Thermal polymerization of acrylamide during the formation of the structures of acrylamide complexes with metal nitrates

The effects of the composition of Mn^II, Co^II or Ni^II nitrate hydrate — acrylamide (AAM) mixtures and of the duration of their aging at ambient temperature on the structurization of acrylamide complexes and on the character of their thermal polymerization have been studied by scanning and isothermic differential calorimetry. Structurization is a rather prolonged step in the synthesis of acrylamide complexes. The peculiarities and rate of this step are determined by the composition of the mixture and by the nature of the complexforming compound; it yields several structural modifications of the AAM complexes. The thermal polymerization of those structural forms of acrylamide complexes that polymerize at low temperatures may be formally described as polymerization in an acrylamide-nitrate-water mixture. The effective activation energy of the polymerization of acrylamide mixed with Mn^II nitrate hydrate is 45 kJ mol^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 679–683, April, 1995.This work was carried out with financial support from the Russian Foundation for Basic Research, Project No. 93-03-4162.     

Detection and assessment of citrus tristeza virus antigens by computer-aided scanning densitometry

Citrus tristeza virus (CTV) extracted from Etrog citron (C.medica L.) was immunoprecipitated. The immunoprecipitate was fractionated by SDS-PAGE and western blotted onto nitrocellulose. The CTV antigens were determined by immunoblot analysis using rabbit anti-CTV IgG, and the protein-band pattern exhibited on the nitrocellulose was assessed by soft-laser scanning densitometry. The densitometric tracing revealed the presence of bands that were not visible to the naked eye. Using the superimposition mode of the instrument, it was also revealed that the protein-band patterns of different CTV samples were not identical. Computer-aided soft-laser scanning densitometry proved to be a powerful approach in the detection and assessment of protein bands revealed on nitrocellulose immunoblots, which we were previously unable to do employing conventional methods.     

聚苯胺纳米点阵列的制备和库仑台阶现象

本文制备了PANI纳米点阵列, 利用导电原子力显微镜(C-AFM)表征了其形貌和导电性能, 在室温下观察到PANI纳米点的库仑台阶(Coulomb staircase)现象, 并利用库仑阻塞效应的理论进行了初步的分析.     

Thermal techniques to study complex elastomer/filler systems

The transfer of heat through an elastomeric matrix is important for both the processing of the material and its subsequent lifetime. Thermal conductivity can be used to evaluate the influence of different polymers and fillers on heat transfer. Additionally, the dispersion of the filler has an effect on heat transfer and thermal conductivity measurements can be used to provide semi-quantitative estimations of filler dispersion. The degradation of sulfur-crosslinked elastomer systems has been studied for many years. The degradation of the crosslinks (changes in sulfur rank) and degradation of the polymer backbone by thermal and/or oxidative processes have been studied extensively using many techniques including thermal analysis (references). However, the degradation of the crosslinked-polymer 'network' is less well understood. The relationship of the crosslink network to this degradation process is a key to both the long term and higher temperature performance of the sulfur-crosslinked elastomer. The changes in physical properties observed upon exposure of sulfur-crosslinked elastomers can be monitored using dynamic mechanical analysis. Subsequently, other thermal techniques can be used to monitor the chemistry that is occurring during these degradations. Thermal desorption/mass spectroscopy and dynamic scanning calorimetry are used to complete the picture of the degradation processes taking place. Examples of these techniques will be provided to illustrate the utility of the analytical approach, the chemistry involved in these degradation processes and the effect of changes in the polymer, cure package and other ingredients. This revised version was published online in August 2006 with corrections to the Cover Date.     

Molecular Basis of the Dynamic Strength of the Sialyl Lewis X—Selectin Interaction

The selectins are Ca²⁺‐dependent cell adhesion molecules that facilitate the initial attachment of leukocytes to the vascular endothelium by binding to a carbohydrate moiety as exemplified by the tetrasaccharide, sialyl Lewis X (sLeX). An important property of the selectin‐sLeX interaction is its ability to withstand the hydrodynamic force of the blood flow. Herein, we used single‐molecule dynamic force spectroscopy (DFS) to identify the molecular determinants within sLeX that give rise to the dynamic properties of the selectin/sLeX interaction. Our atomic force microscopy (AFM) measurements revealed that the unbinding of the selectin/sLeX complexes involves overcoming at least two activation barriers. The inner barrier, which determines the dynamic response of the complex at high forces, is governed by the interaction between the Fuc residue of sLeX and a Ca²⁺ ion chelated to the lectin domain of the selectin molecule, whereas the outer activation barrier can be attributed to interactions involving the sialic acid residue of sLeX. Due to their steep inner activation barriers, the selectin‐sLeX complexes are less sensitive to high pulling forces. Hence, besides its contribution to the bond energy, the Ca²⁺ ion also grants the selectin–sLeX complexes a tensile strength that is crucial for the selectin‐mediated rolling of leukocytes.     

Dynamik linearer Molekülanionen in den Hydrogensulfiden von Natrium,Kalium und Rubidium: Differential-Scanning-Kalorimetrie,Röntgen- und Neutronenbeugung

Dynamical Behaviour of Linear Molecular Anions in the Hydrogensulfides of Sodium, Potassium and Rubidium: Differential Scanning Calorimetry, X-ray and Neutron Diffraction Hydrogensulfides of the alkali metals M ? Na, K, Rb were prepared in autoclaves by the reaction of the corresponding metals with H₂S and D₂S, respectively, in the temperature range from 50°C to 150°C. Differential scanning calorimetry, X-ray and neutron diffraction methods reveal that both, the HS^?-and DS^?-compounds occur in three crystalline modifications with HT ? high-, MT ? medium- and LT ? low-temperature form: The temperatures and enthalpies for the changes of modifications of the H- and D-compounds are given and the atomic arrangements revealed mainly by neutron diffraction data are discussed, in relation to, for example, size of cations.     

Total luminescence intensity (TLI) offers superior early oxidation detection in unstabilised polyethylene but is no better than FT-IR for stabilised polyolefins

In an earlier study, we have shown that chemiluminescence (CL) and the total luminescence intensity (TLI) method are highly sensitive to oxidation in degradable PE. In this study, stabilised PE and PP were characterised with CL in an inert (TLI) and in an oxygen atmosphere (CL-OIT) and the results were compared to those obtained by the commonly used techniques, FT-IR (carbonyl index (CI)) and thermal analysis (DSC-OIT). PE was aged at a low temperature (80 °C) and PP was aged at temperatures between 60 and 120 °C. Non-Arrhenius behaviour was observed in the oxidation of PP. This showed the importance of aging at a low temperature to obtain realistic results. TLI and CI of stabilised PP and most of the stabilised PE gave comparable results with the same sensitivity for oxidation detection. This was in contrast to our previous results for degradable PE. However, TLI of unstabilised PE showed earlier oxidation detection than CI, which agreed with our earlier results. TLI of PE had a higher sensitivity than CL-OIT, and both TLI and CI of PP were sufficiently sensitive to detect the effect of aging at different temperatures, whereas DSC-OIT was not.     

Copper passivity and its breakdown in sodium bicarbonate solutions: A scanning electron microscopy and x-ray photoelectron and auger spectroscopy study

Effect of bicarbonate ions on the copper passivity and its local breakdown is studied by cyclic voltammetry, Auger spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. Comparing data for solutions with various sodium bicarbonate concentrations shows copper to undergo pitting in 0.02 M NaHCO₃, whereas it remains stably passive in 0.10 M NaHCO₃. Independent studies suggest that carbonates play a protective role in stabilization of the oxide-hydroxide passive film on copper.