Optical Spectroscopic Studies on Structural Changes of Helical-rich Proteins in Aqueous Solutions of Ionic Liquids

We have investigated structural changes of myoglobin and cytochrome c, which are helical-rich proteins, in aqueous 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) solutions by Fourier transform infrared and circular dichroism spectroscopy. At low [bmim][Cl] concentrations {X (mol% IL) < 10}, both proteins unfold. Remarkably, at high [bmim][Cl] concentrations (X > 10), myoglobin aggregates whereas cytochrome c refolds its α-helical structure. The tertiary structures of both proteins are disrupted over the entire range of studied [bmim][Cl] concentrations. Our results suggest that, in aqueous solutions at high [bmim][Cl] concentrations, the differences in structural transitions between myoglobin and cytochrome c might be due to the difference in hydration between these proteins.     

Structural Characteristics of Hydrated Complexes of Aminoacids in Aqueous Solutions

Aqueous solutions of nine α-aminoacids were studied in a diluted range of concentrations (from 0. 01 m to 0.1 m) at 5°C, 15°C, 25°C, 35°C, and 45°C based on the model that uses molar adiabatic compressibility (β _s V _m ) as a measure of hydration of the solute. For the resulting hydrated complexes of aminoacids, structural characteristics have been determined. At increased concentrations of the acid, variation of its hydration number is the chief factor affecting the molar compressibility of the hydrated complex.Original Russian Text Copyright © 2004 by V. N. Afanasiev, E. Yu. Tyunina, and V. V. Ryabova__________Translated from Zhurnal Strukturnoi Khimii, Vol. 45, No. 5, pp. 883–888, September–October, 2004.     

Nonelectrolyte-Induced Micellar Shape Changes in Aqueous Solutions of Silicone Surfactants

Dynamic light scattering, cloud point (CP), and surface tension studies have been carried out to examine the influence of six (including two acetates, two alkoxyethanols, and two sugars) nonelectrolyte additives on two silicone surfactants based on poly (dimethyl siloxane)—graft—polyethers in aqueous solutions. The results indicate that the presence of alkoxyethanols induced the oblate ellipsoidal to spherical micellar transition, while sugars increase the size of the ellipsoidal micelles. The effect of cosolvent or additives on critical micelle concentration (CMC) and CP is discussed on the basis of water structure making and breaking effect. The thermodynamic and surface active parameters were calculated from the surface tension isotherm curves.     

Effect of Poly(methacrylic acid) on Photoinduced Processes with Cyanine Dyes in Aqueous Solutions

The cyanine dyes thiamonomethinecyanine (I), thiacarbocyanine (II), and thiadicarbocyanine (III) in an aqueous poly(methacrylic acid) (PMA) solution form different molecular species: monomers anchored to PMA, dimers, and H aggregates, in proportions determined by the [dye]/[PMA] concentration ratio. Each molecular species is characterized by its own probabilities of radiative and nonradiative transitions: the absence of photoisomerization for I–IIIdimers and H aggregates, the absence of fluorescence for H aggregates of IIand III, the capability of transition to a triplet state for PMA-anchored monomers I–III, and considerable enhancement of intersystem crossing to the triplet state for the dimers of II.     

Entropy Changes in Aqueous Solutions of Non-polar Substances and in Bio-complex Formation

The entropy changes, ΔS _app, (i) for dissolution in water of non-polar substances and (ii) for protein-ligand complexation show linear dependences on the logarithm of the absolute temperature. For every compound, the slope m _(S)=ΔC _p for the line ΔS _app=f(ln T) depends on the size of the molecule and is exactly equal to the slope m _(H)=ΔC _p found in the diagram ΔH _app=f(T). This means that the slopes are rigorously proportional (with a ratio m _(S)/n _w=C _p,w) where n _w is the number of involved water molecules as determined from the enthalpy change ΔH _app=f(T). It is also worth noting that the value of n _w is positive (as well as m _(S) and m _(H)) in the dissolution of non-polar substances, whereas it is negative (as well as m _(S) and m _(H)) in bio-complex formation and in micelle formation. The number n _w (n _w>0) involved in the dissolution of non-polar substances depends on the size of the cavity (excluded volume) formed in the structure of water. These water molecules that have been excluded from the structure of the solvent absorb thermal energy that compensates for the negative enthalpy change, whereas the formation of the cavity implies there should be a large negative entropy contribution. The low solubility of non-polar substances in water depends on the highly negative entropy effect due both to cavity formation and to loss of configurational entropy by the gas trapped in a cage of water molecules. In processes involving association, as in micelle formation and in protein complexation, the cavities surrounding the separate units coalesce and the resultant cavity is smaller than the sum of the previous ones. The n _w water molecules (n _w<0) needed to fill the excess cavity return to the structure of the bulk solvent and release thermal energy, which compensates for the endothermic enthalpy. The affinity in the association processes is bound, for the most part, to the entropy produced by occupation of part of the cavity by condensation of water molecules. The association processes are therefore entropy driven.     

Oxidation of Tyrosine Photoinduced by Pterin in Aqueous Solution

Pterins, heterocyclic compounds widespread in biological systems, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. Pterins have been previously identified as good photosensitizers under UV‐A irradiation. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of tyrosine (Tyr) in aqueous solutions. Tyr is an important target in the study of the photodynamic effects of UV‐A radiation because it is oxidized by singlet oxygen (¹O₂) and plays a key role in polymerization and cross‐linking of proteins. Steady UV‐A irradiation of solutions containing Ptr and Tyr led to the consumption of Tyr and dissolved O₂, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H₂O₂) was produced. By combining different analytical techniques, we could establish that the mechanism of the photosensitized process involves an electron transfer from Tyr to the triplet excited state of Ptr. Mass spectrometry, chromatography and fluorescence were used to analyze the photoproducts. In particular, oxygenated and dimeric compounds were identified.     

Structural Specifics of the Concentration Dependence of Sound Velocity in Aqueous Solutions of Electrolytes

The concentration dependences of sound velocity in aqueous solutions of electrolytes are analyzed over a wide range of concentrations (with extrapolation to 100% salt). For many systems, the concentration dependence of the square of sound velocity is linear up to the values corresponding to overcooled salt melts. For these solutions, an expression for partial molar compressibility of salts in infinitely dilute solutions is derived based on the additivity of the squares of sound velocities of the solution components, where the salt component is an overcooled melt. For alkali halides, there is good agreement between the values of calculated in this way and the values obtained by extrapolation of the apparent molar compressibility in the low-concentration region using the concentration dependences of density and the solution compressibility coefficient. For systems of this type, where hydration is the dominant process, sound velocity in hydration shells and complex ion groups is constant (i.e., independent of concentration). The effects are interpreted in terms of the concept of redistribution of the fixed structural groups in two concentration zones of a solution, where 1) the hydration groups are built in the structure of water in a complementary way; 2) the fragments of hydration spheres and the ionic clusters are united with each other in a complementary way.     

Equilibrium and Structural Study of m-Methyl Red in Aqueous Solutions: Distribution Diagram Construction

The UV/Vis spectra of the m-methyl red (m-MR) ({3-[4-(dimethyl-amino) phenylazo] benzoic acid}) were examined in aqueous solutions at various acidities. These were characterized by the overlap of the different bands of m-MR. The thermodynamic acid dissociation constant, K _a2, of the equilibrium between m-H₂MR⁺ (diprotic form) and m-HMR (monoprotic form) was determined as 1.02 × 10^?2 at 25 °C, and that for the equilibrium between m-HMR and MR^? (basic form), K_a3, was determined as 4.94 × 10^?5 at 25 °C. Based on the two observed K _a values, the distribution diagram of the three forms of m-MR in water was constructed. The neutral monoprotic form (m-HMR) has a maximum fraction of 0.883 at pH = 3.14.     

Electrochemistry of Europium in Aqueous and Aqueous Formamide Solutions

Abstract

The electrochemical behavior of the system Eu(III)/Eu(II) in aqueous formamide solutions was studied by current reversal chronopotentiometry at a mercury pool electrode, in 1 M NaClO₄ The results indicate that double layer effects must be considered rather that ion solvation effects when explaining the influence of formamide on the Eu(III)/Eu(II) electrode reaction.     

Changes in the IR Spectra of Aqueous Solutions of Alkali Metal Chlorides during Crystallization

The IR spectra of aqueous solutions of sodium chloride and rubidium chloride with the same concentration of 0.1 M upon freezing are studied in the middle IR region. The changes that occur in the absorption bands of the bending ν₂, compound ν₂ + ν_L, and stretching (ν₁, 2ν₂, and ν₃) vibrations of water molecules with gradual crystallization of the solutions are studied. The obtained spectra of crystallized solutions are compared to the IR spectrum of ice Ih. Analysis allows conclusions about the structure of the investigated frozen crystallized solutions.     

Specific Structural Features of Concentrated Aqueous Solutions of Potassium Iodide in a Wide Temperature Range

The structural characteristics of concentrated aqueous solutions of KI under the conditions of isobaric heating (P 20 MPa, T 298–623 K) were studied by the method of integral equations. As the temperature is increased, the continuous tetrahedral network of hydrogen bonds in the KI:15H₂O solution is destroyed (at 323 K); in the KI: 8H₂O solution, this network is not formed in the entire temperature range. The number of intermolecular H bonds in the systems appreciably decreases on heating. With increasing temperature, the influence exerted by the salt concentration on the intrinsic solvent structure becomes weaker. In both solutions, heating results in significant destruction of the first hydration shell of the K⁺ ion. At the same time, the nearest environment of the I⁻ ion becomes more structured on heating to 473–523 K. On further heating, however, the first hydration shell of the anion is destroyed. Heating exerts virtually no effect on the amount of contact ion pairs in the KI:15H₂O solution but decreases the content of contact associates in the KI:8H₂O solution.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 211–219.Original Russian Text Copyright © 2005 by Fedotova, Gribkov, Trostin.     

Explosivity Conditions of Aqueous Solutions

This paper focuses on the conditions for explosive boiling and gas exsolution of aqueous solutions from a thermodynamic point of view. Indeed, the kinetic nature of these processes, hence their explosivity, can be assessed by considering their relation with the spinodal curve of these liquids. First, the concepts of mechanical and diffusion spinodals are briefly described, which allows us to introduce the notions of superspinodal (explosive) transformations and subspinodal (non-explosive) ones. Then, a quantitative study of spinodal curves is attempted for the binary systems H₂O–CO₂ and H₂O–NaCl using equations of state having a strong physical basis. It is shown that dissolved gaseous components and electrolytes have an antagonist effect: dissolved volatiles (like CO₂) tend to shift the superspinodal region towards lower temperatures, whereas electrolytes (like NaCl) tend to extend the metastable field towards higher temperatures.     

Alkalinity of Electro-Activated Aqueous Solutions

Russian Journal of Electrochemistry - Electro-activated alkaline (basic) solution known as catholyte has been reported to have a number of unique properties. The changes in the pH and alkalinity of...     

Volume Changes in the Course of Neutralization of Nitrilotris(methanephosphonic) Acid with Aqueous Ammonia

Volume changes in the course of neutralization of solutions of nitrilotris(methanephosphonic), hydrochloric, and nitric acids with aqueous ammonia solution were measured dilatometrically at 298 K. The changes in the molar volumes were calculated for various stages of the neutralization. The data were com- pared to the results of titration of nitrilotris(methanephosphonic) acid with strong hydroxyl-containing bases. The specific features of reactions of ammonia with acids and the state of nitrilotris(methanephosphonic) acid in various stages of its dissociation were taken into account in data interpretation.     

Bubble Motion in Aqueous Surfactant Solutions

This paper presents the results of numerical simulations of the unsteady motion of a single bubble that is released or injected into water. The governing equations are solved with a finite difference method using an adaptive boundary-fitted coordinate system. Results are shown for bubbles in the size range 0.72 to 1.5 mm. The effects of surfactants on the motion and shape of the bubble are investigated. When the surfactant concentration is sufficiently small, the bubble attains a maximum velocity before slowing down to its steady-state velocity. Although the steady-state velocity is nearly independent of surfactant concentration, the maximum velocity can be comparable to steady-state velocity in pure water. This behavior is observed even when the bubble is allowed to equilibrate prior to releasing it. The formation of an immobilized surfactant cap is observed soon after the bubble is released. The effect of the injection velocity on the bubble velocity profile is investigated. The effects of the sorption rate constants and the bulk surfactant concentration on the behavior of the bubble are investigated. The feasibility of using experimental measurements and simulations of unsteady bubble velocities to estimate sorption rate constants is discussed. Copyright 2000 Academic Press.     

Dissolution of Cellulose in Aqueous NaOH Solutions

Dissolution of a number of cellulose samples in aqueous NaOH was investigated with respect to the influence of molecular weight, crystalline form and the degree of crystallinity of the source samples. A procedure for dissolving microcrystalline cellulose was developed and optimized, and then applied to other cellulose samples of different crystalline forms, crystallinity indices and molecular weights. The optimum conditions involved swelling cellulose in 8–9 wt % NaOH and then freezing it into a solid mass by holding it at –20°C. This was followed by thawing the frozen mass at room temperature and diluting with water to 5% NaOH. All samples prepared from microcrystalline cellulose were completely dissolved in the NaOH solution by this procedure. All regenerated celluloses having either cellulose II or an amorphous structure prepared from linter cellulose and kraft pulps were also essentially dissolved in the aqueous NaOH by this process. The original linter cellulose, its mercerized form and cellulose III samples prepared from it had limited solubility values of only 26–37%, when the same procedure was applied. The differences in the solubility of the celluloses investigated have been interpreted in terms of the degrees to which some long-range orders present in solid cellulose samples have been disrupted in the course of pre- treatments.     

水介质中吡咯的电化学聚合反应

研究了扫描电位上限对循环伏安法制备聚吡咯膜性能的影响，吡咯在水溶液中于玻碳电极表面化学聚合的起始电位为0．58V，在聚吡咯（Ppy）修饰电极表面聚合的起始电位为0．55V，当聚合电位上限在0．80V以上时，Ppy的氧化还原反应可逆性变差，同时，氧化电位过高将导致Ppy膜导电性能下降；研究了聚合介质对循环伏安法制备导电聚吡咯膜的影响，实时观察了吡咯（Py）聚合过程溶液中质子含量的动态变化，发现Py聚合伴随有质掺杂←→释放过程；结合Ppy膜的元素分析、ESR分析和IR光谱分析，总结出了水介质中电化学聚合高导电性聚吡咯膜的条件。     

Kinetics of Ozone-Olefin Reactions in Aqueous Solutions

The kinetics of the reactions of ozone with 12 substituted olefins in aqueous solutions at 293 K are studied by the stopped-flow method. Second-order rate constants are determined for these reactions. The dependence of the reactivity of olefins in H₂O on their structure can be described by the Taft equation.