Spectroscopic and electrochemical study of Rose Bengal in aqueous solutions of cyclodextrins

The interaction of Rose Bengal (RB) in aqueous solution of LiClO4 0.1 M with alpha-cyclodextrin (alpha-CD), hydroxypropyl-beta-cyclodextrins (HP-beta-CD) and hydroxypropyl-gamma-cyclodextrins (HP-gamma-CD) were studied by spectrophotometric measurements. The presence of Induced Circular Signals and the results of the analysis of the modifications in the absorbance spectra of RB produced by the presence of CDs in solution indicate that RB forms inclusion complexes only with HP-beta-CD and with HP-gamma-CD.     

The effects of thermal quenching on the excited-state intramolecular proton transfer reaction in 3-hydroxyflavones

The 3-hydroxyflavone (3HF) and its derivatives are the classical objects in the studies of the mechanisms of excited-state intramolecular proton transfer (ESIPT) reaction due to very frequent observation of two separate bands in fluorescence emission belonging to reactant and reaction product. Those of them possessing electron-donor groups in 4' position find many applications as fluorescence sensors and probes because of their much higher sensitivity of their two-band ratiometric response to interactions with the environment. We report on the strong differences between 3HF and such derivatives in the behavior of their fluorescence spectra as a function of temperature. The thermal quenching changes the intensity ratio of two bands strongly for 3HF but does not change it for its studied derivatives. These results are interpreted in terms of different kinetic mechanisms of ESIPT reaction. In 3HF the equilibrium between the two excited-state species is not established prior to emission, so that the ESIPT reaction is under kinetic control, but in these derivatives the equilibrium is established faster than the emission and the reaction is under thermodynamic control. We suggest that the thermal perturbation of fluorescence spectra can be an extremely simple and convenient alternative to time-resolved spectroscopy for determining if slow irreversible or fast reversible ESIPT reaction gives rise to two bands of fluorescence spectra of similar magnitude. This is essential for the development of new wavelength-ratiometric fluorescence sensors and probes.     

Chemiluminescence in the reaction of ozone with 1,3-dioxans in aqueous solutions

Chemiluminescence in the visible part of spectrum is found to assist the oxidation of substituted 1,3-dioxans by ozone in aqueous media. The kinetics of the chemiluminescence decay was studied in the temperature interval 323-353 K, and the activation parameters of this process were determined. A kinetic scheme is proposed to explain the results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spectroscopic evidence of monomeric aluminium phthalocyanine tetrasulphonate in aqueous solutions

Aqueous solutions of aluminium phthalocyanine tetrasulphonate (AlPcS(4)) were investigated by means of absorption and fluorescence spectroscopy. The absorption spectrum of AlPcS(4) is independent of concentration in a wide range (from 10(-8) to 10(-4) M). The fluorescence spectrum measured with a standard setup is strongly dependent on AlPcS(4) concentration, and the fluorescence maximum is gradually red-shifted with increasing concentration. Calculations that take into account reabsorption of fluorescence (inner-filter effect) fit the experimental observations at low concentrations (up to 10(-6) M). Disagreement between the calculations and spectra recorded at higher concentrations (above 10(-5) M) shows that the reabsorbed light may be reemitted as fluorescence. The influence of inner-filter effects on the spectral shape was demonstrated by the experiments where a fibre-optic front-face fluorescence setup was applied: Under such conditions the shape of the fluorescence spectra for a high concentration (10(-3) M) coincided with that of a low concentration (10(-8) M). In conclusion, the present spectroscopic results show that AlPcS(4) does not form aggregates and is a very stable compound in aqueous solutions.     

NMR relaxation study of the reaction of 3-hydroxypyridine with macromolecules in solutions

The concentration dependences of the rates of¹³C nuclei spin-lattice relaxation in molecules of 3-hydroxypyridine (HP), pyridine (P), and phenol (Ph) in aqueous solutions of polyethylenimine (PEI) and polyacrylic acid (PAA) were obtained. The average complexing constants of HP with these polymers were calculated with the experimental data and the anisotropy of rotation of molecules of HP, P, and Ph in aqueous solutions and in complexes with PAA and PEI were determined. A comparative analysis of the change in the anisotropy of rotation of the molecules and efficiency of formation of hydrogen bonds by different groups was conducted. The expediency of using HP as a probe for studying the complexing capacity of different molecular groups in the chains of polymers was demonstrated.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2751–2756, December, 1990.     

New products and reaction pathways for the reaction of dimethylsulfoxide with peroxonitrite in aqueous solutions

The first observed basic products of the reaction of dimethylsulfoxide with peroxonitrite were methyl nitrite and methane with pH=7 and methyl nitrate at pH>10. The dependence of the yield of these and other reaction products on pH, concentrations of the substrate and peroxonitrite, and the additives NO ₂ ⁻ and CO₂ was studied. It is proposed that CH₄ and CH₃ONO are formed in the reaction between DMSO and the cis-form of⁻OONO, while DMSO₂ and CH₃ONO₂ are formed in reactions with the trans-form of⁻OONO or the dimer HOONO/OONO respectively. Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 36, No. 1, pp. 41–46, Jaunuary–February, 2000.     

Spectroscopic investigations of the structure of liquid water and aqueous solutions

IR spectroscopy is a powerful tool for investigating the structure of aqueous systems. Changes in vibrational frequencies and intensities of the absorption band provide information about the structure of the associated water molecules. The water molecule has C_2v symmetry when the intermolecular interaction is symmetrical to both OH bonds as in 2:1 complexes. In this case the frequency difference of the two stretching vibrations ν₃ and ν₁ is nearly constant. If the intermolecular interaction is unsymmetric to the OH bonds as in 1:1 complexes the band separation of ν₃ and ν₁ increases markedly in relation to the increase of the unsymmetry. The IR overtone region is more suitable for the study of the structure of liquid water or aqueous solutions than the IR fundamental region. The reason is the higher intensity of the absorption bands of the “free” OH vibration compared to the H-bonded OH groups. The ratio of the intensities is inverse in the fundamental region. Furthermore it is possible to measure quantitatively in the overtone region and there are no experimental difficulties. The results are estimations of the H-bonded and the free OH groups in different aqueous systems.     

A phosphate catalyzed reaction of iodine with hydrazine in aqueous solutions

Pulse radiolysis was utilized to study the kinetics of the iodine-hydrazine reaction in aqueous solutions containing phosphate buffer in the pH range 5.5 to 7. The reaction rate was found to be proportional to and [I^–]^–1, but did not show simple proportionality to [H⁺]^–1 and was considerably higher than that found earlier when the pH of solutions was adjusted with HClO₄ or H₃BO₃. The results are in a formal agreement with the assumption that in phosphate buffered solutions a complex N₂H₄. HPO ₄ ^2– is formed, reacting with I₂ with a rate constant which is greater than that ascribed earlier to the reaction of N₂H₄ with I₂/Ref. 1/.     

Lubrication, adsorption, and rheology of aqueous polysaccharide solutions

Aqueous lubrication is currently at the forefront of tribological research due to the desire to learn and potentially mimic how nature lubricates biotribological contacts. We focus here on understanding the lubrication properties of naturally occurring polysaccharides in aqueous solution using a combination of tribology, adsorption, and rheology. The polysaccharides include pectin, xanthan gum, gellan, and locus bean gum that are all widely used in food and nonfood applications. They form rheologically complex fluids in aqueous solution that are both shear thinning and elastic, and their normal stress differences at high shear rates are found to be characteristic of semiflexible/rigid molecules. Lubrication is studied using a ball-on-disk tribometer with hydrophobic elastomer surfaces, mimicking biotribological contacts, and the friction coefficient is measured as a function of speed across the boundary, mixed, and hydrodynamic lubrication regimes. The hydrodynamic regime, where the friction coefficient increases with increasing lubricant entrainment speed, is found to depend on the viscosity of the polysaccharide solutions at shear rates of around 10(4) s(-1). The boundary regime, which occurs at the lowest entrainment speeds, depends on the adsorption of polymer to the substrate. In this regime, the friction coefficient for a rough substrate (400 nm rms roughness) is dependent on the dry mass of polymer adsorbed to the surface (obtained from surface plasmon resonance), while for a smooth substrate (10 nm rms roughness) the friction coefficient is strongly dependent on the hydrated wet mass of adsorbed polymer (obtained from quartz crystal microbalance, QCM-D). The mixed regime is dependent on both the adsorbed film properties and lubricant's viscosity at high shear rates. In addition, the entrainment speed where the friction coefficient is a minimum, which corresponds to the transition between the hydrodynamic and mixed regime, correlates linearly with the ratio of the wet mass and viscosity at ～10(4) s(-1) for the smooth surface. These findings are independent of the different polysaccharides used in the study and their different viscoelastic flow properties.     

The Soret effect with chemical reaction in aqueous sulfuric acid solutions

The Soret coefficient of aqueous sulfuric acid has been determined conductimetrically from 0.0005 to 0.2m at 25°C. The derived enthalpies of transport increase sharply as the concentration drops, approaching a limiting value near 35 kJ-mol^–1. The increase, in the enthalpy of transport at low concentrations is due to the large intrinsic enthalpies of transport of the hydrogen and sulfate ions that are produced by the dissociation of bisulfate ions. The enthalpy of dissociation of the bisulfate ions reduces the acid's enthalpy of transport by up to 1.8 kJ-mol^–1.     

Mechanism and kinetics of the reaction of ozone with sodium chloride in aqueous solutions

It is shown experimentally that Cl⁻ appreciably accelerates ozone decomposition in water (τ_1/2 = 1.5 h versus 6 h in pure water). The decomposition of ozone in NaCl solutions includes the reversible reaction of ozone with the chloride ion (O₃ + Cl⁻ → O₃⁻ + Cl) as the key step, which is followed by the development of a chain reaction in which chain propagation is performed alternately by the chlorine atom Cl and its monoxide ClO. The current concentrations of the chlorine atom are rather low ([Cl] ∼ 10⁻¹⁴ mol/l). The overall process is satisfactorily described by a first-order rate law with respect to ozone. The decomposition of ozone in aqueous solutions of NaCl is not accompanied by the formation of products other than oxygen. In particular, no noticeable amounts of hypochlorites and chlorates are observed. This is particularly significant for medicinal applications of ozonized isotonic solutions.     

Different channels of ·OH radical reaction with tryptophanol in aqueous solutions: A pulse radiolysis study

The pulse radiolysis technique has been employed to study the reaction of ·OH radical with tryptophanol (TPN). Reactions of specific one-electron oxidants like Br₂· - and N₃· and ·H atom were carried out to understand the contribution of different channels of · OH radical reaction with TPN. The studies were carried out in the pH range 3 to 10. One-electron oxidation of TPN (pH 3) produced radical cation absorbing at 570 nm. However, at higher pH, deprotonation of TPN cation radical takes place from N(1) position and indolyl radical absorbing at 520 nm with a p ^K a value of 3.6 is formed. Redox titration with TMPD, ABTS^2- and MV²⁺ was performed to determine the total yield of oxidizing and reducing radicals produced during ·OH reaction.     

Stereoselectivity ratios in a simple Diels-Alder reaction in aqueous salt solutions of alcohols

This is the first exhaustive report on the variation of stereoselectivity ratios for a simple Diels-Alder reaction between cyclopentadiene and methyl acrylate. The reaction was carried out in aqueous mixtures of methanol, ethanol, propan-1-ol and butan-1-ol in presence of LiClO₄, LiCl, NaCl, KCl, CaCl₂ and MgCl₂. The endo stereoisomer decreases with the increase in carbon chain length of the alcohol. However, LiClO₄, a salting-in agent in water becomes salting-out in aqueous mixtures of alcohols. The solvent properties, thus can be attuned by adjusting the amount of solvents and salts.     

Reactions of reducing radicals with 2- and 3-nitroanilines in aqueous solutions: a pulse radiolysis study

Reactions of 2- and 3-nitro anilines (2- and 3-NA) with e_aq⁻, H-atoms and one-electron reductants have been studied using pulse radiolysis in aqueous solutions. Reactions of e_aq⁻ were found to be quite fast with both 2-NA and 3-NA resulting in their corresponding semi-reduced species which are reducing in nature. Reduction potentials for 2-NA/2-Na^•′ have been estimated to be approx. −0.56 Vvs. NHE and that for 3-NA/3-NA^•− was found to be between −0.185 V and −0.45 Vvs. NHE. Semi-reduced 2-NA has main absorption peak at 300 nm with a shoulder in the 350 nm region and a broad weak band in the 470–500 nm region, whereas semi-reduced 3-NA possesses an absorption peak at 520 nm. Reducing radicals such as (CH₃)₂ C^•OH and CO₂^•− reacted with 2-NA, producing semi-reduced species, whereas reactions of these radicals with 3-NA produced their corresponding radical-adduct species.