Kinetics of oxidation of bilirubin and its protein complex by hydrogen peroxide in aqueous solutions

A comparative study of oxidation reactions of bilirubin and its complex with albumin was carried out in aqueous solutions under the action of hydrogen peroxide and molecular oxygen at different pH values. Free radical oxidation of the pigment in both free and bound forms at pH 7.4 was shown not to lead to the formation of biliverdin, but to be associated with the decomposition of the tetrapyrrole chromophore into monopyrrolic products. The effective and true rate constants of the reactions under study were determined. It was assumed that one possible mechanism of the oxidation reaction is associated with the interaction of peroxyl radicals and protons of the NH groups of bilirubin molecules at the limiting stage with the formation of a highly reactive radical intermediate. The binding of bilirubin with albumin was found to result in a considerable reduction in the rate of the oxidation reaction associated with the kinetic manifestation of the protein protection effect. It was found that the autoxidation of bilirubin by molecular oxygen with the formation of biliverdin at the intermediate stage can be observed with an increase in the pH of solutions.     

Kinetics and mechanism of the reactions of uracil with hydrogen peroxide in aqueous solutions

The kinetics of the reactions of 5-hydroxy-6-methyluracil and 6-methyluracil with hydrogen peroxide in aqueous solutions has been investigated. The reactions proceed via two competitive mechanisms, namely, free-radical and nonradical ones. The iron ions present in water as impurities make possible the free-radical mechanism.     

Dynamics of halide ion-water hydrogen bonds in aqueous solutions: dependence on ion size and temperature

We have carried out a series of molecular dynamics simulations to investigate the dynamics of X(-)-water (X = F, Cl, Br, and I) and water-water hydrogen bonds in aqueous alkali halide solutions at room temperature and also of Cl(-)-water and water-water hydrogen bonds at seven different temperatures ranging from 238 to 318 K. The hydrogen bonds are defined by using a set of configurational criteria with respect to the anion(oxygen)-oxygen and anion(oxygen)-hydrogen distances and the anion(oxygen)-oxygen-hydrogen angle for an anion(water)-water pair. The results of the hydrogen bond dynamics are obtained for two different cutoff values for the angular criterion. In both cases, similar dynamical behavior of the hydrogen bonds is found with respect to their dependence on ion size and temperature. The fluoride ion-water hydrogen bonds are found to break at a much slower rate than water-water hydrogen bonds, while the lifetimes of chloride and bromide ion-water hydrogen bonds are found to be shorter than those of fluoride ion-water ones but still longer than water-water hydrogen bonds. The short-time dynamics of iodide ion-water hydrogen bonds is found to be slightly faster, while its long-time dynamics is found to be slightly slower than the corresponding water-water hydrogen bond dynamics. Correlations of the observed dynamics of anion(water)-water hydrogen bonds with those of rotational and translational diffusion and residence times of water molecules in ion(water) hydration shells are also discussed. With variation of temperature, the lifetimes of both Cl(-)-water and water-water hydrogen bonds are found to show Arrhenius behavior with a slightly higher activation energy for the Cl(-)-water hydrogen bonds.     

Three-dimensional net of hydrogen bonds in liquids and solutions

This paper presents definitions and analysis of the three-dimensional net of hydrogen bonds in liquids, major physicochemical properties of solvents possessing a net of this kind, and processes in solutions. Mechanisms are suggested in order to explain the mobility of particles that comprise the 3D net of bonds and solvophobic effects in nonelectrolyte solutions.     

Kinetics of hydrogen isotope exchange in molecules with strong intramolecular hydrogen bonds

Rate constants and activation energies of hydrogen exchange in solution between methanol and molecules with intramolecular H-bonds have been measured. It has been established that the rate-determining step is the dissociation of this bond.

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Extremely strongly polarizable hydrogen bonds in purine solutions

IR spectra of aqueous purine solutions are plotted for various degrees of protonation. With increasing protonation a continuous absorption occurs. This shows that extremely strongly polarizable NH⁺…N hydrogen bonds are formed between the purine molecules in the range of 0–50% protonation.     

1H NMR spectroscopic study of hydrogen bonds and mobile NH protons in aqueous solutions of porphyrin ions

The concentration and temperature dependence of the chemical shifts of aromatic and -protons of tetra-meso substituted porphyrin ions in aqueous solution have been investigated. A considerable increase in the localization temperatures of interior NH protons of porphyrins dissolved in water compared to solutions of porphyrins in organic solvents was observed. Types of association of the porphyrins studied and the possibility of intra- and intermolecular hydrogen bonds are discussed.State Science Center, Institute of Biophysics, Ministry of Public Health, Moscow 123182. Institute of Biological and Medicinal Chemistry, Russian Academy of Medical Science, Moscow 119832, Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 535–541, April, 1998.     

Helical assembly through charged hydrogen bonds in aqueous solvent

[reaction: see text] Helical capsule-like assembly of [1(2).2(3)] shows a strong Cotton effect from the chirality transfer of the tartaric acid unit to tris(imidazoline) base through charged hydrogen bonds in aqueous solvent.     

Enthalpies of transfer of Mebicarum from water to aqueous solutions of carbamide and its methyl-substituted derivatives at 298.15 K

The standard molar enthalpies of solution of Mebicarum (MbCA) in aqueous solutions of carbamide (CA), 1,3-dimethylcarbamide (1,3-DMCA), and 1,1,3,3-tetramethylcarbamide (TMCA) of various molalities, as well as the enthalpies of dilution of solutions of these compounds, were measured at 298.15 K. The enthalpy of transfer of MbCA from water to aqueous solutions of 1,3-DMCA exhibits an unusual dependence on the concentration of 1,3-DMCA. An analysis of the McMillan-Mayer enthalpy parameters of pair interactions revealed that the hydration of MbCA should be regarded as a superposition of the hydrophobic and hydrophilic mechanisms, with the latter one being predominant.     

Kinetics of lecithin oxidation in liposomal aqueous solutions

The peculiarities of egg phosphatidylcholine (lecithin) oxidation with molecular oxygen in aqueous media are investigated. Under the action of ultrasound, lecithin forms multilamellar liposomes or vesicles in aqueous solutions. Lecithin is oxidized through a chain free-radical mechanism. The nonlinear dependence of the rate of oxygen absorption on the substrate concentration and the imitation of the linear termination of the oxidation chain are observed. The study of the inhibited oxidation of phosphatidylcholine suggests that catecholamines, such as dopamine, noradrenalin, and adrenalin, are markedly more efficient antioxidants than quercetin and α-tocopherol. Phosphate buffer solution is shown to strongly influence the ways of adrenalin transformation in reactions with peroxyl radicals.     

Formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide and sodium acrylate observed by ATR-FTIR spectroscopy

In this paper, the formation and destruction of hydrogen bonds in gels and in aqueous solutions of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) were studied using Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (ATR). Aqueous solutions of NIPA and SA monomers with different pHs were prepared, and the ATR-FTIR spectra were obtained immediately after preparing the solution and after having been stored at room temperature for 6 months. It was found that the IR spectra evidently changed after 6 months due to polymerization, and the viscosity of a solution in the lowest pH system increased with time and finally the solution became a gel. The detailed analysis of the IR spectra indicated that the network of the gel was formed by the formation of hydrogen bonds (without crosslinker). Moreover, this physical gel exhibited the re-swelling transition by increasing the pH of solvent. The transition was caused by the destruction of hydrogen bonds due to the dissociation of carboxyl groups, which was also confirmed by the IR spectroscopy.     

Kinetics of synthesis of monomeric betaines in aqueous solutions

In the nucleophilic addition of N-(3-dimethylaminopropyl)methacrylamide to acrylic acid (1 : 1) in aqueous solutions, forming monomeric β-propiobetaine, the dependence of the initial rate on the starting reagent concentration was found to have a pronounced maximum (whose position does not depend on the temperature at 30–70°С). In the case of the addition of N,N-dimethylaminoethyl methacrylate, the dependence was exponential. The dependences of equilibrium conversions on the starting reagent concentrations were of the same type and had a maximum for both systems. The detected concentration effects are related to the peculiarities of the pre-reaction association of the reagents.     

Characterization of interactions in aqueous solutions of hydroxyethylcellulose and its hydrophobically modified analogue in the presence of a cyclodextrin derivative

The formation of associative networks in semidilute aqueous solutions of hydrophobically modified hydroxyethylcellulose (HM-HEC) is dependent on intermolecular hydrophobic interactions. Addition of hydroxypropyl-beta-cyclodextrin (HP-beta-CD) monomers to the system provides decoupling of these associations via inclusion complex formation with the polymer hydrophobic tails. Results from viscosity, polymer NMR self-diffusion, and dynamic light scattering (DLS) measurements show that the hydrophobic interactions in HM-HEC solutions are effectively suppressed when the level of HP-beta-CD addition increases. Small-angle neutron scattering (SANS) results reveal that the large-scale association complexes in HM-HEC solutions are strongly diminished when the concentration of HP-beta-CD rises. The time correlation data obtained from the DLS experiments unveiled the existence of two relaxation modes: one single exponential at short times followed by a stretched exponential at longer times. The fast mode is always diffusive, whereas the slow mode exhibits progressively stronger wavevector dependence as the intensity of the hydrophobic interactions increases. This feature, as well as the accompanying drop of the stretched exponential beta as the HP-beta-CD concentration decreases, is attributed to enhanced hydrophobic interactions and can be well rationalized in the framework of the coupling model of Ngai.     

Kinetics of wetting and spreading by aqueous surfactant solutions

Interest in wetting dynamics processes has immensely increased during the past 10-15 years. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant, a surface-active polymer, a polyelectrolyte or their mixture are added to a liquid (usually water). The main idea of the paper is to give an overview on some dynamic wetting and spreading phenomena in the presence of surfactants in the case of smooth or porous substrates, which can be either moderately or highly hydrophobic surfaces based on the literature data and the authors own investigations. Instability problems associated with spreading over dry or pre-wetted hydrophilic surfaces as well as over thin aqueous layers are briefly discussed. Toward a better understanding of the superspreading phenomenon, unusual wetting properties of trisiloxanes on hydrophobic surfaces are also discussed.     

Kinetics of silver nanoparticle growth in aqueous polymer solutions

Silver nanoparticles were prepared in aqueous AgNO₃ solution by using hydroquinone and sodium citrate as reducing agents with neutral polymers poly(vinylpyrrolidone) and poly(vinyl alcohol) as stabilizers. The rate of particle formation was determined with a diode array UV-Vis spectrophotometer. The effects of the polymer concentration on the reaction rate, the size, and the size distribution of the particles formed were studied by transmission electron microscopy. Both the reaction rate and the size of silver nanoparticles decreased with increasing polymer concentration in the range 0.07–0.50 w/v%.     

Calorimetric investigations of hydrogen bonding in binary mixtures containing pyridine and its methyl-substituted derivatives. I. The dilute solutions of water

Enthalpies of solution of water in pyridine and its methyl derivatives were investigated at T=298.15 K with a titration calorimeter. The effects of solution are exothermic and increase in the following order: pyridine <3-methylpyridine <4-methylpyridine <2-methylpyridine <2,6-dimethylpyridine <2,4,6-trimethylpyridine. As expected, the negative enthalpies of solution are fairly large, especially those for pyridine derivatives with a methyl group in the ortho position, due to the formation of hydrogen bonds between water and amine. Thus, the energy of the N⋯H–O bond, strengthened by the ortho methyl group, overcomes the hindering effect of that group. The experimental results were correlated with the association energies of the 1:1 (water+amine) complexes and with the net charges on the nitrogen atom reported in the literature. Although semi-quantitative agreement between the correlated quantities is fairly good, a simple model based on the hydrogen bond interactions was found to be too simple to account for the measured heat effects.     

Gas–liquid partitioning of halogenated volatile organic compounds in aqueous cyclodextrin solutions

Gas–liquid partitioning coefficients (K_GL) were measured for halogenated volatile organic compounds (VOCs), namely 1-chlorobutane, methoxyflurane, pentafluoropropan-1-ol, heptafluorobutan-1-ol, α,α,α-trifluorotoluene, and toluene in aqueous solutions of natural α-, β-, and γ-cyclodextrins (CDs) at temperatures from (273.35 to 326.35) K employing the techniques of headspace gas chromatography and inert gas stripping. The binding constants of the 1:1 inclusion complex formation between the VOCs and CDs were evaluated from the depression of the VOCs volatility as a function of CD concentration. The host–guest size matching and the hydrophobic interaction concept were used to rationalize the observed widely different affinity of the VOC–CD pairs to form the inclusion complex. The enthalpic and entropic component of the standard Gibbs free energy of complex formation as derived from the temperature dependence of the binding constant indicate the thermodynamic origin of the binding to vary greatly among the systems studied, but follow the global enthalpy–entropy compensation relationships reported previously in the literature.