Pulse radiolysis study of one-electron oxidation of thionine in aqueous solutions

One-electron oxidation of thionine has been studied using specific oxidizing radicals such as Cl⁻Tl(II) and N₃ generated by pulse radiolysis of aqueous solutions. The semioxidized thionine exhibited threepK’s indicating four conjugate acid-base forms. N₃ radicals were found to be less efficient in oxidizing thionine as compared to Cl ₂ ⁻ , Tl²⁺ and Tl(OH)⁺. The rate constants for electron abstraction from thionine by Cl ₂ ⁻ , Tl²⁺, Tl(OH)⁺, Tl(OH)₂ and N₃ were evaluated. The spectra of different protonated forms of semioxidized thionine and the extinction coefficients at λ_max are presented. Reaction of OH radicals with thionine gave transient products whose spectra and acid-base properties were different from those of semioxidized thionine. The rate constant for formation of the product transient agrees well with competition kinetic value for reaction of OH with thionine reported earlier.     

Rate constants for one-electron oxidation by methylperoxyl radicals in aqueous solutions

Rate constants for one-electron oxidation by the methylperoxyl radicals (CH₃O₂, HOCH₂O₂, ^?O₂CCH₂O₂, and CCI₃O₂) in aqueous solutions have been measured by pulse radiolysis and found to be in the range of 3 × 10⁵ to 6 × 10⁸ M^?1 s^?1 for compounds with redox potentials between 0.6 and 0.1 V. Substitution on the methylperoxyl radical with OH or CO₂^? has only a minor effect on the rate of oxidation but substitution with three chlorines increases the rate constants by two orders of magnitude. The redox potential of the CH₃O₂ radical is estimated to be 0.6–0.7 V.     

Pulse radiolysis study on one-electron oxidation of 1-naphthylamine-4-sulphonic acid in aqueous solutions

Reactions of 1-naphthylamine-1-sulphonic acid (NASA) with hydroxyl radicals and oneelectron oxidants such as N₃, Br₂ ^- and Cl₂ ^- radicals have been studied at various pHs using pulse radiolysis technique. Rate constants for the reaction of N₃ and Br₂ ^-. radicals with NASA at neutral pH were found to be 5 × 10⁹ and 4 × 10⁸ dm³ mol^-1 s^-1 respectively. These reactions led to the formation of a cation radical (semi-oxidized species). OH radical reaction with NASA (k = 7.2 × 10⁹ dm³ mol^-1 s^-1) at neutral pH gave a mixture of species, namely, a semi-oxidized species as well as an adduct species. Cl₂ ^-. radicals reacted with NASA rather slowly (k = 7 × 10⁷ dm³ mol^-1 s^-1) at pH 1 to give the semioxidised species. However, even at pH 1, OH radical reaction with NASA gave a mixture containing semi-oxidized as well as an adduct species. The OH-adduct species having _max at 340 nm decays at acidic pHs to give semi-oxidized species having _max at 370 nm. Electron adduct of NASA was found to be a strong reducing radical.     

Nucleophilic coaction during one-electron oxidation of hydroxyaromatic compounds

Conclusions The role of internal catalysis during one-electron oxidation of hydroxyaromatic compounds by mercuric salts, the possibility of which is determined by the nature of the salt anion, was shown.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2601–2603, November, 1974.     

Nitrogen dioxide multiphase chemistry: uptake kinetics on aqueous solutions containing phenolic compounds

The uptake coefficients of NO2 on aqueous solutions containing guaiacol, syringol and catechol were determined over the pH range from 1 to 13 using the wetted wall flowtube technique. The measured uptake coefficients were used to determine the rate coefficients for the reaction of the physically dissolved NO2 with the neutral and deprotonated forms of phenolic compounds listed above. These organic compounds are ubiquitous not only in biomass burning plumes but also in soils, where they form part of the building blocks of humic acids. The NO2 uptake kinetics on solutions containing guaiacol, syringol or catechol were observed to be strongly pH dependent with uptake coefficients increasing from below 10(-7), under acidic conditions, to more than 10(-5) at pH values above 10. This behaviour illustrates the difference of reactivity between the neutral phenolic species and the phenoxide ions. The corresponding second order rate coefficients were typically observed to increase from 10(5) M(-1) s(-1) for the neutral compounds to a minimum of 10(8) M(-1) s(-1) for the phenoxide ions.     

Pulse radiolysis investigations of one-electron reduction of terephthalonitrile in aqueous solutions

Pulse radiolysis of aqueous solutions of terephthalonitrile (TPN) was carried out to study the redox properties of anions of TPN, by determining the rate constants for the reaction of TPN with specific one-electron reducing species formed on reaction of OH radicals with methyl,ethyl,isopropyl alcohols,tetrahydrofuran and cytosine. Formation of anions of TPN, which absorb at wavelength 340 nm, was followed. From the graph of rate constants and maximum absorbance vs reduction potential of reducing species, reduction potential of TPN was found out to be —0.85 V vs NHE. Reduction of thionine (TH⁺) and methyl viologen (MV²⁺) with the help of radical anions of TPN was carried out to establish its high reduction potential.     

The reorganization energies in one-electron oxidation of ferrous complexes in aqueous solution

The photoelectron emission method was used to determine the reorganization free energy of nuclear coordinates in one-electron oxidation reactions. The experimental results for eight ferrous complexes, some of which have large organic ligands, in aqueous solution are in excellent agreement with the theoretical ones.     

Effect of netropsin on one-electron oxidation of duplex DNA

The effect of netropsin on the oxidative reactions of duplex DNA was examined. One-electron oxidation of DNA creates a radical cation that migrates through duplex DNA and reacts primarily at GG steps. Netropsin is a dication that specifically binds primarily by hydrogen bonding in the minor groove at sites that have four or more contiguous A.T base pairs. We showed that the oxidation potential of netropsin is less than that of any of the four nucleobases. We find that netropsin quenches the oxidative reactions of DNA independent of whether it is specifically bound. Within the Perrin model of static quenching, a netropsin within a rather large fixed volume around the DNA is an effective quencher.     

Effect of pH on concentration of species present in aqueous solutions of carbonyl compounds affected by hydration-dehydration and keto-enol equilibria

Equations have been derived for hydration-dehydration and keto-enol equilibria of carbonyl compounds, showing the changes, with pH, of individual species present in aqueous solutions. An Egtran computer program has been constructed, so that the pH-variation of individual species for widely varying values of equilibrium constants can be evaluated. The treatment enables isolation of individual equilibrium constants from experimental data and identification of the system involved. The ratios of hydrated and non-hydrated, as well as of keto and enol forms, are pH-independent.     

Air oxidation of organic compounds in aqueous solutions. Ecochemical and analytical aspects

The results of static and dynamic laboratory experiments confirm the oxidation of organic compounds in aquatic medium by dissolved oxygen of atmospheric air to be possible under conditions close to normal. Alkylaromatics containing hydrogen atoms in alkyl substituents in α-positions to the aromatic system, as well as simple ethers, are characterised by the highest chemical reactivity in relation to triplet (³Σ _g ^? ) which is consistent with the pattern of free-radical reactions. Principal environmental and analytical consequences of the obtained results are under discussion. Liquid-phase free-radical reactions of atmospheric oxygen may proceed with the participation of atmospheric aerosols, i.e water droplets within clouds. Another example relates to the prevention of accumulation in natural bodies of water stable nonylphenol (predominantly 4-tert-isomers), characterized by endocrine activity. Suggested the possibility of replacing them with secondary alkylphenols, identical in all chemical properties except for the ease of oxidation by air oxygen in aqueous solutions     

Fast pH calculations in aqueous solution chemistry

A technique for computation of the neutralization curves of acid-base solutions, based on an optimization search method, has been developed. The criterion function is the absolute value of the calculated difference between the numbers of positive and negative charges present in the solutions. This technique is generally applicable for solution chemistry, but because of its speed of resolution and its accuracy, it is particularly useful in the control of a real-time process by a computer.     

Acid-base chemistry of omeprazole in aqueous solutions

Omeprazole is a potent anti-acid drug. Its absorption and mode of action are closely related to its prototropic behavior. In the present study, omeprazole samples from different sources and in different forms were studied spectrophotometrically to obtain pK_a values. In the neutral to alkaline pH region, two consistent pK_a values of 7.1 and 14.7 were obtained from various samples. The assignment of these pK_a values was realized by comparison with the prototropic properties of N(1)-methylated omeprazole substituted on the nitrogen at the 1-position of the benzimidazole ring, which was found to have a pK_a of 7.5. The omeprazole pK_a of 14.7 is assigned to the dissociation of the hydrogen from the 1-position of the benzimidazole ring and the pK_a of 7.1 is assigned to the dissociation from the protonated pyridine nitrogen of omeprazole. The results presented are at variance with those of earlier work.     

Effect of pH on radical polymerization of butadiene 1-carboxylic acid in aqueous solutions

Radical polymerization of butadiene 1-carboxylic acid (Bu-1-Acid) was carried out in aqueous solutions at 50°C with ammonium persulfate (APS) as an initiator. Kinetic studies led to the rate equation, R_p = k[APS]^1/2 [Bu-1-Acid]¹ at pH 6.8. The overall activation energy for the polymerization was 16.0 kcal/mole. The polymerization rate R_p of Bu-1-Acid decreased with an increase of pH in the range 2.4–6.8 and increased with an increase of pH in the range 6.8–8.4. Moreover, in the pH range 8.4–13, the rate of polymerization was not affected by the pH of the system. In copolymerization with acrylonitrile, the trends of changes in the monomer reactivity ratios r₁, r₂ and Q-e values caused by changes in pH were similar to trends found in homopolymerization described above. In addition, it was observed that the resultant polymer was extended in alkaline solution and contracted in acidic solution.     

The oxidation of sulphide in aqueous solutions

The purpose of this work was to study the catalytic effect of activated carbon particles 7.5 μm in diameter on the initial oxidation rate of bisulphide, with oxygen as oxidant, in an aqueous solution buffered at a pH of 9.7 and at temperatures of 48.5, 35 and 25 °C. Two reactors were used: a stirred gas—liquid contactor to determine the kinetics and a continuous flow reactor to demonstrate that a continuous process is possible.The liquid phase oxidation of sulphide was found to be much faster in the presence of activated carbon. Kinetic equations for the initial rate of reaction are given separately for homogeneous and heterogeneous oxidation.As activated carbon shows a strong catalytic activity, it can be of interest in the oxidation of sulphide in industrial and other waste waters at low temperatures.     

Gamma-radiation induced oxidation of p-hydroquinone in aqueous solutions

The products of γ-radiation induced oxidation of p-hydroquinone (H₂Q) solutions are p-quinone (Q), 2-hydroxybenzoquinone (2-Q-OH) and hydrogen peroxide (H₂O₂). The effects of dose, [O₂], [H₂Q], pH and different anions on yields have been investigated. Plots of G(Q) and G (H₂O₂) versus pH give titration type curves with pK of ～1.2 and 1.3, respectively. This pK was attributed to protonation of the HO₂ radical. The protonated form can oxidize H₂Q. The neutral form of this radical appears to react with the semiquinone radical (HQ) to form (2-Q-OH) or disproportionate to give H₂O₂. A reaction mechanism is proposed to account for the experimental observations.     

Gamma-radiation induced oxidation of p-hydroquinone in aqueous solutions

P-hydroquinone (H₂Q) has been gamma-irradiated in the presence of O₂ and different alcohols in acidic solutions. In the presence of tert-BuOH, the oxidation products were quinone(q), 2-hydroxybenzoquinone (2-Q-OH) and hydrogen peroxide (H₂O₂). In case of propan-2-ol, ethanol and methanol, the oxidation products were as above in addition to a carbonyl compound. A chain reaction was observed in case of propan-2-ol and this was inhibited by H₂Q. The dependence of G(Q) and G(H₂O₂) on pH is a titration-type curve and the species responsible for this curve is assumed to be the protonated and unprotonated alcohol peroxy radicals.     

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.     

Radiation-induced oxidation of ferrous iron in aqueous solutions

Summary An investigation was made of the dependence of the yield G in the oxidation of aqueous solutions of ferrous iron on various factors. The limits within which the enhanced-G effect is observed and the accuracy with which the effect can be measured were established.