Mechanism of protection of adenosine from sulphate radical anion and repair of adenosine radicals by caffeic acid in aqueous solution

The photooxidation of adenosine in presence of peroxydisulphate (PDS) has been studied by spectrophotometrically measuring the absorbance of adenosine at 260 nm. The rates of oxidation of adenosine by sulphate radical anion have been determined in the presence of different concentrations of caffeic acid. Increase in [caffeic acid] is found to decrease the rate of oxidation of adenosine suggesting that caffeic acid acts as an efficient scavenger of SO ₄ ^• and protects adenosine from it. Sulphate radical anion competes for adenosine as well as for caffeic acid. The quantum yields of photooxidation of adenosine have been calculated from the rates of oxidation of adenosine and the light intensity absorbed by PDS at 254 nm, the wavelength at which PDS is activated to sulphate radical anion. From the results of experimentally determined quantum yields (Φ_expt1) and the quantum yields calculated (Φ_cal) assuming caffeic acid acting only as a scavenger of SO ₄ ^• show that Φ_expt1 values are lower than Φ_expt1 values. The ǵf values, which are experimentally found quantum yield values at each caffeic acid concentration and corrected for ₄ ^• scavenging by caffeic acid, are also found to be greater than Φ_expt1 values. These observations suggest that the transient adenosine radicals are repaired by caffeic acid in addition to scavenging of sulphate radical anions.     

Kinetics and mechanism of protection of thymine from phosphate radical anion under anoxic conditions

The rates of photooxidation of thymine in the presence of peroxydiphosphate (PDP) have been determined by measuring the absorbance of thymine at 264 nm spectrophotometrically. The rates and the quantum yields (φ) of oxidation of thymine by phosphate radical anion have been determined in the presence of different concentrations of dithiothreitol (DTT). An increase in DTT is found to decrease the rate of oxidation of thymine, suggesting that DTT acts as an efficient scavenger of PO₄^·2? and protects thymine from it. Phosphate radical anion competes for thymine as well as DTT; the rate constant for the phosphate radical anion with DTT has been calculated to be 2.21 × 10⁹ dm³ mol^?1 s^?1, assuming the rate constant of phosphate radical anion reaction with thymine as 9.6 × 10⁷ dm³ mol^?1 s^?1. The quantum yields of photooxidation of thymine have been calculated from the rates of oxidation of thymine and the light intensity absorbed by PDP at 254 nm, the wavelength at which PDP is activated to phosphate radical anion. From the results of experimentally determined quantum yields (φ_exptl) and the quantum yields calculated (φ_cl), assuming DTT acts only as a scavenger of PO₄^·2? radicals, show that φ_exptl values are lower than φ_cl values. The φ′ values, which are experimentally found quantum yield values at each DTT concentration and corrected for PO₄^·2? scavenging by DTT, are also found to be greater than φ_exptl values. These observations suggest that the thymine radicals are repaired by DTT in addition to scavenging of phosphate radical anions. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 271–275, 2001     

Electrochemical conversions of pentaphenyl(methoxycarbonyl)cyclopentadiene derivatives

Redox processes of fluctuating pentaphenyl(methoxycarbonyl)cyclopentadiene derivatives on platinum electrode in acetonitrile are studied by cyclic voltammetry. The reduction of derivatives of pentaphenylcyclopentadiene C₅Ph₅M yields a radical anion which decomposes to anion M^- and stable radical C₅Ph ₅ ^• ; the latter reduces to C₅Ph₅. In the first stage of oxidation of these compounds, a stable radical cation forms, with substituent M intact. Reduction of derivatives of pentamethoxycarbonylcyclopentadiene [C₅(CO_2Me)₅]M yields a dianion. Such electrochemical behavior of fluctuating cyclopentadiene compounds reveals additional information on the mechanism of displacement of the element-centered substituents M in them     

Oxidation of caffeine by phosphate radical anion in aqueous solution under anoxic conditions

The photooxidation of caffeine in presence of peroxydiphosphate (PDP) in aqueous solution at natural pH (∼7.5) has been carried out in a quantum yield reactor using a high-pressure mercury lamp. The reactions were followed spectrophotometrically by measuring the absorbance of caffeine at λ_max (272 nm). The rates of reaction were calculated under different experimental conditions. The quantum yields were calculated from the rates of oxidation of caffeine and the intensity of light at 254 nm which was measured by using peroxydisulphate solution as a standard chemical actinometer. The reaction rates of oxidation of caffeine by PDP increase with increase in [PDP] as well as with increase in light intensity, while they are independent of [caffeine]. The quantum yields of oxidation of caffeine by PDP are independent of [PDP] as well as light intensity. However, quantum yields of oxidation of caffeine by PDP increase with increase in caffeine concentration. On the basis of these experimental results and product analysis, a probable mechanism has been suggested in which PDP is activated to phosphate radical anions (PO₄ ^·2−) by direct photolysis of PDP and also by the sensitizing effect of caffeine. The phosphate radical anions thus produced react with caffeine by electron transfer reaction, resulting in the formation of caffeine radical cation, which deprotonates in a fast step to produce C8-OH adduct radicals. These radicals might react with PDP to give final product 1,3,7-trimethyluric acid and PO₄ ^·2− radicals, the latter propagates the chain reaction.     

Effect of electron-withdrawing power of the substituted group on?OH radical reaction with substituted aryl sulphides: A pulse radiolysis study

In neutral aqueous solution of (phenylthio)acetic acid, hydroxyl radical is observed to react with a bimolecular rate constant of 7.2 × 10^-1 dm³ mol⁻s⁻ and the transient absorption bands are assigned to^•OH radical addition to benzene and sulphur with a rough estimated values of 50 and 40% respectively. The reaction of the^•OH radical with diphenyl sulphide (k = 4.3 × 10⁸ dm³ mol⁻¹ s⁻¹) is observed to take place with formation of solute radical cation, OH-adduct at sulphur and benzene with estimated values of about 12, 28 and 60% respectively. The transient absorption bands observed on reaction of^•OH radical, in neutral aqueous solution of 4-(methylthio)phenyl acetic acid, are assigned to solute radical cation (λ_max = 550 and 730 nm), OH-adduct at sulphur (λ_max = 360 nm) and addition at benzene ring (λ_max = 320 nm). The fraction of^•OH radical reacting to form solute radical cation is observed to depend on the electron-withdrawing power of substituted group. In acidic solutions, depending on the concentration of acid and electron-withdrawing power, solute radical cation is the only transient species formed on reaction of^•OH radical with the sulphides studied.     

A CIDEP study of photolysis of duroquinone/hydrogen-donor homogeneous and triton micelle solutions

The CIDEP spectra of transient radicals during photolysis of the duroquinone (DQ)/ethylene glycol (EG) system in acid, basic, and micellar environments were measured with a home-made highly time-resolved ESR spectrometer. In the DQ/EG homogeneous solution, the enhanced emissive CIDEP signal of the neutral durosemiquinone radical DQH^• was observed. When the DQ/EG solution at pH 9 or the DQ/EG/TX-100/H₂O micelle system was photolyzed, the CIDEP signal of the duroquinone anion radical (DQ^•−) was obtained. When the DQ/EG solution at pH 2.5 was irradiated, the CIDEP signal of DQH^• appeared. These experimental results indicate that the neutral radical DQH^• was formed by proton transfer from EG to ³DQ*, and that DQ^•− was formed by dissociation of DQH^• accompanying polarization transfer.     

Kinetics of the ?OH-radical initiated reactions of acetic acid and its deuterated isomers

Kinetics of the ^•OH-initiated reactions of acetic acid and its deuterated isomers have been investigated performing simulation chamber experiments at T = 300 ± 2 K. The following rate constant values have been obtained (± 1σ, in cm³ molecule⁻¹ s⁻¹): k ₁(CH₃C(O)OH + ^•OH) = (6.3 ± 0.9) × 10⁻¹³, k ₂(CH₃C(O)OD + ^•OH) = (1.5 ± 0.3) × 10⁻¹³, k ₃(CD₃C(O)OH + ^•OH) = (6.3 ± 0.9) × 10⁻¹³, and k ₄(CD₃C(O)OD + ^•OH) = (0.90 ± 0.1) × 10⁻¹³. This study presents the first data on k ₂(CH₃C(O)OD + ^•OH). Glyoxylic acid has been detected among the products confirming the fate of the ^•CH₂C(O)OH radical as suggested by recent theoretical studies.     

The role of triplet repulsion in alkyl radical addition to a 7π-C-O bond and alkoxy radical addition to a π-C-C bond

The experimental activation energies of the R^• + O = CR¹R² and RO^• + CH₂ = CHR¹ addition reactions are analyzed within the framework of the parabolic model of the bimolecular addition reaction. The activation energy also depends on the dissociation energy of the forming C-O bond and on the reaction enthalpy: the higher the dissociation energy, the higher the activation energy. The empirical relationshipr _e J..D _e = 0.97 x 10-¹³ m kJ.-¹ mol is found for H^•, Cl^•, Br^{• •} and RO^• radical addition to multiple C=C and C=O bonds (r_e is the distance between the peaks of the intersecting parabolic curves). This is due to the effect of the triplet repulsion on radical addition. The interaction of polar groups and the steric effect also influence the activation energy.     

Enthalpies of addition of radicals to aldehydes, ketones, acids and esters

Enthalpies of the addition of radicals HO^•, HO₂ ^•, CH₃ ^•, CH₃O^•, CH₃O₂ ^• to the C=0 bond of model aldehydes, ketones, acids and esters have been calculated by thermodynamic and quantum chemical methods. It has been shown that the exothermic effect of these reactions decreases in the row HO^•>CH₃O^•>CH₃ ^•>HOO^•>CH₃OO^•.     

Photosensitization and photoprotection properties of nicotinic acid derivatives

The photosensitizing action of nicotinic acid, nicotinamide, and nicotinehydroxymethylamide on the photooxidation of glycyltryptophan (Gly-Trp) in an aqueous solution using UV light (240–410 nm) was found. The photooxidation was monitored by measuring chemiluminescence (CL) resulting from the decay of one of the oxidation products, dioxethane. The photosensitizing action decreases in the following sequence: nicotinamide > nicotine-hydroxymethylamide > nicotinic acid. The addition of benzoquinone (0.01 mmol L⁻¹) results in a substantial decrease in the yield of sensitized CL, which indicates that the superoxide radical anion participates in the photooxidation. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 942–945, May, 1997.     

Simultaneous voltammetric measurement of ascorbic acid and dopamine on poly(caffeic acid)-modified glassy carbon electrode

A poly(caffeic acid) thin film was deposited on the surface of a glassy carbon electrode by potentiostatic technique in an aqueous solution containing caffeic acid. The poly(caffeic acid)-modified electrode was used for the determination of ascorbic acid (AA), dopamine (DA), and their mixture by cyclic voltammetry. This modified electrode exhibited a potent and persistent electron-mediating behavior followed by well-separated oxidation peaks toward AA and DA at a scan rate of 10 mV s⁻¹ with a potential difference of 135 mV, which was large enough to determine AA and DA individually and simultaneously. The catalytic peak current obtained was linearly dependent on the AA and DA concentrations in the range of 2.0 × 10⁻⁵−1.2 × 10⁻³ and 1.0 × 10⁻⁶−4.0 × 10⁻⁵ mol L⁻¹ in 0.15 mol L⁻¹ phosphate buffer (pH 6.64). The detection limits for AA and DA were 9.0 × 10⁻⁶ and 4.0 × 10⁻⁷ mol L⁻¹, respectively. The modified electrode shows good sensitivity, selectivity, and stability and has been applied to the determination of DA and AA in real samples with satisfactory results.     

Radiolytic degradation of 2,4-dichlorophenoxyacetic acid in dilute aqueous solution: pH dependence

The reactions of hydroxyl radical and hydrated electron intermediates of water radiolysis were studied in the radiolytic degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) at pH values of 4, 6 and 8. The hydrated electron reactions are also suggested to contribute to the aromatic ring decomposition in addition to the highly effective hydroxyl radical reactions. The experimental results suggest also some contribution from the O₂^−•/HO₂^• pair to the degradation. The degradation efficiency was found to be the highest at pH 8 and the lowest at pH 6.     

Activity coefficients and pitzer parameters in the systems Na+/Cs+/Cl-/TcO 4 - or ClO 4 - /H2O at 25°C

Isopiestic vapor pressure experiments are performed at 25°C with aqueous NaTcO₄, mixed NaTcO₄-NaCl, and NaCl reference solutions. The solubility of CsTcO₄ is determined in 0–7.4m CsCl and in 0–5.6m NaCl solutions. The osmotic coefficients of the binary and ternary solutions are used to evaluate the binary Pitzer parameters Β⁰, Β^l, and C^Φ for NaTcO₄ and the mixing parameters θ_{TcO 4} ^- /cl^- and ψ_Na ⁺/TcO ₄ ^- /cl^-. The binary Pitzer parameters for the sparingly soluble CsTcO₄ and CsC1O₄ are calculated together with ψ_Cs ⁺/mo ₄ ^- /cl^- from their solubilities in CsCl solution. The solubilities of CsTcO₄ in NaCl and CsClO₄ in NaClO₄ solution are also included in the parametrization of the reciprocal salt systems Na⁺/Cs⁺/Cl^-/MO ₄ ^- . The parameters Β⁰ and Β^l of pertechnetate and perchlorate salts correlate well with the ionic radii.     

Reactions of heme proteins with carbonate radical anion

Reactions of radiolytically generated CO₃ ^•− with some ferric heme proteins, catalase, cytochrome c, and horseradish peroxidase (HRP), were studied. Carbonate radical anion oxidized amino acid residues of these proteins, but did not react directly with heme iron. HRP and catalase lost about 30% and 20% of their activity, respectively, after the reaction with 100 μM of CO₃ ^•−. The rate constants of the reactions of CO₃ ^•− with the investigated proteins measured by the pulse radiolysis method at pH 8–8.4 and 10 varied from 1.0 × 10⁸ M⁻¹ s⁻¹ (for cytochrome c) to 3.7 × 10⁹ M⁻¹ s⁻¹ (for catalase).     

Formation of p-terphenyl excited states in the ionic liquid methyltributylammonium bis[(trifluoromethyl)sulfonyl]imide: pulse radiolysis study

Solutions of 80 mM benzophenone (BP) and up to 14 mM p-terphenyl (TP) in the ionic liquid methyltributylammonium bis[(trifluoromethyl)sulfonyl]imide (R₄NNTf₂) have been investigated by nanosecond pulse radiolysis. The resulting transient absorption spectra of pulse-irradiated argon saturated solutions correspond to the formation of several intermediates derived from BP and TP: benzophenone radical anion [(C₆H₅)₂CO]^•− (BP^•−) converted after ~20 μs into ketyl radicals (C₆H₅)₂C^•OH (BP^•H), a hydrogen adduct to the phenyl ring of benzophenone C₆H₅COC₆H₆^•, p-terphenyl triplet excited state ³TP*, and traces of TP radical ions. ³TP* was formed in two steps, the first immediately during the pulse and the second in pseudo-first order process with a second order reaction rate constant calculated from TP concentration dependence: k = ~2 × 10⁸ dm³ mol⁻¹ s⁻¹.     

Kinetics of oxidation of adenosine by t-butoxyl radical: Protection and repair by caffeic acid

The kinetics of oxidation of adenosine and caffeic acid by t-BuO^• has been studied by the photolysis of t-BuOOH in the presence of t-BuOH. The rates and the quantum yields (φ) of oxidation of caffeic acid by t-BuO^• radicals have been determined in the absence and presence of varying concentrations of adenosine. An increase in the concentration of adenosine has been found to decrease the rate of oxidation of caffeic acid suggesting that adenosine and caffeic acid compete for t-BuO^• radicals. From competition kinetics, the rate constant of t-BuO^•–caffeic acid reaction has been calculated to be 8.15 × 10⁸ dm³ mol⁻¹ s⁻¹. The results of experimentally determined quantum yield (φ_exptl) values of oxidation of caffeic acid and the quantum yield values calculated (φ_cal) by assuming that caffeic acid reacts only with t-BuO^• radicals suggest that caffeic acid not only protects adenosine from t-BuO^• radicals but also repairs adenosine radicals formed by the reaction of t-BuO^• radicals. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 515–521, 2005     

Use of Raman spectroscopy for the identification of radical-mediated damages in human serum albumin

Damages induced by free radicals on human serum albumin (HSA), the most prominent protein in plasma, were investigated by Raman spectroscopy. HSA underwent oxidative and reductive radical stress. Gamma-irradiation was used to simulate the endogenous formation of reactive radical species such as hydrogen atoms (^•H), solvated electrons (e_aq⁻) and hydroxyl radicals (^•OH). Raman spectroscopy was shown to be a useful tool in identifying conformational changes of the protein structure and specific damages occurring at sensitive amino acid sites. In particular, the analysis of the S–S stretching region suggested the radical species caused modifications in the 17 disulphide bridges of HSA. The concomitant action of e_aq⁻ and ^•H atoms caused the formation of cyclic disulphide bridges, showing how cystine pairs act as efficient interceptors of reducing species, by direct scavenging and electron transfer reactions within the protein. This conclusion was further confirmed by the modifications visible in the Raman bands due to Phe and Tyr residues. As regards to protein folding, both oxidative and reductive radical stresses were able to cause a loss in α-helix content, although the latter remains the most abundant secondary structure component. β-turns motifs significantly increased as a consequence of the synergic action of e_aq⁻ and ^•H atoms, whereas a larger increase in the β-sheet content was found following the exposure to ^•OH and/or ^•H attack.     

Significant roles of oxygen and unbound <Superscript>•</Superscript>OH radical in phenol formation during photo-catalytic degradation of benzene on TiO<Subscript>2</Subscript> suspension in aqueous system

Studies on photo-catalytic degradation of benzene using TiO₂ photo-catalyst as a suspension in water is reported. Degradation studies have been carried out using 350 nm UV light. Phenol, a photo-catalytic product of benzene, was monitored under varying experimental conditions such as amount of TiO₂, concentration of benzene, photolysis time, ambient (air, O₂, Ar, N₂O and N₂O–O₂ mixture), etc. The phenol yields in both aerated and O₂-purged systems increased with the photolysis time. In contrast to oxygenated systems, the yields of phenol in deoxygenated (viz. Ar-purged and N₂O-purged) systems were quite low (~30 μM) and remained steady. H₂O₂ yields in all these systems were also monitored, and found lower than an order of magnitude as compared to phenol yields for the respective systems. The rate of phenol production in aerated 1 mM benzene solution containing 0.05% TiO₂ suspension was evaluated at 12.3 μM min⁻¹ which is lower than the rate obtained in an O₂-saturated system (22.4 μM min⁻¹). The low yields of phenol in both Ar- and N₂O-purged systems, and also the increasing trends in oxygenated systems, together reveal that, for the phenol formation with an enhanced rate, oxygen is essential. In the present study, it is implied that the photo-generated hole, which is mainly an ^•OH radical, is either freely available in the aqueous phase or migrates to the aqueous phase from the catalyst surface, to react with benzene to produce HO-adduct radical. Later, following reaction with oxygen, the adduct produces phenol. On the other hand, h⁺ and surface adsorbed ^•OH radical, being trapped/bonded due to rigid association with the catalyst surface, were not able to generate phenol under similar experimental conditions. The mechanism of phenol formation with TiO₂ photolysis in an aqueous system is rechecked, on the basis of present results on h⁺/surface adsorbed ^•OH radical/unbound ^•OH radical scavenging by benzene, collectively with previous reports on various systems.     

Scavenging of reactive oxygen radicals by resveratrol: antioxidant effect

Pulse radiolysis of resveratrol was carried out in aqueous solutions at pH ranging from 6.5 to 10.5. The one-electron oxidized species formed by the N^•₃ radicals at pH 6.5 and 10.5 were essentially the same with λ_max at 420 nm and rate constant varying marginally (k = (5−6.5) × 10⁹ dm³ mol⁻¹ s⁻¹). The nature of the transients formed by NO^•₂, NO^• radical reaction at pH 10.5 was the same as that with N^•₃, due to the similarity in decay rates and the absorption maximum. Reaction of ^•OH radical with resveratrol at pH 7 gives an absorption maximum at 380 nm, attributed to the formation of carbon centered radical. The repair rates for the thymidine and guanosine radicals by resveratrol were approx. 1 × 10⁹ dm³ mol⁻¹ s⁻¹, while the repair rate for tryptophan was lower by nearly an order of magnitude (k = 2 × 10⁸ dm³ mol⁻¹ s⁻¹). The superoxide radical anion was scavenged by resveratrol, as well as by the Cu–resveratrol complex with k = 2 × 10⁷ and 1.5 × 10⁹ dm³ mol⁻¹ s⁻¹, respectively. Its reduction potential was also measured by cyclic voltammetry.     

Density and viscosity of magnesium sulphate in formamide + ethylene glycol mixed solvents

Densities (ρ) and viscosities (η) of different strengths of magnesium sulphate (MgSO₄) in varying proportions of formamide (FA) + ethylene glycol as mixed solvents were measured at room temperature. The experimental values of ρ and η were used to calculate the values of the apparent molar volume, (φ_1,), partial molar volume, (φ₁,^ℴ) at infinite dilution,A- andB-coefficients of the Jones-Dole equation and free energies of activation of viscous flow, (Δμ ₁ ^0* ) and (Δμ ₂ ^0* ), per mole of solvent and solute respectively. The behaviour of these parameters suggests strong ion-solvent interactions in these systems and also that MgSO₄ acts as structure-maker in FA + ethylene glycol mixed solvents.