OH radicals and oxidizing products in the gamma radiolysis of water

The production of OH radicals in the gamma radiolysis of water has been examined with radical scavenger techniques employing formic acid. OH radical yields were found to vary from 2.4 radicals/100 eV at the low scavenger concentration limit to 4.2 at a formic acid concentration of 3 M. An inverse Laplace transform technique was applied to the scavenger concentration dependence to obtain the temporal dependence of OH radicals in pure water. It was found that the relative decrease in OH radical yields from 200 ps to 3 ns was virtually the same for the transform of the scavenger data and the directly measured time-resolved results. The absolute yields for the time-resolved experiments are about 10% higher than expected from the present results with scavengers. The agreement can be considered to be good, and reasons for the observed difference are given. Approximately 40% of the OH radicals produced lead to the formation of hydrogen peroxide, which is the only other major oxidizing species in the gamma radiolysis of water. The net water decomposition for gamma rays was found to vary from an initial value of 5.6 +/- 0.3 molecules/100 eV to 3.8 +/- 0.2 molecules/100 eV at 1 micros.     

Interaction of radicals from water radiolysis with melanin

Melanins are considered to be natural photoprotectors in the melanocytes and keratinocytes of the skin. These pigments have also been suggested to play an important role in protection of melanin-containing cells against ionising radiation. Various mechanisms have been proposed to explain the protective role of melanin which invoke the radical scavenging properties of the polymer. In the present work the reactions of melanins with radicals generated in aqueous media by pulse radiolysis have been studied. Time-resolved changes in absorbance of the melanin or the radical species were recorded at selected wavelengths. Experiments were carried out on synthetic dopa- and 5-S-cysteinyldopa-melanins and a natural melanin in phosphate buffer (pH 7.4). Under the conditions employed, melanin reacted predominantly with either oxidising (OH., N3.) or reducing (eaq-, CO2-) species. We were also able to monitor the interaction of melanin with superoxide radical, which was reducing in this case. Detailed analysis of transient changes in melanin absorbance, detected at different wavelengths, was demonstrated to be a convenient method for studying redox processes of this substance, as shown by model experiments using ferricyanide and dithionite as oxidising and reducing agents, respectively. Among the radicals studied, OH. exhibited the strongest reactivity with melanins. Apparent rate constants for the reactions of radicals with autoxidative dopa-melanin (1.5 X 10(9) M-1 X s-1, 2.6 X 10(8) M-1 X s-1, 1.8 X 10(8) M-1 X s-1, 5 X 10(5) M-1 X s-1, 10(6)-10(7) M-1 X s-1 for OH., eaq-, N.3. O2- and CO2-, respectively) are reported. The reactivity of melanins with radicals from water radiolysis and their effect on pigment properties are discussed in terms of the structure and possible biological role of the pigments.     

Detection of hydroxyl and superoxide radicals in radiolysis of water with time-resolved chemiluminescence

A new method for the detection of low concentrations of hydroxyl and superoxide radicals, formed by water radiolysis, is described in this article. The method used is the time resolved chemiluminescence. It has been performed with an electron beam delivered by a Febetron 707 accelerator. This method allows to measure hydroxyl and superoxide radical concentrations in a large range of concentrations, between 10(-5) and 10(-8) M.     

Scavenging of OH radicals produced in the sonolysis of water

The yield of hydrogen peroxide in the sonication of argon-saturated water was studied in the presence of various solutes. The efficiency of OH radical scavenging is expressed by the reciprocal value of C 1/2, the solute concentration at which the H2O2 yield is decreased by 50 per cent. C 1/2 ranges over several orders of magnitude. It is not related to the specific reactivity towards OH in homogeneous solution. However, it is correlated to the hydrophobicity of the solutes. The competition of I- and a second solute for OH was also studied. The competition between I- and HCO2- follows similar kinetics as in homogeneous solution. However, many other solutes compete in the manner which would be expected if radical scavenging occurred in different phases. The effects are explained in terms of OH radical formation in gaseous argon bubbles, combination of OH radicals to form H2O2 in an interfacial area, and enrichment of hydrophobic solutes in the bubbles.     

Production of superoxide radicals with pulse radiolysis of water with high linear energy transfer

The radiolysis of water with heavy ions of high linear energy transfer (LET) (-dE/dx) is characterized, in deaerated medium, by the production of superoxide anions, the radiolytic yields of which increase with the LET. Radiobiological interest in such radical species comes from the oxidative stress which may be generated by their dismutation in O2 and H2O2 in anoxic medium (radiotherapy with heavy ions). A brief review of the measurements of superoxide free radicals in aqueous solution by indirect or direct methods is presented. Moreover, some experimental results obtained by pulse radiolysis with Ar18+ ions (TEL = 290 keV x microm(-1)), are described. The interpretation of the kinetics takes into account the superoxide absorbance and that of hydrogen peroxide, which is present at the millisecond time scale.     

The production of free radicals during the autoxidation of monosaccharides by buffer ions

The production of free radicals during the autoxidation of simple monosaccharides at 37 degrees has been studied by the electron spin resonance (e.s.r.) technique of spin trapping. In the presence of the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), monosaccharides undergoing autoxidation produced hydroxyl and 1-hydroxyalkyl radical-derived spin adducts, indicating that hydroxyl and hydroxyalkyl free-radicals are involved in the autoxidation of monosaccharides. The pH profile for the production of free radicals from monosaccharides undergoing autoxidation revealed the formation of both hydroxyl and hydroxyalkyl radicals at relatively high pH, whereas at low pH, only the formation of hydroxyalkyl radicals was observed; the transition between these routes for the production of free radicals occurred at pH 8.0-8.5. Glycolaldehyde, glyceraldehyde, dihydroxyacetone, and erythrose are relatively rapidly enolised (to an ene-diol) and autoxidised with the concomitant production of free radicals. Ribose and glucose enolise and autoxidise very slowly without detectable production of free radicals. A comparison of the pH profiles of the rates of enolisation and the pH dependence of the production of free radicals from glyceraldehyde during autoxidation suggests that a change in reaction mechanism occurs at pH 8.2. Below pH 8.2, the rates of enolisation and autoxidation increase with increasing pH, with a concomitant increase in the formation of hydroxyalkyl spin-adducts. Above pH 8.2, glyceraldehyde undergoing autoxidation shows a much higher rate of enolisation than of autoxidation and, although the formation of hydroxyalkyl radicals is decreased, the production of hydroxyl radicals is also observed. A free-radical mechanism for the autoxidation of monosaccharides is proposed, to account for the pH-dependent characteristics of the production of free radicals and the relationships between the production of free radicals, autoxidation, and enolisation of the monosaccharides.     

Pulse radiolysis study of the interaction of retinoids with peroxyl radicals

Vitamin A (retinol) and its derivatives-retinal and retinoic acid-are known for their ability to inhibit lipid peroxidation. Antioxidant actions of retinoids have been attributed to chain-breaking by scavenging of peroxyl radicals. Based on chemical analysis of retinoic acid degradation products formed during microsomal lipid peroxidation, it was previously suggested that retinoids interact with peroxyl radicals forming free carbon-centered radical adducts. However, it can be argued that such a mode of antioxidant action of retinoids is not sufficient to fully explain their effectiveness at inhibiting lipid peroxidation, which in many systems is comparable to, or even exceeds, that of alpha-tocopherol. In order to elucidate the mechanism of interaction of retinoids with peroxyl radicals, (trichloromethyl)peroxyl radical was generated by pulse radiolysis, and its interactions with retinoids solubilized in Triton X-100 micelles were followed by kinetic absorption spectroscopy. All retinoids--retinol, retinal, and retinoic acid--interacted with the peroxyl radical, and at least two transient products were detected. One of these products, absorbing at 590 nm, was identified as retinoid cation radical. Therefore, we postulate that, apart from formation of radical adducts, retinoids may also scavenge peroxyl radicals by electron transfer.     

Yield of OH radicals in water under high-density energy deposition by heavy-ion irradiation

The purpose of the present study was to evaluate the dependence of the OH radical yield on the atomic number and the energy of the heavy ions to understand chemical reactions of aqueous solutions. The total yields of oxidized products from phenol in water increased superlinearly as the incident energy increased from 5 MeV/nucleon to 18 MeV/nucleon for carbon and neon ions. The radiolytic yields of OH radicals produced by the ions were determined by analyzing the relationships of the oxidation yields of phenol to the incident energies up to 18 MeV/nucleon for ions in the range of LET from 110 eV/nm to 550 eV/nm and from 320 eV/nm to 1100 eV/nm for carbon and neon ions, respectively. The yields of the OH radicals increased with the specific energy for the same kind of ion and decreased with the atomic number for different ions used at the same specific energy.     

The reactions of OH radicals with D-ribose in deoxygenated and oxygenated aqueous solution

Aqueous solution ofD-ribose (10^?2M) saturated with N₂O and N₂O/O₂ (4/1) were γ-irradiated (dose rate: 3.85 x 10¹⁸ eV.g^?1.h^?1) at room temperature. The following products were identified:D-ribonic acid (1). D-erythro-pentos-2-ulose (2). D-erythro-pentos-4-ulose (3),D-erythro-pentos-3-ulose (4), D-ribo-pentodialdose (5), 2-deoxy-D-erythro-pentonic acid (6), 2-deoxypentos-3-ulose (7)(7), 4-deoxylpentos-3-ulose (8), 3-deoxypentos-4-ulose (9), 3-deoxypentos-2-ulose (10), 5-deoxypentos-4-ulose (11), erythrose (12), erythro-tetrodialdose (13), erythronic acid (14), threose/erythrulose (15). threonic acid (16), 2-deoxytetrose (17), and glyceraldehyde (18). In deoxygenated solutions, 13, 14, and 16 were absent. In the presence of oxygen, the formation of 6–11 and 17 was suppressed. From quantitative measurements, G-values were calculated for both deoxygenated and oxygenated conditions. Five different, primary, ribosyl radicals are formed which, in deoxygenated solution, undergo disproportionation reactions (to give 1-5), and transformations such as elimination of water and carbon monoxide followed by disproportionation reactions (to give6-12.17). Material-balance considerations indicate the formation of dimers (not measured). In oxygenated solutions, oxygen rapidly adds to the primary ribosyl radicals, thus preventing the transformation reactions, and the main products are 1–5 and 13. Possible mechanistic routes are discussed. The attack of HO radicals on D-ribose involves C-1, ～20%; C-2 and C-4, ～35%: C-3, ～ 20%; and C-5, ～25%     

The reactivity of various free radicals with hyaluronic acid: steady-state and pulse radiolysis studies

The reactions of the primary water radicals with the biopolymer hyaluronic acid have been studied by pulse radiolysis. Bimolecular rate constants, expressed in terms of the disaccharide repeating sub-unit of hyaluronic acid, for OH., H. and eaq- were found to be 7 X 10(8) M-1 X s-1, 5 X 10(7) M-1 X s-1 and less than 5 X 10(6) M-1 X s-1, respectively. By comparing the viscosities of samples, gamma-irradiated in the steady state under a variety of conditions, with unirradiated controls, the efficiencies with which selected radicals cause chain breakage have been determined. Efficiencies of 30%, 15%, 0%, 0.2% and 5% were estimated for OH., H., eaq-, methanol radicals and tert-butanol radicals, respectively. The presence of oxygen during irradiation increased the extent of chain breakage by a factor of 1.75.     

Interactions of thiyl free radicals with oxygen: a pulse radiolysis study

A pulse radiolysis study of glutathione in aqueous solution at pH 5.5 containing N2O/O2 mixtures at various ratios indicates that oxygen rapidly adds to the thiyl glutathione radical yielding a transient absorption, with a maximum at 540 nm, whose characteristics appear to be compatible with assignment to the GSOO. radical. The reaction (Formula: see text) appears to be an equilibrium whose kinetic constants have been estimated (kf = 2.0 X 10(9) dm3 mol-1, kb = 6.2 X 10(5) s-1). Evidence for electron transfer from ascorbate to the GSOO. radical has been obtained and the respective rate constant has been determined to be 1.75 +/- 0.15 X 10(8) dm3 mol-1 s-1.     

The necessary and the desirable production of radicals in biology

The production of radicals within biological systems can be necessary, as in the coupling of light capture or oxygen-hydride reactions to the generation of proton gradients in thylakoid and mitochondrial membranes; or it can be extremely desirable, to produce an effective catalyst centre in, for example, ribonucleotide reductases. To fit their function the radicals are usually of controlled reactivity, either through their redox properties or through confinement to a limited volume. The more indiscriminate generation of radicals can be of great use too in protective devices, for example, in the production of cross-linked polymers, or in the generation of oxygen radicals. The different distribution and release of initiating enzymes for all these radical processes ensures the spatial and temporal control of their reactions.     

Reactions of linoleic acid peroxyl radicals with phenolic antioxidants: a pulse radiolysis study

Linoleic acid peroxyl radicals (LOO.) can be viewed as model intermediates occurring during lipid peroxidation processes. Formation and reactions of these species were investigated in aqueous alkaline solution using the technique of pulse radiolysis combined with kinetic spectroscopy. Irradiation of linoleic acid in N2O/O2-saturated solutions leads to a mixture of peroxyl radical isomers, whereas reaction of 13-hydroperoxylinoleic acid (13-LOOH) with azide radicals in N2O-saturated solution produces 13-LOO. radicals specifically. These peroxyl radicals cannot be observed directly, but their reactions with the two flavonols, kaempferol and quercetin, acting as radical-scavenging antioxidants, produced strongly absorbing aroxyl radicals (ArO.). The same aroxyl radicals were generated by .OH and N3. with rate constants exceeding 10(9) dm3 mol-1 s-1. Applying a reaction scheme that includes competing generation and decay reactions of both LOO. and ArO. radicals, we derived individual rate constants for LOO. reactions with the phenols (greater than 10(7) dm3 mol-1 s-1), with the aroxyl radicals to form covalent adducts (greater than 10(8) dm3 mol-1 s-1), as well as for their bimilecular decay (3.0 X 10(8) dm3 mol-1 s-1). These results demonstrate the high reactivity of both fatty acid peroxyl radicals and the flavone antioxidants in aqueous solution.     

The oxidation of ferrocytochrome c by Br2-, (SCN)2-, N3 and OH radicals studied by pulsed-electron and gamma-ray radiolysis

The reactions of ferrocytochrome c with Br2-, (SCN)2-, N3 and OH radicals were followed by measuring the change in the optical spectra of cytochrome c on gamma-irradiation as well as the rate of change of absorbance upon pulse irradiation. Ferrocytochrome c is oxidized to ferricytochrome c by Br2-, (SCN)2- or N3 radical with an efficiency of about 100% through a second-order process in which no intermediates were observed. The rate constants in neutral solutions at I = 0.073 are 9.7 . 10(8) M-1 . s-1, 7.9 . 10(8) M-1, 1.3 . 10(9) M-1 . s-1 for the oxidation by Br2-, (SCN)2- and N3 radicals, respectively. The rate constants do not vary appreciably in alkaline solutions (pH 8.9). The ionic strength dependence was observed for the rate constants of the oxidation by br2- and (SCN)2-. Those rate constants estimated on the assumption that the radicals react only with the amino acid residues with the characteristic steric correction factors were less than one-tenth of the observed ones. These results suggest that the partially exposed region of the heme is the probable site of electron transfer from ferrocytochrome c to the radical. Hydroxyl radicals also oxidize ferrocytochrome c with a high rate constant (k greater than 1 . 10(10) M-1 . s-1), but with a very small efficiency (5%).