Radical Chain Oxidation Research Articles

DNA Cleavage by UVA Irradiation of NADH with Dioxygen via Radical Chain Processes

Efficient DNA cleaving-activity is observed by UVA irradiation of an O(2)-saturated aqueous solution of NADH (beta-nicotinamide adenine dinucleotide, reduced form). No DNA cleavage has been observed without NADH under otherwise the same experimental conditions. In the presence of NADH, energy transfer from the triplet excited state of NADH ((3)NADH*) to O(2) occurs to produce singlet oxygen ((1)O(2)) that is detected by the phosphorescence emission at 1270 nm. No quenching of (1)O(2) by NADH was observed as indicated by no change in the intensity of phosphorescence emission of (1)O(2) at 1270 nm in the presence of various concentrations of NADH. In addition to the energy transfer, photoinduced electron transfer from (3)NADH* to O(2) occurs to produce NADH(*+) and O(2)(*-), both of which was observed by ESR. The quantum yield of the photochemical oxidation of NADH with O(2) increases linearly with increasing concentration of NADH but decreases with increasing the light intensity absorbed by NADH. Such unusual dependence of the quantum yield on concentration of NADH and the light intensity absorbed by NADH indicates that the photochemical oxidation of NADH with O(2) proceeds via radical chain processes. The O(2)(*-) produced in the photoinduced electron transfer is in the protonation equilibrium with HO(2)(*), which acts as a chain carrier for the radical chain oxidation of NADH with O(2) to produce NAD(+) and H(2)O(2), leading to the DNA cleavage.

Formation of excited uranyl ion in oxidation of U(IV) with xenon trioxide in aqueous H2SO4 and HClO4 solutions: III. Kinetic mode of self-accelerated reaction

Self-acceleration was revealed for chemiluminescent radical-chain oxidation of U(IV) with xenon trioxide in aqueous HClO4 solutions in a reactor made of Teflon (instead of conventionally used glass). In oxidant excess, U(IV) oxidation with XeO3 follows the first-order kinetics only at low (under 10−6 M) concentrations of U(IV) in solution. Self-acceleration is observed at relatively high concentrations of the reactants, when the contribution of the heterogeneous decay of the radicals to chain termination substantially decreases and that of the degenerate branching reactions to the U(IV) oxidation mechanism increases. Oxidation of U(IV) with XeO3 demonstrates a critical phenomenon, uncommon for liquid-phase radical-chain processes: a minor increase in the concentrations of the reactants is responsible for transition from a steady-state mode to self-acceleration of the redox process.

Quantitative antioxidant activity of the ethylacetate extract of Larix sibirica bark and its individual components

The antioxidant [AO] properties of the ethylacetate extract of Larix sibirica bark and the flavonoids quercetin and dihydroquercetin found in it were studied using a model radical-chain oxidation of propan-2-ol in the kinetic regime. The quantitative characteristics of their AO activity were determined as effective rate constants fkln. It was found that dihydroquercetin had the highest AO activity among the studied natural compounds.

Antioxidant Mechanism Studies on Ferulic Acid: Isolation and Structure Identification of the Main Antioxidation Product from Methyl Ferulate

As a part of our research project on the elucidation of the chain-breaking antioxidant mechanism of natural phenolic compounds in food components, ferulic acid, a phenolic acid widely distributed in edible plants, especially grains, was investigated. An antioxidation reaction of ferulic acid methyl ester produced a main product when ethyl linoleate was used as the oxidation substrate. Isolation and structure determination of the main product revealed that it was a dimer having a dihydrobenzofuran moiety. On the basis of the formation pathway for the product, a radical scavenging reaction was suggested to occur between the 5′-position of one of the ferulate radicals and the 2-position of another of the ferulate radicals to terminate the radical chain oxidation of linoleate.

Organotins‐promoted peroxidation of unsaturated fatty acids: A new antioxidative scavenger for promoters

AbstractThe organotin compounds RnSnX4‐n are promoters of lipids peroxidation. The influence of (CH3)2SnCl2, (C2H5)2SnCl2, and SnCl2 upon the radical chain oxidation of oleic acid as model substrate R′H for lipid peroxidation in the simultaneous presence of porphyrins (free bases of meso‐tetrakis(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)porphyrin (R″4PH2) and of meso‐tetraphenylporphyrin (TPPH2)) has been studied. The monitoring of the unsaturated acid peroxidation level has been performed by the determination of the total concentration of isomeric hydroperoxides as well as of the thiobarbituric acid reactive substances, as markers of carbonyl compounds formation following the hydroperoxides decomposition. The organotin compounds demonstrate prooxidative activity. The promoting effect of these compounds decreases in the presence of TPPH2. The free‐base porphyrin R″4PH2, containing the antioxidative phenolic moieties (2,6‐di‐tert‐butylphenol), demonstrates the acute inhibitory effect upon the acid's peroxidation. The analogous results have been achieved when compared with the influence of CH3HgI and HgCl2 upon the acids peroxidation of oleic acid in the presence of porphyrins. This fact points out that meso‐tetrakis(3,5‐di‐tert‐butyl‐4‐hydroxyphenyl)porphyrin shows the activities of both the antioxidant and of the scavenger for metals and might be used as a new antioxidative scavenger preventing lipids peroxidation. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:475–480, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20269

Kinetics and mechanism of the liquid-phase oxidation of tert-butyl phenylacetate

The oxidation of tert-butyl phenylacetate in ortho-dichlorobenzene at 140°C occurs with short chains. The primary nonperoxide reaction products (tert-butyl α-hydroxyphenylacetate, tert-butyl α-oxophenylacetate, and benzaldehyde) are formed by the decomposition of a hydroperoxide (tert-butyl α-hydroperoxyphenylacetate) and (or) by the recombination of peroxy radicals with and without chain termination. Benzaldehyde and tert-butyl α-hydroxyphenylacetate undergo radical chain oxidation in a reaction medium to result in benzoic acid and tert-butyl α-oxophenylacetate. Homolytic hydroperoxide decomposition is responsible for process autoacceleration and results in benzaldehyde, which is also formed from hydroperoxide by a nonradical mechanism, probably, via a dioxetane intermediate. Both of the reactions are catalyzed by benzoic acid. Benzoic acid has no effect on hydroperoxide conversion into tert-butyl α-oxophenylacetate, which most likely occurs as a result of hydroperoxide decomposition induced by peroxy radicals. The rate constants of the main steps of the process and kinetic parameters have been calculated by solving an inverse kinetic problem.

Microwave‐Assisted Fast Cyclohexane Oxygenation Catalyzed by Iron‐Substituted Polyoxotungstates

AbstractUnder microwave irradiation, iron‐substituted polyoxotungstates (Fe‐POMs) catalyze cyclohexane oxygenation to A/K oil with 90–95% selectivity, unmatched turnover frequencies (40–400 h−1), and multi‐turnover regime (>1000 TON). Such a rapid reaction protocol allowed the screening of so‐called inorganic Fe‐synzymes with 1–4 nuclearity, including a family of Krebs‐type isostructural complexes. Product distribution, kinetic analysis, mechanistic probes and fitting calculations, are consistent with a radical chain oxidation propagated by Fe‐catalysed decomposition of organic peroxides which ultimately depends on the redox‐properties and structural arrangement of the iron moiety within the POM cage.

Ageing of a carbon epoxy composite for aeronautic applications

It is possible to simulate the weight loss and the density increase of an epoxy matrix induced by oxidation with a kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion. In this paper, we checked this kinetic model for UD composite samples and showed that it allows the calculation of the weight loss taking into account two diffusion coefficients of oxygen, the first in the parallel direction to fibres and the second in the transverse direction. Our simulation tool is valid in the validity domain of Arrhenius law and if the crack density remains under a critical value. The carbon fibres do not show a specific effect on the weight loss induced by oxidation at the ageing temperature under consideration.

Activity of the Phenolic Compounds of Plant Extracts in Reactions with Diphenylpicrylhydrazyl

As is known [1], antioxidants are capable of inhibiting radical-chain oxidation of organic substances. Previously [2], we suggested that the antioxidant activity of plant extracts, as manifested in the course of oil oxidation test, is determined by interactions of the extract components with radicals guiding the oxidation chains. If this assumption is valid, the activity of extracts in radical reactions would by correlated with their antioxidant activity. One of the simplest and most convenient model systems for evaluation of the level of antioxidant activity (AOA) of a particular substance is its reaction with the free stable diphenylpicrylhydrazyl (DPPH) radical. DPPH is capable of readily dehydrogenating (oxidating) compounds containing labile hydrogen atoms (such as phenols, amines, thiols, etc.), while remaining stable with respect to molecular oxygen [3]. This study was aimed at determining the activity of medicinal plant extracts in their reactions with DPPH, depending on the type and content of phenols in these extracts. EXPERIMENTAL PART The investigation was performed with the extracts of some medicinal plants containing a considerable amount of phenolic compounds, which are known to be potential natural antioxidants. For the comparative evaluation of AOA, we also employed plant materials with a low content of phenols (elecampane inula roots, wild elder flowers) [4]. All plant extracts were obtained using a procedure described previously [2]. The obtained extracts were characterized by the content of phenolic compounds (recalculated to tannin) and the total content of flavonols (recalculated to quercetin), which were determined by means of permanganatometric [5] and photocolorimetric [6] techniques, respectively. These parameters, which are widely used in medicine and biology, can probably be used for characterization of the antioxidant properties of substances.

Chemiluminescence from oxidation of polyamide 6,6: II. The effect of metal salts

Chemiluminescence (CL) emission from isothermal and non-isothermal oxidation of polyamide 6,6 in the presence of Co, Ni, Cu and Zn chlorides, alone or combined with KI, demonstrates that copper salts, irrespective of the oxidation number, are the most efficient stabilizing species. They reduce CL emission of the third stage of observed kinetic runs, ascribed to the chain oxidation of polyamide, most efficiently. Cobalt chloride has pro-oxidative rather than a stabilizing action. Synergism has been observed for chlorides of Ni, Zn and Co and for KI. The kinetic analysis of chemiluminescence against temperature was based on two different pathways of initiation: 1. A fast process via either free radicals entrapped in the polymer or terminal amino groups. 2. A slower process involving the free radical chain oxidation of the polyamide. The presence of metal salts combined with KI leads to a reduction of the rate constants of the slower stage by more than two orders of magnitude whilst the rate constants of the faster stage are increased by the presence of metal salts.

Oleic acid peroxidation in the presence of metallo-porphyrins

The influence of free bases of meso-tetrakis(3,5-di-tert-butyl-4-hydroxy-phenyl)porphyrins and meso-tetraphenylporphyrins and their complexes of Co , Ni , Cu , Pt upon the radical chain oxidation of oleic acid as model substrates for lipid peroxidation has been studied. It was shown that the introduction of the antioxidative phenolic fragment into the porphyrin ring leads to the dual activity of phenolic porphyrins when compared with their tetraphenyl substituted analogues in substrate oxidation. Free base phenolic porphyrins and their nickel, copper, platinum complexes demonstrate acute inhibitory effect upon oleic acid peroxidation due to the key role of 2,6-di-tert-butylphenol moities, whereas cobalt porphyrin exhibits dual activity associated with the presence of both the redox active metal center and phenolic group.

About a quasi-universal character of unstabilised polyethylene thermal oxidation kinetics

Literature values of the induction time t i and maximum rate r S of oxygen absorption for unstabilised polyethylene samples have been compiled. In Arrhenius plots all of their representative points are close to straight lines of parameters: Ln(t i0)≈−24.4; E i ≈116 kJ mol −1 for t i and Ln(r S0 )≈−34.2; E s ≈146 kJ mol −1 for r s It can be concluded that there is a quasi-universal behaviour, for the thermal oxidation of polyethylene, irrespective to branching and other structural irregularities or crystallinity. This result is consistent with the hypothesis that radical chain oxidation of PE is essentially initiated by the bimolecular decomposition of hydroperoxides provided that their initial concentration is not lower than 10 −5 mol l −1 and not higher than 10 −1 mol l −1.

Oxidation induced shrinkage for thermally aged epoxy networks

The matrix oxidation of composite materials involves a weight loss and a density increase and as a consequence, a shrinkage of the skin layer of the material. To simulate this behaviour, we chose a kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion. This model predicts the concentration profile of oxidation products, the weight loss profile and the shrinkage profile in a thick part. When the stress field induced by shrinkage reaches a critical value (the ultimate stress of the oxidized polymer), a “spontaneous” cracking appears in the skin layer of thick parts. In order to predict these critical conditions, we determined the mechanical properties of the oxidized layer of thick parts by studying the ageing of matrix thin films for which oxidation is not controlled by the oxygen diffusion.

CdS Nanoparticle Photocatalysis of the Chain Oxidation of Sulfite Ions by Molecular Oxygen

High photocatalytic activity was found for colloidal cadmium sulfide nanoparticles in the radical chain oxidation of sulfite ions by molecular oxygen in aqueous solution. The kinetics of this reaction was studied and a mechanism was proposed, which is in satisfactory accord with the experimental data.

Competitive Mechanism of the Nonbranched Radical Chain Oxidation of Hydrogen Involving the Free Cyclohydrotetraoxyl Radical [OO•••H•••OO] • , which Inhibits the Chain Process

A nonbranched radical chain mechanism, which does not pass into non-steady-state critical regimes, was proposed for the uncatalyzed oxidation of hydrogen. This mechanism includes competitive reactions with the participation of H•, HO•, and HO 2 • free radicals and the inactive cyclohydrotetraoxyl radical HO 4 • stabilized by inner hydrogen bonding, wherein the cyclohydrotetraoxyl radical inhibits the chain process. A simple rate equation, which describes the nonmonotonic (with a maximum) dependence of the rate of the nonbranched chain process of hydrogen peroxide and water formation on the concentration of molecular oxygen, was derived using the quasi-steady-state approximation. The nonchain reaction paths of free-radical hydrogen oxidation were considered. It was assumed that the cyclic free radical HO 4 • can be an intermediate in the series of conversions of biologically harmful energy of cosmic radiation in the upper atmosphere.

Increasing the ketone selectivity of the cobalt-catalyzed radical chain oxidation of cyclohexane.

A variety of heterogeneous catalysts for the radical chain oxidation of cyclohexane has been prepared by immobilization of the well-defined cobalt acetate oligomers [py(3)Co(3)(mu(3)-O)(OH)(O(2)CCH(3))(5)](PF(6)) (1) and [py(4)Co(2)(OH)(2)(O(2)CCH(3))(3)](PF(6)) (2) on carboxy-modified mesoporous silica supports A-D by carboxylate exchange. The catalytic oxidation of cyclohexane with tert-butyl hydroperoxide (TBHP) in the presence of these homogeneous and immobilized cobalt acetate complexes afforded the corresponding alcohol and ketone in high yield. The immobilization of 1 and 2 results in a significant increase of catalytic activity. TBHP acts as a radical initiator and as source of molecular oxygen, which is also involved in the overall oxidation process. The rate of cyclohexane conversion is limited by the diffusion of molecular oxygen, and steady-state concentrations of cyclohexanone (K, ketone) and cyclohexanol (A, alcohol) are established; these determine the maximum K:A ratio.

Kinetic modelling and simulation of gravimetric curves: application to the oxidation of bismaleimide and epoxy resins

A simplification of a previously presented “closed-loop” kinetic scheme of radical chain oxidation consisting in taking into account the substrate consumption is applied to the thermal oxidation (in oxygen excess) of a polybismaleimide (BMI) and an amine crosslinked epoxy (ACE). The model fits gravimetric curves in the 0–12% mass loss range well, for both polymers and constitutes a noticeable progress compared to the previously established “steady-state” models. The qualitative difference between BMI (presence of an initial maximum) and ACE (continuous mass decrease), is explained by the fact that ACE hydroperoxides are considerably less stable than BMI ones, which can be mechanistically justified. A generalisation of these results leading to a classification of gravimetric oxidation curves according to their shape, is tentatively proposed.

Study of antioxidant properties by voltammetry

A highly attractive, convenient and especially sensitive voltammetric approach for the study of antioxidant properties and determination of their activity is suggested in this work, where antioxidants are substances, which interrupt radical-chain oxidation processes in organic and inorganic molecules. The comparative analysis of the activity of well-known antioxidants such as ascorbic and citric acids, glucose, their compound solutions, some food products (green tea extract, apple vinegar) and pharmaceuticals (haemodesum, polyglucinum, Ringer solution) has been carried out. The character of the antioxidant influence on the oxygen electrochemical reduction has been investigated. Finally the use of these substances for prophylactic purposes has been recommended.

Thermal oxidation kinetics for a poly(bismaleimide)

AbstractThe thermal oxidation of poly(bismaleimide) of the F655‐2 type, supplied by Hexcel‐Genin, was studied by isothermal gravimetry at 180, 210, and 240°C and various oxygen pressures ranging from 0 to 1.2 bar. Comparison of various sample thicknesses and visible microscopy observations on bulk aged samples shows that the whole oxidized layer has a depth of about 75 μm at 240°C, 138 μm at 210°C, and 229 μm at 180°C. An attempt was made to build a kinetic model to predict this depth. It is based on a differential equation in which O2 diffusion and its consumption rate, r(C), are coupled, C being the O2 concentration. Its resolution needs two sets of experiments: the first one to determine the O2 diffusivity and solubility in the polymer, and the second one to determine r(C). The mathematical form of r(C) is derived from a mechanistic scheme of radical chain oxidation in which initiation is mainly due to POOH decomposition. This expression contains two kinetic parameters, α and β, the values of which are determined from the experimental curves of mass loss rate against O2 pressure (in the stationary state). The theoretical predictions, at each temperature under consideration, are in excellent agreement with experimental results. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3418–3430, 2001

Chain oxidation initiated by OH, O( 3P) radicals, thermal electrons, and O 3 in electron beam irradiation of 1,2-dichloroethylenes and air mixtures

Electron beam irradiation of trans- and cis-1,2-dichloroethylenes ( cis-DCEs)-air mixtures was carried out in order to get quantitative information on chain oxidation. Decomposition of cis-DCE was initiated by OH, O( 3P) radicals, and thermal electrons and proceeded by Cl radical chain oxidation, whereas that of trans-DCE was initiated by not only the radicals and the electrons but also O 3. The chain lengths of trans-DCE by O 3 were estimated to be 6.2±0.2 independently of trans-DCE and O 3 concentrations. The decomposition mechanism was also clarified.