Free Radical Chain Oxidation Research Articles

Regioselective oxidations of equilenin derivatives catalyzed by a rhodium(III) porphyrin complex—contrast with the manganese(III) porphyrin

Equilenin acetate and dihydroequilenin acetate were oxidized with iodosobenzene and a rhodium(III) porphyrin catalyst. The selectivity of the reactions differs from that with the corresponding Mn(III) catalyst, or from that of free radical chain oxidation.

Regioisomeric distribution of cholesteryl linoleate hydroperoxides and hydroxides in plasma from healthy humans provides evidence for free radical-mediated lipid peroxidation in vivo

We have previously reported the detection of cholesteryl ester hydroperoxides, consisting mainly of cholesteryl linoleate hydroperoxides (Ch18:2-OOH), at nm levels in plasma from healthy humans (Y. Yamamoto and E. Niki, 1989. Biochem. Biophys. Res. Commun. 165: 988–993). To elucidate their production mechanism in vivo, we examined the distribution of Ch18:2-O(O)H regioisomers in blood plasma from nine healthy young subjects using a sequential method consisting of methanol/hexane extraction in the presence of antioxidant, reductant, and internal standard, solid phase extraction to remove unoxidized cholesteryl linoleate, purification by reversed-phase high-performance liquid chromatography (HPLC), and detection by normal phase HPLC. Furthermore, we confirm that little artifactual oxidation of cholesteryl linoleate occurred during analytical procedures indicated by the absence of oxidation products of cholesteryl 11Z,14Z-eicosadienoate (Ch20:2) when provided as an exogenous substrate to the experimental procedure. We detected nm levels of all free radical-mediated oxidation products, 13ZE-, 13EE-, 9-EZ-, and 9-EE-forms of Ch18:2-O(O)H, in blood plasma, whereas the 13ZE-isomer resulting from enzymatic 15-lipoxygenase oxidation was not evident as a major product. These results indicate that free radical chain oxidation of lipids occurs even in healthy young individuals. —Mashima, R., K. Onodera, and Y. Yamamoto. Regioisomeric distribution of cholesteryl linoleate hydroperoxides and hydroxides in plasma from healthy humans provides evidence for free radical-mediated lipid peroxidation in vivo.

Rapid NO 2 formation in diluted petrol-fuelled engine exhaust—A source of NO 2 in winter smog episodes

Substantially enhanced rates of oxidation of nitric oxide have been found in petrol engine exhaust diluted with air in the dark. The main product of oxidation is nitrogen dioxide, but up to 20% of the oxidised NO can be in the form of other NO y species. Similarly enhanced NO oxidation rates are also found in the presence of petrol vapour. The kinetics of NO oxidation are close to zero order in NO, the rate being appreciably faster than the known thermal reaction 2NO + O 2 → 2NO 2. Attempts to isolate the chemical species responsible for this rate enhancement from petrol vapour and engine exhaust have led to identification of 1-methyl-1,3-cyclopentadiene and 1,3-cyclohexadiene. A mechanism has been developed in which addition of NO 2 to a conjugated diene causes initiation of a free-radical chain oxidation. A simple numerical model incorporating this process is capable of explaining the observed rate enhancements.

Photo-initiated peroxidation of lipids in micelles by azaaromatics.

The monoazaaromatics, pyridine (1), hexyl nicotinate (2), and quinoline (3) and diazaaromatics, pyrimidine (4) and purine (5), readily act as photo-initiators for the peroxidation of methyl linoleate in 0.50 M SDS at 37 degrees C giving free radical chain oxidations of linoleate. Quantitative kinetic runs on the order in substate, RH, and in the rate of chain initiation, Ri, showed that the classical rate law for autoxidation, -d[O2]/dt = (kp/(2 kt 1/2))[RH] x Ri 1/2, is applicable to these photo-initiated oxidations. The oxidizability of methyl linoleate under these conditions is 2.92 x 10(-2) M-1/2 s-1/2. These peroxidations were inhibited by chromanol phenolic antioxidants of the vitamin E class, such as lipid-soluble 2,2,5,7,8-pentamethyl-6-hydroxychroman (PMHC) and water-soluble 2-carboxy- 2,5,7,8-tetramethyl-6-hydroxychroman (Trolox) and derived rate constants for inhibition of peroxidation were kinh (PMHC) = 4.35 x 10(4) M-1 s-1 and k(inh) (Trolox) = 2.81 x 10(4) M-1 s-1 during inhibited oxidation of methyl linoleate photo-initiated by 4. The products from photo-initiated peroxidation of methyl linoleate by 1 through 5 were determined by reduction and high-performance liquid chromatography analyses to be the 9- and 13-positional hydroperoxides of the four geometrical isomers: cis-9, trans-11 (6), trans-10, cis-12 (7), trans-9, trans-11 (8), and trans-10, trans-12 (9)-octadecadienoates typical of the free radical chain mechanism of lipid peroxidation. Products from dye-sensitized oxidation by Methylene Blue or Rose Bengal of methyl linoleate gave a product distribution of six hydroperoxides typical of oxidation by singlet oxygen. Thermal or photo-initiated peroxidation of methyl linoleate in SDS gave some selectivity of oxidation at the 13-position of the linoleate chain. The ratio of 13- to 9-oxidation varied in the range 1.23 to 1.14 as the cis/trans to trans/trans ratio of geometrical isomers varied from 0.44 to 1.25 during photooxidation of increased amounts of linoleate in SDS. This selectivity is attributed to loss of the pseudo symmetry around the pentadienyl system in the lipid chain in the SDS system during the peroxidation.

Mechanisms for the release of halogens from sea‐salt particles by free radical reactions

A possible source of gas phase bromine in the Arctic winter and early spring is the sea‐salt aerosol. In this paper, chemical mechanisms for the release of photochemically active bromine from sea‐salt are examined. The first of these is oxidation of bromide to elemental bromine by peroxymonosulfuric acid (Caro's acid) produced by the free radical chain oxidation of S(IV). The chain reaction could be initiated in the dark by NO2 or, following polar sunrise, by the reaction of ozone with Superoxide. Although the yield of Caro's acid at 298 K is small, the yield may increase at the low temperatures encountered in the Arctic. This could result in the conversion of a large fraction of the initial sea‐salt alkalinity to Caro's acid. Caro's acid is known to oxidize bromide to Br2. Since this mechanism requires low temperatures and high SO2 concentrations, it is only effective during the winter and early spring and should not oxidize significant amounts of halides in the global marine boundary layer. A second possible mechanism is the free radical oxidation of aqueous bromide to bromine by OH and HO2. This may be effective at moderate pH and may contribute to Br cycling on a global scale. The evaluation of both mechanisms is highly uncertain because of incomplete physical‐chemical data. In addition to these primary release mechanisms, there are possible autocatalytic cycles for the release of bromine from sea‐salt. These involve the gas phase production of either BrO or HOBr and the return of these species to the sea‐salt particles where they initiate additional oxidation of bromide. The efficiency of these cycles should depend critically on the relative amounts of HOBr and HBr produced by the gas phase chemistry. These mechanisms do not appear to be effective as sources of photochemically active Cl.

Kinetics and mechanism for the redox reaction between hexaaquathallium(III) and sulfur dioxide in acidic aqueous solution

Oxidation of dissolved sulfur dioxide, SO2·nH2O, HSO3– and SO32–, by hexaaquathallium(III) has been studied in acidic aqueous solution with ionic strength 1.0 mol dm–3 at 25 °C. The stoichiometry of the reaction has been determined: [Tl(H2O)6]3++ SO2(aq)+ 2H2O →[Tl(H2O)6]++ HSO4–+ 3H+. The kinetics has been studied by use of stopped-flow spectrophotometry under pseudo-first-order conditions with either SIV or TlIII in excess. The reaction is first order in both [SIV] and [TlIII] and has a complex [H+] dependence. The rate decreases with increasing [H+] in the range 0.1 < [H+] < 1.0 mol dm–3. The results are interpreted in terms of a reaction mechanism where sulfito complexes are formed by reaction between [Tl(H2O)6]3+ or [Tl(H2O)5(OH)]2+ and HSO3–. These complexes decompose to the products [Tl(H2O)6]+ and HSO4– by intramolecular inner-sphere electron-transfer processes. The thallium(III) sulfito complexes are weak, as indicated by the absence of any transient absorbance peaks in the UV region. Potentiometric measurements of the oxygen concentration and spectrophotometric measurements showed that TlIII does not initiate a free-radical chain oxidation of SIV by dissolved oxygen.

Stannum (IV) chloride stimulates vanadium (IV) catalyzed oxidation of NADH.

Stannum has been found to be an essential ultratrace element, however, the biological functions of stannum remain to be clarified. We found that stannum (IV) chloride stimulates vanadium (IV) catalyzed free radical chain oxidation of NADH. Stannum (IV) chloride, per se, did not catalyze the NADH oxidation. Superoxide, H2O2, and OH. are known to be the key species in vanadium catalyzed NADH oxidation. The inhibition of the vanadium (IV) catalyzed NADH oxidation in the presence of stannum (IV) chloride by catalase, superoxide dismutase (SOD), and hydroxyl radical scavengers indicated that the stannum (IV) chloride stimulated NADH oxidation consisted of almost the same reaction steps as that in the absence of stannum (IV) chloride. The results of inhibition studies on the NADH oxidation with SOD and catalase suggested that the reaction mixture containing stannum (IV) chloride contained a greater amount of H2O2 and a lower amount of O2- than that containing only vanadium (IV). Hydrogen peroxide is the precursor of OH. in the free radical chain reaction. The stimulation of NADH oxidation by stannum (IV) chloride is due to the stimulation of H atom abstraction by OH.. Stannum (IV) chloride might stimulate the generation of OH. by producing H2O2.

Superoxide generated by glutathione reductase initiates a vanadate-dependent free radical chain oxidation of NADH

Vanadate V (V) markedly stimulated the oxidation of NADPH by GSSG reductase and this oxidation was accompanied by the consumption of O 2 and the accumulation of H 2O 2. Superoxide dismutases completely eliminated this effect of V (V), whereas catalase was without effect, as was exogenous H 2O 2 added to 0.1 m m. These effects could be seen equally well in phosphate- or in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid-buffered solutions. Under anaerobic conditions there was no V (V)-stimulated oxidation of NADPH. Approximately 4% of the electrons flowing from NADPH to O 2, through GSSG reductase, resulted in release of O 2 −. The average length of the free radical chains causing the oxidation of NADPH, initiated by O 2 − plus V (V), was calculated to be in the range 140–200 NADPH oxidized per O 2 − introduced. We conclude that GSSG reductase, and by extension other O 2 −-producing flavoprotein dehydrogenases such as lipoyl dehydrogenase and ferredoxin reductase, catalyze V (V)-stimulated oxidation of NAD(P)H because they release O 2 − and because O 2 − plus V (V) initiate a free radical chain oxidation of NAD(P)H. There is no reason to suppose that these enzymes can act as NAD(P)H:V (V) oxidoreductases.

Spectroscopy and decay kinetics of the sulfite radical anion in aqueous solution

The sulfite radical anion, SO–3, has been proposed as a chain carrier in the aerobic free-radical chain oxidation of sulfur dioxide in aqueous solution. The absorption spectrum of SO–3 has been obtained following the laser photolysis of argon-saturated sodium dithionate solutions at 193 nm. Using kinetic spectrophotometric and ionchromatographic techniques, it has been shown that the laser photolysis of aqueous dithionate solutions generates both the sulfite anion, SO2–3, and the sulfite radical anion, SO–3. the decay of SO–3 in aqueous solution is second-order with an overall rate coefficient of (4.0 ± 0.3)× 108 dm3 mol–1 s–1 at 293 K in the zero ionic strength limit. The reaction proceeds by two channels; recombination to produce dithionate and electron or O– transfer to produce SIV and SVI. SO–3+ SO–3 [graphic omitted] S2O2–6, SO–3+ SO–3 [graphic omitted] SO2–3+ SO3.Product analysis gives a value of 0.8 ± 0.2 for the ratio ka/kb.

Effects of vanadate on the oxidation of NADH by xanthine oxidase

Vanadate (V (v)) stimulates the oxidation of NADH by xanthine oxidase and superoxide dismutase eliminates the effect of V (v). Paraquat stimulates both the oxidation of NADH by xanthine oxidase and the V (v) enhancement of that oxidation. Xanthine, which is a better substrate for xanthine oxidase than is NADH, causes a V (v)-dependent cooxidation of NADH which is transient and eliminated by SOD. Urate inhibits the V (v)-stimulated oxidation of NADH by xanthine oxidase or by Rose Bengal plus light. Measurement of rates of both O 2 − production and V (v)-stimulated NADH oxidation showed that many molecules of NADH were oxidized per O 2 −. These chain lengths were an inverse function of overall reaction rate. Minimum chain lengths, calculated on the basis of 100% univalent reduction of O 2 to O 2 −, were smaller than measured average chain lengths by a factor of five. All of these results are in accord with the view that V (v) does not directly affect the activity of the enzyme, but rather catalyzes the free radical chain oxidation of NADH by O 2 −. It was further shown that phosphate was not involved and that the active form of V (v) was orthovanadate, rather than decavanadate.

Tocopherol behavior and membrane constituents in erythrocytes with oxidant stress.

Recently, toxicity caused by oxygen and its active species has received much attention in connection with a variety of pathological events.1 However, the mechanism responsible for the peroxidation of biomembranes is still unclear. Tocopherols (vitamin E) are located in the membranes and act there as an only chain-breaking antioxidant.2,3 When biomembranes are damaged mainly by a free radical chain oxidation,4 the effects of tocopherols will be spotlighted. Nevertheless, the mechanisms by which membrane tocopherols inhibit the peroxidation still require detailed study, partly because the active species and the sites of their generation have not usually been known or controlled.

Interaction of α-tocopherol with iron: antioxidant and prooxidant effects of α-tocopherol in the oxidation of lipids in aqueous dispersions in the presence of iron

The dual functions of α-tocopherol in the oxidation of lipids in aqueous dispersions in the presence of iron were studied, aiming specifically at elucidating the effect of interaction between α-tocopherol and iron. Ferrous ion decomposed hydroperoxide rapidly and induced the free radical chain oxidation of soybean phosphatidylcholine liposomes. α-Tocopherol acted primarily as a radical scavenger in the oxidation induced by ferrous ion and acted as an antioxidant. Ferric ion decomposed hydroperoxide much more slowly than ferrous ion, but it also induced the oxidation of liposomal membranes. α-Tocopherol incorporated into artificial liposomal membranes reduced ferric ion rapidly to give more reactive ferrous ion, and α-tocopherol acted either as an antioxidant or as a prooxidant depending on the experimental conditions. When α-tocopherol was depleted by the interaction with ferric ion, it acted solely as a prooxidant, whereas if some α-tocopherol remained, it acted as an antioxidant. On the other hand, α-tocopherol residing in the intact erythrocyte membranes did not reduce ferric ion in the aqueous region.

A study on the mechanism of the vanadate-dependent NADH oxidation

The mechanism of the vanadate (V( v))-dependent oxidation of NADH was different in phosphate buffers and in phosphate-free media. In phosphate-free media (aqueous medium or HEPES buffer) the vanadyl (V( v)) generated by the direct V( v)-dependent oxidation of NADH formed a complex with V( v). In phosphate buffers V( v) autoxidized instead of forming a complex with V( v). The generated superoxide radical (O 2 −) initiated, in turn, a high-rate free radical chain oxidation of NADH. Phosphate did not stimulate the V( v)-dependent NADH oxidation catalyzed by O 2 −-generating systems. Monovanadate proved to be a stronger catalyzer of NADH oxidation as compared to polyvanadate.

Corrosion in smoke, hydrocarbon and SO 2 polluted atmospheres—I. General behaviour of iron

Iron foil specimens have been exposed for up to 145 h in a modified industrial atmosphere test chamber, to a control air-based environment at 298 K with a relative humidity of 85% and a CO 2 concentration of 5500 ppm. Additions of SO 2 (3.5 ppm), ethane, ethylene or acetylene (8 ppm) and smoke produced by burning kerosene (up to 560 μg m −3 average concentration) have been made singly and in appropriate combinations, so the separate and combined effects could be examined subsequently by weight change, metallography, SEM/EDX etc. The smoke aerosol was characterized by size and shape using TEM, and its influence on the conductivity and pH of water assessed by appropriate methods. The many results are analysed in terms of considerations such as sorption, catalytic activity at active complexes on smoke platelet surfaces and the interaction with moisture and free radical chain oxidation mechanisms, in an attempt to bridge the gap between simple observation and the clearly subtle chemical reactions which can control such atmospheric corrosion.

Free-radical chain oxidation of rat red blood cells by molecular oxygen and its inhibition by α-tocopherol

The oxidation of rat red blood cells (RBC) by molecular oxygen was performed in an aqueous suspension with an azo compound as a free-radical initiator. The RBC were oxidized at a constant rate by a free-radical chain mechanism, resulting in hemolysis. The extent of hemolysis was proportional to the concentration of free radical. α-Tocopherol in RBC membranes suppressed the oxidation and hemolysis to produce an induction period. Tocopherol was constantly consumed during the induction period, and hemolysis developed when tocopherol concentrations fell below a critically low level. Among the membrane lipids, phosphatidylethanolamine, phosphatidylserine, and arachidonic acids were predominantly oxidized in the absence of tocopherol. In the presence of tocopherol, however, such lipid changes were suppressed during a 120-min incubation even when hemolysis started. Membrane proteins as well as lipids were oxidized. The formation of proteins with high molecular weight and concomitant decrease of the lowmolecular-weight proteins were observed on gel electrophoresis with the onset of hemolysis. This study clearly showed the damage of RBC membranes caused by oxygen radical attack from outside of the membranes, and suggested that membrane tocopherol even below a critically low level could suppress lipid oxidation but that it could not prevent protein oxidation and hemolysis.

Free-radical chain oxidation of 2-nitropropane initiated and propagated by superoxide.

The superoxide radical O2.-, whether produced by the xanthine/xanthine oxidase reaction or infused as KO2, solubilized by a crown ether in dry dimethyl sulphoxide, initiated a free-radical chain oxidation of anionic 2-nitropropane. Superoxide dismutase, but not catalase, inhibited oxidation of the nitroalkane. Xanthine oxidase suffered a syncatalytic inactivation, during the co-oxidation of 2-nitropropane, which was reversed by dialysis. Cyanide exacerbated this syncatalytic inactivation and rendered it irreversible. The frequently observed oxidations of nitroalkanes by flavoenzymes now need to be re-examined to clarify the extent to which O2.--initiated free-radical chain oxidation contributed to the overall nitroalkane oxidation.

Free radical chain oxidation and hemolysis of erythrocytes by molecular oxygen and their inhibition by vitamin E.

Erythrocytes of vitamin E-deficient rats and normal rats were oxidized at 37 degrees C by molecular oxygen using a free radical initiator. The erythrocytes were oxidized by a free radical chain mechanism with kinetic chain length considerably larger than 1 and resulted in hemolysis. Vitamin E suppressed both oxidation and hemolysis, but the extent of hemolysis was determined primarily by the extent of oxidation independent of the presence or absence of vitamin E.

Oxidation of biological membranes and its inhibition. Free radical chain oxidation of erythrocyte ghost membranes by oxygen

The oxidation of human and rat erythrocyte ghost membranes by molecular oxygen has been performed in an aqueous suspension at 37°C. A constant rate of oxygen uptake was observed in the presence of radical initiator. α-Tocopherol in the membrane suppressed the oxidation and the induction period was clearly observed. α-Tocopherol decreased linearly during the induction period and when it was depleted the induction period was over and a rapid oxidation started. The rate of oxidation was proportional to the square root of the rate of initial radical generation. The kinetic chain length, the ratio of the rate of propagation to that of initiation, was long, ranging from 7 to 100. These results indicate that the erythrocyte ghost membranes are oxidized by a free radical chain mechanism by molecular oxygen. Among the fatty acids of membrane lipids, polyunsaturated fatty acids were oxidized exclusively. Proteins as well as polyunsaturated fatty acids were oxidized and the formation of the high- and low-molecular-weight proteins and the decrease of protein bands were observed on gel electrophoresis.

ChemInform Abstract: THE OXIDATION OF CFCLCFCL AND CF2CCL2

The oxidation of CFClCFCl and CF2CCl2 were studied at room temperature by chlorine- and oxygen-atom initiation. The chlorine-atom initiated oxidation of CFClCFCl yields CCl2FCF(O) as the exclusive product. Its quantum yield is ∼420, which gives k3a/k3b=210 where reactions (3a) and (3b) are The O(3P)CFClCFCl reaction gives CClFO with a quantum yield of 0.80, polymer, and small amounts of an unidentified product which is probably cyclo-(CFCl)3. Thereaction paths are with k9a/k9=0.80. The overall reaction of O(3P) with CFClCFCl proceed one fifth as fast as the O(3P)-C2F4 reaction. When O2 is also present, the same free-radical chain oxidation occurs by O(3P)initiation as by chlorine-atom initiation. The chlorine-atom initiated oxidation of CF2CCl2 gives CF2ClCCl(O) as the major product, with quantum yields ranging from 42 to 85. Smaller amounts of CF2O and CCl2O are produced in equal amounts with quantum yields of ∼3.5. The reactions responsible for the products are The O(3P)-CF2CCl2interaction yields CF2O and with quantum yields of 1.0 and ∼0.85, respectively. In thepresence of O2 the radical chain products are observed, but the mechanism is different than that for other chloroolefins.

The reaction of O(3P) with C2HCl3

AbstractThe reaction of O(3P), prepared from the Hg photosensitization of N2O, with C2HCl3 was studied at 25°C. The products of the reaction in the absence of O2 were CO, CHCl3, and polymer (as well as N2 from the N2O). The quantum yields of CO and CHCl3 were 0.23 ± 0.01 and 0.14 ± 0.05, is respectively independent of reaction conditions. The reaction mechanism is equation image with k14a/k14 = 0.23, where k14a + k14b. Most of the HCl and CCl2 combine to form CHCl3, but some other products must also be formed to account for the difference in the CO and CHCl3 quantum yields. The C2HCl3O* adduct polymerizes without involving additional C2HCl3 molecules, since the quantum yield of C2HCl3 disappearance, Φ{C2HCl3}, was about 1.0 at high values of [N2O]/[C2HCl3]. The rate coefficient for the reaction of O(3P) with C2HCl3 is 0.10 that for the reaction of O(3P) with C2F4.In the presence of O2 the free radical chain oxidation occurs because of the reaction equation image The main product is CHCl2CCl(O) with smaller amounts of CO and CCl2O, and some CO2. The chain lengths were long and values of  Φ {C2HCl3} up to 90 were observed.