Hydroxyl anion radical

Hydroxyl The term used to describe the anionic radical (OH-) which is responsible for the alkalinity of a solution. 

Indeed, when present in concentrations sufficient to overwhelm normal antioxidant defences, ROS may be the principal mediators of lung injury (Said and Foda, 1989). These species, arising from the sequential one-electron reductions of oxygen, include the superoxide anion radical, hydrogen peroxide, hypochlorous ions and the hydroxyl radical. The latter species is thought to be formed either from superoxide in the ptesence of iron ions (Haber-Weiss reaction Junod, 1986) or from hydrogen peroxide, also catalysed by ferric ions (Fenton catalysis Kennedy et al., 1989). 

In the presence of trace amounts of iron, superoxide can then reduce Fe3+ to molecular oxygen and Fe2+. The sum of this reaction (equation 2) plus the Fenton reaction (equation 1) produces molecular oxygen plus hydroxyl radical, plus hydroxyl anion from superoxide and hydrogen peroxide, in the presence of catalytic amounts of iron - the so-called Haber-Weiss reaction (equation 3) (Haber and Weiss, 1934). 

Nitrosoarenes are readily formed by the oxidation of primary N-hydroxy arylamines and several mechanisms appear to be involved. These include 1) the metal-catalyzed oxidation/reduction to nitrosoarenes, azoxyarenes and arylamines (144) 2) the 02-dependent, metal-catalyzed oxidation to nitrosoarenes (145) 3) the 02-dependent, hemoglobin-mediated co-oxidation to nitrosoarenes and methe-moglobin (146) and 4) the 0 2-dependent conversion of N-hydroxy arylamines to nitrosoarenes, nitrosophenols and nitroarenes (147,148). Each of these processes can involve intermediate nitroxide radicals, superoxide anion radicals, hydrogen peroxide and hydroxyl radicals, all of which have been observed in model systems (149,151). Although these radicals are electrophilic and have been suggested to result in DNA damage (151,152), a causal relationship has not yet been established. Nitrosoarenes, on the other hand, are readily formed in in vitro metabolic incubations (2,153) and have been shown to react covalently with lipids (154), proteins (28,155) and GSH (17,156-159). Nitrosoarenes are also readily reduced to N-hydroxy arylamines by ascorbic acid (17,160) and by reduced pyridine nucleotides (9,161). 

For the lability of alkoxy-type radicals, see Ando, W. (ed.). (1992). Organic Peroxides. Wiley, New York. In the same way, olefin epoxidation with peracids can be simply viewed as an electron transfer, followed by mesolytic cleavage of the peracid anion radical to carboxylate and hydroxyl radical, followed by homolytic coupling and proton loss. See also Nugent, W.A., Bertini, F. and Kochi, J.K. (1974). J. Am. Chem. Soc. 96,4945... 

Noda Y, Anzai K, Mori A, Kohno M, Shinmei M and Packer L. 1997. Hydroxyl, end superoxide anion radical scavenging activities of natural source antioxidants using the computerized JES-FR30 ESR spectrometer system. Biochem Mol Biol Int 42(1) 35—44. 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

The superoxide anion radical and hydrogen peroxide are not particularly harmful to cells. It is the product of hydrogen peroxide decomposition, the hydroxyl radical (HO ), that is responsible for most of the cytotoxicity of oxygen radicals. The reaction can he catalyzed hy several transition metals, including copper, manganese, cohalt, and iron, of which iron is the most ahimdant in the human body (Reaction 2 also called the Fenton reaction). To avoid iron-catalyzed reactions, iron is transported and stored chiefly as Fe(III), although redox active iron can be formed in oxidative reactions, and Fe(III) can be reduced by semiquinone radicals (Reaction 3). 

Carbon dioxide itself can accept e. during radiolysis of water, giving rise to the carbon dioxide anion-radical. This anion-radical can add to carboradicals. Thus, aliphatic alcohols react with the radiolytically generated hydroxyl radicals, rupturing their C—H bonds RCHjOH + OH HjO + RCH OH. These radicals accept the radiolytically generated COj" forming a-hydroxycarboxylic acids RCHjOH + CO2 RCH(OH)COO (Morkovnik and Okhlobystin 1979). 

There are several cases of hydroxylation according to the hidden-radical mechanism, within a solvent cage. As assumed (Fomin and Skuratova 1978), hydroxylation of the anthraquinone sulfonic acids (AQ—SO3H) proceeds by such a reticent pathway, and OH radicals attack the substrate anion-radicals in the solvent cage. Anthraquinone hydroxyl derivatives are the final products of the reaction. In the specific case of DMSO as a solvenf, hydroxyl radicals give complexes with the solvent and lose their ability to react with the anthraquinone sulfonic acid anion-radicals (Bil kis and Shein 1975). The reaction stops after an anion-radical is formed ... 

As for 2a-la dithia cation-radicals, they do not react directly with Oj. Aliphatic thioether cation-radicals as shown in Scheme 3.24 become capable of reacting with O2 only after the addition of hydroxyl anion (Schoeneich et al. 1993). 

Kinetic calculations accord well with the experimental data only if o-DNB anion-radical is considered as the particle undergoing substitution of hydroxyl group for the nitro group (Abe and Ikegame 1978). 

As described, other nucleophilic reactions in the anthraquinone series also involve the production of anion-radicals. These reactions are as follows Hydroxylation of 9,10-anthraquinone-2-sulfonic acid (Fomin and Gurdzhiyan 1978) hydroxylation, alkoxylation, and cyanation in the homoaromatic ring of 9,10-anthraquinone condensed with 2,1,5-oxadiazole ring at positions 1 and 2 (Gorelik and Puchkova 1969). These studies suggest that one-electron reduction of quinone proceeds in parallel to the main nucleophilic reaction. The concentration of anthraquinone-2-sulfonate anion-radicals, for example, becomes independent of the duration time of the reaction with an alkali hydroxide, and the total yield of the anion-radicals does not exceed 10%. Inhibitors (oxygen, potassium ferricyanide) prevent formation of anion-radicals, and the yield of 2-hydroxyanthraquinone even increases somewhat. In this case, the anion-radical pathway is not the main one. The same conclusion is made in the case of oxadiazoloanthraquinone. 

To prevent the dimerization and by-product formations, Rautenstrauch et al. (1981) proposed to protonate the ketone anion-radicals just at the moment of their formation. These anion-radicals contain the negatively charged oxygen atom. They protonate faster and then undergo dimerization. The resulting hydroxyl-containing carboradicals accept electrons faster and then undergo disproportionation or recombination. This leads to the suppression of ketoenol and pinacol formation. 

Methylated aromatic heterocycles (HetCHj) form cation-radicals that are typical n acids and expel a proton. Methylene radicals are formed. These radicals give rise to the corresponding carbocations if an oxidant was taken in excess. Nucleophiles attack the ions, completing the reaction. If water is the reaction medium (the hydroxyl anion is a nucleophile), an alcohol is formed. The alcohol rapidly transforms into an aldehyde on the action of the same oxidant. 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Allopurinol also inhibits reperfusion injury. This injury occurs when organs that either have been transplanted or have had their usual blood perfusion blocked are reperfused with blood or an appropriate buffer solution. The cause of this injury is local formation of free radicals, such as the superoxide anion, the hydroxyl free radical, or peroxynitrite. These substances are strong oxidants and are quite damaging to tissues. 

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