Dioxygen activation free radicals

The ketyl radicals (R1R2C OH) have a weak O—H bond and are very active reducing agents. They react as donors of hydrogen atoms with carbonyl compounds, dioxygen, and free radicals. 

The combustion of hydrocarbons and other organic materials is the best known oxidation process. The burning of fuels for various purposes is the basis of man s industrial activities. Free radical reactions with dioxygen are responsible for the generation of large amounts of heat and the appearance of flame. Selectivity is nonexistent in these spontaneous, high temperature processes. 

Dioxygen Activation by Transition Metal Complexes. Atom Transfer and Free Radical Chemistry in Aqueous Media... 

The peroxide theory of Bach [20] and Engler [23] fixed the phenomenon of peroxide formation as the primary product of hydrocarbon oxidation by dioxygen. However, the problem of the mechanism of peroxide formation remained unsolved. The new stage of successful study of organic compound oxidation began after the discovery of free radicals as active intermediates of many chemical processes. 

The initiating action of ozone on hydrocarbon oxidation was demonstrated in the case of oxidation of paraffin wax [110] and isodecane [111]. The results of these experiments were described in a monograph [109]. The detailed kinetic study of cyclohexane and cumene oxidation by a mixture of dioxygen and ozone was performed by Komissarov [112]. Ozone is known to be a very active oxidizing agent [113 116]. Ozone reacts with C—H bonds of hydrocarbons and other organic compounds with free radical formation, which was proved by different experimental methods. 

Free radicals were found to be generated on the catalyst surface in hydrocarbon oxidation in the absence of hydroperoxide. The activation of absorbed dioxygen was supposed to be the source of radicals [255], The catalytic action of the silver surface on cumene oxidation was supposed to be the result of activation of sorbed dioxygen [265]. 

As a result, the difference in activity of free radical acceptor and dioxygen is not so great in polymer as in the liquid. 

On the other hand, microsomes may also directly oxidize or reduce various substrates. As already mentioned, microsomal oxidation of carbon tetrachloride results in the formation of trichloromethyl free radical and the initiation of lipid peroxidation. The effect of carbon tetrachloride on microsomes has been widely studied in connection with its cytotoxic activity in humans and animals. It has been shown that CCI4 is reduced by cytochrome P-450. For example, by the use of spin-trapping technique, Albani et al. [38] demonstrated the formation of the CCI3 radical in rat liver microsomal fractions and in vivo in rats. McCay et al. [39] found that carbon tetrachloride metabolism to CC13 by rat liver accompanied by the formation of lipid dienyl and lipid peroxydienyl radicals. The incubation of carbon tetrachloride with liver cells resulted in the formation of the C02 free radical (identified as the PBN-CO2 radical spin adduct) in addition to trichoromethyl radical [40]. It was found that glutathione rather than dioxygen is needed for the formation of this additional free radical. The formation of trichloromethyl radical caused the inactivation of hepatic microsomal calcium pump [41]. 

DIOXYGEN ACTIVATION BY TRANSITION METAL COMPLEXES. ATOM TRANSFER AND FREE RADICAL CHEMISTRY IN AQUEOUS MEDIA... 

The present volume is a non-thematic issue and includes seven contributions. The first chapter byAndreja Bakac presents a detailed account of the activation of dioxygen by transition metal complexes and the important role of atom transfer and free radical chemistry in aqueous solution. The second contribution comes from Jose Olabe, an expert in the field of pentacyanoferrate complexes, in which he describes the redox reactivity of coordinated ligands in such complexes. The third chapter deals with the activation of carbon dioxide and carbonato complexes as models for carbonic anhydrase, and comes from Anadi Dash and collaborators. This is followed by a contribution from Sasha Ryabov on the transition metal chemistry of glucose oxidase, horseradish peroxidase and related enzymes. In chapter five Alexandra Masarwa and Dan Meyerstein present a detailed report on the properties of transition metal complexes containing metal-carbon bonds in aqueous solution. Ivana Ivanovic and Katarina Andjelkovic describe the importance of hepta-coordination in complexes of 3d transition metals in the subsequent contribution. The final chapter by Sally Brooker and co-workers is devoted to the application of lanthanide complexes as luminescent biolabels, an exciting new area of development. 

Dinuclear iron centres occur in several proteins. They either bind or activate dioxygen or they are hydrolases. Ribonucleotide reductase (RR) of the so-called class I type contains one such centre in the R2 protein in combination with a tyrosyl radical, both being essential for enzymatic activity which takes place in the R1 protein subunit. The diiron centre activates dioxygen to generate the tyrosyl radicals which in turn initiate the catalytic reaction in the R1 subunit. The interplay between the tyrosyl free radical in R2 and the formation of deoxyribonucleotides in R1 which also is proposed to involve a protein backbone radical is a topic of lively interest at present but is outside the scope of this review. Only a few recent references dealing with this aspect are mentioned without any further discussion.158 159 1 1,161... 

Lehmann, W.D. 1994. Regio- and stereochemistry of the dioxygenation reaction catalyzed by (S)-type lipoxygenases or by the cyclooxygenase activity of prostaglandin H synthases. Free Radical Biol. Med. 16 241-253. 

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