Free radical reactions during oxidation

In spite of a high reactivity of 3-carotene in free radical reactions and marked antioxidant effects in in vitro systems, 3-carotene did not show itself as an effective in vivo antioxidant. Furthermore, recent clinical trials suggested that the administration of 3-carotene may be useless or even harmful to patients with heart and some other diseases, especially to smokers. One might suspect that one of the major reasons of toxic in vivo effects of 3-carotene might be the formation of prooxidative compounds during 3-carotene oxidation. In contrast to... 

However, the existence of an extremely reactive bound hydroxyl radical is questionable because it is difficult to understand why it does not immediately react with adjacent molecules (most of the reactions of hydroxyl radicals proceed with the rates close to a diffusion limit). Therefore, the mechanism proposed by Zhang et al. [7,8] seems to be much more convincing. They suggested that the genuine oxidizing free radical formed during SOD inactivation is the bicarbonate radical anion CO/, which is formed as a result of the oxidation of bicarbonate. It has also been suggested that DMPO OH is formed by the addition of water to an intermediate of the reaction of DMPO with CO/ via a nucleophilic or electron transfer mechanism. 

Two additional systems were exploited in order to confirm the involvement of free-radical processes during vindoline oxidations. These were the enzyme peroxidase and photochemistry. Horseradish peroxidase (HRP) oxidized both vindoline and 16-O-acetylvindoline in the presence of hydrogen peroxide. Vindoline was converted to the enamine dimer 59 (78). During the reaction, the following sequence of redox reactions occurs ... 

The changes of free radical concentrations during the oxidation reaction are assumed to be the steady-state approximation ... 

Tetrahydrofuranes. Cekovic, BoSnjak, and Mihailovic have reviewed their work on the oxidative cyclization of aliphatic alcohols to tetrahydrofuranes by oxidation with lead tetraacetate. The reagent should be free of acetic acid. Anhydrous calcium carbonate can be added to neutralize the acetic acid in the oxidant and that formed during the reaction. The reaction involves selective oxygenation of a 8-carbon the actual mechanism is uncertain, but is probably a radical reaction. Direct oxidation of the alcohol is generally of minor importance, but P-fragmentation can be a problem, particularly when a stable benzyl or allyl radical is formed. 

The antioxidants studied can be classified into two broad types phenolic antioxidants and non-phenolic antioxidants. Phenolic antioxidants have been found to be more promising as they are obtained from dietary sources.Vitamin E (a-tocopherol), the first known chainbreaking antioxidant, is also an o-methoxy phenol. Pulse radiolysis studies of vitamin E and its water-soluble analogue, trolox C, have been reported several years ago. a-tocopherol reacts with almost all the oxidizing free radicals, and the phenoxyl radicals produced during oxidation reactions absorb at -460 nm (Fig. 1). The regeneration reaction of a-tocopherol phenoxyl radicals back to a-tocopherol by water-soluble antioxidant ascorbic acid was also first reported by pulse radiolysis method. The one-electron reduction potential of vitamin E is -0.48 V vs. NHE. Both a-tocopherol and trolox C are used as standards for evaluating the antioxidant ability of new compounds. 

This reaction prevents the recombination of H and OH radicals to form water molecules and hence, increases the rate of other oxidation processes. In general, free radical reactions similar to those shown for water could occur with any organic or inorganic species capable of being present as a vapor during bubble collapse. 

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