Peroxyl Radical Rearrangement

As mentioned earlier, when NO concentration exceeds that of superoxide, nitric oxide mostly exhibits an inhibitory effect on lipid peroxidation, reacting with lipid peroxyl radicals. These reactions are now well studied [42-44]. The simplest suggestion could be the participation of NO in termination reaction with peroxyl radicals. However, it was found that NO reacts with at least two radicals during inhibition of lipid peroxidation [50]. On these grounds it was proposed that LOONO, a product of the NO recombination with peroxyl radical LOO is rapidly decomposed to LO and N02 and the second NO reacts with LO to form nitroso ester of fatty acid (Reaction (7), Figure 25.1). Alkoxyl radical LO may be transformed into a nitro epoxy compound after rearrangement (Reaction (8)). In addition, LOONO may be hydrolyzed to form fatty acid hydroperoxide (Reaction (6)). Various nitrated lipids can also be formed in the reactions of peroxynitrite and other NO metabolites. 

Thus, LOX-catalyzed oxidative processes are apparently effective producers of superoxide in cell-free and cellular systems. (It has also been found that the arachidonate oxidation by soybean LOX induced a high level of lucigenin-amplified CL, which was completely inhibited by SOD LG Korkina and TB Suslova, unpublished data.) It is obvious that superoxide formation by LOX systems cannot be described by the traditional mechanism (Reactions (1)-(7)). There are various possibilities of superoxide formation during the oxidation of unsaturated compounds one of them is the decomposition of hydroperoxides to alkoxyl radicals. These radicals are able to rearrange into hydroxylalkyl radicals, which form unstable peroxyl radicals, capable of producing superoxide in the reaction with dioxygen. 

Oxidation to CO of biodiesel results in the formation of hydroperoxides. The formation of a hydroperoxide follows a well-known peroxidation chain mechanism. Oxidative lipid modifications occur through lipid peroxidation mechanisms in which free radicals and reactive oxygen species abstract a methylene hydrogen atom from polyunsaturated fatty acids, producing a carbon-centered lipid radical. Spontaneous rearrangement of the 1,4-pentadiene yields a conjugated diene, which reacts with molecular oxygen to form a lipid peroxyl radical. 

There are various possibilities of superoxide formation during the oxidation of unsaturated compounds one of them is the decomposition of hydroperoxides to alkoxyl radicals. These radicals are able to rearrange into hydroxylalkyl radicals, which form unstable peroxyl radicals, capable of producing superoxide in the reaction with dioxygen. 

Alternatively a direct rearrangement of the isobutyl peroxyl radical may account for the products ... 

Elimination and rearrangement reactions of the primary radicals (see Sect. 11,1) that are slower than 2 x 10s s-1 are, therefore, suppressed at ordinary concentrations of oxygen (air-saturated [02] 2 x 10-4 M). Similarly, radical-radical reactions (see Sect. 11,2) cannot compete effectively with reaction 36, even at the high dose-rates of pulse radiolysis. Because the hydroxyalkyl radicals are nearly planar, two different peroxyl radicals are generated at optically active centers. 

Figure 2-17. Lipid peroxidation. A hydroxyl radical abstracts a hydrogen from a fatty acid or lipid molecule. After rearrangement to a conjugated structure, the radical reacts with oxygen to form a peroxyl radical. The newly formed peroxyl radical can initiate a chain reaction whereby new peroxyl radicals are formed.

Lipid peroxidation may beinitiated by any primary free radical which has sufficient reactivity to extract a hydrogen atom (Fig. 2.10) from a reactive methylene group of an unsaturated fatty acid. For example, species such as hydroxyl radicals OH, alkoxyl radicals RO peroxyl radicals ROO and alkyl radicals R may be involved. The formation of the initiating species is accompanied by bond rearrangement that results in stabilization by diene conjugate formation. The lipid radical then takes up oxygen to form the peroxyl radical. Peroxyl radicals can... 

The peroxyl radicals M-0-0 thus formed undergo a variety of molecular rearrangements and/or elimination reactions until the final oxidation products are formed. In reality, these oxidation products are interfering with hydroxyl radical attack on M and hence they are complicating the product spectrum considerably. Additionally, the bicarbonate and carbonate radicals may introduce selective oxidation reactions into the degradation cycle (Fig. 6-16). 

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