Radicals, alkoxyl hydroperoxyl

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

In Scheme 7, the peroxidic 0-0 bond of the hydroperoxyl group is broken together with /1-scission of the formed alkoxyl radical, and, further, ring closure of alkyl peroxyl diradical may occur. The process generates a hydroxyl radical, methylcarbonyl terminal groups (-CH2-CO-CH3) and dioxetane. The latter is unstable and decomposes into an excited triplet state of formaldehyde and/or excited triplet state of methylcarbonyls (Scheme 8). 

The alkylhydroxyperoxyl and hydroperoxyl radicals formed from alcohol possess a reducing activity and attack hydroperoxide with the formation of the alkoxyl radical. This radical is very active and propagates the chain reacting with hydrocarbon. 

Compelling evidence suggesting that the breakdown of hydroperoxyl groups is not related to polymer destruction, at least in the initial period of oxidation at temperatures below 400 K, comes from experiments on the initiated oxidation of polymers. It was found that the destruction of polymers develops in parallel with their oxidation from the very onset of the process, but not after a delay related to the accumulation of a sufficient amount of hydroperoxyl groups [129]. These experiments also demonstrated that it is free macroradicals that undergo destruction. Oxidation of polymers gives rise to alkyl, alkoxyl, and peroxyl macroradicals. Which radicals undergo destruction can be decided based on the kinetics of initiated destructive oxidation. 

A) Initial stage of oxidation at moderate temperatures when hydroperoxyl groups are relatively stable. The rates of C—C bonds scission by decomposition of alkoxyl and peroxyl radicals are the following ... 

The selectivity of the trap towards hydroxyl radicals was demonstrated by several control experiments using different radicals, showing that the formation of the respective hydroxylation product, 5-hydroxy-6-0-zso-propyl-y-tocopherol (57), was caused exclusively by hydroxyl radicals, but not by hydroperoxyl, alkylperoxyl, alkoxyl, nitroxyl, or superoxide anion radicals. These radicals caused the formation of spin adducts from standard nitrone-and pyrroline-based spin traps, whereas a chemical change of spin trap 56 was only observed in the case of hydroxyl radicals. This result was independent of the use of monophasic, biphasic, or micellar reaction systems in all OH radical generating test systems, the trapping product 57 was found. For quantitation, compound 57 was extracted with petrol ether, separated by adsorption onto basic alumina and subsequently oxidized in a quantitative reaction to a-tocored, the deeply red-colored 5,6-tocopheryldione, which was subsequently determined by UV spectrophotometry (Scheme 23). 

ROS include oxygen-based free radicals such as superoxide, hydroxyl, alkoxyl, peroxyl, and hydroperoxyl (Table 1). Other ROS, such as hydrogen peroxide and lipid peroxides, can be converted into free radicals by transition... 

The antioxidant role of NO comes Ifom its reaction with oxygen, carbon and nitrogen centred radicals and can be seen to have a scavenger role under a range of conditions [112]. This is because of the unpaired electrons of NO which react rapidly with alkoxyl and alkyl hydroperoxyl radicals at near diffusion reaction rates (2x10 M s [113]). It is these reactions that have been suggested to have a modulatory role in enzyme- or metal-catalysed... 

It is evident that ascorbic acid, which is capable of scavenging active oxygen species, such as hydroxyl, hydroperoxyl, and lipid alkoxyl radicals generated in vitro, may also function as an antioxidant under in vivo conditions (Niki, 1991). 

These free radicals react with hydroperoxyl and alkoxyl radicals of oxidised fatty acids in the termination phase of the reaction ... 

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