Radical peroxyl allyl

All unsaturated fatty acids contain highly reactive allylic positions, which are easily attacked by hydroxyl and peroxyl free radicals. Bielski et al. [3] studied the reactivity of unsaturated acid with perhydroxyl radical by the stopped flow technique. These authors... 

As in the case of linear peroxidation products, the initiation step of the formation of isoprostanes is the abstraction of a hydrogen atom from unsaturated acids by a radical of initiator. Initiation is followed by the addition of oxygen to allylic radicals and the cyclization of peroxyl radicals into bicyclic endoperoxide radicals, which form hydroperoxides reacting with hydrogen donors. 

In the propagation steps, this radical then reacts with oxygen, producing a peroxyl radical, which then abstracts hydrogen from a further molecule of the substrate. The product is thus the hydroperoxide, reaction having occurred at the allylic position of the alkene. Two possible chain-termination steps might... 

As exemplified in Figure 2, Type 1 mechanism, electron transfer from L to sens yields two radicals, the substrate radical, L", and the sensitizer radical anion (sens ). In the next step, the lipid radical may induce a chain peroxidation cascade involving propagation reactions -The sensitizer radical anion may also start a sequential one-electron reduction of 2 generating HO in the presence of reduced transition metals. As a result, this may lead to abstraction of a lipid allylic hydrogen with subsequent generation of a carbon-centered lipid radical, L, that is rapidly oxidized to a peroxyl radical (vide supra). 

In allylperoxyl radicals, allylic rearrangement leads to the 1,3-migration of the peroxyl function, with the corresponding shift of the double bond [reaction (28) Schenck et al. 1958],... 

These peroxyl radicals then attack another molecule of linoleic acid to abstract an allylic hydrogen and produce the conjugated diene hydroperoxide, LOOH, Equation 7. 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

Abbreviations BSO, D.L-buthionine-5, i -sulfoxime L , lipid alkyl radicals LH, lipid LO, lipid alkoxyl radicals LOO, lipid peroxyl radicals L-NAME, 7V -nitro-L-arginine-methyl ester MBI, methylene bridge index (mean number of his-allylic methylene positions per fatty add) NO, nitric oxide NOS, nitric oxide synthase NOy, nitrite N02, nitrogen dioxide NO2CI, nitryl chloride O2 , superoxide OH, hydroxyl radical OL, epoxyallylic radical OLOO, epoxyperoxyl radical 0=NOO , peroxynitrite SNAP, S-nitroso-lV-acetyl-D.L-penicillamine SOD, superoxide dismutase contd. onp. 98, Subcellular Biochemistry, Volume 36 Phospholipid Metabolism in Apoptosis. 

Atom Transfer (hydrogen abstraction) by LOO —> Free Radical Chain Reactions Hydrogen abstraction is the heart of the classic free radical chain reaction schemes (Figure 1). Peroxyl radicals initially formed at any site on a fatty acid pass the unpaired electron to adjacent lipid molecules by abstracting hydrogens from an allylic position or a hydroperoxide, and the process repeats itself indefinitely until the chain is intercepted. 

Both peroxyl and alkoxyl radicals abstract hydrogens much faster from hydroperoxides than from lipid allylic positions, a fact that has been little appreciated previously and can have great consequences to oxidation kinetics and product distributions. 

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