Chain involving peroxyl radicals

The reaction of organic compounds with oxygen, known as autoxidation, is the most common of all organic reactions. The reaction is a free radical chain process involving peroxyl radicals which includes initiation, propagation and termination steps and is the subject of earlier reviews"". For control of these reactions under laboratory conditions, the reaction is usually initiated by azo initiators. The reactions are outlined briefly in equations 2-4. 

Value contributions of steps (11-13,16,17) involving inhibitor and phenoxyl radical with the rate constant sensitive to the reactivity indices Don contain useful information. According to data shown in Figure 7.9, for the nonoptimal inhibitor, /inra-methylphenol, step (11) and its reverse reaction (17) have the most significant contributions. And in the case of the optimum inhibitor,/ flra-N-dimethylaminophenol, value contributions of steps (11) and (17) for the equilibrium (7.19) decrease substantially. Hence it follows that the efficient inhibitor that has the minimmn magnitude of Don, mainly provides for the maximum displacement of equilibrium (7. 19) to the right from the chain carrier (peroxyl radical) to the formation of the phenoxyl radical. 

The chemical mechanisms involved in the action of antioxidants have been discussed in standard texts [9,63-66] and the reader is directed to these and the references they contain for more detailed information. Two complementary antioxidant mechanisms are frequently used synergistically in polyolefins. The first is the kinetic chain-breaking hydrogen donor, (CB-D) mechanism in which chain-propagating peroxyl radicals (POO ) are preferentially reduced to hydroperoxide by the antioxidant (AH). 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

The reactions described so far do not require the involvement of the apo-B protein, neither would they necessarily result in a significant amount of protein modification. However, the peroxyl radical can attack the fatty acid to which it is attached to cause scission of the chain with the concomitant formation of aldehydes such as malondialdehyde and 4-hydroxynonenal (Esterbauer et al., 1991). Indeed, complex mixtures of aldehydes have been detected during the oxidation of LDL and it is clear that they are capable of reacting with lysine residues on the surface of the apo-B molecule to convert the molecule to a ligand for the scavenger receptor (Haberland etal., 1984 Steinbrecher et al., 1989). In addition, the lipid-derived radical may react directly with the protein to cause fragmentation and modification of amino acids. 

The reaction proceeds as a chain process involving the peroxyl radical and superoxide ion [284],... 

Of these reactions, the reaction of the peroxyl radical with phosphite is the slowest. The rate constant of this reaction ranges from 102 to 103 L mol 1 s 1 which is two to three orders of magnitude lower than the rate constant of similar reactions with phenols and aromatic amines. Namely, this reaction limits chain propagation in the oxidation of phosphites. Therefore, the chain oxidation of trialkyl phosphites involves chain propagation reactions with the participation of both peroxyl and phosphoranylperoxyl radicals ... 

Nitroxyl radicals are formed as intermediates in reactions of polymer stabilization by steri-cally hindered amines as light stabilizers (HALS) [30,34,39,59]. The very important peculiarity of nitroxyl radicals as antioxidants of polymer degradation is their ability to participate in cyclic mechanisms of chain termination. This mechanism involves alternation of reactions involving alkyl and peroxyl radicals with regeneration of nitroxyl radical [60 64],... 

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). 

The question why the aminyl radicals ensure cyclic chain termination in those systems in which the hydroperoxyl and hydroxyalkylperoxyl radicals are formed, but not in the oxidation of hydrocarbons where alkylperoxyl radicals are the chain-propagating species deserves special attention [22-24]. Indeed, the disproportionation of the aminyl and peroxyl radicals that involve both the —H and C—H bonds is, in principle, possible... 

Another type of one-electron transfer reaction that contributes to the formation of oxyradicals involves the quenching of carbon center radicals (R ) by molecular oxygen. This reaction leads to the formation of peroxyl radicals (R-00 ), which generally have quite different reactivities from those of the parent R species. As a result of the reactivity of these species toward unsaturated fatty acids, the propagation steps of lipid peroxidation follow the initiation step. These propagation steps occur at membrane hydrophobic sites, and the length of the chain reaction is determined by the availability of reactants, PUFA and O2, and of chain-breaking antioxidants such as a-tocopherol, carotenoids, and ubiquinone. 

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