Peroxyl chain reactions, involving

Other reagents, including lead tetraacetate (LTA) or HgO, can be used instead of DIB, although the latter is usually more effective [72-74], In all cases an excess of the reagent must be used (typically 1.5 equiv for DIB) since a chain reaction involving abstraction of the hydrogen atom from hydroxyl cannot be maintained by the peroxyl radical. 

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

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],... 

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. 

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. 

Moreover, formation of radical transients with S.-.O bonds is kinetically preferred, but on longer time scale they convert into transients with S.-.N bonds in a pH dependent manner. Ultimately transients with S.-.N bonds transform intramolecularly into C-centred radicals located on the C moiety of the peptide backbone. Another type of C-centred radicals located in the side chain of Met-residue, a-(aikylthio)alkyl radicals, are formed via deprotonation of MetS +. C-centred radicals are precursors for peroxyl radicals (ROO ) that might be involved in chain reactions of peptide and/or protein oxidation. Stabilization of MetS +through formation of S.-.O- and S.-.N-bonded radicals might potentially accelerate oxidation and autooxidation processes of Met in peptides and proteins. Considering that methionine sulfoxide, which is the final product coming from all radicals centred on sulphur, is restored by the enzyme methionine sulfoxide reductase into MetS, stabilization of MetS +appears as a protection against an eventual peroxidation chain that would develop from a carbon centred radical. 

The photooxidation of polypropylene involves the initiation of free radical chain reaction by the photolysis of hydroperoxides, producing peroxyl radicals as well as alkoxy radicals. In the last decades many studies dealing with the types of stabilizers used to delay the polypropylene photodegradation were reported as well as several studied on the experimental methods for assessing their photostability effectiveness [73-78]. 

The scheme of oxidation in the presence of the inhibitor InH, which reacts with peroxyl radicals, and the inhibitor Q, which is an acceptor of alkyl radicals, includes several reactions involving radicals In- formed from the inhibitor, namely, reactions of radical decay (In- + In-, In- - RO-2) and reactions of chain propagation of the type In- -I- RH, In- + ROOH, and decomposition of In- to the molecule and radical capable of propagating the chain. This can be exemplified by the reaction... 

Being a reducing agent, the inhibitor is oxidized by not only the peroxyl radical (this reaction results in chain termination) but also oxygen and hydroperoxide, which results in the consumption of the inhibitor without chain termination. Below we present the list of reactions involving an inhibitor the reactions of R- and RO-2 in which an inhibitor does not participate are considered in Section 11.1 (R OOR is the initiator). 

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. 

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 ... 

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). 

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