Peroxidation chain

Diarylamiaes fuactioa as mbber antioxidants by breaking the peroxidative chain reactions leading to mbber deterioration. Nearly all commercial synthetic mbbers (see Elastomers, synthetic), including neoprene, butyl, styrene—butadiene, and the acrylonitrile—butadiene mbbers, can be protected with about 1—2% of an alkylated diphenylamine. DPA itself is not used as a mbber antioxidant. An objectionable feature of these antioxidants is that they cause discoloration and staining which limits their use to applications where this is not important. 

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. 

A second difference from the continuum model is that large stresses near the reaction center should undergo thermally activated relaxation. According to the molecular mechanism of stress relaxation proposed above, such irreversible, or plastic, deformations occur in UP when the two decyl radicals back away from the reaction center by rotational translation along their long axes. In the process of making more room for the two new C02 molecules, each radical chain is driven into the adjacent interface between two layers of peroxide molecules. Introduction of a defect or a hole at the end of the peroxide chain should facilitate this motion and allow efficient relaxation of the stress. 

There was no indication of a peroxide chain, the yields being below unity and independent of pressure of a fixed hydrocarbon-oxygen mixture. It is therefore likely that hydroperoxide was formed by a reaction such as... 

Reactive alkyl radicals and nonradical products generated by lipid peroxidation chain reactions are potential alkylating agents. Reactive methyl radicals can also arise by the irradiation or oxidation of methyl compounds such as methylhydrazine (33). 

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. 

As an effective bona fide antioxidant in both plasma and intracellular membrane, Pycnogenol can significantly contribute to the maintenance of cellular redox homeostasis, in particular in the course of events that are likely to overwhelm the capacity to cope with an increased production of RONS, such as in chronic inflammations, thereby reducing Ae possibility of cellular damage at different targets. The ability of Pycnogenol to act as a lipid peroxidation chain breaker is also likely to reduce the toxic consequences of a free-radical-induced cellular stress. 

Oxygen is omnipresent in our environment and the deterioration of rubbers and plastics by peroxidation is the normal cause of property deterioration in most polymers under ambient conditions. Peroxidation is a free radical chain reaction, shown in summary in reactions 3.1 and 3.2. Under normal conditions it is initiated by hydroperoxides that are formed in each cycle of the peroxidation chain sequence (reaction 3.1). Hydroperoxides are very unstable compounds due to the weakness of the peroxide bond which readily undergoes thermolysis when heated (reaction 3.3). This reaction is powerfully catalysed by transition metal ions. 

In the presence of an antioxidant such as vitamin E (a-tocopherol), the peroxyl radical abstracts the hydrogen atom from the antioxidant, and the radical chain reaction is interrupted [7], Hydrogen abstraction from the antioxidant itself generates a radical. The antioxidant radical, however, has greatly increased stability compared to the fatty acid peroxyl such that it fails to initiate another peroxidation chain reaction. The more efficient at transferring a hydrogen atom to the peroxyl radical, the better the antioxidant. Importantly, antioxidants do not intercept the carbon-centered fatty acyl radical but only the peroxyl radical. 

Polymers with a peroxide bond are interesting since they are very reactive. They are used as fuels and initiators. Kishore and coworkers [61] analyzed PSOX, a poly (alpha-methylstyrene peroxide), by SEC and found Mr, = 2.5 kDa and M = 4.0 kDa. The MALDI spectrum of PSOX displayed an acceptable spectral quality, with more than 50 peaks, due to poly(alpha-methylstyrene peroxide) chains. However, the upper limit of the mass range is very low, about 3.0 kDa, and the strongest peaks fall at 1.5 kDa. It was quite apparent that MALDI strongly underestimates the molar mass averages. In fact, the calculation (Equations 45.1 and 45.2) yielded... 

Nitric oxide can both promote and inhibit lipid peroxidation (Hogg and Kalyanaraman (1999). By itself, NO acts as a potent inhibitor of the lipid peroxidation chain reaction by scavenging propagatory lipid peroxyl radicals (formula [81]). It can also inhibit many potential initiators of lipid peroxidation, such as peroxidase enzymes. In the presence of 02 , NO forms peroxynitrite (see equation [46]), a powerful oxidant capable of initiating lipid peroxidation and oxidising lipid soluble antioxidants. 

As noted above, the primary products of the oxidative degradation (the peroxidation chain reaction) of polyolefins are hydroperoxides, which are unstable and undergo thermolysis or photolysis with chain scission. The products are lower molar mass materials including carboxylic acids, alcohols, aldehydes, and ketones (14,15). Depending on the amoimts of antioxidant and other stabilizers that are present, and on the nature of the environment in which they are discarded, it may take a few years or even decades before conventional polyolefins undergo sufficient oxidative degradation to become brittle and disintegrate. 

Fig. 6.5 Model of lipid peroxidation chain. Lipid peroxidation chain associated with salicylic acid (SA) acts as an electron donor during the regeneration of the ferric catalase. The resulting SA radical (SA ) initiates the lipid peroxidation chain giving malondialdehyde (MDA). Modified from Anderson et al. (1998) and Buege and Aust (1978).

Reaction (4) is rate controlling. The kinetics of the peroxidation chain reaction has been discussed in many reviews and standard texts and the reader is directed to these for further information . 

Unlike the peroxidation of the hydrocarbon polymers, the oxidation of lignin occurs by a stoichiometric process and not a chain reaction. Because phenols are antioxidants, the phenoxyl radicals formed are too stable to participate in a peroxidation chain reaction and the aromatic system is converted to quinoid compounds and ultimately humus. Both abiotic transition metal ion catalysed peroxidation and biological oxidation ire involved in the conversion of lignin to hiunus. 

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