Dienes with hydroxyl radicals

Polysaccharides that are mainly found in the gel are also a group of compounds that exhibit antioxidant activities. It was demonstrated that APS-1 (mainly composed b mannose glucose in ratio 18 5) was effective in scavenging superoxide anion radical (dose-dependant fashion), hydroxyl radical, suppressed conjugated diene formation from LDL oxidation induced by Cu, and exhibited a protective effect on hydrogen peroxide-induced injury in PC12 cells 64). Also Kardosova et al. (65) showed that in vitro experiments with acidic and neutral polysaccharides were able to prevent lipid peroxidation by scavenging hydroxyl radicals. 

The radiolytic yields of formation of hydroperoxides (G(hydro)) are determined from the slope ofthe initial tangent ofthe curves (concentration versus radiation dose, as in Fig. 4). An important lipid concentration dependency is observed in micellar medium where, above the cmc, the yields of oxidized products formation increase steeply with PUFA concentration. This means that a chain-mechanism is involved in the formation of hydroperoxides and conjugated dienes. As an example, in the case of a 25 mM linolenate concentration (Fig. 4), the G-values are 20.0 x 10 mol J" and 9.3 x 10" mol J" for hydroperoxides and conjugated dienes, respectively. These yields are considerably larger than G.oh = 2.8 x 10" mol J since the hydroxyl radicals are initiators ofthe oxidation chain mechanism. 

Isoprene is a eonjugated diene (2-methyl-buta-1,3-diene), volatile and hardly soluble in water under normal pressure it boils at 34 C (Merck, 1999) and dissolves up to 1.47x10 M at 21.5 C, with a Henry s constant of 0.027 mole kg atm at 25 C (NIST, 2001). Isoprene is a metabolite in plants, microbes, animals and humans, and a major biogenic trace compound emitted to the atmosphere. It is very reactive towards atmospheric gas-phase oxidants such as hydroxyl and nitrate radicals or ozone. At higher concentrations, 220 - 7000 ppm, it is carcinogenic to rodents and possibly carcinogenic to humans (Melnick and Sills, 2001). 

Peroxy-linked dimers are also formed from linoleate hydroperoxides in the presence of free radical initiators and copper palmitate, and carbon-carbon linked dimers in the presence of copper catalysts. Decomposition of methyl linoleate hydroperoxides at 210°C under nitrogen produces mainly carbon-carbon linked dimers (82%), monomers with loss of diene conjugation, volatile compounds (4-5%) and water. The resulting dimers contain carbonyl and hydroxyl groups and double bonds scattered between carbon 8 and carbon 10. Linoleate hydroperoxides can dimerize by one of the termination reactions discussed in Chapter 1. The termination reactions involving combination of alkyl, alkoxyl, or peroxyl radical intermediates produce dimers with carbon-carbon, carbon ether, or peroxy links. The carbon-carbon and carbon-oxygen linked dimers are favored at elevated temperatures and the peroxy-linked dimers at ambient temperatures. The peroxy-linked dimers may also decompose to the ether-linked and carbon-carbon linked dimers via the corresponding alkyl and alkoxyl radical intermediates. 

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