Peroxyls, carotenoid radical reactions

From Table 14.6 it can be seen that, with the exception of astaxanthin (ASTA), the rate constants for the electron transfer reactions decrease for each carotenoid in the order 9-phenanthryl peroxyl > 1-naphthyl peroxyl > 2-naphthyl peroxyl. This order of reactivity should be related to the reduction potentials of the radicals, with 9-phenanthryl peroxyl having the highest reduction potential. The same order of reactivity for these three arylperoxyl radicals reacting with Trolox was shown by Neta and coworkers (Alfassi et al. 1995). The reactivities of all the carotenoids studied are similar... 

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. 

Some oxy-radicals lead to both electron transfer reactions and addition to the carotenoid. The most well studied is the trichloromethylperoxyl radical CCI3OO-, a peroxyl radical which is known to cause hepatotoxity and other types of tissue injury (Packer et al, 1981 Hill et al, 1995). Packer et al. (1981) showed that, in the presence of/3-carotene, there was a fast bleaching of the carotene ground state absorption with a rate constant of 1.5 x lO M s". The loss of absorption at 450 nm was accompanied by an increase in absorption in the near infrared region (950-1000 nm), indicating that the reaction produces the /3-carotene radical cation. 

Three such peroxyl radicals (9-phenanthrylperoxyl, 1-naphthylperoxyl and 2-naphthylperoxyl) have been studied in methanol and the rate constants obtained for their electron transfer reaction with the carotenoids zeaxanthin and lutein are given in Table 2. 

To understand the mechanism of antioxidant activity of carotenoids it is important to analyze the oxidation products that are formed during their action as antioxidants. " The most important part of the molecule involved in those reactions is the polyene chain. This is a highly reactive electron rich system, susceptible to attack by electrophilic reagents such as peroxyl radicals, and thus responsible for the instability of the carotenoids toward oxidation. 

Recent work in the area has concentrated on the reactions of carotenoids with peroxyl radicals, generated mainly by the thermal decomposition of azo-initiators that lead to a variety of products. " Most of these products seem to be apocarotenals or apocarotenons of various chain lengths produced by cleavage of a double bond in the polyene chain, such as P-apo-12 -carotenal, P-apo-14 -carotenal, P-apo-lO-carotenal, and P-apo-13-carolenone. Kennedy and Liebler " reported that 5,6-epoxy-p,p-carotene and 15,15 -epoxy-P,P-carotene and several unidentified polar products were formed by the peroxyl radical oxidation of P-carotene by the peroxyl radicals. 

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