Biomolecules common in blood plasma, including 2-methyl-1,4-naphthoquinone (vitamin K-0, 2), 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone-0, 3), bilirubin, 4, and urocanic acid, 5, were used as photoactivators for the photooxidation of methyl linoleate (ML) in 0.50 M sodium dodecyl sulfate micelles to mimic a bioenvironment. UV irradiation of 2 in this system initiated H-atom abstraction from ML (Type-I mechanism). The evidence includes kinetics of oxygen uptake, inhibition of oxidation by an antioxidant ((R)-( )-6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid [Trolox], 7) and the analysis of four geometric hydroperoxides formed (cis, trans to trans, trans ratio of 0.5). In contrast, irradiation with a singlet-oxygen sensitizer, 3,5-di-t-butyl-1,2-benzoquinone, 1, formed six isomers by a Type-II mechanism, yielding a cis, trans to trans, trans isomer ratio of 6. Peroxidation activated by 3 or 4 with visible light occurred by a singlet-oxygen pathway (Type-II mechanism), as shown by kinetics of oxygen uptake and the effect of quenchers. In contrast, peroxidation in the presence of 5 in this system initiated H-atom abstraction from ML as shown by oxygen uptake and inhibition by Trolox. A comparison of thermal free-radical peroxidation with direct photooxidation of human blood plasma samples showed important differences. Blood plasma resisted thermal peroxidation because of natural antioxidants or on the addition of Trolox. In contrast, direct photooxidation involved singlet oxygen, according to the effect of quenchers and the lack of inhibition by antioxidants.
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Vol. 78 • No. 3