Kinetic Study of the Aroxyl-Radical-Scavenging Activity of Five Fatty Acid Esters and Six Carotenoids in Toluene Solution: Structure-Activity Relationship for the Hydrogen Abstraction Reaction.

A kinetic study of the reaction between an aroxyl radical (ArO•) and fatty acid esters (LHs 1-5, ethyl stearate 1, ethyl oleate 2, ethyl linoleate 3, ethyl linolenate 4, and ethyl arachidonate 5) has been undertaken. The second-order rate constants (ks) for the reaction of ArO• with LHs 1-5 in toluene at 25.0 °C have been determined spectrophotometrically. The ks values obtained increased in the order of LH 1 < 2 < 3 < 4 < 5, that is, with increasing the number of double bonds included in LHs 1-5. The ks value for LH 5 was 2.93 × 10-3 M-1 s-1. From the result, it has been clarified that the reaction of ArO• with LHs 1-5 was explained by an allylic hydrogen abstraction reaction. A similar kinetic study was performed for the reaction of ArO• with six carotenoids (Car-Hs 1-6, astaxanthin 1, ß-carotene 2, lycopene 3, capsanthin 4, zeaxanthin 5, and lutein 6). The ks values obtained increased in the order of Car-H 1 < 2 < 3 < 4 < 5 < 6. The ks value for Car-H 6 was 8.4 × 10-4 M-1 s-1. The ks values obtained for Car-Hs 1-6 are in the same order as that of the values for LHs 1-5. The results of detailed analyses of the ks values for the above reaction indicated that the reaction was also explained by an allylic hydrogen abstraction reaction. Furthermore, the structure-activity relationship for the reaction was discussed by taking the result of density functional theory calculation reported by Martinez and Barbosa into account.