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

4 years ago

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

Shin-ichi Nagaoka, Masaharu Ishikura, Kazuo Mukai, Maya Yoshimoto

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 (k_s) for the reaction of ArO^• with LHs 1–5 in toluene at 25.0 °C have been determined spectrophotometrically. The k_s 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 k_s 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, β-carotene 2, lycopene 3, capsanthin 4, zeaxanthin 5, and lutein 6). The k_s values obtained increased in the order of Car-H 1 < 2 < 3 < 4 < 5 < 6. The k_s value for Car-H 6 was 8.4 × 10^–4 M^–1 s^–1. The k_s 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 k_s 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.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcb.7b04570

DOI: 10.1021/acs.jpcb.7b04570