Development of a Flow-Injection ESR System for Evaluating the Lipophilic Alkoxyl Radical Eliminating Capacities (L-AREC) of Tocopherol Analogues and Unsaturated Fatty Acids

A new lipophilic spin-trapping flow-injection electron spin resonance (LFI-ESR) system was developed for quantitative detection of DMPO spin-adduct of alkoxyl radical (RO•), which was produced by thermal decomposition of an azo-initiator (AIBN) in ethyl acetate at 70 °C. The 50% and 75% inhibition dose (ID50 and ID75) of a series of lipophilic antioxidants, such as tocopherol derivatives (αToc, βToc, γToc, δToc, Toc, αTocM, βTocM, γTocM, TocM, TRX, and 7TocM), a lipophilic phenol (BHT), and unsaturated fatty acids (Ole, Lin, and Lic), were determined by monitoring the ESR signal intensity of the DMPO spin-adducts of RO• radical (DMPO/OR). On the basis of the observed ID50 and ID75 values, the lipophilic alkoxyl radical eliminating capacities (L-AREC) of these lipophilic antioxidants were determined using TRX as a standard. The evaluated L-AREC values are directly connected to the second order rate constants for the RO• radical induced hydrogen atom abstractions from the hydroxyl groups of the phenolic antioxidants, as well as from the allylic methylene groups of unsaturated fatty acids. The L-AREC values estimated by LFI-ESR method are concluded to be a practical and reliable indicator that reflect the inhibiting abilities of lipophilic antioxidants towards the radical chain reaction involved in the autoxidation processes of unsaturated fatty acids.