Mechanism of lower oxidizability of eicosapentaenoate than linoleate in aqueous micelles

The aerobic oxidation kinetics of methyl eicosapentaenoate (20:5n‐3) and methyl linoleate (18:2n‐6) were compared in homogeneous chlorobenzene solution and in Triton X‐100 aqueous micelles at 37°C. The rate of disappearance of 20:5n‐3 was two times faster than that of 18:2n‐6 in chlorobenzene, while the former was five times slower than the latter in aqueous micelles. It was also observed that ΔO 2 =Δ18:2n‐6 and ΔO 2 =2Δ20:5n‐3 in aqueous micelles. In the oxidation of a 1∶1 mixture of 20:5n‐3 and 18:2n‐6 in micelles, the rate of disappearance of 20:5n‐3 was 3.6 times faster than that of 18:2n‐6, and the rate of total substrate disappearance was reduced by a factor of 5 as compared with 18:2n‐6 oxidation. These data suggest that the peroxyl radical derived from 20:5n‐3 is more polar than that from 18:2n‐6, and the former is likely to diffuse from the core to the micelle surface. This lowers the oxidizability for 20:5n‐3 in aqueous micelles by enhancing the termination reaction rate for peroxyl radicals and by reducing the rate of propagation since there may be more 20:5n‐3 peroxyl radicals at the surface than in the micelle core.