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NIRE Annual Report
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 1999

Specific Oxidizabilities of Polyunsaturated Fatty Acids in Aqueous Micelles

Hydrocarbon Research Division
Energy Resources Department
Objectives
Polyunsaturated fatty acids (PUFA) are particularly susceptible to autoxidation since they have chemically labile, bisallylic hydrogens, and accordingly the oxidizability of PUFA in homogeneous solution is dependent on the number of bisallylic positions present in the molecules. On the contrary, the oxidizability of PUFA in aqueous micelles is reported to decrease with an increasing number of bisallylic sites. The reason for this reversed relationship in reactivity has not been adequately explained. In order to answer this question we compare the oxidation kinetics of methyl docosahexaenoate (22:6n-3), methyl eicosapentaenoate (20:5n-3), methyl arachidonate (20:4n-6), methyl α-linolenate (18:3n-3), methyl g-linolenate (18:3n-6), and methyl linoleate (18:2n-6) both in homogeneous chlorobenzene solution and in Triton X-100 aqueous micelles at 37°C using both water- and lipid-soluble azo initiators to obtain a constant rate of free radical production. In addition, the effect of antioxidants on the oxidation of those PUFA in aqueous micelles was examined by measuring both the rate of oxygen uptake and the decay of antioxidants.
Results
The oxidizabilities for PUFAs in solution increased in the order 18:2n-6 < 18:3n-3, 18:3n-6 < 20:4n-6 < 20:5n-3, 22:6n-3, and this order substantially corresponds with the order of increasing degrees of unsaturation. On the contrary, the oxidation rate in aqueous micelles decreased in the order 18:2n-6 > 18:3n-3, 18:3n-6 > 20:4n-6 > 20:5n-3 > 22:6n-3. The ratio DO2/DPUFA increased with increasing degrees of unsaturation, suggesting that the polarity of the peroxyl radicals derived from PUFAs should increase with increasing degree of unsaturation. In the cooxidation of 18:2n-6 and other more unsaturated PUFAs in aqueous micelles, the reductions in the rate of total substrate consumption increased with increasing degrees of unsaturation of the PUFAs replacing 18:2n-6 although more unsaturated PUFAs are more prone to oxidation than 18:2n-6. The time course of oxygen uptake during the aerobic oxidation of PUFAs suggested that the inhibitory effect of 2,6-di-tert-butyl-4-methylphenol (BMP) would decrease with increasing degrees of unsaturation. The rate of disappearance of BMP during the oxidation of PUFA in aqueous micelles decreased in the order of 18:2n-6, 18:3n-3 > 20:4n-6 > 20:5n-3, 22:6n-3 and this order substantially corresponds with the order of increasing degrees of unsaturation (Fig.1). These results strongly suggest that the peroxyl radicals derived from highly unsaturated PUFA are not located in the core of micelle where BMP resides, rather at the surface of the micelle. The kinetic analysis for the aerobic oxidation of PUFAs in aqueous micelles suggested that the diffusion of the peroxyl radicals derived from highly unsaturated PUFAs to micelle surface would play a significant role in terminating autoxidation.
Fig.1
Fig. 1 Disappearance of BMP during 2 mM AMVN-initiated, aerobic oxidation of 2 mM PUFA in 50mM Tris-HCl buffer (pH 7.4) containing 10 mM Triton X-100 at 37°C.

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