Autoxidation of Docosahexaenoate with Different Chain Length of Alcohol

Four esters of docosahexaenoic acid, methyl (DHA-ME), ethyl (DHA-EE), propyl (DHA-PE) and isopropyl docosahexaenoate (DHA-IE), were synthesized and autoxidized at 25°C to determine how the ester structure provides stability to highly unsaturated fatty acid (HUFA) against oxidation. Stability was assessed based on oxygen absorption during autoxidation. Monohydroperoxide (MHPO) isomers were examined by high-performance liquid chromatography (HPLC). DHA-ME was found to undergo oxidation most rapidly, whereas DHA-IE was most stable toward this process. Induction period of DHA esters was longer with greater alcohol chain length. HPLC demonstrated the presence of ten MHPO positional isomers (4-,7-,8-,10-,11-,13-,14-,16-,17-and 20-MHPO) during autoxidation of the DHA esters. 20-MHPO was the main constituent of any one of these isomers, constituting 20% of total MHPO regardless of the DHA esters. Peroxide had virtually no effect on MHPO isomer compostion. 4-MHPO was produced in the least amount from autoxidized DHA-ME (8%), though more so from autoxidized DHA-IE (17%). More 4-MHPO was obtained from total MHPO with increase in alcohol chain length. MHPO produced from during autoxidation of DHA-EE, DHA-PE and DHA-IE was stable, during its incubation at 25°C, whereas only 4-MHPO of DHA-ME was unstable. Ester structure is thus shown to provide stability toward oxidation and products obtained from the oxidation of DHA.

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