Oxidation of all‐ cis ‐7,10,13,16,19‐docosapentaenoic acid ethyl ester. Hydroperoxide distribution and volatile characterization

Abstract

The isomeric hydroperoxide distribution and the composition of volatiles generated by oxidation of all-cis-7,10,13,16,19-docosapentaenoic acid ethyl ester (DPA Et) were determined. DPA Et was prepared by using seal blubber oils as raw material and purified by urea complexation and reverse-phase high-performance liquid chromatography (HPLC). The DPA Et of over 96% purity thus obtained was dissolved in methanol and subsequently divided into two portions. One portion was added with methylene blue and exposed to a tungsten bulb light at 5 °C for photosensitized oxidation. The other portion was added with 2,2'-azobis (2,4-dimethylvaleronitrile) as an azo-radical initiator and kept in the dark at room temperature for autoxidation. Positional isomers of hydroperoxides generated by autoxidation or photosensitized oxidation of DPA Et were separated by normal-phase HPLC and detected by a fluorescence detection system as well as UV absorption. The peak components were identified by gas chromatography-mass spectrometry (GC-MS). Eight isomeric hydroperoxides, including certain amounts of 7-, 10-, 11-, 13-, 14-, 16-, 17-, and 20-hydroperoxy docosapentaenoate, were generated by autoxidation of DPA Et. The photosensitized oxidation of DPA Et yielded not only the above eight hydroperoxide isomers but also two additional isomeric hydroperoxides, 8- and 19-hydroperoxy docosapentaenoate, which are characteristic hydroperoxide isomers generated by singlet oxygen-mediated oxidation. Volatiles formed by autoxidation of DPA Et at 50 °C were collected and analyzed by solid-phase micro-extraction and GC/GC-MS. A number of aldehydes, ketones, alcohols, acids, furans and hydrocarbons were identified. The formation mechanisms of certain volatiles are discussed.