Autoxidation of Polyunsaturated Lipids. Factors Controlling the Stereochemistry of Product Hydroperoxides

Abstract

The mechanism of the autoxidation of linoleic acid and phospholipid esters of this acid was investigated. The products of autoxidation, 13-hydroperoxy-9-m,11-trans-octadecadienoic (4), 13-hydroperoxy-9-ircms,11-trans-octadecadienoic (5), 9-hydroperoxy-10-irans,12-octadecadienoic (6), and 9-hydroperoxy-10-trans, 12-rrcms-octadecadienoic (7) acids, were analyzed by LC after reduction to the corresponding hydroxy fatty acids. The ratio of trans,cis/trans,trans products, (4 + 6/5 + 7), formed during the initial stages of oxidation (<2% for the free acids) was dependent on temperature and the concentration of linoleic acid. This trans,cis/trans,trans ratio varied from 4.2 (with neat linoleic acid oxidations at 10 °C) to 0.23 (0.24 M linoleic acid in benzene at 50 °C). A similar product distribution was found in emulsion oxidation of mixtures of 1,2-dilinoleoylglycerolphosphatidylcholine and 1,2-dipalmitoylglycerophosphatidylchlorine with the trans,cis/trans,trans product ratio depending on the ratio of dilinoleoyllecithin to dipalmitoyllecithin. Mixtures of linoleic acid and p-methoxyphenol give trans,cis/trans,trans product ratios dependent on the concentration of added phenol. A kinetic scheme consistent with these observations is presented. © 1980, American Chemical Society. All rights reserved.