Autoxidation of Conjugated Linoleic Acid Methyl Ester in the Presence of α-Tocopherol: The Hydroperoxide Pathway

Autoxidation of conjugated linoleic acid (CLA) methyl ester follows at least partly Farmer’s hydroperoxide theory. A mechanism for this hydroperoxide pathway has been proposed based on autoxidation of 9- cis ,11- trans -CLA methyl ester. This investigation aims at confirming and further clarifying the mechanism by analyzing the hydroperoxides produced from 10- trans ,12- cis -CLA methyl ester and by theoretical calculations. Five methyl hydroxyoctadecadienoates were isolated by HPLC and characterized by UV, GC-MS, and 1D- and 2D-NMR techniques. In addition, an HPLC method for the separation of the intact hydroperoxides was developed. The autoxidation of 10- trans ,12- cis -CLA methyl ester in the presence of high amount of α-tocopherol (20%) was diastereoselective in favor of one geometric isomer, namely Me 9-OOH-10 t ,12 c , and produced new positional isomers 10- and 14-hydroperoxides (Me 10-OOH-11 t ,13 t ; Me 14-OOH-10 t ,12 c ; and Me 14-OOH-10 t ,12 t ). Importantly, one of these new isomers, which was characterized as an intact hydroperoxide, had an unusual cis , trans geometry where the cis double bond is adjacent to the hydroperoxyl-bearing methine carbon. Further insight to the mechanism was provided by calculating the relative energies for different conformations of the precursor lipid, the allylic carbon–hydrogen bond dissociation enthalpies, and the spin distributions on the intermediate pentadienyl radicals. As a result, a better understanding of the isomeric distribution of the product hydroperoxides was achieved and a modified mechanism that accounts for these calculations is presented.