A1 Refereed original research article in a scientific journal
Autoxidation of conjugated linoleic acid methyl ester in the presence of alpha-tocopherol: The hydroperoxide pathway
Authors: Pajunen TI, Johansson MP, Hase T, Hopia A
Publisher: SPRINGER HEIDELBERG
Publication year: 2008
Journal:: Lipids
Journal name in source: LIPIDS
Journal acronym: LIPIDS
Volume: 43
Issue: 7
First page : 599
Last page: 610
Number of pages: 12
ISSN: 0024-4201
DOI: https://doi.org/10.1007/s11745-008-3195-0
Abstract
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 alpha-tocopherol (20%) was diastereoselective in favor of one geometric isomer, namely Me 9-OOH-10t,12c, and produced new positional isomers 10- and 14-hydroperoxides (Me 10-OOH-11t,13t; Me 14-OOH-10t,12c; and Me 14-OOH-10t,12t). 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.
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 alpha-tocopherol (20%) was diastereoselective in favor of one geometric isomer, namely Me 9-OOH-10t,12c, and produced new positional isomers 10- and 14-hydroperoxides (Me 10-OOH-11t,13t; Me 14-OOH-10t,12c; and Me 14-OOH-10t,12t). 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.
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