Autoxidation of conjugated linoleic acid methyl ester in the presence of alpha-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 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.
Department of Chemistry, University of Helsinki, A.I. Virtasen aukio 1, Helsinki, 00014, Finland. tainahamalainen@yahoo.co.uk
Taina I
TI
Pajunen
Mikael P
MP
Johansson
Tapio
T
Hase
Anu
A
Hopia
D002851:Chromatography, High Pressure Liquid; D008401:Gas Chromatography-Mass Spectrometry; D006861:Hydrogen Peroxide / Q000737:chemistry*; D008041:Linoleic Acids / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011789:Quantum Theory; D024502:alpha-Tocopherol / Q000737:chemistry*
D008041:Linoleic Acids; C005575:methyl linoleate; D006861:Hydrogen Peroxide; D024502:alpha-Tocopherol