Stereochemistry of the hydroperoxides formed during autoxidation of CLA methyl ester in the presence of α‐tocopherol

Abstract

The initial steps in the autoxidation of CLA methyl ester are poorly understood. The aim of this study was to determine the stereochemistry of the hydroperoxides formed during autoxidation of CLA methyl ester in the presence of a good hydrogen atom donor. For this purpose, 9-cis,11-trans CLA methyl ester was autoxidized in the presence of alpha-tocopherol under atmospheric oxygen at 40 degrees C in the dark. The CLA methyl ester hydroperoxides were isolated, reduced to the corresponding hydroxy derivatives, and separated by HPLC. The stereochemistry of seven hydroxy-CLA methyl esters was investigated. The position of the hydroxy group was determined by GC-MS. The geometry as well as the position of the double bonds in the alkyl chain was determined by NMR. In addition, the 13C NMR spectra of six hydroxy-CLA methyl esters were assigned using COSY, gradient heteronuclear multiple bond correlation, gradient heteronuclear single quantum correlation, and total correlation spectroscopy experiments. The autoxidation of 9-cis,11-trans CLA methyl ester in the presence of a good hydrogen atom donor is stereoselective in favor of one geometric isomer, namely the 13-(R,S)-hydroperoxy-9-cis,11-trans-octadecadienoic acid methyl ester. Three types of conjugated diene hydroperoxides are formed as primary hydroperoxides: trans,trans hydroperoxides (12-OOH-8t,10t and 9-OOH-10t,12t), a cis,trans hydroperoxide with the trans double bond adjacent to the hydroperoxide-bearing carbon atom (13-OOH-9c,11t), and a new type of cis,trans lipid hydroperoxide with the cis double bond adjacent to the hydroperoxide-bearing carbon atom (8-OOH-9c,11t). In addition, three nonkinetic hydroperoxides (13-OOH-9t,11t, 8-OOH-9t,11t, and 9-OOH-10t,12c) are formed. This study supports the theory that CLA methyl ester autoxidizes at least partly through an autocatalytic free radical reaction. The complexity of the hydroperoxide mixture is due to formation of two different pentadienyl radicals. Moreover, the stereoselectivity in favor of one geometric isomer can be explained by the selectivity of the two previous steps: the preferential formation of a W-conformer of the pentadienyl radical over the Z-conformer, and regioselectivity of the oxygen addition to the pentadienyl radical.

Use of propolis extracts as antioxidant in dairy beverages enriched with conjugated linoleic acid

“…These authors also indicated that heptanal can be used as a marker of oxidation progress in CLA-rich oils and it could come from 12-hydroperoxides formed in oxidized cis-9, trans-11 CLA [31]. The aldehydes octanal, nonanal, t-2-octenal and t-2-nonenal were also found as oxidation products in Tonalin ® oil [30].…”

NMR properties of conjugated linoleic acid (CLA) methyl ester hydroperoxides

“…The hydroperoxides were, when necessary for the determination of the positional isomers, hydrogenated to hydroxystearates, and derivatized to trimethylsilyl ethers according to the procedures described earlier (Hämäläinen et al, 2002). The trimethylsilyl ethers were analyzed on an Agilent 5973 mass spectrometer (Palo Alto, CA, USA) connected to a Hewlett Packard gas chromatograph (HP 6890 Series GC System, Wilmington, DE, USA).…”

“…Although the biological activity of CLA is linked to specific geometric and positional isomers, most of the CLA oxidation studies are performed with complex mixtures of isomers. Perhaps partly due to this, CLA was discovered only recently to autoxidize at least partly through the hydroperoxide pathway (Hämäläinen et al, 2001(Hämäläinen et al, , 2002Pajunen et al, 2008). Because of the possible biological importance of these oxidation products, we set out to produce hydroperoxides from the two biologically active CLA isomers and study their NMR properties.…”

Differences in Oxidation Kinetics Between Conjugated and Non‐Conjugated Methyl Linoleate

“…Other reactions such as dimerization and polymerization, although not evaluated, were not ruled out by those authors. In contrast to these assumptions, Hämäläinen et al supported that hydroperoxides are one type of primary oxidation products formed during autoxidation of CLA [17,18]. The controversy over this issue warrants further investigations on the oxidation mechanisms of CLA.…”