Hydroperoxide formation during autoxidation of conjugated linoleic acid methyl ester

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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.

Section: Methodsmentioning

confidence: 56%

mentioning

confidence: 94%

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

Hämäläinen

Sundberg

Hase

et al. 2002

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“…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.…”

Section: Introductionmentioning

confidence: 99%

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

Luna

Fuente

Salvador

et al. 2007

The oxidation kinetics of conjugated methyl linoleate was compared with that of non-conjugated methyl linoleate under mild oxidation conditions (30 degrees C in the dark). Samples of methyl 9-cis,11-trans-linoleate, methyl 10-trans,12-cis linoleate and methyl 9-cis,12-cis linoleate were assayed separately and in mixtures. For comparative purposes, methyl alpha-linolenate and methyl oleate were also used. Two complementary analytical approaches were selected to monitor the progress of oxidation, (1) the traditional follow-up of residual substrate by gas liquid chromatography, and (2) an analytical procedure by high-performance size-exclusion chromatography (HPSEC) for direct measurement of the oxidation compounds formed. The HPSEC method enabled us to quantitate oxidized monomers, dimers and polymers concomitantly in a rapid and direct analysis. Results showed that conjugated methyl linoleate samples oxidized later than their non-conjugated counterparts, and showed a very different oxidation pattern. Thus, formation of oxidized monomers was negligible and the first and major compounds formed were polymerization products. Also, under the conditions used, non-conjugated and conjugated methyl linoleate samples in 1:1 mixtures led to decreased oxidation rate of non-conjugated methyl linoleate and increased oxidation rate of conjugated methyl linoleate. This study supports the view that oxidation kinetics of conjugated dienes differ substantially from that of methylene-interrupted dienes.

“…Yurawecz et al (2) found that CLA in methanol/water mixtures was oxidized to furan FA when exposed to air, and a subsequent study investigated the autoxidation products of furan fatty esters derived from CLA (3). Hämäläinen et al (4) obtained the 1 H and 13 C NMR spectra of autooxidized CLA methyl ester and showed that hydroperoxides were formed. Lie Ken Jie and Pasha (5) studied the action of m-chloroperoxybenzoic acid and potassium peroxymonosulfate on CLA isomer, methyl 9(Z),11(E)-octadecadienoate.…”

mentioning

confidence: 99%

Isolation of unsaturated diols after oxidation of conjugated linoleic acid with peroxygenase

Piazza

Núñez

Foglia

2003

Oat seeds are a rich source of peroxygenase, an iron heme enzyme that participates in oxylipin metabolism in plants. An isomer of CLA, 9(Z),11 (E)-octadecadienoic acid (1), believed to have anticarcinogenic activity, was used as a substrate for peroxygenase in an aqueous medium using t-butyl hydroperoxide as the oxidant. After acidification of the reaction medium, the products were extracted with ethyl ether, converted to their methyl esters, and characterized using HPLC. Major products after reaction for 24 h showed resonances from 1H NMR spectroscopy that were further downfield than the expected epoxides and were thought to be diol hydrolysis products. However, analyses by HPLC with atmospheric pressure chemical ionization MS (APCI-MS) of the putative allylic diols or their bis-trimethylsilyl ether derivatives gave incorrect M.W. The M.W. of the diols could be obtained by APCI-MS after removal of unsaturation by hydrogenation or by EI-MS after conversion of the allylic 1,2-diols to cyclic methyl boronic esters. Data from MS in conjunction with analyses using 1H and 13C NMR showed that the methylated products from 1 were methyl 9,10(threo)-dihydroxy-11 (E)-octadecenoate, methyl 9,10(erythro)-dihydroxy-11(E)-octadecenoate, methyl 9,12(erythro)-dihydroxy-10(E)-octadecenoate, and methyl 9,12(threo)-dihydroxy-10(E)-octadecenoate. Solid-phase extraction without prior acidification and conversion of the products to methyl esters allowed identification of the following epoxides: methyl 9,10(Z)-epoxy-11 (E)-octadecenoate (6M), methyl 9,10(E)-epoxy-11 (E)-octadecenoate, and methyl 11,12(E)-epoxy-9(Z)-octadecenoate. At times of up to at least 6 h, 6M accounted for approximately 90% of the epoxide product. Product analysis after the hydrolysis of isolated epoxide 6M showed that hydrolysis of epoxide 6 could largely account for the diol products obtained from the acidified reaction mixtures.