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

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Lipids

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 α-tocopherol under atmospheric oxygen at 40°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 stereoslectivity 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.

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Abbreviations

BSTFA:

bis(trimethylsilyl)-trifluoroacetamide

gHMBC:

gradient heteronuclear multiple bond correlation

gHSQC:

gradient heteronuclear single quantum correlation

methyl linoleate:

9-cis,12-cis-octadecadienoic acid methyl ester

8-OH-9c,11t :

8-(R,S)-hydroxy-9-cis,11-trans-octadecadienoic acid methyl ester

8-OH-9t :

11t, 8-(R,S)-hydroxy-9-trans,11-trans-octadecadienoic acid methyl ester

9-OH-10t,12c :

9-(R,S)-hydroxy-10-trans,12-cis-octadecadienoic acid methyl ester

9-OH-10t,12t :

9-(R,S)-hydroxy-10-trans,12-trans-octadecadienoic acid methyl ester

12-OH-8t,10t :

12-(R,S)-hydroxy-8-trans,10-trans-octadecadienoic acid methyl ester

13-OH-9c,11t :

13-(R,S)-hydroxy-9-cis,11-trans-octadecadienoic acid methyl ester

13-OH-9t,11t :

13-(R,S)-hydroxy-9-trans,11-trans-octadecadienoic acid methyl ester

TMCS:

trimethylcholorosilane

TOCSY:

total correlation spectroscopy

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Authors and Affiliations

  1. Department of Chemistry, Laboratory of Organic Chemistry, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, 00014, Finland

    Taina I. Hämäläinen & Tapio Hase

  2. Department of Applied Chemistry and Microbiology Food Chemistry Division, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, 00014, Finland

    Susanna Sundberg & Anu Hopia

  3. Department of Chemistry, Laboratory of Analytical Chemistry, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, 00014, Finland

    Susanna Sundberg

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  1. Taina I. Hämäläinen
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Hämäläinen, T.I., Sundberg, S., Hase, T. et al. Stereochemistry of the hydroperoxides formed during autoxidation of CLA methyl ester in the presence of α-tocopherol. Lipids 37, 533–540 (2002). https://doi.org/10.1007/s11745-002-0929-8

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  • DOI: https://doi.org/10.1007/s11745-002-0929-8

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