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Stereochemistry of the hydroperoxides formed during autoxidation of CLA methyl ester in the presence of α‐tocopherol
Author(s) -
Hämäläinen Taina I.,
Sundberg Susanna,
Hase Tapio,
Hopia Anu
Publication year - 2002
Publication title -
lipids
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.601
H-Index - 120
eISSN - 1558-9307
pISSN - 0024-4201
DOI - 10.1007/s11745-002-0929-8
Subject(s) - chemistry , autoxidation , double bond , heteronuclear molecule , stereochemistry , conjugated system , nuclear magnetic resonance spectroscopy , proton nmr , medicinal chemistry , organic chemistry , polymer
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 13 C 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‐8 t ,10 t and 9‐OOH‐10 t ,12 t ), a cis,trans hydroperoxide with the trans double bond adjacent to the hydroperoxide‐bearing carbon atom (13‐OOH‐9 c ,11 t ), and a new type of cis,trans lipid hydroperoxide with the cis double bond adjacent to the hydroperoxide‐bearing carbon atom (8‐OOH‐9 c ,11 t ). In addition, three nonkinetic hydroperoxides (13‐OOH‐9 t ,11 t , 8‐OOH‐9 t ,11 t , and 9‐OOH‐10 t ,12 c ) 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.