Use of EPR and ENDOR spectroscopy in conjunction with the spin trapping technique to study the high-temperature oxidative degradation of fatty acid methyl esters

Free radicals produced during the autoxidation of unsaturated edible oils are extremely short-lived, but are able to react with spin traps to produce adducts with sufficient stability for spectroscopic characterisation at (near) cooking temperatures (353-443 K). EPR spectra have shown that the model esters methyl oleate, linoleate and linolenate each formed three distinct radical adducts with N-tert-butyl-αa-phenylnitrone (PBN). These adducts have been further characterised by obtaining spectra under conditions of limited oxygen availability and in the presence of α-tocopherol; two of these adducts corresponded to peroxyl and alkyl radical adducts of PBN, whereas the other was an alkyl adduct of 2-methyl-2-nitrosopropane (MNP), which was formed as a result of decomposition of the PBN peroxyl radical adduct. The origins of the various ¹H hyperfine splittings have been determined by using selectively and fully deuterated PBN and selectively deuterated oleate and some of their magnitudes have been confirmed by ENDOR spectroscopy. The results obtained clearly confirm the high temperature oxidation of fatty acid esters to proceed via a different mechanism from that observed at low temperature, and point to significant differences in oxidation mechanisms of monounsaturated fatty acid esters relative to polyunsaturated.