Effects of α‐tocopherol and ascorbyl palmitate on the isomerization and decomposition of methyl linoleate hydroperoxides

Abstract

In order to study antioxidant action on lipid hydroperoxide decomposition, the effects of alpha-tocopherol (TOH) and ascorbyl palmitate on the decomposition rate and reaction sequences of 9- and 13-cis,trans methyl linoleate hydroperoxide (cis,trans ML-OOH) decomposition in hexadecane were studied at 40 degrees C. Decomposition of cis,trans ML-OOH as well as the formation and isomeric configuration of methyl linoleate hydroxy and ketodiene compounds were followed by high-performance liquid chromatographic analysis. TOH effectively inhibited the decomposition of ML-OOH. The decomposition rate was two times slower at 0.2 mM and more than 10 times slower at 2 and 20 mM of TOH. Ascorbyl palmitate (0.2, 2, and 20 mM) slightly accelerated the decomposition of ML-OOH. Both compounds had an effect on the reaction sequences of ML-OOH decomposition. At high levels TOH inhibited the isomerization of cis,trans ML-OOH to trans,trans ML-OOH through peroxyl radicals and increased the formation of hydroxy compounds. Further, the majority of the hydroxy and ketodiene compounds formed had a cis,trans configuration, indicating that cis,trans ML-OOH decomposed through alkoxyl radicals without isomerization. These results suggest that when inhibiting the decomposition of hydroperoxides, TOH can act as a hydrogen atom donor to both peroxyl and alkoxyl radicals. In the presence of ascorbyl palmitate, cis,trans ML-OOH decomposed rapidly but without isomerization. In contrast to TOH, the majority of hydroxy compounds were cis,trans, but the ketodiene compounds were trans,trans isomers. This indicates that ascorbyl palmitate reduced cis,trans ML-OOH to the corresponding hydroxy compounds. However, the simultaneous formation of trans,trans ketodiene compounds suggests that ML-OOH decomposition, similar to the control sample, also occurred in these samples. Thus, under these experimental conditions, the reduction of ML-OOH to more stable hydroxy compounds did not occur to an extent significant enough to inhibit the radical chain reactions of ML-OOH decomposition.

Dihydroperoxidation facilitates the conversion of lipids to aldehydic products via alkoxyl radicals

“…At high concentrations, a-tocopherol inhibits reactions of fatty acid hydroperoxides via peroxyl radicals but does not prevent their conversion to hydroxy-or ketodiene derivatives via alkoxyl radicals (Makinen & Hopia, 2000). By extension, we considered that under such conditions, there should be (i) negligible conversion of PC-LA-OOH 1 to aldehydes 4-7 if they are exclusively formed via peroxyl radical addition and (ii) enhanced relative importance of aldehydes formed via alkoxyl radicals, such as 3.…”

α-, γ- and δ- Tocopherols as inhibitors of isomerization and decomposition ofcis,trans methyl linoleate hydroperoxides

“…Methyl linoleate was oxidized in the dark at 40 °C in the presence of 5% α-tocopherol to produce only cis,trans ML-OOH [8,11]. Cis,trans ML-OOH were purified from non-oxidized ML and α-tocopherol with solid-phase extraction and the purity of the ML-OOH fraction was checked by thin layer chromatography [8].…”

“…It has been previously shown that α-tocopherol inhibited the decomposition of methyl linoleate hydroperoxides (ML-OOH) when studied in hexadecane at 40 °C, most likely by acting as a hydrogen atom donor to both peroxyl and alkoxyl radicals [8][9]. Further, α-tocopherol had an effect on the decomposition reaction sequences of ML-OOH, since it inhibited the isomerization of cis,trans to trans,trans ML-OOH and had an effect on the distribution of ML-OOH decomposition products [8]. Koskas et al [10] showed that the addition of α-, γand δ-tocopherols increased the stability of both 13-cis,trans and 13trans,trans hydroperoxides of linoleic acid in an aqueous system.…”

Role of lipid reactions in quality of oat products

“…the presence of antioxidants and metal-ions or other radical forming compounds, phase distribution and presence of amino acids and sugars (Nishiike et al 1999, McClements and Decker 2000, Mäkinen and Hopia 2000. Certain antioxidants slow the decomposition of hydroperoxides, which can reduce the overall rate of lipid oxidation, as the decomposition of hydroperoxides will provide less radicals for the initial H * abstraction from the fatty acid.…”