Investigation of Phenols Activity in Early Stage Oxidation of Edible Oils by Electron Paramagnetic Resonance and 19F NMR Spectroscopies Using Novel Lipid Vanadium Complexes As Radical Initiators

Abstract

A novel dynamic method for the investigation of the phenols activity in early stage oxidation of edible oils based on the formation of a-tocopheryl radicals initiated by oil-soluble vanadium complexes is developed. Two new vanadium complexes in oxidation states V and IV were synthesized by reacting 2,2'-((2-hydroxyoctadecyl)azanediyl)bis(ethan-1-ol) (C18DEA) with [VO(acac)(2)] and 1-(bis(pyridin-2-ylmethyl)amino)octadecan-2-ol (C18DPA) with VOCl2. Addition of a solution of either complex in edible oils resulted in the formation of alpha-tocopheryl radical, which was monitored by electron paramagnetic resonance (EPR) spectroscopy. The intensity of the alpha-tocopheryl signal in the EPR spectra was measured versus time. It was found that the profile of the intensity of the a-tocopheryl signal versus time depends on the type of oil, the phenolic content, and the storage time of the oil. The time interval until the occurrence of maximum peak intensity be reached (t(m).), the height of the maximum intensity, and the rate of the quenching of the alpha-tocopheryl radical were used for the investigation of the mechanism of the edible oils oxidation. F-19 NMR of the F-19 labeled phenolic compounds (through trifluoroacetate esters) and radical trap experiments showed that the vanadium complexes in edible oil activate the one electron reduction of dioxygen to superperoxide radical. Superperoxide reacts with the lipids to form alkoperoxyl and alkoxyl lipid radicals, and all these radicals react with the phenols contained in oils.