Antioxidant interaction between α-tocopherol and quercetin in homogeneous solutions of peroxidating methyl linoleate

Interaction between α-tocopherol and quercetin as antioxidants was studied by quantification of both compounds in air-saturated tert-butyl alcohol as a hydrogen-bonding solvent during oxidation of dissolved methyl linoleate initiated by lipophilic α,α´-azobis-isobutyronitrile (AIBN) at 50 °C. The main question of the study was, if α-tocopherol and quercetin regenerate each other from their one-electron oxidized radical forms when they both are present during lipid autoxidation. α-Tocopherol with an initial concentration of 0.14 mM was by HPLC-analysis found to be depleted first, indicating that α-tocopherol is a more effective antioxidant. The concentration of quercetin, also initially 0.14 mM as followed spectrophotometrically, remained constant until α-tocopherol was consumed. The rate of α-tocopherol depletion was found to be independent of the presence of quercetin. Cyclic voltammetry showed that α-tocopherol is more easily oxidized than quercetin. Quercetin has previously been reported to regenerate the more reducing α-tocopherol during oxidation of lipid substrates in polar hydrogen-bonding solvents based on measured oxygen consumption rates. Our results further indicate that regeneration reactions between α-tocopherol and quercetin as antioxidants are of little if any importance, most likely due to the low extent of quercetin oxidation when they both are present. This conclusion was further supported by simulation of time profiles, from which an upper limit of 400 M⁻¹·s⁻¹ was estimated for the second-order rate constant for α-tocopherol regeneration of quercetin.