Defining the role of flavonoid structure on cottonseed oil stabilization: Study of A- and C-ring substitution

Structure-activity relationships of flavonoids against cottonseed oil oxidation are the objective of this work, which focuses on the contribution of C-ring and 5,7-di-OH A-ring. Flavonoids with a catecholic B-ring, namely quercetin, fisetin, luteolin, taxifolin, (+)-catechin and eriodictyol were added in increasing concentrations to cottonseed oil and the induced decrease in the rate constant of peroxide formation (k) and peroxide values (PV) were determined. The flavonoid depletion was monitored during the autoxidation with high-performance liquid chromatography-diode array detector (HPLC-DAD) analysis. The fully substituted C-ring was established as the most significant element for maximal antioxidant activity; quercetin even decreased k by 86% in comparison with the control and presented the highest decrease of PV per mmole of antioxidant. Flavonoids missing the 2,3-double bond or the 3-OH (taxifolin, luteolin) resulted in a smaller decrease of k, and the lack of both of the above structural elements (eriodictyol) resulted in the smallest decrease. Lack of the 4-carbonyl in the C-ring had the least negative effect on antioxidant activity since (+)-catechin presented the strongest protection after quercetin and fisetin. The activity of the latter flavonoid proved that A-ring hydroxyls, despite the unfavorable m-configuration, participate in the stabilization of the lipid substrate, although their significance was secondary.