Investigations on the binding and antioxidant properties of the plant flavonoid fisetin in model biomembranes

Plant flavonoids are emerging as potent therapeutic drugs for free radical mediated diseases, for which cell membranes generally serve as targets for lipid peroxidation and related deleterious effects. Screening and characterization of these ubiquitous, therapeutically potent polyphenolic compounds, require a clear understanding regarding their incorporation and possible location in membranes, as well as quantitative estimates of their antioxidative and radical scavenging capacities. Here, we demonstrate the novel use of the intrinsic fluorescence characteristics of the plant flavonoid fisetin (3,3^′,4^′,7-OH flavone) to explore its binding and site(s) of solubilisation in egg lecithin liposomal membranes. Spectrophotometric assays have been used to obtain quantitative estimates of its antioxidative capacity. Furthermore, our quantum mechanical semi-empirical calculations provide a quantitative measure for the free radical scavenging activity of fisetin from the OH (at 3, 3^′, 4^′, 7 positions of the molecule)-bond dissociation enthalpies. Implications of these findings are discussed.