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Flavonols are plant pigments that are ubiquitous in nature. Quercetin (3,3',4',5,7-pentahydroxyflavone) and other related plant flavonols have come into recent prominence because of their usefulness as anticancer, antitumor, anti-AIDS, and other important therapeutic activities of significant potency and low systemic toxicity. Quercetin is intrinsically weakly fluorescent in aqueous solution, showing an emission maximum at approximately 538 nm. Upon binding to human serum albumin (HSA), quercetin undergoes dramatic enhancement in its fluorescence emission intensity, along with the appearance of dual emission behavior, consisting of normal and excited-state proton transfer (ESPT) fluorescence. In addition, the occurrence of a third emitting species has been noted for the first time. This is attributed to a electronic ground-state complex formed in the protein environment. High values of the fluorescence anisotropy (r) are obtained in the presence of HSA for the ESPT tautomer (r = 0.18), as well as the complex species (r = 0.37) of quercetin, indicating that the precursor ground-state molecules for both these emitting species of quercetin molecules are located in the motionally constrained sites of HSA. The steady-state emission data suggest that quercetin binds to two distinct sites in HSA from which the emissions from the normal tautomer and complex species take place. The preliminary results of studies on emission decay kinetics are also reported herein. Studies by far-UV circular dichroism spectroscopy reveal that binding of quercetin induces no significant perturbation in the secondary structure of HSA.

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 0 ,4 0 ,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 0 , 4 0 , 7 positions of the molecule)-bond dissociation enthalpies. Implications of these findings are discussed.