An alternate mode of binding of the polyphenol quercetin with serum albumins when complexed with Cu(II)

Polyphenols find wide use as antioxidants, cancer chemopreventive agents and metal chelators. The latter activity has proved interesting in many aspects. We have probed the binding characteristics of the polyphenol quercetin–Cu(II) complex with human serum albumin (HSA) and bovine serum albumin (BSA). Fluorescence studies reveal that the quercetin–Cu(II) complex can quench the fluorescence of the serum albumins. The binding constant (K_b) values are of the order of 10⁵ M^?1 which increased with rise in temperature in case of HSA and BSA interacting with the quercetin–Cu(II) complex. Displacement studies reveal that both the ligands bind to site 1 (subdomain IIA) of the serum albumins. However, thermodynamic parameters calculated from temperature dependent studies indicated that the mode of interaction of the complexes with the proteins differs. Both ΔH° and ΔS° were positive for the interaction of the quercetin–Cu(II) complex with both proteins but the value of ΔH° was negative in case of the interaction of quercetin with the proteins. This implies that after chelation with metal ions, the polyphenol alters its mode of interaction which could have varying implications on its other physicochemical activities.