Evaluation of binding and thermodynamic characteristics of interactions between a citrus flavonoid hesperitin with protein and effects of metal ions on binding

The mechanism of interaction of a non-glycosidic citrus flavonoid, hesperitin (HES) with bovine serum albumin (BSA) was studied by UV-vis absorption, fluorescence, FT-IR, circular dichroism, fluorescence anisotropy and synchronous fluorescence spectroscopy in phosphate buffer of pH 7.4. Fluorescence data revealed that the fluorescence quenching of BSA by HES was the result of the formed complex of HES-BSA. The binding constants and thermodynamic parameters at four different temperatures, the location of binding, and the nature of binding force were determined. The hydrogen bonds interactions were found to be the predominant intermolecular forces to stabilize the complex. The conformation of BSA was discussed by synchronous fluorescence and CD methods. The alterations of protein secondary structure upon complexation with HES were evident from the gradual decrease in a-helicity. The distance between the donor (BSA) and acceptor (flavonoid) was calculated from the fluorescence resonance energy transfer and found to be 1.978 nm. Common ions viz., Zn ²⁺, K ⁺, Cu ²⁺, Ni ²⁺, Mn ²⁺ and Co ²⁺ were found to influence the binding of flavonoid to protein.