Spectroscopic studies on the interaction characteristics between norethisterone and bovine serum albumin

The interaction characteristics between norethisterone (NET) and bovine serum albumin (BSA) were studied by fluorescence spectroscopy combined with UV–vis spectrophotometric techniques under simulative physiological conditions. The influence of Cd(II) and/or Se(IV) ions on the interaction between NET and BSA was also investigated. The fluorescence quenching rate constants and binding constants for BSA–NET system were determined at different temperatures. The mechanism of BSA fluorescence quenched by NET was discussed according to the Stern–Volmer equation. The results of thermodynamic parameters, ΔG, ΔS and ΔH, indicated that van der Waals interaction and hydrogen bonding played a major role for NET–BSA association. The results of competitive experiments demonstrated that the primary binding site of NET within subdomain IIA of BSA, and the second binding site within subdomain IIIA. The distance between BSA and NET is estimated to be 3.65 nm based on the Förster resonance energy transfer theory. The conformational change of BSA was observed in the existence of NET, Cd(II) or/and Se(IV) ions by synchronous fluorescence and three-dimensional fluorescence spectra.