Daidzein interaction with human serum albumin studied using optical spectroscopy and molecular modeling methods

In this work, fluorescence anisotropy, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy and molecular modeling methods were used to characterize optical properties of the Daidzein-HSA complex and to gain information on the binding mechanism at molecular level. Daidzein is weakly fluorescent in aqueous buffer medium, with an emission maximum at 466 nm. Binding of Daidzein with HSA leads to dramatic enhancement in the fluorescence intensity and anisotropy (r), along with significant changes in the fluorescence excitation and emission profiles. The binding constant (K = (5.9 ± 0.6) × 10⁴ M^-1) and the standard free energy change (ΔG ≈ -27.5 kJ/mol) for Daidzein-HSA interaction have been calculated from the relevant fluorescence data. The secondary structure compositions of free HSA and its Daidzein complexes were estimated by the FT-IR spectra and the curve-fitted results of amide I band, which are in good agreement with the analyses of CD spectra. Furthermore, the displacement experiments indicated that Daidzein can bind to the site I of HSA which is also in agreement with the result of molecule modeling study.