The interaction of silibinin with human serum albumin: A spectroscopic investigation

Silibinin is a well-known naturally occurring compound used for the treatment of a wide range of hepatitic diseases. In this study, we report the binding of silibinin to the carrier protein, human serum albumin (HSA) under physiological conditions. UV–Vis and fluorescence quenching methods in combination with Fourier transformed infrared (FT-IR) and circular dichroism (CD) spectroscopy have been used to study the nature and mode of binding. The binding parameters were determined from tryptophan fluorescence quenching by a Scatchard plot and the results were found to be consistent with those obtained from a modified Stern–Volmer equation. The association constant is of the order of 105. The change in enthalpy (ΔH°) and entropy (ΔS°) due to the interaction were found to be −6.76 kJ mol−1 and 73.69 J mol−1 K−1, respectively. These values suggest that apart from an initial hydrophobic association, electrostatic interactions play a decisive role during complexation of silibinin with HSA. Observations from FT-IR and CD spectra upon ligand binding indicate changes in the secondary structure of HSA. Docking studies that corroborate our experimental results reveal that the silibinin molecule lies within hydrogen bonding distance of Trp 214 and Asp 451 residues of subdomains IIa and IIIa of HSA, respectively.