Structural and Thermodynamic Investigation into the Protein-Binding Properties of a Natural Product Crytotanshinone

Crytotanshinone (CTSO) is a Chinese herbal medicine active ingredient isolated from Salvia miltiorrhiza. In this work, the interaction of CTSO and human serum albumin (HSA) was studied by fluorescence spectra, ultraviolet spectra, circular dichroism (CD) spectra, molecular probe and molecular modeling methods. The results showed that the endogenous fluorescence of HSA was quenched by CTSO through a static mechanism. The number of binding sites, equilibrium constants, and thermodynamic parameters of the reaction were calculated at three different temperatures. The positive enthalpy change (Δ H θ ) and entropy change (Δ S θ ) revealed that the interaction was an endothermic as well as an entropy-driven process, where hydrophobic power played the major role in stabilizing the structure of the new complex. Site-selective binding experiments were carried out using warfarin and ibuprofen as probes, which proved that CTSO binds to Sudlow’s site II in subdomain IIIA of the HSA molecule. Circular dichroism (CD) spectra was employed to detect the α -helix and β -strand contents in HSA before and after the binding of CTSO. Based on the experimental results, the structure of the CTSO–HSA complex was calculated by docking CTSO to the proven site using molecular modeling. The study obtained comprehensive information on structure and thermodynamics, which is essential for understanding the bioaffinity, delivery process and pharmacological mechanism.