Differential binding modes of anti-cancer, anti-HIV drugs belonging to isatin family with a model transport protein: A joint refinement from spectroscopic and molecular modeling approaches

The present contribution reports a detailed characterization of the binding interaction of two potential anticancer, anti-HIV drugs isatin (IST) and 1-methylisatin (MI) with model transport protein Bovine Serum Albumin (BSA). Thermodynamic parameters e.g., ΔH, ΔS and ΔG for the binding phenomenon have been evaluated on the basis of van’t Hoff equation to understand the force behind the binding process. A combined application of steady-state and time-resolved fluorescence spectroscopic techniques substantiate the observed drug-induced quenching of intrinsic tryptophanyl fluorescence of the protein to proceed through a static mechanism. Circular dichroic (CD), synchronous fluorescence and excitation–emission matrix fluorescence spectroscopic techniques have been exploited to delineate the secondary and tertiary conformational changes in the protein structure induced by the binding of drugs (IST/MI). The probable binding location of the drug molecules within the protein cavity (hydrophobic subdomain IIIA) has been explored from AutoDock-based blind docking simulation. Examination of drug–protein binding kinetics using stopped-flow fluorescence technique reveals that the association constants (ka) for IST-BSA and MI-BSA interactions are 1.09 × 10−3 s−1 (±5%) and 1.73 × 10−3 s−1 (±5%), respectively, at the experimental temperature (T) of 298 K. The present study also delves into the effect of drug-binding on the esterase activity of the protein which is found to be reduced in the drug–protein conjugate system in comparison with the native protein.