Interplay of Multiple Interaction Forces: Binding of Norfloxacin to Human Serum Albumin.

Herein, the binding interaction of a potential chemotherapeutic antibacterial drug norfloxacin (NOF) with a serum transport protein, human serum albumin (HSA), is investigated. The prototropic transformation of the drug (NOF) is found to be remarkably modified following interaction with the protein as manifested through significant modulations of the photophysics of the drug. The predominant zwitterionic form of NOF in aqueous buffer phase undergoes transformation to the cationic form within the protein-encapsulated state. This implies the possible role of electrostatic interaction force in NOF-HSA binding. This postulate is further substantiated from the effect of ionic strength on the interaction process. To this end, the detailed study of the thermodynamics of the drug-protein interaction process from isothermal titration calorimetric (ITC) experiments is found to unfold the signature of electrostatic as well as hydrophobic interaction forces underlying the binding process. Thus, interplay of more than one interaction forces is argued to be responsible for the overall drug-protein binding. The ITC results reveal an important finding in terms of enthalpy-entropy compensation (EEC) characterizing the NOF-HSA binding. The effect of drug-binding on the native protein conformation has also been evaluated from circular dichroism (CD) spectroscopy which unveils partial rupture of the protein secondary structure. In conjunction to this, the functionality of the native protein (in terms of esterase-like activity) is found to be lowered as a result of binding with NOF. The AutoDock-based docking simulation unravels the probable binding location of NOF within the hydrophilic subdomain IA of HSA. The present program also focuses on exploring the dynamical aspects of the drug-protein interaction scenario. The rotational-relaxation dynamics of the protein-bound drug reveals the not-so-common "dip-and-rise" pattern.