Biophysical investigation of thymoquinone binding to 'N' and 'B' isoforms of human serum albumin: exploring the interaction mechanism and radical scavenging activity

Thymoquinone (TQ) is the main constituent of Nigella sativa and is traditionally used as a folk medicine. Our aim was to investigate the binding mechanism of TQ to human serum albumin (HSA) isoforms ('N' form at pH 7.4 and 'B' form at pH 9.0) using biophysical methods such as intrinsic tryptophan fluorescence quenching, isothermal titration calorimetry (ITC), circular dichroism (CD), dynamic light scattering (DLS), Forster resonance energy transfer (FRET) and antioxidant activity in the absence and presence of TQ. We have calculated the binding and thermodynamic parameters from spectroscopic and calorimetric methods. CD and DLS were respectively used to monitor the changes in the secondary structure and hydrodynamic radii of HSA as a result of its interaction with TQ. The esterase and antioxidant or radical scavenging activities of both the isoforms of HSA were investigated in the absence/presence of TQ. The antioxidant activity of TQ was remarkably enhanced upon its interaction with HSA. Therefore, the efficiency of HSA to scavenge the free radical ions was increased in the presence of TQ which is generated in the body by various metabolic processes.