Investigating the site selective binding of busulfan to human serum albumin: Biophysical and molecular docking approaches

We have studied the binding of busulfan (BN) to human serum albumin (HSA) at physiological pH 7.4 by using fluorescence, UV-vis and circular dichroism (CD) spectroscopic tools, as well as dynamic light scattering (DLS) measurements and molecular simulation approaches. HSA fluorescence quenching experiments showed that BN reduces the HSA native fluorescence intensity through the static mechanism. In addition, a single binding site on the HSA is occupied by BN with a binding constant at 298 K of 1.84 × 103 M−1. The enthalpy change (ΔH) and entropy change (ΔS) of BN-HSA interaction were calculated as −1.40 kcal mol−1 and +10.14 cal mol−1 K−1 respectively, which suggest the possible interaction mode as hydrophobic and hydrogen bonding. Moreover, the secondary structure alteration of HSA following its complexation with BN was studied and showed that α-helical content of HSA gets increased on interacting with BN. Ligand binding site to HSA was further investigated by site-specific markers in fluorescence measurements as well molecular modeling approach which indicated that BN bind to the nearby sudlow site II of HSA through hydrophobic as well as hydrogen bonding interaction. The present study will be helpful for understanding the binding mechanism of BN to human serum albumin.