Spectroscopic investigation on the binding of bioactive pyridazinone derivative to human serum albumin and molecular modeling

The interaction between a novel promising pyridazinone derivative (5-chloro-2-nitro-N-(4-(6-oxo-1,4,5,6-tetrahydropyridazin-3-yl)phenyl)benzamide (CNPB)) and human serum albumin (HSA) under physiological conditions has been investigated systematically by fluorescence spectroscopy, UV absorption spectroscopy, circular dichroism (CD) and molecular modeling. From the spectra obtained, it was observed that CNPB had a strong ability to quench the intrinsic fluorescence of HSA through a static quenching procedure. The site binding constants (Kb) were 4.22 × 104 and 3.32 × 104 M−1 at 290 and 300 K, respectively. The alterations of protein secondary structure in the presence of CNPB were qualitative and quantitative calculated by the results from CD and synchronous fluorescence. In addition, the thermodynamic standard enthalpy (ΔH) and standard entropy (ΔS) for the reaction were calculated to be −17.35 kJ mol−1 and 9.57 J mol−1 K−1, respectively. These results showed that the binding of CNPB to HSA was mainly of hydrophobic interaction, but the hydrogen bonding and electrostatic interaction could not be excluded. Furthermore, the study of molecular modeling also indicated that CNPB could strongly bind to the site I (subdomain IIA) of HSA mainly by hydrophobic interaction and there were hydrogen bond interactions between CNPB and the residue His242.