Effects of 2-amino-8-hydroxyquinoline interaction on the conformation of physiological isomers of human serum albumin

The methods of synthetic chemistry create small molecules rapidly for screening, and ligand–protein interaction studies provide information on how a potential drug interacts with target or carrier proteins such as serum albumin. In this work, we investigate the interaction of amino derivative of 8-hydroxyquinoline, 2-amino-8-hydroxyquinoline (A8HQ), and the effects of its binding on the conformation of different isomers of human serum albumin (HSA) using multispectroscopic techniques and molecular modeling. We found that B isomer, which exists at pH 9, bound A8HQ ( K a = 1.92 ± 0.07 × 10 5 M −1 at 298 K) more strongly as compared with N isomer ( K a = 1.19 ± 0.04 × 10 5 M −1 at 298 K) of HSA, which is known to exist around pH 6. The binding constant at physiological pH (7.4) was also determined, and the value ( K a = 1.38 ± 0.05 × 10 5 M −1 at 298 K) was found to fall between those for N and B isomers, suggesting that both the N and B isomers exist in an equilibrium in plasma. We also determined the thermodynamic parameters such as changes in enthalpy, entropy , and free energy of binding by measuring the binding at four different temperatures. Based on molecular modeling and thermodynamic studies, we propound that the A8HQ–HSA binding involves mainly hydrophobic interactions and hydrogen bonding. Site-specific marker displacement experiments and molecular modeling showed that the molecule preferably binds in subdomain IIA close to Trp214. A8HQ binding to HSA isomers was found to cause both secondary and tertiary structural alterations in the protein.