Binding interaction of four azo linked copper (II) complexes with Human Serum Albumin (HSA): Spectroscopic and molecular docking explorations

Four copper (II) complexes [Cu(L1)2] (1), [Cu(L2)2] (2), [Cu(L3)2] (3) and [Cu(L4)2] (4) with azo linked O,O donar ligands viz 2-hydroxy-5 (phenyldiazenyl)benzaldehyde (HL1), 1-(2-hydroxy-5-(phenyldiazenyl)phenyl) ethanone (HL2), 2-Hydroxy-5-p-tolylazo-benzaldehyde (HL3), 1-(2-Hydroxy-5-p-tolylazo-phenyl)-ethanone (HL4) have been prepared and characterized by different spectroscopy methods and published earlier by our research group. We investigated the interaction of Human Serum Albumin (HSA) with complexes 1-4 under physiological condition in phosphate buffer solution at pH 7.4 using various spectroscopic techniques. The fluorescence titration spectrum disclosed that the complex 1-4 quench the intrinsic fluorescence of HSA robustly through a static quenching mechanism. Binding constants (Kb) and the number of binding sites (n approximate to 1) were evaluated using modified Stern-Volmer equations. Binding constants were found to be 1.9, 6.6, 0.99 and 1.2x 105 (M-1) for complex 1, 2, 3 and 4 respectively. The CD spectra of free HSA and HSA with complexes 1-4 showed that the complexes have negligibleimpact on the secondary structure of HSA as theyremain helical even after the addition of complexes. Molecular docking study was performed to analyse the binding mode of complexes 1-4with HSA. Docking study revealed that hydrophobic and Vander waals interactions were considered to be the main interaction forces involved in the binding of HSA with complexes 1-4.