Study of the Interaction between Bovine Serum Albumin and Three Copper(II)-imine Complexes by Electronic Absorption Spectroscopy

Three complexes: bis(2,2′-bipyridine)-, bis(1,10-phenanthroline)- and (2,2′-bipyridine)(1,10-phenanthroline)-chlorocopper(II) chloride with molecular formula of [Cu(bpy)2Cl]Cl, [Cu(phen)2Cl]Cl and [Cu(bpy)(phen)Cl]Cl, having trigonal-bipyramidal geometry were prepared according to the reported procedure via the reaction of CuCl2 with bpy and phen ligands. The interaction of these complexes with bovine serum albumin (BSA) was studied by electronic absorption spectroscopy at two temperatures of 296 and 310 K in Tris buffer medium (pH = 7) containing sodium chloride (10 mM). These interactions include the determination of the binding constant of each metal complex to serum albumin (K), the midpoint concentration of each metal complex that can denature half of the BSA molecules present in the medium (L1/2), and the thermodynamic studies. The binding constant values obtained in the range of 2.08 ‒ 65.7 × 104 M‒1 indicate the proper interaction of each complex with BSA. In the titration of BSA with each of the complexes, it was found that the conformation of this protein changed upon the addition of these compounds. It is because the absorbance of BSA at 280 nm showed a significant decrease with increasing the concentration of each compound. While studying thermodynamic parameters: the negative values of Gibbs free energy changes in the interaction of all three complexes with BSA, confirmed the spontaneous nature of the reaction between them. The positive values of standard enthalpy changes for the [Cu(bpy)2Cl]Cl and [Cu(bpy)(phen)Cl]Cl and the negative value for the [Cu(phen)2Cl]Cl indicate that these processes are endothermic and exothermic, respectively. In addition, the positive values of the standard entropy changes for the [Cu(bpy)2Cl]Cl and [Cu(bpy)(phen)Cl]Cl indicate the irregularity of the system (entropy driven) and its negativity for the [Cu(phen)2Cl]Cl indicates ordering. The positive values of ΔH° and ΔS° for the [Cu(bpy)2Cl]Cl and [Cu(bpy)(phen)Cl]Cl and the negative values for [Cu(phen)2Cl]Cl complexes indicate hydrophobic and H-binding + van der Waals interactions, respectively.