Synthesis, characterization and DFT study of new water-soluble aluminum(III), gallium(III) and indium(III) Schiff base complexes: effect of metal on the binding propensity with bovine serum albumin in water

In this work, three new water-soluble aluminum(III), gallium(III) and indium(III) Schiff base complexes; [M(L)NO3], where L denotes; N,N′-bis(5-sulfosalicyliden)-1,2-phenylendiamin (5-SO3-1,2-salophen); were synthesized and characterized by UV–Vis spectroscopy, 1H NMR, FT-IR, thermal gravimetry and C.H.N analyses. Also, the optimized structure and the energy level of the frontier orbitals for the complexes were computed using B3LYP/LANL2DZ method and 6–311g basis set. The interaction of these complexes with bovine serum albumin (BSA) was studied by fluorescence and UV–Vis techniques. The results showed that the intrinsic fluorescence of BSA was quenched by adding the complexes, which was rationalized in terms of the dynamic quenching mechanism. Binding constants, Stern–Volmer constants, number of binding sites and binding stoichiometry were determined by fluorescence method. Furthermore, the thermodynamic parameters calculating by van’t Hoff equation, indicated that the binding mode is entropy driven and enthalpically disfavored. Site marker competitive experiments revealed that these complexes can bind to the subdomain IIIA (site II) of BSA. Based on the Förster theory of non-radiation energy transfer, the efficiency of energy transfer and the distance between the donor (BSA) and the acceptor (complex) were obtained. Also, the synchronous fluorescence spectra displayed that the microenvironment of the tyrosine residues was not apparently changed. Finally, the binding of the complexes to BSA was modeled by molecular docking method. Excellent agreement was found between the experimental and theoretical results with respect to the mechanism of binding and binding constants. Results indicated that the order of propensity of the complexes to BSA was as follows: [In(L)NO3] > [Ga(L)NO3] > [Al(L)NO3].