Interaction of Merocyanine 540 with serum albumins: photophysical and binding studies.

Photophysical studies on binding interactions of a negatively charged anti-tumor photosensitizer, Merocyanine 540 (MC 540), with serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA), have been performed using absorption and steady-state as well as time-resolved fluorescence techniques. Formation of ground state complex has been confirmed from the detailed studies of absorption spectra of MC 540 in presence of SAs producing isosbestic points. Binding between the proteins and MC 540, which perturbs the existing equilibrium between the fluorescent monomer and its non-fluorescent dimer, induces a remarkable enhancement in fluorescence anisotropy and intensity of MC 540 along with a red shift of its maximum. The binding stoichiometry of MC 540 and SAs are more than 1.0 which depicts that two types of complexes, i.e., 1:1 and 2:1 are formed with addition of varied concentration of protein. Both the steady-state and time-resolved fluorescence results show that in 2:1 complex one of the MC 540 molecules is exposed towards aqueous environment with a greater extent when bound with HSA compared to BSA due to the structural flexibility of that protein. Thermodynamic analyses using van't Hoff plot indicate that the binding between MC 540 and individual SA is an entropy-driven phenomenon. The probable hydrophobic binding site has been located by denaturation of proteins, micropolarity measurement and Förster resonance energy transfer and that is further supported by molecular docking studies. Changes in circular dichroism spectra of BSA in presence of MC 540 depict secondary structural changes of the protein. The induced-CD shows that BSA due to its rigid structure generates chirality in MC 540 much more efficiently compared to HSA.