Curcumin and salicylic acid are both phenolic compounds and they can both affect cancer treatment efficacy. In this study, the effects of methylene blue-curcumin (CU-MB) and methylene blue-salicylic acid (SA-MB) ion pair complexes on MDA-MB-231 human breast cancer cells are studied. According to the thermodynamic parameters, the stability of curcumin and salicylic acid complexes ion pair complexes was compared. The free energy of ion pair interactions was calculated based on binding constants. A comparison of the free energies of the complexes (CU-MB: ∆G°b1 = - 21.11 kJ/mol and ∆G°b2 = - 8.37 kJ/mol, SA-MB: ∆G°b1 = - 12.92 kJ/mol and ∆G°b2 = - 9.02 kJ/mol) indicates that the interaction of methylene blue in first binding interaction with curcumin is greater than that of methylene blue with salicylic acid. Electrostatic interactions are the main forces in the binding of both compounds to methylene blue. All forces are inter-molecular physical interactions. The results of cellular experiments show that ion pairing has enhanced the reduction of cell viability. By increasing molecular stability and prevention of dimerization of methylene blue, the cell killing potential of methylene blue increases and it subsequently causes enhancement of photodynamic efficacy.
Keywords: Biothermodynamics; Molecular stability; Photodynamic therapy; Photosensitizer; Polyphenolics.