Interaction of esculetin with aluminium ion by spectroscopic studies and isothermal titration calorimetry: a probable molecule for chelation therapy

Abstract

The use of aluminium has made significant impact in our life by virtue of its attractive properties. The lack of essentiality of aluminium in biosphere indicated that its accumulation above certain level is undesirous. Esculetin (6,7-dihydroxy coumarin) is an excellent aluminium ion chelator and the chelation interaction was studied by exploiting the absorption and fluorescence behavior of esculetin. In presence of aluminium ion, the absorption band of esculetin was shifted from 350 to 380 nm suggesting the possibility of complex formation. The fluorescence intensity of esculetin at 466 nm was significantly quenched in presence of aluminium ion. The fluorescence quenching was interpreted in terms of chelation-quenched fluorescence (CHQF) mechanism where the strong Lewis acid character of aluminium ion accepts electrons from the chelating catechol moiety of the excited esculetin. From the absorption and fluorescence changes the association constant was estimated in the order of 105 M −1. The association constant was further evaluated by isothermal titration calorimetry (ITC) and there was close agreement to that of obtained from spectroscopic studies. Form ITC studies, the binding enthalpy and binding entropy were estimated as −20.6 kcal/mol and −46.7 cal/mol/K respectively. The complex was less toxic compared to the individual complexing agents when studied in Chinese hamster ovary cells. Considering the present investigation, esculetin can be a probable molecule for chelation therapy where rapid complex formation ability of esculetin will help to reduce the aluminium accumulation through chelation and water soluble nature of the complex will help for faster elimination from the system. Communicated by Ramaswamy H. Sarma