Binding Interaction of Captopril with Metal Ions: A Fluorescence Quenching Study

The binding interaction of captopril (CPL) with biologically active metal ions Mg(2+), Ca(2+), Mn(2+), Co(2+), Ni(2+) Cu(2+) and Zn(2+) was investigated in an aqueous acidic medium by fluorescence spectroscopy. The experimental results showed that the metal ions quenched the intrinsic fluorescence of CPL by forming CPL-metal complexes. It was found that static quenching was the main reason for the fluorescence quenching. The quenching constant in the case of Cu(2+) was highest among all quenchers, perhaps due to its high nuclear charge and small size. Quenching of CPL by metal ions follows the order Cu(2+) > Ni(2+) > Co(2+) > Ca(2+) > Zn(2+) > Mn(2+) > Mg(2+). The quenching constant K(sv), bimolecular quenching constant K(q), binding constant K and the binding sites "n" were determined together with their thermodynamic parameters at 27 and 37 degrees C. The positive entropy change indicated the gain in configurational entropy as a result of chelation. The process of interaction was spontaneous and mainly Delta S-driven.