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

The binding interaction of captopril (CPL) with biologically active metal ions Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+and Zn2+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 Cu2+was highest among all quenchers, perhaps due to its high nuclear charge and small size. Quenching of CPL by metal ions follows the order Cu2+>Ni2+>Co2+>Ca2+>Zn2+>Mn2+>Mg2+. The quenching constantKsv, bimolecular quenching constantKq, binding constantKand the binding sites "n" were determined together with their thermodynamic parameters at 27 and 37°C. The positive entropy change indicated the gain in configurational entropy as a result of chelation. The process of interaction was spontaneous and mainly ΔS‐driven.