Interaction of the copper(II) macrocyclic complexes with DNA studied by fluorescence quenching of ethidium

The fluorescence quenching of DNA bound ethidium ion by copper(II) macrocyclic complexes has been investigated. The binding constant indicates that the stable assembly can be formed between the DNA and the metal complex. The replacement of the fluorophore by the metal complexes results in a decrease of the ethidium moles intercalating to the molar DNA base pair and the binding constant of the ethidium to the DNA. The quenching of the ethidium excited state by the Cu(II) complex follows linear Stern-Volmer behavior. The quenching constant decreases regularly with an increase of the ratio of the concentration of the bound ethidium to that of DNA base pair. The dependence of the quenching constant on the ratio can be used to estimate binding constants for the fluorescent molecule and metal complexes to DNA. This method does not depend strongly on the type and the extent of interaction of metal complexes with DNA, allowing for the determination of binding constants for metal complexes that exhibit small changes in absorption spectra upon binding. The precision of this method will ultimately be governed by outstanding agreement between the quenching constant measured and that calculated by using a correlation function of the ratio or the DNA bound concentration of ethidium with quenching constant.