Synthesis and copper-mediated nuclease activity of a tetracationic tris(2,2'-bipyridine) ligand

We report herein the synthesis of a novel tetracationic tris(2,2'-bipyridine) ligand 4. We show that this ligand metalated with copper(II), and in the presence of ascorbate as a reducing agent, strongly damages pUC18 plasmid DNA. Copper complex formation was demonstrated by ESI-MS (electrospray ionization-mass spectrum) at a 1:3 ligand to metal ratio. Binding of both 4 and its copper(II) complex to CT-DNA (calf thymus DNA) was characterized by viscosimetry, thermal denaturation and fluorescence-based competition assays. The viscosimetric data indicated that 4 and its copper(II) complex bind DNA through partial intercalation and thermal denaturation studies revealed a significant increase of duplex DNA stability in the presence of these species (DeltaT(m)=16.4 and 18.3 degrees C, respectively). Moreover, 4 and its copper(II) complex were found to effectively compete with ethidium bromide for the intercalative binding sites of DNA. Overall, the copper(II)-4 complex constitutes a very efficient DNA cleaving agent in the presence of ascorbate. Experiments with scavengers further suggest that the generation of Cu(I), hydrogen peroxide, superoxide, hydroxyl radical and singlet oxygen-like species contributes to the DNA breakage induced by the Cu(II) complex of 4.