Influence of Mg+2 and Cu+2 on the interaction between quinolone and calf thymus DNA

Mg(+2) and Cu(+2) have different binding capacities to quinolone drugs and have different binding modes with calf thymus DNA. Using the method of absorption and fluorescence spectroscopy, the influence of Mg(+2) and Cu(+2) on the binding between calf thymus DNA and each of four quinolone drugs has been studied. The results show that both Mg(+2) and Cu(+2) can bind with the four drugs. In the absence of divalent metal ions, quinolone drugs interact with DNA double helix by forming hydrogen bonds between the carboxyl and carbonyl groups of the drugs and the phosphate groups of the DNA bases, and the binding capacity shows a close relationship with the drug structures. The two metal ions show different influences on the binding between the drug and DNA, which depends on the type of ion, concentration of the metal ions and the structure of drugs. Mg(+2) in lower concentrations (0.01 mM to 3.0 mM) can act as a bridge between the carboxyl group/carbonyl group of the drug and the phosphate group of the DNA by electrostatic interaction, while Cu(+2) can act as an intermediary ion between carboxyl group/carbonyl group of the drug and the DNA bases by a co-ordinate bond. Both actions can increase the interaction of the π electron between the condensed rings of the drugs and the DNA bases. In some conditions, Cu(+2) can weaken the binding between the drug and the DNA by competitive inhibition if there is a site on the drug that can directly bind both DNA bases and Cu(+2).