1H NMR study of heteroassociation of ethidium homodimer and propidium iodide in water

ID and 2D H-1 NMR spectroscopy (500 MHz) was used to study heteroassociation of the bifunctional intercalator ethidium homodimer (EBH) with a phenanthridine dye propidium iodide (PI) in an aqueous salt (0.1 mol/l NaCl). A physical model of equilibrium between various associated forms of ethidium homodimer and propidium in solution is suggested, the most probable forms being 1:1 and 1:2 complexes of P1with the EBH monomer and dimer. The chemical shifts of the EBH molecule in heterocomplexes, as well as the equilibrium constants and the thermodynamic parameters (enthalpy, entropy) of heteroassociations, were calculated from the concentration and temperature dependences of the proton chemical shifts. It is concluded that the propidium molecule is built (intercalated) into the EBH monomer and dimer to form 1:1 and 1:2 complexes mainly stabilized by dispersive and hydrophobic interactions.