A 1H NMR-spectroscopic study of equilibrium associates of ethidium homodimer in aqueous solutions

The self-association of a bifunctional intercalator (ethidium homodimer (EBH)) in an aqueous salt solution (0.1 mol/l NaCl) was studied using one- and two-dimensional H-1 NMR spectroscopies (500 MHz). A physical model of the equilibrium of the different associated homodimer forms in the solution was suggested. The most probable forms are open and folded conformations, dimers, and trimers. The chemical shifts for the protons of the EBH monomer and associates, as well as the equilibrium constants and thermodynamic parameters (enthalpy and entropy) of the self-association, were obtained from the concentration and temperature dependences of the apparent proton chemical shifts. A conclusion was made that the EBH dimers and trimers are stabilized by dispersion and hydrophobic interactions.