1H NMR determination of the self-association of an acridine homodimer and its complexation with ethidium bromide in aqueous solution

1H NMR spectroscopy (500 MHz) has been used to investigate the self-association in aqueous buffered solution of a bis-intercalator, acridine homodimer (AcrH), and its hetero-association with the aromatic dye, ethidium bromide (EB). The equilibrium constants and thermodynamical parameters (enthalpy and entropy) of self-association have been determined from the observed concentration and temperature dependences of chemical shifts of AcrH protons and the results are consistent with a model consisting of at least four distinct forms of AcrH molecules in solution: unfolded (U), folded (F), a dimer formed from two folded molecules (F2) and a trimer formed from three folded molecules (F3). It has also been shown that ethidium bromide complexes strongly to the dimer form (F2) of the bis-acridine molecule, AcrH. Comparison of the calculated association parameters of AcrH with the previously studied ethidium homodimer (EBH) revealed a correlation between the effectiveness of complexation and the length of chain connecting the chromophores of a bis-intercalator.