¹H NMR investigation of the hetero-association of aromatic molecules in aqueous solution: factors involved in the stabilization of complexes of daunomycin and acridine drugs

Hetero-association of the anthracycline drug, daunomycin (DAU), with typical mutagens, the acridine dyes proflavine (PF) and acridine orange (AO), has been studied by 500 MHz 1H NMR spectroscopy as a function of concentration and temperature in 0.1 mol dm−3 phosphate buffered aqueous solutions at pD = 7.1. The results have been analysed in terms of a statistical-thermodynamical model of hetero-association of aromatic molecules, described previously [Davies, D. B., Veselkov, D. A., and Veselkov, A. N., 1999, Molec. Phys., 97, 439], but generalized in this work, so that there is no limitation on the magnitudes of the self-association constants of the interacting molecules. Expressions suitable for the analysis of NMR parameters of both components in the mixed solution have been developed enabling both the structural and thermodynamic properties of hetero-association to be determined. The magnitude of the equilibrium constant for hetero-association of PF + DAU is found to be substantially higher than the self-association constants of these molecules, whereas that for hetero-association of AO + DAU is intermediate between the equilibrium constants of self-association of AO and DAU. Intermolecular cross-peaks observed in 2D-ROESY spectra of PF + DAU mixed solutions are consistent with formation of a hetero-association complex in which an intermolecular hydrogen bond can form between either of the 3,6-diamino groups of the PF chromophore and the 9-MeCO group of DAU, which is in contrast to AO + DAU hetero-association, where such hydrogen bonds are unable to form. Quantitative structural and thermodynamical analysis of PF + DAU complexation is consistent with an intermolecular hydrogen bond contributing to the stability of the hetero-complex in aqueous solution. The NMR results show that hydrophobic interactions play a substantial role in the stabilization of the AO-DAU complex, characterized by a relatively small entropy change on complexation, compared to the PF-DAU hetero-complex, which is mainly stabilized by hydrogen bond and dispersive van der Waals interactions.