A proton NMR study of heteroassociation between caffeine and proflavine in water

The molecular mechanism of the action of caffeine (CAF) as a complex-forming agent intercepting aromatic ligands that form intercalates with DNA is considered for the example of a typical ligand proflavine (PF, an acridine dye). Heteroassociation between CAF and PF was studied by one- and two-dimensional NMR spectroscopy (500 MHz), The concentration (at 298 and 308 K) and temperature dependences of the proton chemical shifts of the molecules in an aqueous solution were studied with the use of a model taking into account the formation of aggregates with infinite numbers of monomer units in both self- and heteroassociation reactions, the equilibrium constants for heteroassociation between CAF and PF were calculated, and the limiting proton chemical shifts of aromatic ligands incorporated into associates were determined. The most probable structure of the 1 : 1 CAF-PF heterocomplex in an aqueous solution was calculated. Calculations were also performed to determine the relative contents of associates of various types in a mixed solution containing CAF and PF. The special features of dynamic equilibrium between CAF-PF heteroassociates were analyzed as depending on the concentration of caffeine in mixed solutions and temperature. The heteroassociation between CAF and PF molecules decreases the effective concentration and, accordingly, the mutagenic activity of the acridine dye.