H-1 NMR study of heteroassociation of caffeine with acridine orange in aqueous solution

The molecular mechanism of the action of caffeine (CAF) as a complexing int erceptor of aromatic ligands intercalated in DNA is considered using a typi cal intercalant - acridine orange (AO) dye. Hetero association of CAF and A O was investigated by one- and two-dimensional H-1 NMR spectroscopy (500 MH z). The concentration (at 298 and 308 K) and temperature dependences of the proton chemical shifts of molecules in aqueous solution were measured. The equilibrium constants of the CAF-AO heteroassociation reactions at differe nt temperatures and the limiting chemical shifts of the protons of the arom atic ligands of the associates were determined. The most plausible structur e of the 1:1 CAF-AO heterocomplex in aqueous solution is suggested based on the calculated values of the induced proton chemical shifts of the molecul es and the quantum mechanical screening curves for CAF and AO. The thermody namic parameters of CAF-AO heterocomplex formation are calculated. The stru ctural and thermodynamic analyses indicate that dispersion fomes and hydrop hobic interactions play a significant role in heterocomplex formation in aq ueous salt solution. The relative contents of different types of associate in a mixed solution containing CAF and AO are estimated. The equilibrium of CAF-AO heteroassociates in solution is characterized in relation to temper ature. Heteroassociation of CAF and AO molecules leads to decreased effecti ve concentration of intercalant in solution and hence to decreased mutageni c activity of the dye.