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

Abstract

The molecular mechanism of the action of caffeine (CAF) as a complexing interceptor of aromatic ligands intercalated in DNA is considered using a typical intercalant — acridine orange (AO) dye. Hetero-association of CAF and AO was investigated by one- and two-dimensional H NMR spectroscopy (500 MHz). 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 hetero-association reactions at different temperatures and the limiting chemical shifts of the protons of the aromatic ligands of the associates were determined. The most plausible structure of the 1:1 CAF-AO helerocomplex in aqueous solution is suggested based on the calculated values of the induced proton chemical shifts of the molecules and the quantum mechanical screening curves for CAF and AO. The thermodynamic parameters of CAF-AO helerocomplex formation art calculated. The structural and thermodynamic analyses indicate that dispersion forces and hydrophobic interactions play a significant role in heterocomplex formation in aqueous 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 temperature. Heteroassociation of CAF and AO molecules leads to decreased effective concentration of intercalant in solution and hence to decreased mutagenic activity of the dye.