1H-NMR analysis of heteroassociation of caffeine with antibiotic actinomycin D in the aqueous solution

The molecular basis of the action of caffeine as a complex forming agent, an interceptor of aromatic drugs intercalating into DNA was studied by the example of the an anticancer antibiotic actinomycin D examined. The hetero-association of caffeine and actionomycin D was studied by one- and two-dimensional 1H-NMR spectroscopy (500 MHz). Concentration and temperature dependences of the proton chemical shifts of molecules in aqueous solution were measured. The equilibrium reaction constant of hetero-association of caffeine with actinomycin D (K = 246 +/- 48 M-1), the limiting chemical shifts of caffeine protons in complexes were determined. The most favourable structure of the 1:1 caffeine-actinomycin D hetero-complex in aqueous solution was constructed using the calculated values of the induced proton chemical shifts of molecules and the quantum-mechanical iso-shielding curves for caffeine and actinomycin D. The thermo-dynamical parameters of the hetero-complex formation between caffeine and actinomycin D were also determined. The structural and thermo-dynamical analysis showed that dispersive forces and hydrophobic interactions play the major role in hetero-association of caffeine and actinomycin D in aqueous-salt solution. The relative content of different complexes in mixed solutions containing caffeine and actinomycin D was calculated and distinctive features of the dynamic equilibrium of caffeine-actinomycin D hetero-associates were revealed as a function of concentration and temperature. It is concluded that hetero-association of caffeine and actinomycin D molecules a lowers the effective concentration of the drug in solution and hence the pharmacological activity of actinomycin D.