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

The molecular basis of the action of caffeine as a complex forming agent, a n interceptor of aromatic drugs intercalating into DNA was studied by the e xample of the an anticancer antibiotic actinomycin D examined. The heteroas sociation of caffeine and actionomycin D was studied by one- and two-dimens ional H-1-NMR spectroscopy (500 MHz). Concentration and temperature depende nces of the proton chemical shifts of molecules in aqueous solution were me asured. The equilibrium reaction constant of heteroassociation of caffeine with actinomycin D (K 246+/-48 M-1), the limiting chemical shifts of caffei ne protons in complexes;were determined. The most favourable structure of t he 1:1 caffeine-actinomycin D heterocomplex in aqueous solution was constru cted using the calculated values of the induced proton chemical shifts of m olecules and the quantum-mechanical isoshielding curves for caffeine and ac tinomycin D. The thermodynamical parameters of the heterocomplex formation between caffeine and actinomycin D were also determined. The structural and thermodynamical analysis showed that dispersive forces and hydrophobic int eractions play the major role in heteroassociation of caffeine and actinomy cin D in aqueous-salt solution. The relative content of different complexes in mixed solutions containing caffeine and actinomycin D was calculated an d distinctive features of the dynamic equilibrium of caffeine-actinomycin D heteroassociates were revealed as a function of concentration and temperat ure. It is concluded-that heteroassociation of caffeine and actinomycin D m olecules a lowers the effective concentration of the drug in solution and h ence the pharmacological activity of actinomycin D.