Biopolymers and cell. 2000; 16 (3): 195 - 204

'H-NMR analysis of hetero-association of caffeine with phenanthridinium dye propidium iodide in aqueous solution

D. A. Veselkov, V. A. Sigaev, S. A. Vysotsky, L. N. Djimant, D. B. Davies, A. N. Veselkov

Molecular basis of the caffeine (CAP) action as a complex forming agent — interceptor of aromatic drugs intercalating into DNA, using as an example a typical intercalator, a phenanthridinium dye propidium iodide (PI), has been examined. Hetero-association of ^AF and PI has been studied by one- and two-dimensional H-NMR spectroscopy (500 MHz). Concentration and temperature dependences of the proton chemical shifts of the molecules in aqueous solution have been measured. The equilibrium reaction constant of the hetero-association of CAP with PI at T-298 K(K~ -28 ± 5 M ), the limiting chemical shifts of the protons of caffeine in the complexes have been determined. The most favourable structure of 1:1 CAF-P1 hetero-complex in aqueous solution has been constructed using the calculated values of the induced proton chemical shifts of the molecules and the Quantum-mechanical isoshielding curves for CAP and PI. Thermodynamical parameters of the hetero-complex formation between CAP and PI have been also determined. Structural and thermodynamical analysis has shown that dispersive forces and hydrophobic interactions play the major role in the hetero-association of CAP and PI in aqueous-salt solution. The relative content of different complexes in the mixed solution containing CAP and PI has been calculated and distinctive features of the dynamic equilibrium of the CAP-PI hetero-associates have been revealed as a function of concentration and temperature. It is concluded that hetero-association of CAP and PI molecules leads to lower effective concentration of the dye in solution and respectively to lower biological activity of PI.