Quantitative study on the non-covalent interactions between ATP and caffeine, theophylline and theobromine in aqueous solution

An investigation is hereby presented on the possible formation of a stacking (pi-pi) complex between adenosine-5'-triphosphate (ATP) and three xanthines (L), namely caffeine (CD, theophylline (Tp) and theobromine (Tb), at different ionic strengths (0.05 <= I/mol L-1 <= 1) in NaCl(aq) and temperature (T=283.15, 298.15 and 313.15 K), by using magnetic resonance spectroscopy H-1 NMR. The values of log beta of the [ATP(L)](4-) species were: 1.32, 1.24 and <= 0.7 at I=0.05 mol L-1 and T=298.15 K for Cf, Tp and Tb, respectively. The stability constant of [ATP(Cf)](4-) species shown a linear dependence on the temperature, and the values of log beta obtained were: 1.18,1.32 and 1.47 at T=283.15, 298.15 and 313.15 K, respectively. By using the values of log beta obtained at different temperatures a value of Delta H=-16 +/- 3 kJ mol(-1) was calculated for the formation of the stacking (pi-pi) complex species. The three systems were also studied potentiometrically at different ionic strengths (0.05 <= I/mol L-1 <= 1) in NaCl(aq) and T=298.15 K. For all the investigated systems, the formation of [(ATP)(L)H-i]((i-4)) species (i=1-3), with high yields (60-80%) was found: the stability trend obtained for the protonated complex species was (ATP-Tb)>(ATP-Tp)>(ATP-Cf). The stability constants of all the [(ATP)(L)H-i]((i-4)) species proved fairly dependent on the ionic strength that was studied by using the Debye-Huckel-type equation. Furthermore, by comparison with the stability of the species of analogous systems, such as ATP-aliphatic amines, ATP-aromatic amines and pyrophosphate-aliphatic amines, an empirical relationship between the stability of the complexes and the total basicity of the ligand was found. (C) 2011 Elsevier B.V. All rights reserved.