Thermodynamic study of the non covalent interactions of phytate with xanthine derivatives and histamine in aqueous solution

The interactions between some xanthine derivatives: caffeine (Cf), theophylline (Tp), theobromine (Tb), and histamine (Hm) with phytate (Phy) were studied at T=298.15 K and I=0.5 mol L−1 in aqueous NaCl solution using ISE-[H+] potentiometry. For all of the investigated systems, [(Phy)(L)Hq](q−7) species (q=4 to 9, depending on the maximum protonation degree of the ligand) were formed in high yields (30–90%). The stability constants of the systems varied; higher stability was observed for the [(Phy)(Tb)Hq](q−7) (5≤q≤8) species. For the [(Phy)(Tb)H8]+ and [(Phy)(Tb)H7] species,we obtained values of log K8=8.1 and log K7=5.58, respectively. The formation ΔH and TΔS parameters were determined for the Phy–Cf system at I=0.5 mol L−1 in NaCl(aq) by calorimetric titrations. As an example, for the [(Phy)(Cf)H5]2− species, we obtained the following values: ΔH=−4.0±0.2 kJ mol−1, −ΔG=14.6±0.5 kJ mol−1 and TΔS=10.5±0.8 kJ mol−1.