Complexation studies on inositol-phosphates. V. Cu2+, Zn2+, Fe2+ and Fe3+ complexes of some myo-inositol triphosphates

Potentiometry and 31P NMR spectroscopy have been used to study the protonation and complexation properties of some myo-inositol triphosphates (Ins(1,2,6)P3, Ins(1,3,5)P3, and Ins(2,4,6)P3) with Cu2+, Zn2+, Fe2+, and Fe3+. The study was performed for all the ligands at 25°C in a 0.1 M tetrabutylammonium bromide solution (medium 1) and in addition, for Ins(1,2,6)P3, at 37°C in a 0.2 M KCl medium (medium 2). The position of the phosphate groups around the inositol ring largely influences the basicity of the ligand. Log β011 has the highest value for Ins(1,2,6)P3 which displays three vicinal phosphates. When no interfering cations such as K+ are present, M2HL, M2L, MHL, ML, and MOHL complexes were found for most of the systems. Although the stability of the complexes follows the Irving-Williams rule, i.e., Cu(II) > Zn(II) > Fe(II), no real selectivity of complexation is observed. A 31P NMR titration was performed for the Zn2+-H+-Ins(1,2,6)P3 system. The resulting titration curves, compared with those obtained in the absence of metal showed two opposite effects: i) a downfield shift due to the competition of the metal with the proton, ii) an upfield shift corresponding to the binding of Zn2+. From these curves it can, in addition, be concluded that the cation in the mononuclear species is mainly coordinated to P1 and P2 whereas the second cation of the homodinuclear complex is stabilized by P6.