Conformational preferences and protonation sequence of myo-inositol hexaphosphate in aqueous solution; potentiometric and multinuclear magnetic resonance studies

Potentiometric methods and multinuclear NMR spectroscopy have been used to determine the protonation sequence and conformational preferences of myo-inositol (cyclohexane-1,2,3,4,5,6-hexol) hexaphosphate over the range pH 0–12. The 1H, 31P and 13C resonances have been assigned from both homo- and hetero-nuclear coupling constants obtained by one- and two-dimensional NMR spectroscopy. The phosphate exists in aqueous solution in either of two conformations, axial and equatorial, as a result of intramolecular hydrogen-bond formation between phosphate groups which leads to stabilization of the equatorial form up to pH 10 and of the axial at pH > 10. Analysis of lanthanide-induced shift data shows unambiguously that the last protonation occurs on the P2 phosphate group in strongly acidic media.