31P nuclear magnetic resonance-pH titrations of myo-inositol hexaphosphate

With the use of 31P n.m.r. spectroscopy, the separate pKa values of each of the six phosphoric monoester groups of myo-inositol hexaphosphate were determined. The range of hydrogen-ion concentrations covered extended from that required for the phosphonium salts to that for the full dodecyl anion, and the determinations were carried out in the presence of sodium and tetrabutylammonium cations. The pKa for each phosphate grouping in the transition from the free acid forms of each group to the monoanion form of each group was determined to be: 1.1, C-2; 1.5, C-1 and C-3; 2.1, C-4 and C-6; and 1.7, C-5. In the mono- to di-anion transition, the pKa values were: 6.85, C-2; 7.60, C-5; 5.70 and 12.0, C-1 and C-3; and 10.0, C-4 and C-6. These data and the appearance of the 31P hexaphosphate n.m.r. multiplet are discussed in terms of conformations of myo-inositol hexaphosphate.