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The 31 P nuclear magnetic resonance (NMR) spectrum of myo ‐inositol hexakisphosphate ( P 6 ‐inositol) in solution consists of four resonances which shift and broaden as a function of pH. From the pH dependence of the resonance positions we calculated an overall hydrogen ion titration curve which corresponds very well with the potentiometric titration data. This implies that the same quantity, namely the degree of dissociation of protons from the phosphate groups, is determined in both experiments. The titration curve covers a pH range of 6.5 and exhibits a pseudo‐equivalence point. It is shown that electrostatic interaction between the six phosphate groups on P 6 ‐inositol, becoming doubly charged at high pH, accounts for the observed extended titration range. A thermodynamic model from which the overall titration curve and the pH dependence of the phosphorous magnetic resonances can be calculated is presented. These calculations demonstrate that the pseudo‐equivalence points observed in the NMR and potentiometric titration data are a result of differences in electrostatic interaction between opposing and adjacent phosphate groups in the P 6 ‐inositol molecule. The pH‐dependent line broadening of the phosphorous resonances is analysed in terms of chemical exchange. It is shown that the proton diffusion rate is enhanced and the hydroxyl‐ion diffusion rate is reduced due to the high charge present on the P 6 ‐inositol molecule. The pH dependence of the 31 P NMR spectrum of myo ‐inositol pentakisphosphate isolated from chicken blood, is discussed in relation to that of P 6 ‐inositol.
Zuiderweg et al. (Thu,) studied this question.