Conformational preferences and intramolecular interactions of myo-inositol hexakisphosphoric acid by1H and31P NMR studies

The long-standing issue of the conformational change of myo-inositol hexakisphosphoric acid (H12inhp), commonly called phytic acid, was resolved by low-temperature pH* studies and NMR spectroscopy. Low-temperature experiments on phytic acid, in a suitable mixed solvent and in the appropriate pH* range (10.0 < pH* < 11.3), allowed the detection of separate NMR lines corresponding to two conformers. Chemical shift variations as a function of pH* reveal that at pH* >10.2, the mono-, di- and triprotonated species, and also the entirely deprotonated species, are stabilized in the axial form (five C—O in the axial position and one C—O in the equatorial position) and at pH* <11.3, in the equatorial form for other protonated species of phytic acid (one C—O in the axial position and five C—O in the equatorial position). From the NMR spectra, we conclude that the structural change is triggered by the ninth acid dissociation of phytic acid. We suggest that the stabilization of the axial and equatorial conformations is due partly to the presence of C—H … O—P through space interactions, and partly to trans-annular hydrogen bonding between all the phosphate groups.