Conformational studies of myo-inositol phosphates

The discovery of the second messenger role of myo-inositol 1,4,5-trisdihydrogenphosphate [Ins(1,4,5)P3] has triggered tremendous interest in investigating the structure, metabolism, and biological roles of inositol phosphates. Although the conformation of phytic acid [(myo-inositol hexakisdihydrogenphosphate), Ins P6] has been the subject of much study, the conformations of lower inositol phosphates such as inositol-pentakis-, tetrakis-, and tris-dihydrogenphosphates have not been investigated. We investigated, by 1H NMR spectroscopy, the conformations of inositol phosphates (Ins P5, Ins P4, Ins P3, Ins P2, and Ins P1) and monitored the influence of pH on conformational preferences. Ins P6 adopts the sterically stable 1ax/5eq (one phosphate in the axial position and five phosphates in the equatorial position) conformation in the pH range 0.5-9.0, and the sterically hindered 5ax/1eq (five phosphates in the axial position and one phosphate in the equatorial position) conformation above pH 9.5. At pH 9.5, both conformations are in dynamic equilibrium. Ins(1,2,3,4,6)P5 and Ins(1,2,3,5,6)P5 adopt the 1ax/5eq form in the pH range 1.0-9.0; in the pH range 9.5-13.0, the 1ax/5eq and 5ax/1eq conformations are in dynamic equilibrium. In contrast to Ins P6 and Ins P5, all the lower inositol phosphates (Ins P4 to Ins P1) investigated adopt the 1ax/5eq conformation over the entire pH range, 1.0-13.0. Preference for the 5ax/1eq conformation by Ins P6 and Ins P5 is probably due to decreased electrostatic repulsion between negatively charged vicinal equatorial phosphates in the 1ax/5eq conformation and stabilization of the sterically hindered 5ax/1eq conformation by hydrogen bonding and/or sodium counter-ions bonding between the syn-oriented phosphates. On the basis of conformations adopted by the inositol phosphates (Ins P6 to Ins P1) at different pH, we conclude that the presence of four or five equatorial phosphates on the inositol ring induces a change in the conformation from the sterically unhindered 1ax/5eq structure to the sterically hindered 5ax/1eq conformation, at high pH. This investigation illustrates that the conformational preferences of inositol phosphates at different pH is unique to the particular isomer and does not parallel the behaviour of phytic acid.