Inframolecular protonation process of myo-inositol 1,4,5-tris(phosphate) and related compounds:: Dynamics of the intramolecular interactions and evidence of C-H•••O hydrogen bending

The intramolecular protonation process of myo-inositol 1.3,5-tris(phosphate) (1, Ins(1,4,5)P-3) and the closely related analogues myo-inositol 1,4,6-tris(phosphate) (2, Ins(1,4,6)P-3), and 3-deoxy-myo-inositol 1,4,5-tris(phosphate) (3) and the latter's epimer, 3-deoxy-muco-inositol 1,3,5-tris(phosphate) (4). were explored by performing P-31 and H-1 NMR titration experiments. The microprotonation scheme for compounds 1, 2, and 4 were quantitatively derived. The influence of the configuration of the functional groups and of the presence of the hydroxyls on the P-31 and H-1 chemical shifts and phosphate basicity was discussed. Thus, the basicity increase of the phosphates and the shielding of the related phosphorus nuclei observed upon deletion of a vicinal hydroxyl is mainly attributed to solvation changes around the phosphate groups. A concerted wrongway shift of some protons and phosphorus nuclei provides information on the conformational dynamics of the phosphates upon protonation. These wrongway shifts may be the result of electrostatic interactions between a ring proton and a doubly negatively charged phosphate group in a trans diequatorial configuration. According to G. R. Desiraju, such an interaction may be considered as a C-H ... O hydrogen bond. The necessary condition to observe this bond is a constraint of the phosphate group such that it closely approaches the ring proton. This may occur as shown by molecular modeling studies when two phosphates strongly repel each other either directly or via the relaying effect of an intervening equatorial hydroxyl.