myo-Inositol 1,4,5-triphosphate and related compounds′ protonation sequence: potentiometric and 31P NMR studies

Abstract

The protonation sequence of myo-inositol 1,4,5-triphosphate [Ins(1,4,5)P₃], of its dehydroxylated analogue, Cyhx(1,2,4) P₃, of two diphosphorylated inositol phosphates, Ins(1,4)P₂ and Ins(4,5)P₂ and of one inositol monophosphate, Ins(1)P₁, have been determined. Potentiometric and ¹¹P NMR studies have been performed in a 0.1 mol dm^–3 solution of tetraethylammonium perchlorate 25 °C (medium 1) and, in addition, for Ins(1,4,5)P₃, in a 0.2 mol dm^–3 KCl solution at 37 °C (medium 2). In the case of the inositol diphosphate, microconstants related to each individual phosphate could be calculated and interpreted according to the position of the phosphates around the inositol ring. Cyhx(1,2,4)P₃ bears an independent phosphate (P4) and two additional phosphates (P1 and P2) equally sharing the bound protons. For Ins(1,4,5)P₃, strong interactions between the phosphate groups are possible, due to the presence of the hydroxy groups. The chemical shifts of the monoanionic and dianionic phosphate forms of the studied compounds are discussed. The superimposition of binding data with the NMR titration curves of Ins(1,4,5)P₃ emphasizes the particular importance of P5 in binding of Ins(1,4,5)P₃ to its receptor.