Insight into the protonation and K(I)-interaction of the inositol 1,2,3-trisphosphate as provided by 31P NMR and theoretical calculations

Animal cells contain a pool of inositol phosphates whose biological function is still under current investigation. Ins(1,2,3)P3 is probably an important safe chelator of iron cations not strongly bound to proteins. In order to clarify its biological functions, Ins(1,2,3)P3 chemistry under physiological conditions must be completely elucidated. The protonation and complexation behaviour of Ins(1,2,3)P3 has been recently studied under these conditions by potentiometry. Under simulated physiological conditions it forms the protonated species H₂L^4- and H₃L^3-. The presence of high concentrations of potassium in intracellular compartments causes the formation of two predominant Ins(1,2,3)P3 complexes: [K(HL)]^4- and [K(H₂L)]^3-, in the absence of iron. In this work we expand part of this macroscopic knowledge to the inframolecular level, by ³¹P NMR measurements and focusing on the protonation and complexation of this biologically relevant molecule to potassium. We complete this study with theoretical calculations which lead us to predict the geometries of every form of the ligand and their relative stabilities. The influence of the ring conformation in protonated and complexed forms is also discussed.