Interaction of myo-inositol hexakisphosphate with alkali and alkaline earth metal ions: Spectroscopic, potentiometric and theoretical studies

myo-Inositol hexakisphosphate (InsP6) interacts strongly with cations, forming soluble complexes and solids, both of which are biologically relevant. We have recently reported advances in this area, showing the importance of the interaction with Mg2+ and Ca2+. We now extend the data to Sr2+ and Ba2+, which behaved similarly to Mg2+ and Ca2+, but giving even less soluble systems. We also describe the interactions with Li+, Na+, K+, Mg2+, Ca2+, Sr2+, and Ba2+ in a non-interacting medium: these depended strongly on cation charge but only weakly on ionic radius within each series. In addition, we probed structural aspects of InsP6 compounds with alkaline earth cations, by means of IR and Raman spectroscopy of the solids and computational calculations on the 1:1 complexes of Mg2+. Both approaches suggested direct and multidentate phosphate-metal coordination. In [Mg(H2L)(H2O)3]8−, three oxygen atoms from two phosphate groups and three water molecules were predicted to form a roughly octahedral coordination shell; alternate or vicinal phosphate groups were predicted to chelate Mg2+ depending on whether InsP6 is in the 5-axial/1-equatorial or the 1-axial/5-equatorial conformation, respectively.