Stepwise hydration of [CH3COOMg]+ studied by cold ion trap infrared spectroscopy: insights into interactions in the magnesium channel selection filters

International audienceThe magnesium channel controls Mg2+ concentration in the cell and plays an indispensable role inbiological functions. The crystal structure of the Magnesium Transport E channel suggested that Mg2+hydrated by 6 water molecules is transported through a selection filter consisting of COO groups on twoAsp residues. This Mg2+ motion implies successive pairing with OOC-R and dissociation mediated by watermolecules. For another divalent ion, however, it is known that RCOO Ca2+ cannot be separated evenwith 12 water molecules. From this discrepancy, we probe the structure of Mg2+(CH3COO)(H2O)4–17 clustersby measuring the infrared spectra and monitoring the vibrational frequencies of COO with the help ofquantum chemistry calculations. The hydration by (H2O)6 is not enough to induce ion separation, andpartially-separated or separated pairs are formed from 10 water molecules at least. These results suggestthat the ion separation between Mg2+ and carboxylate ions in the selection-filter of the MgtE channel notonly results from water molecules in their first hydration shell, but also from additional factors includingwater molecules and protein groups in the second solvation shell of Mg2+