Ion Binding Site Structure and the Role of Water in Alkali Earth EDTA Complexes

The interactions between molecular hosts and ionic guests and how the chemical environment in aqueous solutions changes these interactions are challenging to disentangle from solution data alone. The vibrational spectra of cold complexes of ethylenediaminetetraacetic acid (EDTA) chelating alkaline earth dications in vacuo encode structural characteristics of these complexes and their dependence on the size of the bound ion. The correlation between metal binding geometry and the relative intensities of vibrational bands of the carboxylate groups forming the binding pocket allows us to characterize changes in molecular geometry by interaction with other molecules. The evolution of these structural markers from bare ions to water adducts to aqueous solution illustrates the role of water for the structure of ion binding sites in chelators. The results show that the binding pocket of EDTA opens up in aqueous solution, bringing the bound ion closer to the mouth of the binding site, and leading to an increased exposure of the bound ion to the chemical environment.