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

Abstract

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.

Influence of transition metal electron configuration on the structure of metal-EDTA complexes

“…The spectrum of [Ca(II)-EDTA] 2provides the starting point for our discussion. As described previously, 7,9 this is a complex with C2 symmetry, where the metal ion is hexa-coordinated by the EDTA binding pocket. The antisymmetric OCO stretching motions of the EDTA moiety form four linear combinations, which give rise to partially resolved features in the higher frequency range.…”

“…In the condensed phase, the environment (e.g., protein or solvent) influences the interactions of ions with ion receptors and chelators such as EDTA. [7][8][9] In addition, the presence of solvent molecules can mask the spectroscopic response of the complex under study, and the spectroscopic signatures can be rather broad. 7,9 Experiments on mass selected ions circumvent many of these problems, allowing one to gauge the intrinsic interactions between ion and chelator.…”

“…[7][8][9] In addition, the presence of solvent molecules can mask the spectroscopic response of the complex under study, and the spectroscopic signatures can be rather broad. 7,9 Experiments on mass selected ions circumvent many of these problems, allowing one to gauge the intrinsic interactions between ion and chelator. Cryogenic preparation of the target ions serves to further simplify their spectroscopic response.…”

“…The lower frequency range mainly contains the signatures of linear combinations of the symmetric OCO stretching vibrations, while linear combinations of the antisymmetric stretching motions are found in the higher frequency range. 7 . Note the break of the wavenumber axis (1420 -1570 cm -1 , no significant IR signal was found in this range).…”

“…The shoulder at 1615 cm -1 is the signature of a mode which combines the motions of equatorial ligands radially in phase, with the axial ligands opposite in phase (see Figure S2 in ref. 7 ). The peak at 1365 cm -1 contains the four unresolved analogous vibrational modes consisting of linear combinations of the symmetric OCO stretching vibrations.…”