Conformation, Coordination and Ion Selectivity of the Ionophore X206

The conformation and the cation coordination of the ionophore X206, which selectively transports potassium over sodium, are evaluated in an effort to correlate the structural features of the ionophore with its ion selectivity. For this purpose, the crystal structure of the Na⁺‐complex of X206 has been determined by X‐ray diffraction and is compared with those of the free acid form of X206, its Ag⁺‐complex and the K⁺‐complex of the closely related ionophore alborixin. The ionophore conformation is very similar in all four structures. The carboxylic acid end is essentially invariant, but small changes in torsion angles of up to 26° at the other end of the molecule, combined with differences in the positions of the cations, result in very different coordination geometries for the cations. The conformations of the Ag⁺‐ and K⁺‐complexes resemble the free acid conformation more closely than does that of the Na⁺‐complex. The K⁺ ion is 8 coordinate while the Ag⁺ and Na⁺ ions are 6 and 5 coordinate, respectively. Empirical calculations of the bond strength on the basis of the coordination bond lengths show a stronger binding of K⁺ than of Na⁺ which is consistent with the greater selectivity for binding of the former.