Ferrocenyl-bearing cyclopseudopeptides as redox-switchable cation receptors

A family of ferrocenyl-bearing cyclopseudopeptides (1-10) designed for redox-switchable receptors of cations was synthesized. Circular dichroism (CD) and cyclic valtammetry (CV) studies of cation binding properties in both the reduced (K-1) and oxidized (K-2) forms revealed that the binding preference is mainly governed by the charges and radius of the guest cation as well as by the suitability of the host to accommodate the guest. Particularly worth mentioning is the fact that some synthesized cyclopseudopeptides showed high binding affinity and selectivity toward alkaline-earth ions. For example, the K-1 of compound 2 binding with Ca2+ is 4.37 x 10(6) mol(-1).L and its Ca2+/K+ selectivity is 3.1 x 10(1):1, both values are much greater than those of an excellent natural ionophore, valinomycin (1 x 10(5) mol(-1).L and 0.33:1, respectively). The linear relationship between the shifts of half-wave potentials (DeltaE(1/2)) and the radius/charge [r/(+)] ratios suggests that the sensitivity of electrochemical responses to cation complexation be dominated by repulsion factors between the redox center and the incoming cation guest.