Cation Binding by Natural and Modified Ionophores: From Natural Ionophore to Synthetic Ionophore

This chapter describes relationships between molecular structures and cation binding properties of several natural ionophores. Natural ionophores all function through biological processes on similar principles, though they vary widely in chemical composition and molecular size. Natural ionophores of the podand-type have "preorganized" molecular cavities for cation binding. The cation binding properties of synthetic crown ethers are generally determined by size fitting between crown ring and guest cation, nature of ligating interactions and three-dimensional ligand topologies. Neutral ionophores such as valinomycin and nonactin have been successfully applied to new biomimetic membrane transport systems, in which amino acid derivatives are transported as anionic guest species. Natural ionophores of the podand-type are recognized as efficient carriers for unnatural guest species in biomimetic transport systems. Natural ionophores have other advantages and can be used in optical resolution of racemic organic substrates. From the standpoint of host-guest chemistry, primarily two families of natural ionophores were discussed: crown-type nonactins and podand-type monensins.