ABSTRACT
This chapter explores useful parallels between anion and cation receptors. It deals with charge-charge interactions in general and the magnitude of the effects to be expected. The chapter illustrates how electrostatic interactions alone have been exploited for binding of anions. It examines anionic ligands for cation binding in which electrostatic effects act in conjunction with other modes of cation binding. The chapter examines cases where charged ligands are essential components of membrane transport systems. It considers ligands with several charged sites directly adjacent to the crown ether binding site. The chapter is concerned with applications of charged ligands for the extraction and transport of cations that explicitly or implicity exploit the charged nature of the ligand. It also examines the possibilities for controlling the transport cycle from the aqueous phases, by control of the driving force for the cycle. The binding of ionic species by synthetic receptors is always an electrostatic interaction.
