This chapter discusses the contributions that have been made by synthetic molecular receptors of the crown ether type to the development of enzyme mimics and analogues. A complementary steric and electronic relationship between a substrate species and a synthetic molecular receptor is desirable if strong and highly ordered complex formation is to occur. Because the potential binding sites in the smaller substrate are divergent, the larger molecular receptor has to be designed with matching convergent binding sites. This relationship is a reminiscent of the lock and key model, which was proposed many years ago to describe the interaction between an enzyme and its substrate. A more recent nomenclature system advocated by Cram and co-workers, that employs the terms “host” and “guest” respectively to refer to the crown ether molecular receptor and the substrate, has now gained popular acceptance. In the context of crown ether hosts, non-covalent bonds of pole–pole, pole–dipole, and dipole–dipole types can be employed in the formation of host–guest complexes.
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