Ditopic crown ether–guanidinium ion receptors for the molecular recognition of amino acids and small peptides

A series of ditopic synthetic receptors based on a crown ether–guanidinium ion recognition motif is reported. The compounds show binding affinity to selected amino acids, including important neurotransmitters. The effect of the distance of the ammonium and the carboxylate ion, the rigidity of the spacer, and the use of pre-organized pyrrole– and pyrene–guanidinium groups on binding affinity and selectivity are discussed.