Antibiotic Ligands and Model Compounds

As we have seen, X-ray studies of the ionophorous antibiotics and their cation complexes were able to explain many of the steric factors that determine the selectivity patterns shown by these ligands. However, more systematic investigations on the relationship between host-cavity size and guest-ion radius could only be carried out using simpler synthetic ligands as models. In 1967, Pedersen reported the synthesis and complexing properties of a new class of compounds named crown ethers which are able to mimic effectively their natural counterpieces. 

Since the discovery in 1964 that the antibiotic valinomydn exhibited alkali cation specificity in rat liver mitochondria, a new area of research has developed, based not only on biological systems but also on model systems such as crown ethers.484 The ability of neutral compounds to form lipid-soluble alkali and alkaline earth complexes was observed in 1951. The structure of the corresponding ligand, the anion of the antibiotic nigericin (78), was characterized as its silver salt in 1968.488 486 Silver was used as a heavy atom crystaUographically, since the Ag+ cation had a radius between that of Na+ and K+, which were the two alkali cations with which nigericin was most active. 

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