Nonactin alkali metal complexes

Nonactin, isolated from actinomyces, is a macrotetrolide, (VI), which complexes alkali metal salts in the sequence Li < < Na+, Cs+< Rb+< K+. The crystal structures of its complex with potassium thiocyanate and of the uncomplexed molecule have been determined. 

Until the late 1960s, whereas there had been considerable interest in the transition metal complexes of natural and synthetic macrocyclic ligands (1—4), relatively few reports described complexes of alkaline earth and more particularly alkali metal cations. Research in this area was stimulated by the recognition of the importance of the biological role of Na+, K, Ca2 , and Mg2 and also the discovery and characterization of the natural antibiotic ionophores (5, 6). These macrocyclic antibiotics, such as valinomycin and nonactin, were shown to complex alkali metal cations with remarkable selectivity (7-9). 

It was the first naturally occurring crown ether, and the earliest for which the antibiotic activity could be traced to its ionophoric properties (4). The actins give one-to-one complexes with many alkali and alkaline earth metal ions, with nonactin showing selectivity in the order NH4+ > K+ = Rb+ > Cs+ > Na+ > Ba2+ (5). The first X-ray crystal structure of an ionophore-metal complex was obtained from the nonactin-potassium thiocyanate complex (6). The potassium is bound by coordination to all four tetrahydrofuran oxygens and to the four ester carbonyl oxygens, which creates overall a "tennis ball seam" conformation to the carbon framework as it wraps around the metal. 

A classic temperature jump investigation from Eigen s laboratory resulted in suggestions that the specificity of carriers for membrane translocation of certain ions rested with the rate of dissociation of the complex formed. Several antibiotics act as specific carriers of alkali metal ions across membranes. The cyclic polyether nonactin, for instance, which can form complexes with either Na or K, will selectively transport only the latter. Diebler et al. (1969) found that the association rate constants for... 

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