Nonactin-potassium thiocyanate

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. 

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. 

Figure 2 The complex cation in potassium thiocyanate-nonactin, viewed along the 2-fold axis. Torsion aisles about the bonds marked a and b are approximately 63° and 150°. Carbon atoms are shown as open circles, oxygen atoms as shaded circles, and the potassium ion as a heavy circle...

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