Nonactin-potassium thiocyanate complex

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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