Macrotetrolides Cubic Cages for Alkali Ions

In the cases of valinomycin and enniatin depsipeptides described in Section 2. the explanation for the structural origins of the more or less pronounced ion selectivities exhibited by these antibiotics was, though being plausible, somewhat tentative, as completed X-ray analyses were available only for complexes with a single ion species, i.e. with for valinomycin and Ba for beauvericin. However, a detailed discussion of the structural features that lead to metal ion selectivities should be based on a whole set of comparable data on complex structures with various metal ions of 

The macrotetrolide antibiotics are 32-membered cyclic tetralactons which can be isolated from various actinomyces species. Five homologs of the general formula indicated in Fig. 9 are known, which are referred to as nonactin, monactin etc. depending on the number of methyl replaced by ethyl groups. The compounds are built up by four (o-hydroxycarboxylic acid subunits of alternating enantiomerism condensated to each other by esterification. 

Spectroscopic investigations of the corresponding solution conformations showed that free macrotetrolides display appreciable rotational freedom of the macrocyclic skeleton. Most probably, all types of conformers found in the crystalline state are among those present in solution. 

Upon complex formation, the ionophores undergo considerable conformational rearrangements, mainly resulting in a turning of the ester carbonyl oxygens toward the interior of the molecule. Just as in the case of valinomycin, but in contrast to beauvericin, the central ligand cavity serves as binding site in all the ion complexes 

if the main spatial structures of all the complexes are similar, how can their different stabilities be rationalized It is in fact the structural niceties that are responsible for the cation selectivity. In the complexes the distances 

---