Isolated alkali cation complexes of the antibiotic ionophore nonactin: correlation with crystalline structures.

The antibiotic activity of nonactin is sustained by its ability to transport K+ across lipophilic phases, e.g., the cell membranes. Such a feature can be traced back to a specific ionophoric behavior and to a balanced hydrophobicity modulated by the formation of a cation complex. In this study, the dominant conformations and coordination arrangements in the alkali cation complexes (Na+, K+, Cs+) of nonactin are characterized by means of action vibrational spectroscopy and quantum chemical computations. The low energy conformers of the complexes comprise compact inclusion structures, in which the cation interacts with a varying number of oxygen atoms of the carbonyl and oxolane ring groups of the nonactin macrocycle. The spectroscopy experiments indicate that the three alkali complexes explored are formed in a S4 conformation. This is in contrast with previous crystallography studies, which concluded that the symmetry of the most stable conformer of the complex changes qualitatively with the cation size, from C2 for Na+ to S4 for K+ and Cs+. Computations with different hybrid density functionals lead to contradictory predictions that appear to be quite sensitive to the modelling of the long range interactions in the coordination arrangements. The stabilization of the nonactin-Na+ complex in the C2 or S4 forms emerges as a subtle feature that may be tuned with an appropriate control of the environmental conditions, and constitutes a challenging benchmark to confront novel computational methods for supramolecular systems.