Nonactin is a polyketide antibiotic produced by Streptomyces griseus ETH A7796 and is an ionophore that is selective for K+ ions. It is a cyclic tetraester generated from two monomers of (+)-nonactic acid and two of (-)-nonactic acid, arranged so that nonactin has S4 symmetry and is achiral. To understand why achiral nonactin is the naturally generated diastereoisomer, we generated two alternate diastereoisomers of nonactin, one prepared solely from (+)-nonactic acid and one prepared solely from (-)-nonactic acid, referred to here as 'all-(+)-nonactin' and 'all-(-)-nonactin', respectively. Both non-natural diastereoisomers were 500-fold less active against Gram positive organisms than nonactin confirming that the natural stereochemistry is necessary for biological activity. We used isothermal calorimetry to obtain the K-a, Delta G, Delta H, and Delta S of formation for the K+, Na+, and NH4+ complexes of nonactin and all-(-)-nonactin; the natural diastereoisomer bound K+ 880-fold better than all-(-)-nonactin. A picrate partitioning assay confirmed that all-(-)-nonactin, unlike nonactin, could not partition K+ ions into organic solvent. To complement the thermodynamic data we used a simple model system to show that K+ transport was facilitated by nonactin but not by all-(-)-nonactin. Modeling of the K+ complexes of nonactin and all-(-)nonactin suggested that poor steric interactions in the latter complex precluded tight binding to K+. Overall, the data show that both enantiomers of nonactic acid are needed for the formation of a nonactin diastereoisomer that can act as an ionophore and has antibacterial activity.