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Eight derivatives of monensin with a modified C25-C26 moiety were synthesized. Their ionophore properties were studied on human erythrocytes by measuring Na+ influx with 23Na NMR and concomitant K+ and H+ efflux by potentiometry. Modification of OH-26 led to inversion of selectivity of transport in favor of K+/Na+ in comparison with monensin. This selectivity disappeared by suppression of the C26-OH moiety. Finally the ionophore ability was lost if the head-to-tail chelation of the monensin skeleton was prevented by blocking the terminal OH-25 and -26 functions. All the compounds were inactive on Gram-negative bacteria and fungi. MIC measured on Bacillus cereus showed that derivatives with increased K+/Na+ selectivity were clearly the most active against Bacillus growth. Most of the compounds showed potential antimalarial properties in the nanomolar range when tested in vitro against Plasmodium falciparum. The IC50S measured were correlated with the whole Na+ and K+ transport efficiency rather than with the ionic selectivity. In both cases determination of initial fluxes of transport for both cations (Na+ and K+) was necessary to investigate the relationship between biological and ionophore properties.

Ionophores isolated from bacterial strains, and especially A.23187, are efficient antibiotics against Gram‐positive bacteria and devoid of activity on Gram‐negative species. This difference in activity was attributed to the outer membrane of Gramnegative bacteria which is presumably impermeable to these very hydrophobic compounds. In this context, the partition of the calcium ionophore A.23187 between bacteria and the medium was studied on Escherichia coli (Gram‐negative) and Bacillus cereus (Gram‐positive) using, on the one hand, fluorimetric measurements and, on the other hand, radioautographic analysis of bacteria incubated with the [3H]‐labelled ionophore. Although the first method did not give a definitive answer, the second one clearly showed that the tritiated metabolite was only incorporated into B. cereus.

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