Ionic permeation of lipid bilayer membranes mediated by a neutral, noncyclic Li⁺-selective carrier having imide and ether ligands. I. Selectivity among monovalent cations

We have found that Simon's neutral, noncyclic, Li⁺-selective complexone, which has imide and ether ligands, renders lipid bilayer membranes selectively permeable to certain cations and anions. The present paper characterizes the ability of this molecule to carry monovalent cations; and we show it to be most selective for Li⁺ among the alkali cations, the first reconstitution of Li⁺-selective permeation in lipid bilayer membranes. This complexone acts as an "equilibrium-domain" carrier for Ag⁺> Li⁺>Tl⁺>Na⁺>NH₄⁺>Rb⁺>Cs⁺ over a wide range of experimental conditions. The major type of membrane-permeating species formed is a 2:1 carrier/cation complex dominant except at the lowest salt and carrier concentrations where a 1:1 carrier/cation, with a similar selectivity sequence, can be detected. Among the group Ia cations the selectivity sequence in bilayers, Li⁺>Na⁺>K⁺>Rb⁺>Cs⁺, is similar to that previously found for this molecule in thick solvent-polymer membrane electrodes. We find this carrier to be more selective to Ag⁺ than to any other monovalent cation yet studied. This high Ag⁺ selectivity is used, together with the dependence of the selectivity on the nature of the N-amide substitutents, to argue that the imide oxygens play a major role as ligands.