Ion Channel Formation from a Calix[4]arene Amide That Binds HCl

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Abstract
The ion transport activity of calix[4]arene tetrabutylamide 1,3-alt2 was studied in liposomes, planar lipid bilayers, and HEK-293 cells. These experiments, when considered together with 1H NMR and X-ray crystallography data, indicate that calix[4]arene tetrabutylamide 2 (1) forms ion channels in bilayer membranes, (2) mediates ion transport across cell membranes at positive holding potential, (3) alters the pH inside liposomes experiencing a Cl- gradient, and (4) shows a significant Cl-/SO42- transport selectivity. An analogue, calix[4]arene tetramethylamide 1, self-assembles in the presence of HCl to generate solid-state structures with chloride-filled and water-filled channels. Structure−activity studies indicate that the hydrophobicity, amide substitution, and macrocyclic framework of the calixarene are essential for HCl binding and transport. Calix[4]arene tetrabutylamide 2 is a rare example of an anion-dependent, synthetic ion channel.

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