An earlier study showed that a calixarene could function as a central relay unit to form an ion conductance pathway through a phospholipid bilayer membrane. The present study expands the range of compounds from calixarene to calixarene and incorporates them either as central units or as headgroups, substituting one or more diaza-18-crown-6 residues in functioning hydraphiles. Ion release was assayed by detecting either Na+ or Cl– release from phospholipid vesicles. The ion transport activity for calixarenes in either position is modest, but is almost non-existent when calix residues were incorporated either as head groups or central relay units. The poor activity of the calixarenes may result from an inability to penetrate to the midplane of the bilayer or pass entirely through it to form a conductance pathway. The transmembrane flip-flop may result from high polarity or steric bulk, or both. A hydraphile incorporating a single -NHCOC6H4OCH2CONH- as a central relay proved to be an excellent Na+ conductor, but less selective for Cl–. The fact that this new hydraphile molecule shows selectivity for Na+ over Cl– transport and possesses two secondary amide residues in the central relay suggests a means to control ion selectivity in synthetic ion transporters.
Synthetic cation transporters incorporating crown ethers and calixarenes as headgroups and central relays: A comparison of sodium and chloride selectivity
New J. Chem. 2008, 32, 878-890.