Selective Na+ transport through phospholipid bilayer membrane by a synthetic calix[4]arene carrier

Alkali cation transport by tetrakis(ethoxycarbonylmethyl ether) 1 and tetrakis(methyl ether) 2 of p-tert-butylcalix[4]arene through soybean phospholipid bilayer membranes has been investigated by measurements of electric currents and dynamic Na-23 NMR spectra. When the calix[4]arene ether 1 was incorporated into a planar phospholipid bilayer membrane which separated two chambers filled with 100 mM NaCl solutions, electric currents resulting from sodium ion flues across the planar bilayer membrane were generated upon applying external voltages between two chambers. On the other hand, incorporation of the methyl ether 2 into the planar bilayer did not generate electric currents resulting from sodium ion fluxes under the same experimental condition with 1. The current-voltage (I-V) relationships in XCl/XCl (X = Li, Na, K, Rb, Cs) systems showed that the calix[4]arene 1 selectively transported sodium ions through the phospholipid bilayer membrane. From the measurement of reversal potentials in XCl/NaCl systems, it was found that the permeability of sodium ions was much larger than that of other alkali cations, by a factor of 17 (for K+) or higher (for Li+, Rb+, Cs+). Incorporation of 1 into large unilamellar vesicles (LUVs) in NaCl aqueous solution gave rise to dynamic Na-23 NMR spectra arising from 1-mediated Na+ exchange across the LUV bilayer membrane. The rates (1/tau(Na+in)) of Na+ transport by 1 were ca. 4-fold smaller than that by a naturally occurring ionophore, monensin. The kinetics for 1-mediated Na+ transport follows a facilitated diffusion model in which one calix[4]arene molecule complexes with one sodium ion at the water-membrane interface and sodium ions are transported to the opposite interface by the diffusion of cationic Na+-1 complexes.