Selective transport of potassium ions across a planar phospholipid bilayer by a calix[4]arene-crown-5 as a synthetic carrier

The ion transport ability in a planar phospholipid bilayer, of two p-tert-butylcalix[4]arene-crown-5 derivatives (1, 2), a p-tert-butylcalix[5]arene ester 3, dibenzo-18-crown-6, and valinomycin have been investigated using a voltage clamp method. Membrane current measurements showed that the synthetic calixarene ionophores except for dibenzo-18-crown-6, show ion transport activities for K+ in the bilayer membranes. The order of K+ transport activities of the compounds was valinomycin >1 > 3 > 2. From the measurements of reversal potentials, the relative ion permeability across the bilayer was determined for 1, 3 and valinomycin. Both 1 and 3 showed ion transport selectivity for K+, while valinomycin showed ion transport selectivity for Rb+. Among the calixarene ionophores, compound 1 showed the highest K+ conductivity and K+/Na+ selectivity. Although the K+/Na+ selectivity of 1 is less than that of valinomycin by a factor of ca. 2, calix[4]arene-based ionophore 1 has the potential for use as a synthetic K+ carrier in phospholipid bilayer membranes.