Ion transfer across lipid membranes in the presence of gramicidin A: II. The ion selectivity

A detailed examination has been carried out of the ion selectivity of gramicidin A. Membranes of pure lipids and of various thicknesses have been used, and electrolyte concentrations have been varied over a wide range. It was first confirmed that anion permeability is low, even at very acid pH. Only in CaCl₂ was the transference number of the cation found to be less than unity (0.8). The measurement of biionic potentials was used to give the selectivity sequence for univalent cations. The results are compared with those already in the literature and with the conductivity data for the gramicidin single channel reported in a previous paper. The latter comparison suggests that a complex theoretical model is required to account for the kinetics of ion transfer.