Binding constants of Li+, K+, and Tl+ in the gramicidin channel determined from water permeability measurements

In an open circuit there can be no net cation flux through membranes containing only cation-selective channels, because electroneutrality must be maintained. If the channels are so narrow that water and cations cannot pass by each other, then the net water flux through those "single-file" channels that contain a cation is zero. It is therefore possible to determine the cation binding constants from the decrease in the average water permeability per channel as the cation concentration in the solution is increased. Three different methods were used to determine the osmotic water permeability of gramicidin channels in lipid bilayer membranes. The osmotic water permeability coefficient per gramicidin channel in the absence of cations was found to be 6 x 10(-14) cm3/s. As the cation concentration was raised, the water permeability decreased and a binding constant was determined from a quantitative fit to the data. When the data were fitted assuming a maximum of one ion per channel, the dissociation constant was 115 mM for Li+, 69 mM for K+, and 2 mM for Tl+