Alkali metal ion selectivity of the hemocyanin channel

The selectivity of the hemocyanin channel was measured for alkali metal ions and ammonium. Permeability ratios relative to K+ measured from biionic potentials were: NH4+ (1.52) greater than Rb+ (1.05) greater than K+ (1.0) greater than Cs+ (0.89) greater than Na+ (0.81) greater than Li+ (0.35). Single-channel ion conductance was a saturating function of ion concentration regardless of the cation present in the bathing medium. Maximal conductances were 270, 267, 215, 176, 170 and 37 ps for K+, Rb+, NH4+, Cs+, Na+ and Li+, respectively. Current-voltage curves for the different monovalent cations were measured and described using a three-barrier model previously used to explain the voltage dependence of the "instantaneous" channel conductance (Cecchi, Alvarez & Latorre, 1981). In this way, binding and peak energies were estimated for the different ions. Considering the energy peaks as transition states between the ion and the channel, it is concluded that they follow Eisenman's selectivity sequences XI (cis peak, i.e., Li+ greater than Na+ greater than K+ greater than Rb+ greater than Cs+; highest field strength), VII (central peak) and II (trans peak). The cis side was that to which hemocyanin was added and was electrically ground. The binding energies, on the other hand, follow Eisenman's series XI for strong electric field sites. Binding of NH4+ to the cis-well suggests that the orientation of the ligands in the site is tetrahedric.