Ion selectivity of the channels formed by pardaxin, an lonophore, in bilayer membranes

Using the method of bilayer membranes at the tip of a patch pipette, the properties of the ionic channels produced by the ionophore toxin pardaxin were investigated. At low toxin concentrations, voltage‐dependent, single‐channel events were measured. The current‐voltage curves were non‐linear when determined in Tris‐CI solution, but were linear in K⁺‐HEPES solution. Using asymmetric ion solutions, the ionic selectivity of pardaxin channels was estimated from the reversal potentials obtained. The sequence of the relative permeabilities for monovalent cations was Tl⁺ > Rb⁺ > Cs⁺ > K⁺,NH4⁺ > methylamine⁺ > Li⁺ > dimethylamine⁺ > Na⁺. Except for Li⁺, the selectivity sequence fitted the cations relative hydrated size. For bivalent ions the permeability of Ba²⁺, Sr²⁺, and Mn²⁺ relative to Mg²⁺ changed according to the relative hydrated size. For anions the selectivity sequence was I⁻ > NO₃⁻ > Br⁻ > CI⁻ > CIO₄⁻ > SCN⁻ > BF⁻ > HCOO⁻ > F⁻ > CH₃COO⁻. The selectivity sequence for the small anions (I⁻, NO₃⁻, Br⁻, CI⁻) was different from their hydrated size. Pardaxin channel showed a modest ion selectivity between small anions and cations (P_K:P_CI:P_Na = 1.28:1.00:0.56). Pardaxin is proposed as a biophysical model to study ionic channel selectivity. © 1995 Wiley‐Liss, Inc.