Synthetic multifunctional pores: deletion and inversion of anion/cation selectivity using pM and pH
We report the characterization of multifunctional rigid-rod β-barrel ion channels with either internal aspartates or arginine–histidine dyads by planar bilayer conductance experiments. Barrels with internal aspartates form cation selective, large, unstable and ohmic barrel-stave (rather than toroidal) pores; addition of magnesium cations nearly deletes cation selectivity and increases single-channel stability. Barrels with internal arginine–histidine dyads form cation selective (PK+/PCl− = 2.1), small and ohmic ion channels with superb stability (single-channel lifetime > 20 seconds). Addition of “protons” results in inversion of anion/cation selectivity (PCl−/PK+ = 3.8); addition of an anionic guest (HPTS) results in the blockage of anion selective but not cation selective channels. These results suggest that specific, internal counterion immobilization, here magnesium (but not sodium or potassium) cations by internal aspartates and inorganic phosphates by internal arginines (but not histidines), provides access to synthetic multifunctional pores with attractive properties.