Nanoscale Ion Sequestration To Determine the Polarity Selectivity of Ion Conductance in Carriers and Channels

The nanoscale spacing between a tethered lipid bilayer membrane (tBLM) and its supporting gold electrode can be utilized to determine the polarity selectivity of the conduction of ion channels and ion carriers embedded in a membrane. The technique relies upon a bias voltage sequestering or eliminating ions, of a particular polarity, into or out of the aqueous electrolyte region between the gold electrode and the tethered membrane. A demonstration is given, using ac swept frequency impedance spectrometry, of the bias polarity dependence of the ionophore conductance of gramicidin A, a cationic selective channel, and valinomycin, a potassium ion selective carrier. We further use pulsed amperometry to show that the intrinsic voltage dependence of the ion conduction is actually selective of the polarity of the transported ion and not simply of the direction of the ionic current flow.