Planar lipid bilayers were formed from a solution of synthetic 1-palmitoyl-2-oleoyl-glycero-3-phosphoco line (POPC) and 1-palmitoyl-2-oleoyl-glycero-3-phosphoet hanolaminein (POPE, Avanti Polar Lipids, Birmingham, AL) in ratio 3:1 in n-decane (Aldrich). The solution was used to paint a bilayer in an aperture of ~150 μm diameter in a Delrin cup (Warner Instruments, Hamden, CT) between symmetric aqueous bathing solutions of 150 mM KCl, 20 mM Hepes, pH 7.2, at 22°C. All salts were ultrapure (>99%) (Aldrich).