The effects of a cyclic polyether on the electrical properties of phospholipid bilayer membranes

The cyclic polyether XXXII, a neutral, lipid soluble molecule, produces large increases in the conductance of bilayer membranes formed from a variety of lipids. The conductance increases linearly with the concentration of alkali metal cation but with the square, and at higher concentrations the cube, of the polyether concentration. This implies that two or three polyether molecules combine with a single cation to carry it across the membrane. In the presence of XXXII the bilayer is permeable solely to cations and the membrane potential is described by an equation of the Goldman-Hodgkin-Katz type. The permeability ratios determined from potential measurements are independent of salt concentration, decrease in the sequence Cs>Rb>K>NH4>Na>Li(1.0,0.25, 0.15, 0.075, 0.007, 0.0013) and are equal to the conductance ratios at low (e.g. 10(-3) M) salt concentration. At higher salt concentrations, the permeability and conductance ratios are not equal and maxima in the conductancevs. salt concentration curves are observed. Both these phenomena are postulated to be caused by the formation of relatively impermeant 1ν1 polyether cation complexes in the aqueous phase. The 1ν1 aqueous association constants deduced from bilayer measurements decrease in the sequence K>Rb>Na>NH4>Cs>Li (120, 34, 26, 19, 12, 4 liters per mole) and agree quantitatively with the literature values for the more water soluble polyether XXXI, which lacks only thet-butyl groups of XXXII.