PERMEATION OF PROTONS, POTASSIUM-IONS, AND SMALL POLAR-MOLECULES THROUGH PHOSPHOLIPID-BILAYERS AS A FUNCTION OF MEMBRANE THICKNESS

Two mechanisms have been proposed to account for solute permeation of lipid bilayers. Partitioning into the hydrophobic phase of the bilayer , followed by diffusion, is accepted by many for the permeation of wat er and other small neutral solutes, but transient pores have also been proposed to account for both water and ionic solute permeation. These two mechanisms make distinctively different predictions about the per meability coefficient as a function of bilayer thickness, Whereas the solubility-diffusion mechanism predicts only a modest variation relate d to bilayer thickness, the pore model predicts an exponential relatio nship. To test these models, we measured the permeability of phospholi pid bilayers to protons, potassium ions, water, urea, and glycerol. Bi layers were prepared as liposomes, and thickness was varied systematic ally by using unsaturated lipids with chain lengths ranging from 14 to 24 carbon atoms, The permeability coefficient of water and neutral po lar solutes displayed a modest dependence on bilayer thickness, with a n approximately linear fivefold decrease as the carbon number varied f rom 14 to 24 atoms. In contrast, the permeability to protons and potas sium ions decreased sharply by two orders of magnitude between 14 and 18 carbon atoms, and leveled off, when the chain length was further ex tended to 24 carbon atoms. The results for water and the neutral perme ating solutes are best explained by the solubility-diffusion mechanism . The results for protons and potassium ions in shorter-chain lipids a re consistent with the transient pore model, but better fit the theore tical line predicted by the solubility-diffusion model at longer chain lengths.