Lipid phase transition in planar bilayer membrane and its effect on carrier- and pore-mediated ion transport
Using mixed-chain lipids, we have recorded cooling and heating curves of planar bilayer membranes while they passed the lipid phase transition range. With unmodified planar bilayers, spontaneous current fluctuations are observed near the lipid phase transition temperature (tc approximately 29 degrees C). This effect coincides with the expected and measured decrease in membrane capacitance. Carrier (valinomycin)-modified planar bilayers exhibit near tc an abrupt change from a high-conducting state above tc to the state of bare membrane conductance below tc. In contrast to this behavior, planar bilayers modified by pore-forming antibiotics (gramicidin A, alamethicin) do not show any peculiar effect at tc. However, at 22--23 degrees C a pronounced maximum in pore-induced conductance is seen. Whereas the gramicidin A pore abruptly stops stepwise fluctuations below approximately 16 degrees C, with alamethicin a few long-lasting pore and pore state fluctuations persist down to 10 degrees C. It is suggested that the carrier may freeze out into the membrane/water interface. The effects observed with pore-forming substances, on the other hand, are interpreted in terms of lateral phase separation into pure lipid and lipid/antibiotic domains.