Lipid phase transition in planar bilayer membrane and its effect on carrier- and pore-mediated ion transport.

Abstract

Using mixed-chain lipids, cooling and heating curves of planar bilayer membranes were recorded while they passed the lipid phase transition range. With unmodified planar bilayers, spontaneous current fluctuations are observed near the lipid phase transition temperature (tc .apprxeq. 29.degree. 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. At 22-23.degree. C a pronounced maximum in pore-induced conductance is seen. Whereas the gramicidin A pore abruptly stops stepwise fluctuations below .apprxeq. 16.degree. C, with alamethicin a few long-lasting pore and pore state fluctuations persist down to 10.degree. C. The carrier apparently may freeze out into the membrane/water interface. The effects observed with pore-forming substances are interpreted in terms of lateral phase separation into pure lipid and lipid/antibiotic domains.