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

Abstract

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 t 290C). 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 ofbare 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 22-230C a pronounced maximum in pore-induced conductance is seen. Whereas the gramicidin A pore abruptly stops stepwise fluctuations below 16'C, with alamethicin a few long-lasting pore and pore state fluctuations persist down to 10'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. Phase changes are well-known phenomena in artificial lipid/ water systems (1) and biological systems (2). These phase transitions, which may play a role in triggering biological processes (3), can be induced by temperature changes or by interaction of ions with charged membrane lipids (4, 5). A great number of publications report on phase transition phenomena in pure and protein-loaded vesicles and liposomes. Due to the instability of planar bilayer membranes in the solid state, there are so far only two reports on electrochemical measurements in the freezing and melting range of planar lipid bilayer membranes. Experiments carried out on membranes from a 1:1 (wt/wt) mixture of dipalmitoylglycerol and distearoylglycerol in n-decane led to the interpretation that ion carriers became frozen and thus immobile within the membrane phase (6). On the other hand, the ionic conductance induced by the pore-former gramicidin was found to remain unchanged at the transition temperature tc of 41'C. Recently, ion-conducting channels were reported to appear in unmodified planar bilayer membranes at the phase tr of 59°C (7). Membranes were formed from a 1,2-distearoyl-glycero-3-phosphocholine/decane solution. There is also a paper, based on optical reflectivity measurements on membranes from monostearoylglycerol in n-hexadecane, which demonstrated an ;70% increase in membrane thickness when the system was cooled below the tc of 55°C (8).Using saturated mixed-chain lipids with a tc of ;290C, we succeeded in forming virtually solvent-free planar bilayer membranes below and above tc. In this paper we report our investigations on pure and ionophore-modified planar bilayers of this type in the 10-40°C temperature range.

Monolayer black membranes from bipolar lipids of archaebacteria and their temperature-induced structural changes

“…The capacitance behavior is similar to that described by Boheim et al [5] for a 1-stearoyl-3-myristoyl-glycero-2-phosphocholine membrane formed by the Montal and Mueller method. The decrease in capacitance corresponds to a variation in length due to stretching of the chains upon freezing.…”

“…At T= 35 ~ C valinomycin is immobilized, indicating a completely frozen state of the membrane. Such a behavior has already been observed in usual bilayers [5,23]. The relative maximum that is noticed in the warming curve is likely to be ascribed to an increase of the bare membrane conductance.…”

“…In several cases, valinomycin was shown to act as a probe of phase transition, as well as of secondary structural changes in lipids [5,22,23]. In spite of the very unusual lipid structure, valinomycin is still acting as a carrier in the present system [13].…”

“…Results are compared with previous data on phase transitions in usual lipid bilayers [2,5,23,37]. The analysis is extended to bipolar lipid membranes dispersed in various solvent systems.…”

Giant vesicles in electric fields

“…This decrease was experimentally confirmed to be between 10 and 15%, corresponding well to the change in the membrane thickness. 77 Thus, the characteristic charging time for membranes in the gel phase as determined from eqn (2) should be shorter. Following this approach, the estimated critical transmembrane potential for poration of membranes in the gel state is V c = 1.5 ¡ 0.2 V. This value is higher than V c measured for fluid membranes.…”