Effect of n-alcohols and glycerol on the pretransition of dipalmitoylphosphatidylcholine

We have systematically investigated the effect of short-chain n-alcohols and glycerol on the pretransition of 1,2-dipalmitoylphosphatidylcholine (DPPC) by spectrophotometry. It is found that the n-alcohols and glycerol remove the pretransition above a critical concentration for each ligand. In addition, the short-chain n-alcohols below the critical concentration decrease the pretransition temperature. The longer the aliphatic chain length of the n-alcohol (up to butanol) (a) the greater the decrease in the pretransition temperature, and (b) the lower the concentration necessary to remove the pretransition. However, glycerol differs from the short-chain n-alcohols in that it has no significant effect on either the pretransition or the main transition, but it is also capable of removing the pretransition above a critical concentration. It has previously been shown that alcohols have a biphasic effect on the main transition temperature of phosphatidylcholines (Rowe, E.S. (1983) Biochemistry 22, 3299–3305). At high alcohol concentrations, the main transition is not thermodynamically reversible (Rowe, E.S. (1985) Biochim. Biophys. Acta 813, 321–330). Recently, Simon and McIntosh (Biochim. Biophys. Acta (1984) 773, 169–172) have identified that at high ethanol concentration DPPC exists in the interdigitated phase. The critical ligand concentration at which the pretransition disappears coincides with the induction of main transition hysteresis and the biphasic alcohol effect in the main transition. These three effects appear to correlate with the induction of the interdigitated gel state by alcohols and glycerol.     

Studies of the ethanol-induced interdigitated gel phase in phosphatidylcholines using the fluorophore 1,6-diphenyl-1,3,5-hexatriene

It is now well established that a number of amphiphilic molecules such as ethanol can induce the formation of the fully interdigitated gel phase in phosphatidylcholines. We have shown earlier that alcohols such as ethanol induce biphasic melting behavior in phosphatidylcholines [Rowe, E. S. (1983) Biochemistry 22, 3299-3305] but not in phosphatidylethanolamines [Rowe, E. S. (1985) Biochim. Biophys. Acta 813, 321-330]. Simon and McIntosh [(1984) Biochim. Biophys. Acta 773, 169-172] showed that the alcohol-induced biphasic melting behavior in phosphatidylcholines is a consequence of acyl chain interdigitation. In the present study we demonstrate the detection of the transition of DPPC and DSPC to the interdigitated phase in the presence of ethanol using the fluorescence properties of the commonly used fluorophore 1,6-diphenyl-1,3,5-hexatriene (DPH). By correlating fluorescence and X-ray diffraction results, we have demonstrated the use of fluorescence to study the phase transition from the noninterdigitated to the interdigitated phase. Using this method, we have investigated the temperature and ethanol concentration dependence of the induction of the interdigitated phase in DSPC and DPPC and shown that the induction of interdigitation by ethanol is temperature dependent, with higher temperature favoring interdigitation. The temperature-ethanol phase diagrams have been determined for DPPC and DSPC.     

Thermodynamic reversibility of phase transitions. Specific effects of alcohols on phosphatidylcholines

The gel-to-fluid phase transitions of several phosphatidylethanolamines (PE's) and phosphatidylcholines (PC's) have been investigated in the presence of three short-chain alcohols. The effects of the alcohols on the thermodynamic reversibility of these transitions was studied and it was found that the transitions for PC's are not thermodynamically reversible at relatively high alcohol concentrations. The PE transitions are thermodynamically reversible for all alcohol concentrations, and the PE's do not exhibit the biphasic effects of alcohol on the transition temperature previously reported for the PC's (Rowe, E.S. (1983) Biochemistry 22, 3299-3305). The biphasic transition temperature effects and the thermodynamic irreversibility of PC transitions at high alcohol concentrations appear to be correlated with the induction of a fully interdigitated gel phase recently reported in the literature (Simon, S.A. and McIntosh, T.J. (1984) Biochim. Biophys. Acta 773, 169-172). The biological significance of these findings is discussed.     

The effect of n-alcohols on vesicular permeability induced at the lipid phase transition temperature: a 1H-NMR study

The effect of a series of n-alcohols on the permeability of small, unilamellar dipalmitoyl phosphatidylcholine (DPPC), dimyristoyl phosphatidylcholine (DMPC) and distearoyl phosphatidylcholine (DSPC) vesicles at the gel-to-liquid crystal phase transition temperature was investigated. It was found that the permeability took the form of the transient lysis of a fraction of the population of vesicles. The effect on this lysis of the n-alcohols was seen to be very chain-length dependent, with a minimum at n = 8 (octan-1-ol) for DPPC vesicles. A similar minimum was observed in the presence of 0.1 mM Triton X-100, but the detergent could then interact with certain of the alcohols to produce permanent channels. The results are discussed in terms of the semi-empirical model of Brasseur et al. (1985) Biochim. Biophys. Acta 814, 227-236, for the interaction of the n-alcohols with a DPPC membrane. The effect of various n-alcohols on the outer and inner monolayers of DPPC vesicles was also studied and the results related to their fluidising effect, allowing channels to open at the phase transition temperature.     

Effect of alcohols on the phase transitions of dihexadecylphosphatidylcholine

We have systematically investigated the effect of short chain alcohols (methanol to n-propanol) on the phase transitions of 1,2-dihexadecylphosphatidylcholine (DHPC), a lipid that forms a stable interdigitated gel phase (L beta I) in aqueous solution. The temperature of the low-temperature L beta I to P beta' phase transition of DHPC was found to increase with alcohol concentration, showing that alcohol interacts preferentially with the interdigitated phase relative to the non-interdigitated gel. The main transition of DHPC exhibited a biphasic effect of alcohol concentration similar to that previously observed with DPPC (Rowe, E.S. (1983) Biochemistry 22,3299-3305). As alcohol concentration is increased the lower L beta I to P beta' and main P beta' to L alpha transitions of DHPC merge at the threshold concentration of the biphasic effect, so that above this concentration there is one phase transition from L beta I directly to L alpha. This is analogous to DPPC above its biphasic threshold. Similar to DPPC, the transition between L beta I and L alpha exhibits marked hysteresis.     

Effects of pentanol isomers on the phase behavior of phospholipid bilayer membranes

Differential scanning calorimetry (DSC) was used to analyze the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of pentanol isomers. The concentration of each pentanol isomer needed to induce the interdigitated phase was determined by the appearance of a biphasic effect in the main transition temperatures, the onset of a hysteresis associated with the main transition from the gel-to-liquid crystalline phase, and the disappearance of the pretransition. Lower threshold concentrations were found to correlate with isomers of greater alkyl chain length while branching of the alkyl chain was found to increase biphasic behavior. The addition of a methyl group to butanol systems drastically decreased threshold concentrations. However, as demonstrated in the DPPC/neopentanol system, branching of the alkyl chain away from the -OH group lowers the threshold concentration while maintaining a biphasic effect.     

Interactions between pressure and ethanol on the formation of interdigitated DPPC liposomes: a study with Prodan fluorescence

Steady-state fluorescence of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) has been employed to study the interacting effects between ethanol and pressure on the formation of the fully interdigitated dipalmitoylphosphatidylcholine (DPPC). At 1 atm and 20 degrees C, a dramatic change in the emission spectrum of Prodan fluorescence is observed at about 1.1-1.3 M ethanol. The emission maximum shifts to longer wavelengths, and the intensity ratio of Prodan fluorescence at 435 nm to that at 510 nm, F435/F510, decreases abruptly with increasing ethanol content. The spectral changes are correlated to the ethanol-induced phase transition of DPPC from the noninterdigitated gel state to the fully interdigitated gel state [Rowe, E.S. (1983) Biochemistry 22, 3299-3305; Simon, S.A., & McIntosh, T.J. (1984) Biochim. Biophys. Acta 773, 169-172]. The spectral changes are attributed to the probe relocation from a less polar environment to a more polar environment due to lipid interdigitation. This relocation is either due to the bulky terminal methyl group of the lipids or due to the partition of Prodan into the bulk solution or both. The present study demonstrates that Prodan is a useful probe in monitoring the formation of the ethanol-induced fully interdigitated DPPC gel phase. Pressure is found to produce spectral changes similar to those induced by ethanol when the ethanol content amounts to 0.8-1.1 M. At lower (e.g., less than 0.4 M) and higher ethanol (e.g., greater than 2.4 M) concentrations, pressure is unable to induce such spectral changes. The critical ethanol concentrations for the formation of the fully interdigitated DPPC gel phase (Cr) have been determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Surfactant partition between bulk water and DPPC vesicle membrane: solid-gel vs. liquid-crystalline membrane

The main phase transition temperature, Tm, of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was measured in the presence of the cationic surfactants tetradecyltrimethylammonium bromide and hexadecyltrimethylammonium bromide. Variation of the perturbing effect of these surfactants on Tm with the lipid concentration was analyzed according to the theory recently proposed by Kaminoh et al. (Y. Kaminoh, C. Tashiro, H. Kamaya and I. Ueda (1988) Biochim. Biophys. Acta 946, 215-220), and the partition coefficients of the surfactant into solid-gel and liquid-crystalline membranes were estimated.     

Alcohol interaction with high entropy states of macromolecules: critical temperature hypothesis for anesthesia cutoff.

Nerve excitation generates heat and decreases the entropy (review by Ritchie and Keynes (1985) Q. Rev. Biophys. 18, 451-476). The data suggest the existence of at least two thermodynamically identifiable states: resting and excited, with a thermotropic transition between the two. We envision that nerve excitation is a transition between the two states of the excitation machinery consisting of proteins and lipids, rather than the sodium channel protein alone. Presumably, both proteins and lipids change their conformation at excitation. We proposed (Kaminoh et al. (1991) Ann. N.Y. Acad. Sci. 625, 315-317) that anesthesia occurs when compounds have a higher affinity to the resting state than to the excited state of excitable membranes, and that there is a critical temperature above which the affinity to the excited state becomes greater than to the resting state. When the temperature exceeds this critical level, compounds lose their anesthetic potency. We used thermotropic phase-transition of macromolecules as a model for the excitation process. Anesthetic alcohols decreased the main transition temperature of dipalmitoylphosphatidylcholine (DPPC) membranes and also the temperature of the alpha-helix to beta-sheet transition of poly(L-lysine). The affinity of alcohols to the high- and low-temperature states of the DPPC membranes were separately estimated. The difference in the affinity of n-alcohols to the liquid (high-temperature) and solid (low-temperature) states correlated with their anesthetic potency. It is not the total number of bound anesthetic molecules that determines the anesthesia, rather, the difference in the affinity between the higher and lower entropy states determines the effects. The critical temperatures of the long-chain alcohols were found to be lower than those of the short-chain alcohols. Cutoff occurs when the critical temperature of long-chain alcohols is below the physiological temperature, such that the anesthetic potency is not manifested in the experimental temperature range.     

Membrane fluidity as affected by the organochlorine insecticide DDT

Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to study the interaction of DDT with model and native membranes. DDT decreases the phase transition midpoint temperature (Tm) of liposomes reconstituted with dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines (DMPC, DPPC and DSPC), and broadens the thermotropic profile of the transition. The effects of DDT are concentration dependent and are more pronounced in bilayers of short-chain lipids, e.g., DMPC. The insecticide fails to alter DPH polarization in the fluid phase of the above lipids. Similar effects were observed in binary mixtures of DMPC plus DPPC. Furthermore, DDT alters the single broad transition of the equimolar mixture of DMPC plus DSPC into a biphasic transition. The lower temperature component has a midpoint at 25 degrees C, i.e., a value close to the Tm of DMPC. DDT inhibits to some extent the cholesterol-induced ordering in DMPC bilayers and high cholesterol concentrations (greater than or equal to 30 mol%) do not prevent insecticide interaction, conversely to the effect observed for lindane (Antunes-Madeira, M.C. and Madeira, V.M.C. (1989) Biochim. Biophys. Acta 982, 161-166). Apparently, the bilayer order is not disturbed by DDT in fluid native membranes of mitochondria and sarcoplasmic reticulum, but moderate disordering effects are noticed in membranes enriched in cholesterol, namely, brain microsomes and erythrocytes.     

Effect of 2,4-dichlorophenol on DPPC/water liposomes studied by X-ray and freeze-fracture electron microscopy

The effect of 2,4-dichlorophenol (DCP) was studied on the fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)--water liposomes. The structure and the thermotropic phase behaviour of the liposomes was examined in the presence of DCP (DCP/DPPC molar ratio, varied from 2x10(-2) up to 1) using small- and wide-angle X-ray scattering (SAXS, WAXS) and freeze-fracture electron microscopy. The structural behaviour of the DPPC/DCP/water system was strongly dependent on the concentration of the DCP. In the pretransition range the DCP molecules (at 2x10(-2) DCP/DPPC molar ratio) induced the interdigitated phase beside the parent (gel and rippled gel) phases, locally which can be form at higher DCP concentration. When the DCP/DPPC molar ratio was increased the pretransition disappeared and the main transition was shifted to lower temperatures. In the molar ratio range from 2x10(-1) up to 5x10(-1), a coexistence of different phases was observed in the wide temperature range from 20 up to 40 degrees C. With a further increase of the DCP/DPPC molar ratio (6x10(-1) to 1) only the interdigitated gel phase occurred below 25 degrees C. A schematic phase diagram of DPPC/DCP/water system was constructed to summarise the results.     

Effects of alcohols on lipid bilayers with and without cholesterol: the dipalmitoylphosphatidylcholine system

Differential scanning calorimetry is a useful method to study the thermotropic phase transitions of a phospholipid bilayer. In the present study DSC is used to determine the effects of methanol and ethanol on DPPC and DPPC/2 mol% cholesterol bilayers. The biphasic effect of the main transition and the presence of an extra peak on the DSC cooling scans were observed above certain alcohol concentrations. In the presence of 2% cholesterol, the concentration at which the biphasic effect occurs is increased by both short-chain alcohols. 1,6-Diphenyl-1,3,5-hexatriene (DPH) is used as a fluorescent probe to directly determine the onset of interdigitation in these systems as reflected by a drop in the DPH fluorescence intensity.     

Induction of lateral phase separations in binary lipid mixtures by alcohol

It has previously been shown that alcohol has different effects on the gel to liquid-crystal phase transition of phosphatidylcholines (PC's) and phosphatidylethanolamines (PE's) [Rowe, E. S. (1985) Biochim. Biophys. Acta 813, 321-330]. In this investigation, the thermotropic properties of binary PE-PC mixtures were studied in the presence of ethanol in order to determine whether the differential interactions of alcohol with PC and PE would lead to lateral phase separations. Phase diagrams of the dilaurylphosphatidylethanolamine-dipalmitoylphosphatidylcholine [PE(12:0)-PC(16:0)] system were constructed in the presence and absence of ethanol. It was shown that lateral phase separations occur in the gel phase over a certain composition range in the presence of 100 mg/mL ethanol. In the absence of alcohol these two lipids are miscible in both the gel and liquid-crystal states. The data suggest that in the presence of ethanol these lateral phase separations involve the coexistence of regular bilayer gel and the fully interdigitated gel phase, which has previously been shown to occur in pure PC(16:0) under these conditions [Simon, S. A., & McIntosh, T. J. (1984) Biochim. Biophys. Acta 773, 169-172]. The biological implications of these findings are discussed.     

Thermodynamic elucidation of solute-induced lipid interdigitation phase: lipid interactions with hydrophobic versus amphipathic species.

Comparative thermodynamic studies on the interactions of aqueous dispersions of dipalmitoyl phosphatidylcholine (DPPC) bilayer vesicles with hydrophobic and amphipathic species were conducted to elucidate the nature of the solute-induced interdigitated lipid phase. Cyclohexanol, a strong hydrophobic species, lowers the temperature (tm) of the lipid main phase transition from the gel to the liquid-crystalline phase. Unlike ethanol (an amphipathic species), as reported previously, cyclohexanol does not exert a biphasic effect on tm (lowering tm at lower concentrations and raising tm at higher concentrations). At cyclohexanol greater than or equal to 15.4 mg/ml or 0.154 M, the thermogram of DPPC vesicles exhibits a small transition adjacent to the main phase transition but at a lower temperature. In contrast, ethanol does not promote such a small transition. Furthermore, the enthalpy (delta H) of the transition is increased in the presence of cyclohexanol. The sign of the enthalpy change (delta H-delta Ho) is positive and that of the free energy change (delta G-delta Go) is negative, a characteristic of solute-solute hydrophobic interaction. In contrast, DPPC bilayer vesicles exhibit both (delta H-delta Ho) and (delta G-delta Go) greater than 0 in the presence of ethanol in a concentration range where lipid vesicles exist in an interdigitated phase. To support the above distinct thermodynamic observations, fluorescence steady-state polarization (P) measurements were also performed. At the temperature below tm, the value of P decreases as cyclohexanol concentration increases, while a biphasic effect on P was found in the presence of ethanol. These findings support the postulation that the solute-induced interdigitated lipid phase requires the solute molecule to be amphipathic in nature.     

High pressure antagonism of alcohol effects on the main phase-transition temperature of phospholipid membranes: biphasic response.

The combined effects of high pressure (up to 300 bar) and a homologous series of 1-alkanols (ethanol C2 to 1-tridecanol C13) were studied on the main phase-transition temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. It is known that short-chain alkanols depress and long-chain alkanols elevate the main transition temperature. The crossover from depression to elevation occurs at the carbon-chain length about C10-C12 in DPPC vesicle membranes coinciding with the cutoff chain-length where anesthetic potency suddenly disappears. Alkanols shorter than C8 linearly decreased the transition temperature and high pressure antagonized the temperature depression. Alkanols longer than C10 showed biphasic dose-response curves. High pressure enhanced the biphasic response. In addition, alkanols longer than the cutoff length depressed the transition temperature under high pressure at the low concentration range. These non-anesthetic alkanols may manifest anesthetic potency under high pressure. At higher concentrations, the temperature elevatory effect was accentuated by pressure. This biphasic effect of long-chain alkanols is not related to the 'interdigitation' associated with short-chain alkanols. The increment of the transition temperature by pressure was 0.0242 K bar-1 in the absence of alkanols. The volume change of the transition was estimated to be 27.7 cm3 mol-1. This value stayed constant to the limit of the present study of 300 bar.     

A new monofluorinated phosphatidylcholine forms interdigitated bilayers.

16-Fluoropalmitic acid was synthesized from 16-hydroxypalmitic acid using diethylaminosulfur trifluoride. This monofluorinated fatty acid then was used to make 1-palmitoyl-2-[16-fluoropalmitoyl]-phosphatidylcholine (F-DPPC) as a fluorinated analog of dipalmitoylphosphatidylcholine (DPPC). Surprisingly, we found that the phase transition temperature (Tm) of F-DPPC occurs near 50 degrees C, approximately 10 degrees C higher than its nonfluorinated counterpart, DPPC, as judged by both differential scanning calorimetry and infrared spectroscopy. The pretransition observed for DPPC is absent in F-DPPC. A combination of REDOR, rotational-echo double-resonance, and conventional solid-state NMR experiments demonstrates that F-DPPC forms a fully interdigitated bilayer in the gel phase. Electron paramagnetic resonance experiments show that below Tm, the hydrocarbon chains of F-DPPC are more motionally restricted than those of DPPC. X-ray scattering experiments confirm that the thickness and packing of gel phase F-DPPC is similar to that of heptanetriol-induced interdigitated DPPC. F-DPPC is the first phosphoglyceride containing sn-1 and sn-2 ester-linked fatty acyl chains of equal length that spontaneously forms interdigitated bilayers in the gel state in the absence of inducing agents such as alcohols.     

Thermal effects on motion and orientation of the cholestane spin label in planar multibilayers of dimyristoylphosphatidylcholine and dipalmitoylphosphatidylcholine

A detailed picture of the orientation and restricted motion of the cholestane spin label (3-spiro-doxyl-5α-cholestane) in planar multibilayers of dipalmitoylphosphatidylcholine and dimyristoylphosphatidylcholine has been recorded by simultaneous simulation of ESR spectra obtained with the magnetic field parallel and perpendicular to the bilayers (Shimoyama, Y., Eriksson, L.E.G. and Ehrenberg, A. (1978) Biochim. Biophys. Acta 508, 213–235). The analysis has been made over the temperature range ?30°C to 60°C on samples containing 20 to 22% water. At low temperatures the cholestane spin label is tilted with respect to the lipid bilayer normal by an angle of approx. 30° which disappears at the pretransition. In this low temperature range the restricted twisting motion has an activation energy of 5.5 kJ·mol^?1. Above the main transition the twisting motion is unrestricted and has the activation energy 20 kJ·mol^?1. From below the pretransition to above the main transition the velocity of the twisting motion increases by an order of magnitude. The amplitude of the wobbling motion increases abruptly from 0° to 35° at the main transition.     

Ethanol induces interdigitated gel phase (L beta I) between lamellar gel phase (L beta') and ripple phase (P beta') in phosphatidylcholine membranes: a scanning density meter study

Effects of ethanol on dipalmitoylphosphatidylcholine (DPPC) and distearoylphosphatidylcholine (DSPC) dispersions were investigated with an automated scanning density meter and a differential scanning calorimeter (DSC). The temperature-dependent profile of specific volume measured by the density meter clearly exhibited phase transitions of the DPPC and the DSPC dispersions as drastic changes in the thermal expansion coefficients. On increasing the ethanol concentration in the DPPC dispersions, the pretransition temperature was reduced faster than the main transition temperature was. An interdigitated gel phase (L beta I) appeared as a region of lower specific volume at the pretransition temperature when the ethanol concentration reached 40 mg/ml. The L beta I phase spread both its ends in an ethanol-dependent fashion, and the high-temperature end merged to the main transition at 50 mg/ml of ethanol. The temperature-ethanol phase diagram has been determined for DPPC. The transitions L beta' to L beta I and from L beta I to P beta' were also observed on the thermograms of DSC measurements. In the DSPC dispersions, the L beta I phase was induced between the L beta' and the P beta' phases by a lower ethanol concentration (about 20 mg/ml).     

Membrane fluidity as affected by the insecticide lindane

Fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to study the interaction of lindane with model and native membranes. Lindane disorders the gel phase of liposomes reconstituted with dimyristoyl-, dipalmitoyl- and distearoylphosphatidylcholines (DMPC, DPPC and DSPC), since it broadens and shifts the main phase transition, but no apparent effect is detected in the fluid phase. These effects of lindane are more pronounced in bilayers of short-chain lipids, e.g., DMPC. In equimolar mixtures containing DMPC and DSPC, lindane preferentially interacts with the more fluid lipid species inducing lateral phase separations. However, in mixtures of DMPC and DPPC, the insecticide only broadens and shifts the main phase transition, i.e., an effect similar to that observed in bilayers of pure lipids. Lindane has no apparent effect in DMPC bilayers enriched with high cholesterol content (greater than or equal to 30 mol%), whereas disordering effects can still be detected in bilayers with low cholesterol (less than 30 mol%). Apparently, lindane does not perturb the fluid phase of representative native membranes, namely, mitochondria, sarcoplasmic reticulum, myelin, brain microsomes and erythrocytes in agreement with the results obtained in fluid phospholipid bilayers, despite the reasonable incorporation of the insecticide in these membranes, as previously reported (Antunes-Madeira, M.C. and Madeira, V.M.C. (1985) Biochim. Biophys. Acta 820, 165-172).     

The permeability and the effect of acyl-chain length for phospholipid bilayers containing cholesterol: theory and experiment.

The model of Cruzeiro-Hansson et al. (Biochim. Biophys. Acta (1989) 979, 166-1176) for lipid-cholesterol bilayers at low cholesterol concentrations is used to predict the thermodynamic properties and the passive ion permeability of lipid bilayers as a function of acyl-chain length and cholesterol concentration. Numerical simulations based on the Monte Carlo method are used to determine the equilibrium state of the system near the main gel-fluid phase transition. The permeability is calculated using an ansatz which relates the passive permeability to the amount of interfaces formed in the bilayer when cholesterol is present. The model predicts at low cholesterol contents an increase in the membrane permeability in the transition region both for increasing cholesterol concentration and for decreasing chain length at a given value of the reduced temperature. This is in contrast to the case of lipid bilayers containing high cholesterol concentrations where the cholesterol strongly suppresses the permeability. Experimental results for the Na+ permeability of C15PC and DPPC (C16PC) bilayers containing cholesterol are presented which confirm the theoretical predictions at low cholesterol concentrations.