Optical properties of black lecithin films

Studies of the conductance changes induced in bimolecular lipid membranes by alamethicin

The addition of alamethicin to lecithin bilayers results in both voltage-dependent and voltage-independent conductance changes. In the voltage-dependent region, the slope of the conductance-voltage curve varies with the charge of the cation present in the aqueous phase. It may be shown that these effects may be accounted for by a kinetic model which incorporates the following suppositions: (1) alamethicin molecules are adsorbed at the membrane-water interface; (2) the effect of the potential is to redistribute alamethicin-cation complexes between the two surfaces of the bilayer; (3) conduction through the bilayer follows the surface interaction of approximately six alamethicin molecules; and (4) there is an assymetry in the rate constants for corresponding transitions on opposite sides of the bilayer.The effects of alamethicin are found to be approximately the same at neutral and low pH and are unchanged when bilayers are formed from phosphatidyl serine rather than lecithin. These findings are discussed in relation to current hypotheses of the molecular nature of the conduction mechanism.

The thickness, composition and structure of some lipid bilayers and natural membranes

It has been shown that the capacitance, thickness and composition of black lipid films may depend strongly on the hydrocarbon solvent used in their formation. By the use of n-hexadecane, films have been formed which contain effectively no solvent and which are comparable to the leaflets of the mesomorphic phase of the pure lipid. These films have capacitances of ca. 0.6 μF/cm(2) and hydrocarbon thicknesses of ca. 31 Å. Thinner black films of higher capacitances are also described.The capacitances of biological membranes are, in contrast, nearer to 1 μF/cm(2), and it is suggested that the hydrocarbon region in these membranes may often be thinner than in the lipid leaflets. This suggestion is consistent with some X-ray and lipid composition data. It is pointed out that if the membranes contain abnormally thin lipid leaflets, the area per polar head group of the phospholipid must be increased, and that hydrocarbon is thereby exposed to the aqueous phases. Non-polar protein residues could then interact with these hydrocarbon areas, thus tending to stabilize the expanded leaflet.

Phospholipid free thin liquid films with grafted poly(ethylene glycol)-2000: formation, interaction forces and phase states

Free thin liquid films (foam films) formed from aqueous dispersions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylethanolamine with covalently bound poly-(ethylene glycol) of molecular weight 2000 (DPPE-PEG-2000) were studied by the thin liquid film microinterferometric technique of Scheludko and Exerowa in the temperature range 14-36 degrees C. The surface tension kinetics of the dispersions were studied in order to ensure equilibration of the foam films. These measurements showed that the rate of surface coverage depends slightly on the temperature and does not reach equilibrium values within reasonable time intervals for the dispersions containing only one amphiphile (DPPE-PEG-2000). The destruction of the vesicles at the air/(aqueous dispersion) interface was much faster for the dispersions containing DMPC/DPPE-PEG-2000 mixtures above 23 degrees C, the temperature of the chain-melting phase transition of the main lipid component (DMPC). The dependence of the equilibrium thickness of the foam films on the electrolyte concentration was measured for 1 and 9 mol% DPPE-PEG-2000 at 28 degrees C in the range 10-3 to 0.5 M NaCl. These results indicate that at the low electrolyte concentrations the electrostatic and van der Waals interactions are dominant similar to the foam films stabilized with DMPC alone. At the high electrolyte concentrations the steric repulsion of the PEG layers becomes dominant. The temperature-composition dependence of the bilayer thickness was measured for the foam bilayers at 0.14 M NaCl. The data for the foam bilayer thickness and the comparison with the phase diagrams of PC/PE-PEG dispersions, show that the DMPC/DPPE-PEG-2000 foam bilayers are able to exist in two phase states characterised by different conformations (mushroom and brush) of the grafted polymer.

Double-cone internal reflection as a basis for polarization detection in fish

Some species of fish are able to discriminate, in addition to intensity and wavelength (color), the direction of polarization of visible light. Optical experiments on axially oriented retinal cones from trout and sunfish with use of two types of polarization microscope indicate anisotropic light transmission through paired cones. The measured linear birefringence of paired cone ellipsoids is consistent with the presence of membranous partitions. It is proposed that the partition between the two members of a paired cone, which often appears extensive and flat, functions as a dielectric mirror and that polarization-dependent reflection and refraction at this partition constitutes the underlying mechanism in the transduction of polarization into intensity variation at the photoreceptor's outer segments. We support this hypothesis with linear birefringence and linear dichroism measurements, histological evidence, large-scale optical model measurements, and theoretical calculations based on Fresnel's formulas.

Calculation of deformation energies and conformations in lipid membranes containing gramicidin channels

In this paper we calculate surface conformation and deformation free energy associated with the incorporation of gramicidin channels into phospholipid bilayer membranes. Two types of membranes are considered. One is a relatively thin solvent-free membrane. The other is a thicker solvent-containing membrane. We follow the approach used for the thin membrane case by Huang (1986) in that we use smectic liquid crystal theory to evaluate the free energy associated with distorting the membrane to other than a flat configuration. Our approach is different from Huang, however, in two ways. One is that we include a term for surface tension, which Huang did not. The second is that one of our four boundary conditions for solving the fourth-order differential equation describing the free energy of the surface is different from Huang's. The details of the difference are described in the text. Our results confirm that for thin membranes Huang's neglect of surface tension is appropriate. However, the precise geometrical form that we calculate for the surface of the thin membrane in the region of the gramicidin channel is somewhat different from his. For thicker membranes that have to deform to a greater extent to accommodate the channel, we find that the contribution of surface tension to the total energy in the deformed surface is significant. Computed results for the shape of the deformed surface, the total energy in the deformed surface, and the contributions of different components to the total energy, are presented for the two types of membranes considered. These results may be significant for understanding the mechanisms of dimer formation and breakup, and the access resistance for ions entering gramicidin channels.

Range of the solvation pressure between lipid membranes: dependence on the packing density of solvent molecules

Well-ordered multilamellar arrays of liquid-crystalline phosphatidylcholine and equimolar phosphatidylcholine-cholesterol bilayers have been formed in the nonaqueous solvents formamide and 1,3-propanediol. The organization of these bilayers and the interactions between apposing bilayer surfaces have been investigated by X-ray diffraction analysis of liposomes compressed by applied osmotic pressures up to 6 X 10(7) dyn/cm2 (60 atm). The structure of egg phosphatidylcholine (EPC) bilayers in these solvents is quite different than in water, with the bilayer thickness being largest in water, 3 A narrower in formamide, and 6 A narrower in 1,3-propanediol. The incorporation of equimolar cholesterol increases the thickness of EPC bilayers immersed in each solvent, by over 10 A in the case of 1,3-propanediol. The osmotic pressures of various concentrations of the neutral polymer poly(vinylpyrrolidone) dissolved in formamide or 1,3-propanediol have been measured with a custom-built membrane osmometer. These measurements are used to obtain the distance dependence of the repulsive solvation pressure between apposing bilayer surfaces. For each solvent, the solvation pressure decreases exponentially with distance between bilayer surfaces. However, for both EPC and EPC-cholesterol bilayers, the decay length and magnitude of this repulsive pressure strongly depend on the solvent. The decay length for EPC bilayers in water, formamide, and 1,3-propanediol is found to be 1.7, 2.4, and 2.6 A, respectively, whereas the decay length for equimolar EPC-cholesterol bilayers in water, formamide, and 1,3-propanediol is found to be 2.1, 2.9, and 3.1 A, respectively. These data indicate that the decay length is inversely proportional to the cube root of the number of solvent molecules per unit volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence depolarization measurements on oriented membranes

We describe the theory and experimental application of fluorescence depolarization measurements on small molecules bound to oriented phospholipid bilayers. The results yield insight into both the orientation and the rotational motion of fluorophores in a membrane environment. To accomplish this the angular distribution of polarized fluorescence intensities is measured on a membrane preparation consisting of stacked phospholipid bilayers oriented in a known coordinate system. Considerably more information is available from this data than in comparable solution phase measurements. Three parameters are derived from the data: the rate of rotational diffusion and the second and fourth degree order parameters. These latter two parameters provide an assessment of the average distribution of fluorophore orientation in the membrane bilayer. The data have been carefully examined for systematic experimental artifacts and new protocols are presented which help to eliminate errors that have not been amply treated in the past. We present data for two types of fluorescent molecules: (a) conventional membrane probes like diphenylhexatriene, perylene and anthroyloxy fatty acids; and (b) the anticancer agent adriamycin and several congeneric anthracycline antibiotics. The results show that the hydrocarbon core of membranes is more rigid than previously thought, particularly above the thermal phase transition temperature. We also show that the orientation of small molecules is sensitive to both the phospholipid composition and to the interaction of specific functional groups with the lipid bilayer. The results are discussed in terms of energetic models describing the general patterns for the binding of small molecules to biological membranes.

The attractive forces between polar lipid bilayers

Long-range attractive forces between lipid bilayers are not well described by the Lifshitz theory of Van der Waals forces between macroscopic media. It is shown that when correlations between polar headgroups are taken into account, the predicted forces take a qualitatively different form consistent with the measured data.

Direct evidence for the formation of a monolayer from a bilayer. An ellipsometric study at the nitrogen-water interface

Direct evidence for the formation of a monolayer from a bilayer was measured by ellipsometry after spreading unilamellar vesicles of dioleoyl phosphatidylcholine (DOPC) at the nitrogen-water interface. The ellipsometric isotherms of DOPC vesicles and DOPC spread from an organic solvent were compared and found similar. From the observed ellipsometric angle (delta delta) in the plateau region (-1.04 degrees) and literature data for refractive indices of an anisotropic film similar to DOPC, we have calculated a thickness of 20 +/- 1 A. These results strongly suggest that, similarly to DOPC spread from an organic solvent, DOPC vesicles form a monolayer when spread at the nitrogen-water interface.

Deformation free energy of bilayer membrane and its effect on gramicidin channel lifetime

The deformation free energy of a lipid bilayer is presented based on the principle of a continuum theory. For small deformations, the free energy consists of a layer-compression term, a splay-distortion term, and a surface-tension term, equivalent to the elastic free energy of a two-layer smectic liquid crystal with surface tension. Minimization of the free energy leads to a differential equation that, with boundary conditions, determines the elastic deformation of a bilayer membrane. When a dimeric gramicidin channel is formed in a membrane of thickness greater than the length of the channel, the membrane deformation reduces the stability of the channel. Previously this effect was studied by comparing the variation of channel lifetime with the surface tension of bilayers (Elliott, J. R., D. Needham, J. P. Dilger, and D. A. Hayden, 1983, Biochim. Biophys. Acta, 735:95-103). The tension was assumed to pull a dimer for a distance z before the channel loses ion conductivity. To account for the data, z was found to be 18 A. With the deformation free energy, the data can be accounted for with z less than or approximately to 1 A, which is consistent with the breaking of hydrogen bonds in a dimer dissociation. Increasing the strength of lipid-protein interactions is not the only consequence of the complete free energy compared with the previous discussions. It also changes the shape of membrane deformation around an embedded channel from convex to concave, and increases the range of deformation from less than 10 A to greater than 20 A. Clearly these will be important factors in the general considerations of lipid-protein interactions and membrane-mediated interactions between proteins. In addition, thermal fluctuations of a membrane are calculated; in particular, we calculate the relations between the intrinsic thickness and the experimentally measured values. The experimental parameters of monoolein-squalene membranes are used for quantitative analyses.

Phase transitions in monoglyceride bilayers. A light scattering study

Thermotropic phase transitions in single planar bilayers of glycerol mono-oleate have been investigated using quasi-elastic light scattering from thermally excited membrane fluctuations. In certain cases both spectroscopic and intensity information were derived from the observations. For solvent-free bilayers transitional changes were observed in several membrane parameters: in tension, viscosity and thickness, in a combination of lipid orientational order parameter and dielectric anisotropy, and in the lateral compression modulus. These changes, particularly those in membrane thickness and in the anisotropy/order combination, were clearly indicative of a chain-melting transition in the lipid molecules. The chain-melting transition temperature was identified as 16.6 +/- 0.03 degrees C (delta T 1/2 = 1.5 degrees C). The other changes tended to cluster around 12.5 and 16.6 degrees C, suggesting that a two-stage transition was involved. Analysis of pretransitional fluctuations in membrane viscosity, based on a Landau approach, suggested that at the transition the membrane was close to a critical point (T = 12.7 degrees C). Less information was accessible for membranes containing n-decane within their structure. In this case, the change in membrane tension was much smaller than in the solvent-free case and the transition was considerably broadened. These effects accord with an increase in 'interactive volume' within the bilayer due to solvent inclusion.

A self-consistent chain model for the phase transitions in lipid bilayer membranes

We presented a mechanical model of a lipid bilayer membrane. The internal conformations of a polar head group and double hydrocarbon chains in a lipid molecule were described on the basis of the isomeric bond-rotation scheme. The thermodynamic properties of the lipid membranes were represented by a density matrix that described the rotational isomeric states of the head groups and chains. The parameters that determined the density matrix were obtained in the presence of the intermolecular interactions, which depend on the conformation of the molecules. The interchain interaction was given by the Kihara potential, which depends on the shape of the chains. The Coulomb interaction between the polar head groups and the lateral pressure were considered. The calculation was made for the three lipid molecules corresponding to DMPC, DPPC, and DSPC. The model agreed well with the following experimental results: the temperature, the latent heat of the gel-to-liquid crystalline phase transition, the temperature dependencies of (a) the intermolecular distance, (b) the number of gauche bonds in a hydrocarbon chain, (c) the order parameter for the bond orientation, (d) the volume of the membrane, (e) the thermal expansion coefficients, and (f) the birefringence.

Tensions and free energies of formation of "solventless" lipid bilayers. Measurement of high contact angles

A method is described for the accurate measurement of the interfacial tension of lipid bilayer membranes containing little or no solvent. The tensions were obtained from the interfacial tensions of the equilibrium film-forming solution in the Plateau-Gibbs border, measured by conventional techniques, and the contact angle between the border and the bilayer. The contact angles in these systems are large (greater than 10 degrees) and were estimated by a new method that involved the injection of small known volumes of lipid solution into the bilayer so as to form a lens. Results have been obtained for monoolein-triolein, monoolein-squalene, and monoolein-squalene-decane systems. Half bilayer tensions in these systems were up to approximately 1 mN m-1 less than the single interface tensions. Although bilayer tension tended to increase with bilayer thickness, the interdependence of these quantities varied with the alkane solvents present. In the monoolein-squalene-decane systems, small concentrations of decane have a larger effect on tension than on thickness. Free energies of formation of the near-solventless bilayers were much greater than estimated from the simple application of Lifshitz theory.

Photon correlation spectroscopy of bilayer lipid membranes

Light scattering by thermal fluctuations on simple monoglyceride bilayer membranes has been used to investigate the viscoelastic properties of these structures. Spectroscopic analysis of these fluctuations (capillary waves) permits the nonperturbative measurement of the interfacial tension and a shear interfacial viscosity acting normal to the membrane plane. The methods were established by studies of solvent and nonsolvent bilayers of glycerol monooleate (GMO). Changes in the tension of GMO/n-decane membranes induced by altering the composition of the parent solution were detected and quantified. In a test of the reliability of the technique controlled variations of the viscosity of the aqueous bathing solution were accurately monitored. The technique was applied to solvent-free bilayers formed from dispersions of GMO in squalane. The lower tensions observed attested to the comparative absence of solvent in such bilayers. In contrast to the solvent case, the solvent-free membranes exhibited a significant transverse shear viscosity, indicative of the enhanced intermolecular interactions within the bilayer.

Thickness fluctuations in black lipid membranes

Because a black lipid membrane is compressible, there will be spontaneous fluctuations in its thickness. Qualitative arguments are given that the preferred configuration of the membranes is flat and that thickness fluctuations are smaller in amplitude than the differences in mean thickness observed using different hydrocarbon solvents. Fluctuations with short characteristic lengths will not be large as a result of the large amounts of oil-water contact these would entail. Quantitative analysis based on an extension of the treatment for soap films, predicts that the root mean square (rms) amplitude for fluctuations of wavelength longer than approximately 10 nm is negligible for glyceryl monooleate membranes with squalene (less than 3%) but may be approximately 20% with n-decane. rms fluctuations of 20% would lead to a discrepancy between the rms thickness of the core and the mean reciprocal thickness of only 6%.

The thickness of monoolein lipid bilayers as determined from reflectance measurements

Measurements of the reflectance of monoolein/n-alkane and monoolein/squalene lipid bilayers have been made. The total thickness of the bilayer was calculated from the dependence of reflectance on the refractive index of the aqueous salt or sucrose solution surrounding the bilayer. The total thickness was then compared to the thickness of the hydrocarbon chain region as determined from capacitance measurements. From this comparison, we found that the thickness of each polar region of the bilayers in salt solutions was 0.5 +/- 0.1 nm, independent of the hydrocarbon solvent used. When the aqueous solutions contained sucrose, each polar region was approx. 0.9 nm thick. When n-tetradecane and n-hexadecane were used as solvents, microlenses of solvent trapped in the monoolein bilayer increased the reflectance. After about one hour, the coalescence of microlenses into larger lenses allowed the reflectance of the bilayer alone to be measured. The use of reflectance to measure the thickness of monoolein bilayers appears to be consistent with other methods and to give useful information about the structure of lipid bilayers.

The adsorption of nonpolar molecules into lipid bilayer membranes

Thermodynamic considerations show that the adsorption of nonpolar molecules into lipid bilayer membranes should depend upon the curvature of the membranes. Estimations of the differences in adsorption of a small n-alkane between a planar phospholipid bilayer and liposomal vesicles have been attempted. For spherical multilamellar liposomes exposed to saturated solutions of alkane in water the adsorption is calculated to be 17-65% of the value for the planar bilayer, depending on the assumptions in the model.

Voltage-induced reflectivity relaxation of bilayer lipid membranes: on changes of bilayer thickness

Temporal and voltage-induced changes of reflectivity (R), the optical phase difference in transmitted polarized light, of tension and total capacity of bilayer lipid membrane (BLM) were studied. The membranes were mainly formed from total brain phospholipids (TP) in n-alkanes. 1) Reflectivity of "black" regions of films made of TP in decane and hexadecane decreases by several percent with a time constant (tauR) of about 30 min, whereas that of membranes with hexane and heptane does not depend on time (with an accuracy up to 1--2%). The BLM tension decreases appreciably in the course of time and reaches its steady-state value in tens of minutes after complete blackening of the membrane. 2) Under prolonged (up to tens of minutes) action of voltage (V) no R changes of BLMs with hexane, heptane, and hexadecane were revealed at a noise level of 0.2%. Blms with decane usually respond to voltage application, first by a rapid (jump-like) and then by a slow decrease of R with a value spread from 0.2% to 3%. 3) With higher amplitude and temporal resolutions of the signal (signal averaging method) it can be seen that after voltage jump R decreases down to a new steady-state value: at V = 100 mV, deltaR/R = -(2--4) . 10(-4) and tauR approximately 0.1 msec for BLMs from TP in heptane, and deltaR/R = -(3--6) . 10(-2) and tauR approximately 2 msec for BLMs from oxidized cholesterol in decane. It is shown in the latter case that the great value of deltaR/R is due to the contribution of invisible microlenses. In all the cases deltaR approximately V2. 4) It is concluded that at voltage jump a bilayer first becomes thinner due to volumic compression of its hydrocarbon core; then it spreads with a time constant of the order of 0.1 msec, getting thinner until a new equilibrium state is reached. Complete change of bilayer thickness is detah/h approximately -10(-4) at 100 mV.

Theoretical calculation of the dielectric constant of a bilayer membrane

The dielectric constant (epsilon) and refractive index (n) of a bilayer lipid membrane is determined from the known values of the polarizabilities of the carbon-carbon and carbon-hydrogen bonds. It is assumed that the hydrocarbon chains are hexagonally arranged in an all-trans conformation perpendicular to the plane of the membrane. The only variable in the calculation is the average separation between the chains and the theory relates epsilon to this separation. The calculation and results differ significantly from those presented in a 1968 publication by Ohki. It is shown that a thin membrane is not homogeneously polarized by the applied field. This effect is analysed and the dependence of epsilon on the membrane thickness is determined. The theoretical results are in good quantitative agreement with experimental measurements on bulk paraffins and on oriented multilayers of saturated fatty acids. The most important conclusion is that the dielectric constant for an applied field perpendicular to the membrane (which is the appropriate value for capacitance measurements) differs by only a few percent from the value for the macroscopic (bulk) liquid hydrocarbon. Thus the dielectric constant of a bilayer membrane can be approximated by the value for the appropriate bulk hydrocarbon.

Simultaneous measurements of optical and electrical properties of artificial membranes composed of mitochondrial lipids and their interaction with cytochrome c

A newly constructed cell, which allows simultaneous measurements of optical and electrical properties, was used to study bimolecular black membranes composed of beef heart mitochondrial lipids and their interaction with cytochrome c. The results show that these highly charged membranes are stable only in relatively limited ranges of boundary conditions. In 0.1 n KCl their maximum direct current (dc) resistance is 7 X 10(8) Ohm cm2 +/- 10%; the series capacity at 1 kHz is 0.43 muF/cm2 +/- 3%; the entire thickness, determined by optical reflectivity, is 5.8 +/- 0.2 nm. The interaction between oxidized cytochrome c and these lipid membranes is primarily of electrostatic nature, and dependent on the presence of highly charged phospholipids, such as diphosphatidyl glycerol (cardiolipin) and phosphatidyl ethanolamine. The attachment of cytochrome c maximally causes a 2.5-fold increase in reflectivity, without any noticeable change in the capacity. This leads to a subsequent instability of the membrane (i.e., rupture) preceded by a rapid increase of the dc conductivity. This behavior is far less pronounced with reduced cytochrome c.

Nonelectrolyte diffusion across lipid bilayer systems

The permeability coefficients of a homologous series of amides from formamide through valeramide have been measured in spherical bilayers prepared by the method described by Jung. They do not depend directly on the water:ether partition coefficient which increases regularly with chain length. Instead there is a minimum at acetamide. This has been ascribed to the effect of steric hindrance on diffusion within the bilayer which increases with solute molar volume. This factor is of the same magnitude, though opposite in sign to the effect of lipid solubility, thus accounting for the minimum. The resistance to passage across the interface has been compared to the resistance to diffusion within the membrane. As the solute chain length increases the interface becomes more important, until for valeramide it comprises about 90% of the total resistance. Interface resistance is also important in urea permeation, causing urea to permeate much more slowly than an amide of comparable size, after allowance is made for the difference in the water:ether partition coefficient. Amide permeation coefficients have been compared with relative liposome permeation data measured by the rate of liposome swelling. The ratios of the two measures of permeation vary between 3 and 16 for the homologous amides. The apparent enthalpy of liposome permeation has been measured and found to be in the neighborhood of 12 kcal mol-1 essentially independent of chain length. Comparison of the bilayer permeability coefficients with those of red cells shows that red cell permeation by the lipophilic solutes resembles that of the bilayers, whereas permeation by the hydrophilic solutes differs significantly.

Phase transitions in planar bilayer membranes

Temperature-dependent structural changes in planar bilayer membranes formed from glycerol monooleate (GMO) dispersed in various n-alkane solvents (C12-C17) have been studied using precise measurements of specific geometric capacitance (Cg). Cg generally increases as temperature (T) decreases. A change in the slope of Cg(T) occurs between 15 and 18 degrees C for all solvent systems examined. Measurements of the interfacial tension (gamma) of the bulk GMO-alkane dispersions against 0.1 M NaCl show that gamma generally decreases with decreasing temperature. The data can be fitted with two straight lines of different slope which intersect on the average at 17 degrees C. Pagano et al. (1973, Science (Wash. D.C.). 181:557) have shown using calorimetry that GMO has a phase transition at about 15 degrees C. Thus, the changes in Cg and gamma with temperature are likely to result from a GMO phase transition. A second structural change is observed to occur between 5 and 10 degrees C which has not been detected calorimetrically. Calculations of Cg based on various estimates of the hydrocarbon dielectric coefficient (epsilon-b) and/or hydrocarbon thickness (delta-b) leads to models for the structure of the bilayer above and below the phase transition temperature.

Phase transitions and heterogeneity in lipid bilayers

The optical reflectivity of several well-characterized lipid bilayer systems has been correlated with calorimetric studies of the membrane components. There is a large increase in mean membrane thickness when a bilayer is cooled below the transition temperature of the membrane lipid. Similar studies on membranes generated from a mixture of two lipids possessing different degrees of unsaturation suggest that between the characteristic transition temperatures of the two lipids, the bilayer contains clusters of gel and liquid crystalline lipid which coexist within the plane of the membrane.

Formation of bimolecular membranes from lipid monolayers and a study of their electrical properties

Bimolecular membranes are formed from two lipid monolayers at an air-water interface by the apposition of their hydrocarbon chains when an aperture in a Teflon partition separating two aqueous phases is lowered through the interface. Formation of the membrane is monitored by an increase of the electrical capacity, as measured with a voltage clamp. Electrical resistance of the unmodified membrane is analogous to that of conventional planar bilayers (black lipid membranes) prepared in the presence of a hydrocarbon solvent, i.e., 10(6)-10(8) ohm cm(2); the resistance can be lowered to values of 10(3) ohm cm(2) by gramicidin, an antibiotic that modifies the conductance only when the membranes are of biomolecular thickness. In contrast to the resistance, there is a significant difference between the capacity of bilayers made from mono-layers and that of hydrocarbon-containing bilayers made by phase transition; the average values are 0.9 and 0.45 muF cm(-2), respectively. The value of 0.9 muF cm(-2) approximates that of biological membranes. Assuming a dielectric constant of 2.1 for the hydrocarbon region, the dielectric thickness, as calculated from a capacity of 0.9 muF cm(-2), is 22 A. This value is 6-10 A smaller than the actual thickness of the hydrocarbon region of bilayers and cell membranes, as determined by x-ray diffraction. The difference may be due to a limited penetration of water into the hydrocarbon region near the ester groups that would lower the electrical resistance of this region and reduce the dielectric thickness. Asymmetric membranes have been formed by adjoining two lipid monolayers of different chemical composition.