Polar group orientation and the electrical properties of lecithin bimolecular leaflets

Toward Realistic Large-Area Cell Membrane Mimics: Excluding Oil, Controlling Composition, and Including Ion Channels

Capacitance measurements provide unique insights into the thickness, compressibility, and composition of large-area membrane bilayers and are used here in addition to demonstrate the successful incorporation of model ion channels. The simultaneous ability to control the bilayer size, manipulate tension, and optically monitor and electrically stimulate freestanding membranes enables precise determination of their specific capacitance and thickness across a wide range of areas. We confirm that membranes formed by this recently developed technique have capacitive properties similar to those formed by existing protocols, including solvent-free approaches, and discuss the effect using either hexadecane or squalene as the oil solvent. The results obtained here are relevant for other methods where lipid membranes are reconstituted from a bulk oil solvent. Because biological membranes have a diverse phospholipid profile, we show that the technique can successfully reconstitute membranes with binary composition mixtures. As an outlook, we show the capability of model membrane proteins, specifically α-hemolysin and alamethicin, to be incorporated into the formed bilayers and measure ion transport.

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.

The effects of the macrotetralide actin antibiotics on the electrical properties of phospholipid bilayer membranes

This paper, the last in a series of three, characterizes the electrical properties of phospholipid bilayer membranes exposed to aqueous solutions containing nonactin, monactin, dinactin, and trinactin and Li(+), Na(+), K(+), Rb(+), Cs(+), and NH 4 (+) ions. Not only are both the membrane resistance at zero current and the membrane potential at zero current found to depend on the aqueous concentrations of antibiotic and ions in the manner expected from the theory of the first paper, but also these measurements are demonstrated to be related to each other in the manner required by this theory for "neutral carriers". To verify that these antibiotics indeed are free to move as carriers of cations, cholesterol was added to the lipid to increase the "viscosity" of the interior of the membrane. Cholesterol decreased by several orders of magnitude the ability of the macrotetralide antibiotics to lower the membrane resistance; nevertheless, the permeability ratios and conductance ratios remained exactly the same as in cholesterolfree membranes. These findings are expected for the "carrier" mechanism postulated in the first paper and serve to verify it. Lastly, the observed effects of nonactin, monactin, dinactin, and trinactin on bilayers are compared with those predicted in the preceding paper from the salt-extraction equilibrium constants measured there; and a close agreement is found. These results show that the theory of the first paper satisfactorily predicts the effects of the macrotetralide actin antibiotics on the electrical properties of phospholipid bilayer membranes, using only the thermodynamic constants measured in the second paper. It therefore seems reasonable to conclude that these antibiotics produce their characteristic effects on membranes by solubilizing cations therein as mobile positively charged complexes.

Measurement of axonal membrane conductances and capacity by means of a varying potential control voltage clamp

A new mode of voltage clamping in the squid giant axon is introduced and its advantages are analyzed, tested, and utilized to investigate membrane conductances and capacity. This method replaces the constant command potentials of the standard voltage clamp with potentials which vary with time. Some of the advantages in using the varying potential clamp are: (1) slowly varying potentials generate practically pureI K ; (2) rapidly varying potentials generate practically pureI Na; (3) triangular waves generate, under proper conditions, pure capacity currents and easy-to-analyze leakage currents; (4) the method gives direct, on-line display of sodium or potassium I-V characteristics within milliseconds; (5) it enables rapid and accurateE Na andE K determinations; and (6) it enables simple and accurate determination ofC m. The method was utilized to study the effects of various ions on membrane conductances and the effects of ionic composition, ionic strength, and temperature on membrane capacity. Membrane capacity was found to be practically independent of frequency in the 200 to 2,000 Hz range. Replacement of external sodium by Ca(++), by impermeable Tris(+), or even by dextrose or sucrose (low ionic-strength solutions) had negligible effects onC m.C m showed a small, positive temperature coefficient of 1.39% per °C in the 3 to 21°C range, and little change with temperature in the 20 to 40°C range. Above 40°C, bothC m andg L increased considerably with temperature.

Specific binding of chloride ions to lipid vesicles and implications at molecular scale

Biological membranes composed of lipids and proteins are in contact with electrolytes like aqueous NaCl solutions. Based on molecular dynamics studies it is widely believed that Na(+) ions specifically bind to 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membranes, whereas Cl(-) ions stay in solution. Here, we present a careful comparison of recent data from electrophoresis and isothermal titration calorimetry experiments as well as molecular dynamics simulations suggesting that in fact both ions show very similar affinities. The corresponding binding constants are 0.44(±0.05) M(-1) for Na(+) and 0.40(±0.04) M(-1) for Cl(-) ions. This is highlighted by our observation that a widely used simulation setup showing asymmetric affinities of Na(+) and Cl(-) for POPC bilayers overestimates the effect of NaCl on the electrophoretic mobility of a POPC membrane by an order of magnitude. Implications for previous simulation results on the effect of NaCl on polarization of interfacial water, transmembrane potentials, and mechanisms for ion transport through bilayers are discussed. Our findings suggest that a range of published simulations results on the interaction of NaCl with phosphocholine bilayers have to be reconsidered and revised and that force field refinements are necessary for reliable simulation studies of membranes at physiological conditions on a molecular level.

NaCl interactions with phosphatidylcholine bilayers do not alter membrane structure but induce long-range ordering of ions and water

It is generally accepted that ions interact directly with lipids in biological membranes. Decades of biophysical studies on pure lipid bilayer systems have shown that only certain types of ions, most significantly large anions and multivalent cations, can fundamentally alter the structure and dynamics of lipid bilayers. It has long been accepted that at physiological concentrations NaCl ions do not alter the physical behavior or structure of bilayers composed solely of zwitterionic phosphatidylcholine (PC) lipids. Recent X-ray scattering experiments have reaffirmed this dogma, showing that below 1 M concentration, NaCl does not significantly alter bilayer structure. However, despite this history, there is an ongoing controversy within the molecular dynamics (MD) simulation community regarding NaCl/PC interactions. In particular, the CHARMM and GROMOS force fields show dramatically different behavior, including the effect on bilayer structure, surface potential, and the ability to form stable, coordinated ion-lipid complexes. Here, using long-timescale, constant-pressure simulations under the newest version of the CHARMM force field, we find that Na⁺ and Cl⁻ associate with PC head groups in a POPC bilayer with approximately equal, though weak, affinity, and that the salt has a negligible effect on bilayer structure, consistent with earlier CHARMM results and more recent X-ray data. The results suggest that interpretation of simulations where lipids interact with charged groups of any sort, including charged proteins, must be carefully scrutinized.

Changes in phosphatidylcholine headgroup tilt and water order induced by monovalent salts: molecular dynamics simulations

The association between monovalent salts and neutral lipid bilayers is known to influence global bilayer structural properties such as headgroup conformational fluctuations and the dipole potential. The local influence of the ions, however, has been unknown due to limited structural resolution of experimental methods. Molecular dynamics simulations are used here to elucidate local structural rearrangements upon association of a series of monovalent Na(+) salts to a palmitoyl-oleoyl-phosphatidylcholine bilayer. We observe association of all ion types in the interfacial region. Larger anions, which are meant to rationalize data regarding a Hofmeister series of anions, bind more deeply within the bilayer than either Cl(-) or Na(+). Although the simulations are able to reproduce experimentally measured quantities, the analysis is focused on local properties currently invisible to experiments, which may be critical to biological systems. As such, for all ion types, including Cl(-), we show local ion-induced perturbations to headgroup tilt, the extent and direction of which is sensitive to ion charge and size. Additionally, we report salt-induced ordering of the water well beyond the interfacial region, which may be significant in terms of hydration repulsion between stacked bilayers.

Dielectric Relaxation on the Intermediate Layer in a Bipolar Membrane under the Water Splitting Phenomenon

In this study, we investigated the impedance spectra of bipolar membranes. Under the application of a reverse-biased voltage, the spectra showed a double dielectric relaxation profile due to the heterogeneous structure and it was analyzed in accordance with the three-layered dielectric model. It is defined that one of the compositions of the heterogeneous structure is situated at the membrane interface region between the negatively and the positively charged membrane with a thickness of less than several micrometers, which has an extraordinarily large electric capacity with a magnitude of sub-microfarads. It is concluded that this layer is identified with the intermediate layer in which the water splitting phenomenon occurs on the bipolar membrane.

Changes in Friend murine erythroleukaemia cell membranes during induced differentiation determined by electrorotation

We used electrorotation measurements to investigate alterations in the plasma membranes of DS19 murine erythroleukaemia cells that accompanied erythropoietic differentiation induced by hexamethylene bisacetamide (HMBA). Following 3 days of HMBA treatment, the mean cell membrane specific capacitance determined from electrorotation spectra of individual, viable cells at physiological tonicity (300 mosmol/kg) fell from 1.74 to 1.53 microF/cm2, in agreement with trends observed earlier by dielectrophoretic measurements on bulk cell populations. Scanning and transmission electron microscopy revealed that the relatively high values found for cell membrane capacitance (> 1 microF/cm2) reflected the large area of plasma membrane associated with complex surface morphology including numerous microvilli. Furthermore, it demonstrated that the fall in membrane capacitance during HMBA treatment correlated with a reduction in the density of these complex surface features. Differences in the mechanical characteristics of the cell membranes of untreated and treated cells were then examined by exposing cells to osmotic stress. The intricacy of membrane morphology intensified with increasing osmolality of the suspending medium and this was reflected in higher specific capacitance values. When the osmolality was increased from 210 to 450 mosmol/kg, the mean membrane capacitance of untreated DS19 cells changed from 1.58 to 2.05 microF/cm2 while that for HMBA-treated cells changed from 1.47 to 1.72 microF/cm2, a significantly smaller response. This demonstrated that cells exposed to 72 h of differentiation treatment had an enhanced mechanical resilience as compared with their untreated counterparts, evidencing the early stages of the development of the membrane skeleton which becomes fully developed in mature erythrocytes. Our findings demonstrate the value of electrorotation measurements as a method for the non-invasive characterisation of viable leukaemic cells and their responses to stimuli and show that the membrane capacitance values so derived reflect membrane morphology.

Voltage-dependent capacitance as a probe for albumin adsorption onto a solid surface

The process of adsorption of bovine serum albumin onto a platinum electrode was monitored through the measurement of a nonlinear electrochemical property. The principle of the new method is that a sinusoidal voltage source is applied to a test solution and the waveform of the output current is analyzed by Fourier transformation. It was found that the intensities of the higher harmonics in the Fourier transformation change depending on the concentration of albumin and with time. From the higher harmonics, voltage dependence of the capacitance was quantitatively evaluated. The change of the state of albumin adsorbed onto the platinum plate was also monitored from the pattern of 'crack' of adsorbed albumin by using scanning electron microscopy. These results were discussed in relation to the mechanism of bimodal adsorption of albumin.

Characteristic response to taste stimuli of the intensities of higher harmonics in an electrochemical oscillatory system

In general, the electrochemical characteristics of solid/liquid or liquid/liquid interfaces are highly nonlinear, i.e., the capacitance changes markedly according to the applied voltage. In this paper, we propose a novel method for evaluating these nonlinear characteristics quantitatively. That is, a sinusoidal voltage source is applied to a test solution and the waveform of the output current is analyzed by Fourier transformation. It is shown theoretically that higher harmonic components in the Fourier transformation afford us useful information on nonlinear behavior. It is stressed that our technique is entirely different from the classical impedance method, i.e., nonlinear components of the impedance can be evaluated in our method, having been ignored previously in the classical impedance measurement. As an application of this method, we have studied the effect of taste compounds on the intensities of the higher harmonics, using an electrochemical cell containing an aqueous solution of sodium oleate. It has been found that the intensities of the higher harmonics exhibit characteristic changes upon the addition of taste compounds, the change being dependent upon the taste category. The characteristic response to taste compounds in the electrochemical nonlinearity is discussed in relation to the experimental trend of the dynamic isotherm for oleic acid at an air/water interface.

Surface potential changes in lipid monolayers and the 'cut-off' in anaesthetic effects of N-alkanols

The effects of 0.09 saturated solutions of the n-alkanols n-hexanol to n-tridecanol on the surface (compensation) potential of lipid monolayers have been examined. Actions on monolayers spread from pure egg phosphatidylcholine have been compared with effects on a system containing 2:1 mole ratio of egg phosphatidylcholine and cholesterol. The mean compensation potential for the pure phospholipid system was 475 +/- 9 mV; addition of cholesterol increased the potential to 503 +/- 10 mV. All n-alkanols tested reduced the surface potential in both systems. The reduction was larger in the pure phospholipid system but the difference in effect between lipid systems declined as the n-alkanol chainlength increased, becoming negligible by n-tridecanol. These results are considered in relation to the 'cut-off' in biological activity of n-alkanols around n-tridecanol.

The membrane dipole potential in a total membrane potential model. Applications to hydrophobic ion interactions with membranes

The total potential energy profile for hydrophobic ion interactions with lipid bilayers can be written as the sum of four terms: the electrical Born, image and dipole contributions, and a neutral energy term. We introduce a specific model for the membrane dipole potential, treating it as a two-dimensional array of point dipoles located near each membrane-water interface. Together with specific theoretical models for the other energy terms, a total potential profile is developed that successfully describes the complete set of thermodynamic parameters for binding and translocation for the two hydrophobic ion structural analogues, tetraphenylphosphonium (TPP+) and tetraphenylboron (TPB-). A reasonable fit to the data is possible if the dipole potential energy has a magnitude of 5.5 + 0.5 kcal/mol (240 + 20 mV), positive inside, and if the neutral energy contribution for TPP+ and TPB- is -7.0 + 1.0 kcal/mol. These results may also have important implications for small ion interactions with membranes and the energetics of charged groups in membrane proteins.

Effect of lipid phase transition on the binding of anions to dimyristoylphosphatidylcholine liposomes

Temperature dependence of the electrophoretic mobility of multilamellar liposomes prepared from dimyristoylphosphatidylcholine was measured in the presence of salts with different anions in aqueous solutions. It was established that specific binding of anions to liposome surface induced a pronounced zeta potential (electrostatic potential at the hydrodynamic plane of shear). A combination of Langmuir, Gouy-Chapman, and Boltzmann equations was used to describe the dependence of the zeta potential on the concentration of anions. The values of binding constants (K) and maximum numbers of binding sites per unit area (sigma max) were determined by this method. The sequence for anion affinities to liposome surface was found to be as follows: trinitrophenol greater than ClO-4 greater than I- greater than SCN- greater than Br- greater than NO-3 greater than Cl- approximately or equal to SO2-4. A sharp increase in the negative zeta potential was detected at the temperature of phase transition of the lipid from the gel to liquid-crystalline state. It was found that the parameter K did not change at lipid phase transition and the shifts in zeta potential might be due to alterations of sigma max. The binding sites were considered as defects in the package of lipid molecules in membranes.

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.

Alterations of the carrier-mediated transport of an anionic solute, methotrexate, by charged liposomes in Ehrlich ascites tumor cells

Interaction of positively charged liposomes with Ehrlich ascites tumor cells increases the bidirectional transmembrane fluxes of the anionic folic acid analog, methotrexate. Negative liposomes reduce methotrexate influx. Stimulation of methotrexate influx by positively charged liposomes is time and concentration dependent, requiring at least a 5-min incubation with 2.5 mM phosphatidylcholine containing 20% stearylamine for maximum effect. Stimulation is not appreciably reversed by washing the cells. Similar increases are observed for influx and efflux so that there is no change in the steady-state methotrexate electrochemical-potential difference across the cell membrane. The increase influx appears to be a stimulation of the carrier-mediated transport process for methotrexate since both control and stimulated influx are abolished by the competitive inhibitor, 5-formyltetrahydrofolate or the sulfhydryl group inhibitor, p-chloromercuriphenylsulfonic acid and the Q10 of the system remains unchanged. Influx of 5-methyltetrahydrofolate, which shares the same transport carrier as methotrexate, is also stimulated. However, the transport of folic acid, which is structurally similar to methotrexate but does not utilize the carrier, is unaffected. The kinetic change induced by positively charged liposomes is an increase in the Vma in, while the Kt in remains unchanged. Trans-stimulation of methotrexate influx by 5-formyltetrahydrofolate occurs to the same extent in the presence or absence of positively charged liposomes. The liposomes have no apparent effect on the intracellular water, the extracellular space, or the chloride distribution ratio. The data suggest that interaction of positively charged liposomes with Ehrlich ascites tumor cells accelerates the rate of transposition of the membrane carrier system for methotrexate, altering the kinetics of transport without a change in transport thermodynamics.

The adsorption of divalent cations to phosphatidylcholine bilayer membranes

Electrophoretic mobility and 31P NMR measurements were combined to test whether the combination of the Henry, Boltzmann and Grahame equations is capable of describing the adsorption of divalent cations of phosphatidylcholine membranes. Cobalt was chosen for this study because, of all the common divalent cations, its effects on the 31P NMR spectrum of phosphatidylcholine membranes are easiest to interpret. Both the 31P NMR data on the adsorption of cobalt and the zeta potential data calculated from the electrophoretic mobility in the presence of cobalt are well described by the combination of these three equations. Electrophoretic mobility measurements were also performed with a number of other divalent cations and the zeta potentials were, in all cases, well described by the combination of these three equations. The binding deduced from such measurements decreases in the sequence: Mn2+, Mg2+, Ca2+, Co2+, Ni2+, Sr2+, Ba2+. If we assume that a lipid molecule occupies an area of 60 A2 and that there is a 1 : 1 stoichiometry for the binding of the divalent ions to phosphatidylcholine, the dissociation constants are, respectively: 0.3, 1.0, 1.0, 1.2, 1.2, 2.8, 3.6 M.

Non-isothermal potential of phospholipid bilayer films. Influence of cholesterol and macrocyclic carrier effects

The effect of cholesterol on the ion selective behavior of phoshpolipid (phosphatidylcholine or phosphatidylethanolamine) bilayer films is studied through the measurement of the membrane non-isothermal potential. It is shown how the mixed phosphatidylcholine-cholesterol membrane can be either cation of anion permselective according to the film composition (cationic behavior is met in the 0--10% cholesterol composition range while anionic selectivity appears in the 20--50% range). On the contrary, mixed phosphatidylethanolamine-cholesterol membranes show the absence of ionic selectivity already met with pure phosphatidylethanolamine films. The presence of a cationic carrier as Dibenzo-18-crown-6 in the film transforms all the studied films (cationic, anionic and no selective bilayers) into ideally cationic selective membranes. These results are discussed on the basis of the current ideas on the charge distribution through the bilayer membranes. Moreover, the role of the permeating ions as potential determining species is stressed.

The effect of synthetic polymers on the electrical and permeability properties of lipid membranes

1. The effect of two series of hydrophilic and hydrophobic polymers on the stability, conductivity and permeability towards water and leucine of black lipid membranes and liposomes is reported. 2. The changes in properties of these membrane preparations is related to bulk phase viscosity and dielectric measurements together with monolayer studies. 3. The hydrophobic polymers dramatically increase membrane stability, had no effect on conductivity, but increased the permeability coefficient of leucine. 4. The hydrophilic polymers produced minor, but significant changes to membrane properties. 5. It is concluded that not only basic polymers but also neutral and acidic macromolecules can interact strongly with lipid membranes.

Non-isothermal potential of phospholipid bilayer films

The thermal electrical potential generated in isochemical conditions by a temperature gradient at the two sides of lipid bilayer leaflets is measured. The experimental results agree rather well with the theoretical predictions. The cephaline (from sheep brain) bilayer behaves like a film with zero charge while the phosphatidylcholine (from egg yolk) film performs like a charged membrane. The results presented suggest that the measurement of electrical thermal potential is an interesting method to investigate the electrical behaviour of bilayer membranes.

Alternating current studies of charge carrier transport in lipid bilayers. Pentachlorophenol in lecithin-cholesterol membranes

Surface and interior electrical properties of lecithin-cholesterol bilayer membranes treated with the uncoupler pentachlorophenol have been determined on the basis of a.c. measurements over a wide range of frequencies (0.02 to 1000 kHZ). The method used depends on accurately determining the resistance of the aqueous solution in series with each individual membrane by extrapolating admittance data into infinite frequency. Loss tangent vs. frequency curves are corrected by subtracting out a loss contribution which is present in untreated membranes and is due, presumably, to dielectric relaxation. The results, which are useful below 100 kHZ, can be fitted to loss tangent curves computed for a three-element equivalent circuit consisting of frequency independent conductance-capacitance pairs, arranged in series to represent surface and interior properties of membranes. Interior conductances agree with net conductances obtained from d.c. measurements. The pH and concentration dependence of surface conductance is consistent with a scheme of transport in which a fixed number of surface binding sites are filled preferentially with neutral pentachlorophenol molecules, which in turn dissociate to supply protons to the aqueous phase. Surface capacitances range from 15 to 90 times that of interior capacitance and show a systematic increase with pentachlorophenol concentration at high pH, and a decrease with concentration at low pH.