The variation of capacitance and conductance of bimolecular lipid membranes with area

Direct in situ measurement of specific capacitance, monolayer tension, and bilayer tension in a droplet interface bilayer

Thickness and tension are important physical parameters of model cell membranes. However, traditional methods to measure these quantities require multiple experiments using separate equipment. This work introduces a new multi-step procedure for directly accessing in situ multiple physical properties of droplet interface bilayers (DIB), including specific capacitance (related to thickness), lipid monolayer tension in the Plateau-Gibbs border, and bilayer tension. The procedure employs a combination of mechanical manipulation of bilayer area followed by electrowetting of the capacitive interface to examine the sensitivities of bilayer capacitance to area and contact angle to voltage, respectively. These data allow for determining the specific capacitance of the membrane and surface tension of the lipid monolayer, which are then used to compute bilayer thickness and tension, respectively. The use of DIBs affords accurate optical imaging of the connected droplets in addition to electrical measurements of bilayer capacitance, and it allows for reversibly varying bilayer area. After validating the accuracy of the technique with diphytanoyl phosphatidylcholine (DPhPC) DIBs in hexadecane, the method is applied herein to quantify separately the effects on membrane thickness and tension caused by varying the solvent in which the DIB is formed and introducing cholesterol into the bilayer. Because the technique relies only on capacitance measurements and optical images to determine both thickness and tension, this approach is specifically well-suited for studying the effects of peptides, biomolecules, natural and synthetic nanoparticles, and other species that accumulate within membranes without altering bilayer conductance.

Ion-induced defect permeation of lipid membranes

We have explored the mechanisms of uncatalyzed membrane ion permeation using atomistic simulations and electrophysiological recordings. The solubility-diffusion mechanism of membrane charge transport has prevailed since the 1960s, despite inconsistencies in experimental observations and its lack of consideration for the flexible response of lipid bilayers. We show that direct lipid bilayer translocation of alkali metal cations, Cl(-), and a charged arginine side chain analog occurs via an ion-induced defect mechanism. Contrary to some previous suggestions, the arginine analog experiences a large free-energy barrier, very similar to those for Na(+), K(+), and Cl(-). Our simulations reveal that membrane perturbations, due to the movement of an ion, are central for explaining the permeation process, leading to both free-energy and diffusion-coefficient profiles that show little dependence on ion chemistry and charge, despite wide-ranging hydration energies and the membrane's dipole potential. The results yield membrane permeabilities that are in semiquantitative agreement with experiments in terms of both magnitude and selectivity. We conclude that ion-induced defect-mediated permeation may compete with transient pores as the dominant mechanism of uncatalyzed ion permeation, providing new understanding for the actions of a range of membrane-active peptides and proteins.

The effect of bacterial signal indole on the electrical properties of lipid membranes

Indole is an important biological signalling molecule produced by many Gram positive and Gram negative bacterial species, including Escherichia coli. Here we study the effect of indole on the electrical properties of lipid membranes. Using electrophysiology, we show that two indole molecules act cooperatively to transport charge across the hydrophobic core of the lipid membrane. To enhance charge transport, induced by indole across the lipid membrane, we use an indole derivative, 4 fluoro-indole. We demonstrate parallels between charge transport through artificial lipid membranes and the function of complex eukaryotic membrane systems by showing that physiological indole concentrations increase the rate of mitochondrial oxygen consumption. Our data provide a biophysical explanation for how indole may link the metabolism of bacterial and eukaryotic cells.

Determining membrane capacitance by dynamic control of droplet interface bilayer area

By making dynamic changes to the area of a droplet interface bilayer (DIB), we are able to measure the specific capacitance of lipid bilayers with improved accuracy and precision over existing methods. The dependence of membrane specific capacitance on the chain-length of the alkane oil present in the bilayer is similar to that observed in black lipid membranes. In contrast to conventional artificial bilayers, DIBs are not confined by an aperture, which enables us to determine that the dependence of whole bilayer capacitance on applied potential is predominantly a result of a spontaneous increase in bilayer area. This area change arises from the creation of new bilayer at the three phase interface and is driven by changes in surface tension with applied potential that can be described by the Young-Lippmann equation. By accounting for this area change, we are able to determine the proportion of the capacitance dependence that arises from a change in specific capacitance with applied potential. This method provides a new tool with which to investigate the vertical compression of the bilayer and understand the changes in bilayer thickness with applied potential. We find that, for 1,2-diphytanoyl-sn-glycero-3-phosphocholine membranes in hexadecane, specific bilayer capacitance varies by 0.6-1.5% over an applied potential of ±100 mV.

Black lipid membranes stabilized through substrate conjugation to a hydrogel

Recent research in stabilizing lipid bilayer membranes has been directed toward tethering the membrane to a solid surface or contacting the membrane with a solid support such as a gel. It is also known that the solvent annulus plays an important role in lipid bilayer stability. In this work, the authors set out to stabilize the solvent annulus. Glass substrates with approximately 500 mum apertures were functionalized with 3-methacryloxypropyltrimethoxysilane to allow cross-linking with a surrounding polyethyleneglycol dimethacrylate hydrogel. The hydrogel makes a conformal mold around both the lipid bilayer and the solvent reservoir. Since the hydrogel is covalently conjugated with the glass substrate via vinyl groups, the solvent annulus is prevented from leaving the aperture boundary. Measurements of a membrane created with this approach showed that it remained a stable bilayer with a resistance greater than 1 GOmega for 12 days. Measurements of the ion channel gramicidin A, alpha-hemolysin, and alamethicin incorporated into these membranes showed the same conductance behavior as conventional membranes.

Gramicidin channels

Gramicidin channels are mini-proteins composed of two tryptophan-rich subunits. The conducting channels are formed by the transbilayer dimerization of nonconducting subunits, which are tied to the bilayer/solution interface through hydrogen bonds between the indole NH groups and the phospholipid backbone and water. The channel structure is known at atomic resolution and the channel's permeability characteristics are particularly well defined: gramicidin channels are selective for monovalent cations, with no measurable permeability to anions or polyvalent cations; ions and water move through a pore whose wall is formed by the peptide backbone; and the single-channel conductance and cation selectivity vary when the amino acid sequence is varied, even though the permeating ions make no contact with the amino acid side chains. Given the amount of experimental information that is available--for both the wild-type channels and for channels formed by amino acid-substituted gramicidin analogues--gramicidin channels provide important insights into the microphysics of ion permeation through bilayer-spanning channels. For the same reason, gramicidin channels constitute the system of choice for evaluating computational strategies for obtaining mechanistic insights into ion permeation through the complex channels formed by integral membrane proteins.

Permeability increase in black lipid membrane induced by compound 48/80

When compound 48/80, a potent histamine liberator, was added in the aqueous phase facing the black lipid membrane, the conductivity of the membrane was remarkably increased. Although valinomycin displayed a distinct selectivity for K+ movement, such selection for ionic permeability was not observed in the case of compound 48/80.

Effect of 3-phenylindole on lipophilic ion and carrier-mediated ion transport across bilayer lipid membranes

The physical effects of 3-phenylindole, an antimicrobial compound which interacts with phospholipids, on ion transport across phosphatidylcholine-cholesterol bilayers have been investigated using three lipophilic ions and one ion-carrier complex. It was found that 3-phenylindole increased membrane electrical conductance of positively charged membrane probes and decreased electrical conductance of negatively charged probes. The enhancement of conductance detected by nonactin-K+ complex and tetraphenylarsonium+ was several orders of magnitude, whereas the suppression of conductance due to tetraphenylborate- and dipicrylamine- was less than a factor of ten. Presence of 3-phenylindole in aqueous phase slightly decreased adsorption of tetraphenylborate- and dipicrylamine- at the membrane surface. From the voltage dependence of the steady-state conductance it was shown that 3-phenylindole induced kinetic limitation of membrane transport of potassium mediated by nonactin. No such limitation was found in the case of tetraphenylarsonium+ transport. These results are shown to be consistent with the present concept of ion diffusion in membranes and the assumption that 3-phenylindole decreases the electric potential in the membrane interior. The asymmetry of the effect of 3-phenylindole on the magnitude of conductance changes for positively and negatively charged membrane permeable ions is also discussed as a reflection of the discreteness of both the absorbed 3-phenylindole and lipid dipoles.

Chloride flux in bilayer membranes: the electrically silent chloride flux in semispherical bilayers

High resistance semispherical bilayer membranes of areas as large as 0.3 cm-2 were formed from a decane solution of synthetic diphytanoylphosphatidylcholine. These bilayers had a specific resistance of about 10-9 omega cm-2 and a specific capacitance of 0.38 mu F cm- minus 2 at 20 degrees in 0.1 M KCL. Under these conditions, chloride permeability was 6.8 times 10- minus 8 cm/sec. This electrically silen 36-Cl flux was found to be about 10-3-fold larger than the chloride current calculated from the electrical parameters of the system. The chloride flux in the bilayer was independent of the applied electrical field and was unaltered by addition of reducing agents to the ambient aqueous solutions. It was, however, substantially reduced when NO3 minus was substituted for Cl minus on the side of the bilayer initially free of 36-Cl, or if I minus was added to the aquesous phases in the concentration range of 0.001-0.1 M. These results strongly suggested that the electrically silent flux of 36-Cl is primarily a carrier mediated diffusion process in which phosphatidylcholine acts as the carrier species.

Electrical characteristics of sphingomyelin bilayer membranes

Current-voltage characteristics and the conductivity temperature dependence of sphingomyelin bilayer membranes have been determined. The resistances were of the order of 10(8) Omega-cm(2) and exhibited ohmic behavior up to approximately 25 mv followed by increasing conductivity with applied voltage. The current is found to be proportional to a hyperbolic sine function of the voltage. The temperature dependence indicates a thermally activated conduction mechanism. The observed behavior closely follows a kinetic model involving a barrier modified by the applied electric field, the rate-limiting process being the surmounting of the barrier by the impinging ions. The model allows predictions to be made over a wide range of conditions.

Resistance changes in lipid bilayers: immunological applications

The electrical resistance of a bimolecular lipid membrane in 0.1 molar NaCl decreases if antibody and complement are present on one side of the membrane and the homologous antigen is added to the other side. The reaction occurs within minutes and requires less than 0.1 microliter of antiserum.

The effect of valinomycin on the ionic permeability of thin lipid membranes

Optically black membranes prepared from sheep red cell lipids have a high electrical resistance (1-3 x 10(8) ohm-cm(2)). The ionic transference numbers (T(i)) for cations (Na(+) or K(+)) are equal to each other but at least four to five times greater than for Cl(-). The cyclic depsipeptide valinomycin produces a striking decrease in the membrane resistance when K(+), but not when Na(+) is in the solutions bathing the membrane. The ratio T(Na)/T(K), estimated from membrane voltages in the presence of ionic concentration gradients, approaches zero. The order of membrane monovalent cation selectivity, in the presence of valinomycin, is H(+) > Rb(+) > K(+) > Cs(+) > Na(+). Addition of the antibiotic to one side of a membrane which separates identical solutions of NaCl produces a substantial (up to 80 mV) membrane voltage (side opposite valinomycin negative). These data are consistent with the hypothesis that valinomycin can interact with appropriately sized cations (hydrated diameter ??? 6 A) to increase their membrane permeability, perhaps by forming hydrogen bonds between the solvation shell of the cations and carbonyl oxygens in the valinomycin molecule which are directed toward the aperture of the ring.

The formation and properties of thin lipid membranes from HK and LK sheep red cell lipids

Lipids were obtained from high potassium (HK) and low potassium (LK) sheep red cells by sequential extraction of the erythrocytes with isopropanol-chloroform, chloroform-methanol-0.1 M KCl, and chloroform. The extract contained cholesterol and phospholipid in a molar ratio of 0.8:1.0, and less than 1% protein contaminant. Stable thin lipid membranes separating two aqueous compartments were formed from an erythrocyte lipid-hydrocarbon solution, and had an electrical resistance of approximately 10(8) ohm-cm(2) and a capacitance of 0.38-0.4 microf/cm(2). From the capacitance values, membrane thickness was estimated to be 46-132 A, depending on the assumed value for the dielectric constant (2.0-4.5). Membrane voltage was recorded in the presence of ionic (NaCl and/or KCl) concentration gradients in the solutions bathing the membrane. The permeability of the membrane to Na(+), K(+), and Cl(-) (expressed as the transference number, T(ion)) was computed from the steady-state membrane voltage and the activity ratio of the ions in the compartments bathing the membrane. T(Na) and T(K) were approximately equal ( approximately 0.8) and considerably greater than T(Cl) ( approximately 0.2). The ionic transference numbers were independent of temperature, the hydrocarbon solvent, the osmolarity of the solutions bathing the membranes, and the cholesterol content of the membranes, over the range 21-38 degrees C. The high degree of membrane cation selectivity was tentatively attributed to the negatively charged phospholipids (phosphatidylethanolamine and phosphatidylserine) present in the lipid extract.