Millimeter microwave effect on ion transport across lipid bilayer membranes

The effects of millimeter microwaves in the frequency range of 54–76 GHz on capacitance and conductance of lipid bilayer membranes (BLM) were studied. Some of the membranes were modified by gramicidin A and amphotericin B or by tetraphenylboron anions (TPhB_?). The millimeter microwaves were pulse-modulated (PW) at repetition rates ranging from 1 to 100 pps, PW at 1000 pps, or unmodulated continuous waves (CW). The maximum output power at the waveguide outlet was 20 mW. It was found that CW irradiation decreased the unmodified BLM capacitance by 1.2% ± 0.5%. At the same time, membrane current induced by TPhB transport increased by 5% ± 1%. The changes in conductance of ionic channels formed by gramicidin A and amphotericin B were small (0.6% ± 0.4%). No “resonance-like” effects of mm-wave irradiation on membrane capacitance, ionic channel currents, or TPhB transport were detected. All changes in membrane capacitance and currents were independent of the modulation employed and were equivalent to heating by approximately 1.1 °C. © 1995 Wiley-Liss, Inc.     

The effect of uncouplers of oxidative phosphorylation on lipid bilayer membranes: Carbonylcyanidem-chlorophenylhydrazone

Summary Detailed experimental data for conductivity and membrane potentials are presented for lecithin/cholesterol/decane bilayers in the presence of the uncoupler carbonylcyanidem-chlorophenylhydrazone (CCCP). These compare favorably with a theoretical model derived to explain the mechanism of action of uncouplers on bilayers. The model assumes that the weak acid uncoupler HA and its anion A⁻ are the sole species which permeate the membrane. Its key feature is the recognition of the existence of unstirred aqueous layers on either side of the membrane. The model accounts for, among other things, a maximum in the transmembrane conductivity at a pH to the alkaline side of the uncoupler pK _a and saturating current-voltage characteristics at high pH, both phenomena being found for CCCP. From a quantitative fit of model to data, values of 2.0×10⁻³ and 11 cm/sec are deduced for the permeability coefficients of the CCCP anion and the undissociated CCCP molecule, respectively.     

Photoelectrospectrometry of bilayer lipid membranes

Three different bilayer lipid membrane systems were studied under visible and ultraviolet illumination. The first system consisted of a bilayer lipid membrane formed with a mixture of phospholipids and cholesterol, to one side of which purple membrane fragments from Halobacterium halobium were added. The second system consisted of a membrane formed from spinach chloroplast extract. When either of these membrane systems was illuminated with ultraviolet and visible radiation, photopotentials were observed and photoelectric action spectra were recorded (the technique is termed photoelectrospectrometry). Each spectrum had a definite structure which was characteristic of each of the modified membranes. The third system studied consisted of an otherwise photoinactive membrane formed with a mixture of phospholipids and cholesterol, to one side of which chymotrypsin was added. When the membrane was illuminated with visible light no photoresponse was observed. On the other hand, a photopotential which increased with incubation time was observed when the membrane was illuminated with ultraviolet light. Since, in our systems, the photoresponses have been observed to be due to certain species incorporated into the membrane, it appears that photoelectrospectrometry is a useful tool for studying lipid-protein interactions, constituent organization and energy transfer in membranes.     

Cell-mediated tumor-killing effect studied by using bilayer lipid membranes

The bilayer lipid membrane (BLM) system was used to investigate the tumor killing effect of natural killer (NK) cells under various experimental conditions. It was found that NK cells interact specifically with BLMs made from lipids and proteolipids isolated from target K562 cells inducing an increase of the membrane conductance. This effect was more pronounced when the NK cells were pretreated with interferon. A similar effect was observed when NK cells were pretreated with sodium selenite. The results suggest that changes in membrane conductance and permeability are involved in the mechanism of the tumor-killing effect mediated by NK cells.     

The nonelectrolyte permeability of planar lipid bilayer membranes

The permeability of lecithin bilayer membranes to nonelectrolytes is in reasonable agreement with Overton's rule. The is, Pd alpha DKhc, where/Pd is the permeability coefficient of a solute through the bilayer, Khc is its hydrocarbon:water partition coefficient, and D is its diffusion coefficient in bulk hydrocarbon. The partition coefficients are by far the major determinants of the relative magnitudes of the permeability coefficients; the diffusion coefficients make only a minor contribution. We note that the recent emphasis on theoretically calculated intramembranous diffusion coefficients (Dm'S) has diverted attention from the experimentally measurable and physiologically relevant permeability coefficients (Pd'S) and has obscured the simplicity and usefulness of Overton's rule.     

The structure of melittin in lipid bilayer membranes

0.15 M inorganic phosphate dramatically increased the α-helix content of melittin in aqueous solution.When melittin interacted with egg yolk phosphatidylcholine liposomes in the absence of inorganic phosphate, it was converted to an α-helix rich form, as postulated by Dawson et al. (Dawson, C.R., Drake, A.F. Helliwell, J. and Hider, R.C. (1978) Biochim. Biophys. Acta 510, 75–86).     

The interaction of detergents with bilayer lipid membranes

Detergents are widely used for extracting and purifying membrane proteins. Four such detergents have been studied to find the extent to which they alone can alter black lipid film conductances. The slope of the plot of conductivity versus concentration for Triton X-100 is 4.54 in the range 0.025–0.15 mM; dodecyl sulphate 0.82 in the range 0.01–1 mM; sodium deoxycholate 1.03 in the range 0.01–1 mM and sodium cholate 1.37 in the range 0.1–10 mM. These ranges are below the respective critical micelle concentrations; above these concentrations the membranes break. Bilayer lipid membrane conductivity measured at constant detergent concentration increases with the conductivity of the bathing salt solution with a slope greater than 1, indicating an effect on the putative pore structures induced by detergents.     

The breakdown of bilayer lipid membranes by dendrimers

The BLM-system for studying the electrophysical properties of bilayer lipid membranes (BLM) was applied to investigate interactions between polyamidoamine (PAMAM) dendrimers and lipid bilayers. The cationic PAMAM G5 dendrimer effectively disrupted planar phosphatidylcholine membranes, while the hydroxyl PAMAM-OH G5 and carboxyl PAMAM G4.5 dendrimers had no significant effect on them.     

The effect of prymnesin on the electric conductivity of thin lipid membranes

Summary A proteolipidic toxin, prymnesin, when added to the aqueous solutions around thin lipid membranes causes a marked increase in membrane conductance. The toxin-treated membrane is cation-permselective. The extent of cation permselectivity is dependent upon ionic strength of the aqueous solutions in a fashion similar to the dependence of cation permselectivity of a cation exchanger containing about 100mm of fixed negative sites. Dose-response relationship studies reveal a linear relation between log prymnesin concentration and log membrane conductance. The slope of the curve is around 3 if the toxin is applied to one side of the membrane and is around 7 if the toxin is applied to both sides of the membrane. The membrane treated with toxin on one side only is clearly asymmetric in its properties. These characteristics are expressed by an asymmetric current-voltage relationship, and by asymmetric sensitivity of membrane conductance to pH and to salt concentration. The conductance of the toxin-treated membrane is inversely proportional to temperature. It is suggested that aggregates of toxin moieties assemble in the membrane to form negatively charged aqueous pores. There is roughly a good correlation between the increase in membrane conductance and the increase in membrane permeability to urea if both were attributed to the formation of aqueous channels in the membrane.     

Self-assembling bilayer lipid membranes on solid support

Solid-supported bilayer lipid membranes (s-BLMs) that possess some properties similar to those of conventional BLMs can be self-assembled on a freshly cleaved metal wire by a two-step procedure: (i) The tip of a Teflon-coated platinum wire, while immersed in a lipid solution, is cut off with a scalpel; (ii) the new tip of the wire, having become coated with lipid solution, is transferred into 0.1 M KCl. After a few minutes, a stable lipid bilayer forms spontaneously on the tip of the wire, as verified by electrical measurements. An application of such a supported BLM (s-BLM) is reported for the detection of Pb2+ ions. The s-BLM is liquid-crystalline in structure, which makes it amenable to modification for basic studies, as well as for technological applications such as biosensors and molecular electronic devices.     

Effects of uranyl ions on lipid bilayer membranes

Uranyl ions (UO₂²⁺) stabilize black lipid membranes (BLM's) as inferred from the doubling of the breakdown voltage and from a considerable increase in the lifetime of the BLM's. These effects are observed also in BLM's made of mono-olein and of oxidized cholesterol. The lytic effect of lysolecithin is significantly reduced in the presence of UO₂²⁺. Uranyl ions adsorb to the interface of BLM's made of phosphatidylcholine (PC) with a dissociation constant of about 3 : 10^?6 M and thereby charge the interface of the membrane and attain almost stoichiometric binding of one molecule of uranyl ion per one molecule of PC at 1 M ionic strength and 20 μM of UO₂²⁺. The membrane conductance induced by ionophores is considerably reduced by UO₂²⁺ and it is inferred by various tests that this is due to the charging of the interface and not to changes in membrane fluidity.     

The interaction of anionic detergents with lipid bilayer membranes

Summary The time course of the reaction of anionic surfactants with lipid bilayers is followed and analyzed. The distribution of detergents in the membrane phase gives rise to an asymmetry potential followed by a diffusion potential. Detergent-doped membranes are cation-permselective. It is postulated that a variable profile of mobile charges in the membrane account for the cation-permselectivity, the intercation selectivity, and the voltage-dependent gating phenomena observed in excitable membranes.Supported by a grant from the Medical Research Council of Canada.I thank Mr. G. Beauchesme for his technical assistance in part of this work.     

The interaction of hydrophobic ions with lipid bilayer membranes

Summary Electrical relaxation studies have been made on lecithin bilayer membranes of varying chain length and degree of unsaturation, in the presence of dipicrylamine. Results obtained are generally consistent with a model for the transport of hydrophobic ions previously proposed by Ketterer, Neumcke, and Läuger (J. Membrane Biol. 5:225, 1971). This model visualizes as three distinct steps the interfacial adsorption, translocation, and desorption of ions. Measurements at high electric field yield directly the density of ions adsorbed to the membrane-solution interface. Variation of temperature has permitted determination of activation enthalpies for the translocation step which are consistent with the assumption of an electrostatic barrier in the hydrocarbon core of the membrane. The change of enthalpy upon adsorption of ions is, however, found to be negligible, the process being driven instead by an increase of entropy. It is suggested that this increase may be due to the destruction, upon adsorption, of a highly ordered water structure which surrounds the hydrophobic ion in the aqueous phase. Finally, it is shown that a decrease of transient membrane conductance observed at high concentration of hydrophobic ions, previously interpreted in terms of interfacial saturation, must instead be attributed to a more complex effect equivalent to a reduction of membrane fluidity.Research performed while on sabbatical leave April-September, 1974.     

The interaction of bee melittin with lipid bilayer membranes

The influence of melittin and the related 8-26 peptide on the stability and electrical properties of bilayer lipid membranes is reported. Melittin, unlike the 8-26 peptide, has a dramatic influence on lipid membranes, causing rupture at dilute concentrations. The circular dichroism of melittin demonstrated that under physiological conditions, in water, melittin is in extended conformation, which is enhanced in aqueous ethanol. However in 'membrane-like' conditions it is essentially alpha-helical. Secondary structure predictions were used to locate possible alpha-helical nucleation centres and a model of melittin was built according to these predictions. It is postulated that melittin causes a wedge effect in membranes.