The dependence of the conductance of phosphatidylcholine bilayers upon the concentration and composition of the external electrolyte

The Effect of Cholesterol on the Dielectric Structure of Lipid Bilayers

Cholesterol plays an important role in regulating the properties of phospholipid bilayers and many mechanisms have been proposed to explain why cholesterol is so ubiquitous within biological membranes of animals. Here we present the results of studies on the effect of cholesterol on the electrical/dielectric properties of lipid membranes tethered to a solid substrate. These tethered bilayer lipid membranes tBLM were formed on a commercially available chemically modified gold substrate. These lipid bilayers are very robust. Very high-resolution electrical impedance spectroscopy (EIS) was used to determine the dielectric structure of the lipid bilayers and associated interfaces. The EIS data allowed the dielectric substructure of the lipid bilayers to be determined. The results showed that when cholesterol was present in the tethered membranes at a concentration of 10% (mol/mol); the thickness of the tBLMs increased and the membrane conductance decreased. However, when cholesterol was present in the tethered membrane at more than 30% (mol/mol) the effect of cholesterol was dramatically different; the membranes then became thinner and possessed a much larger electrical conductance. The EIS allowed a distinction to be made between a hydrophobic region in the center of the bilayer and another hydrophobic region further out towards the polar head region, in addition to the polar head region itself. Cholesterol was found to have the largest effect on the inner, hydrophobic region, although the outer hydrophobic region was also affected.

The Effect of Benzyl Alcohol on the Dielectric Structure of Lipid Bilayers

Molecularly tethered lipid bilayer membranes were constructed on a commercially available chemically modified gold substrate. This is a new and promising product that has allowed the construction of very robust lipid bilayers. Very high resolution electrical impedance spectroscopy (EIS) was used to determine the dielectric structure of the lipid bilayers and associated interfaces. The EIS data were modelled in terms of the dielectric substructure using purpose developed software. The hydrophobic region, where the lipid tails are located, was revealed by the EIS in the frequency range of (1-100) Hz and its thickness was calculated from the capacitance of this region and found to be approximately 3-4 nm. The hydrophilic region, where the polar heads are located, was revealed at higher frequencies and its thickness was estimated to be approximately 1-2 nm. The effect of the local anaesthetic benzyl alcohol (BZA) on the tethered lipid bilayers was investigated. The effect of BZA on the membrane capacitance and conductance allowed the changes in the thickness of the polar head and hydrophobic tails regions to be determined. It was found that the addition of BZA caused a significant increase in the capacitance (corresponding to a decrease in the thickness) of the hydrophobic region and an increase in the membrane electrical conductance. The EIS allowed a distinction between a hydrophobic region in the centre of the bilayer and an outer hydrophobic region. Benzyl alcohol was found to have the largest effect on the outer, hydrophobic region, although the inner hydrophobic region was also consistently affected.

Influence of torus on the capacitance of asymmetrical phospholipid bilayers

We have measured the specific capacitance of phosphatidylethanolamine/phosphatidylserine membranes formed from monolayers. The membranes were built across Teflon films whose thickness varied from 6 to 25 mum. The building up of the membranes was followed by recording the capacitance of the membranes during the entire process of their formation. It is observed that the specific capacitance increases by about 10% as the thickness of the film is increased. Furthermore, during formation of the membrane it is observed that the capacitance values increased markedly immediately before the membranes are completely formed and then suddenly decrease to their normal values when formed (closing-off phenomenon). These results have led us to propose that the transition region known to surround membranes formed from monolayers may affect the capacitance values measured for such membranes. However, this effect is indirect in the sense that the composition (most likely in the form of inverted micelles) of the transition region will ultimately modify the hydrophobic/hydrophilic barrier in the bilayer by affecting the lateral tension known to exist in such membranes and, as a consequence, the average molecular area occupied by a phospholipid molecule in the bilayer. By such a mechanism, one can rationalize our experimental finding that the membrane capacitance varies as a function of the partition thickness across which they are prepared. It also rationalizes the large fluctuations in capacitance values usually found in the literature for such membranes as well as, at least in part, the closing-off phenomenon.

Modulation of properties of phospholipid membranes by the long-chain polyprenol (C(160))

The electrical measurements of phospholipid bilayers and the studies of phospholipid vesicles by using the transmission electron microscopy (TEM) showed that dotriacontaprenol (C(160)) isolated from leaves of Spermatophyta influences some properties of membranes. The current-voltage characteristics, the membrane conductance-temperature relationships, the membrane breakdown voltage and the membrane capacitance have been measured for different mixtures of C(160)/DOPC. The membrane conductance, the activation energy of ion migration across the membrane and the membrane thickness were determined. Dotriacontaprenol decreases the membrane breakdown voltage, the activation energy and the membrane capacitance, and increases the membrane conductance and the membrane hydrophobic thickness. The analysis of TEM micrographs shows several characteristic structures, which have been described. The results indicate that dotriacontaprenol increases the membrane elasticity and modulates the surface curvature of the membranes by the formation of fluid microdomains. We suggest that the long polyprenols facilitate the formation of transmembrane, ions-conductive pores.

pH-dependent lipid packing, membrane permeability and fusion in phosphatidylcholine vesicles

We have studied the rate of membrane fusion, the lipid dynamics and order and the membrane permeability of phosphatidylcholine vesicles as a function of pH. Acidification induced very different effects depending on the state of the bilayer. In liquid-crystalline bilayers, acidification decreased the rate of membrane fusion, the acyl chain motion and disorder and the rate of K+ release, whereas in solid bilayers acidification increased the rate of membrane fusion, the lipid acyl chain disorder and the rate of K+ release. These pH-dependent modifications are interpreted in terms of conformational and/or packing changes of the phosphatidylcholine head group in the membrane. In solid bilayers, these changes are not easily accommodated by the rigid structure, and the resulting stress leads to an unstable bilayer.

Permeability of water and polar solutes in lipid bilayers

The three commonly used formalisms to describe water and solute permeation in lipid bilayers (namely, solubility-solute properties, activated rate processes and the thermodynamics of the irreversible process theory) are analyzed in the light of experimental results. These approaches are based on the consideration of the lipid bilayer as a composite membrane containing a hydrocarbon core, an H-bonded interfacial network and a fluctuating structure in which pores can appear. The particular structure of the lipid bilayer (i.e., a hydrophobic-hydrophilic leaflet) makes the permeation process of polar solutions more complicated than that occurring in inert polymeric membranes. Thus, the permeation theories of Fick, Henry and Kedem and Katchalsky should be adapted to introduce interfacial and elastic phenomena. A critical analysis of the experimental results available in the current literature opens the possibility to formulate a broader formalism for permeation in lipid membranes.