Electrical properties of BLM

The electrical properties of black lipid membranes (BLM s) have probably been studied more than those of other lipid systems because of the great similarity between BLM s and cell membranes. The electrical properties of BLM s were reviewed extensively by other authors (32, 33, 34, 35), and we shall therefore describe the electrical and physical properties of lipids which are not generally touched upon in connection with BLM s. We also concentrate on those properties which are intimately related to the different states of order in lipid systems. 

It should be pointed out that, first of all, unmodified planar lipid bilayers (i.e., BLMs formed from common phospholipids or oxidized cholesterol dissolved in -octane) in 0.1 M KCl will typically have the following electrical properties greater than 10 cm. Cm of about 0.4 /xF/cm, about 0, V, about 200 50 mV, and current/voltage (I/V) curves obeying Ohm s Law. However, incorporating a host of materials such as pigments, dyes, polypeptides, membrane proteins, organic metals, and semiconductor particles can drastically alter the electrical properties of BLMs. 

A solution of brain lipids was brushed across a small hole in a 5-ml. polyethylene pH cup immersed in an electrolyte solution. As observed under low power magnification, the thick lipid film initially formed exhibited intense interference colors. Finally, after thinning, black spots of poor reflectivity suddenly appeared in the film. The black spots grew rapidly and evenutally extended to the limit of the opening (5, 10). The black membranes have a thickness ranging from 60-90 A. under the electron microscope. Optical and electrical capacitance measurements have also demonstrated that the membrane, when in the final black state, corresponds closely to a bimolecular leaflet structure. Hence, these membranous structures are known as bimolecular, black, or bilayer lipid membranes (abbreviated as BLM). The transverse electrical and transport properties of BLM have been studied usually by forming such a structure interposed between two aqueous phases (10, 17). 

We believe that the type of investigations that are outlined briefly above provide interesting new information on the properties of lipids from both technical and biophysical points of view. Measurements on electrolyte-lipid semiconductor systems should provide useful information complementary to that obtained from BLM investigations. Furthermore, the gas sensitivity of the electrical properties of lipids could be utilized in practical devices. 

The fact that the majority of in vivo processes occur on the surface of or within the membrane and that electrical phenomena are very important in membranes such as those found in the chloroplast, muscle fibres, nerve fibres, mitochondria, etc., has recently led to intensive study of the electrical properties of bilayer lipid membranes (BLM) in an attempt to reproduce a model of the cell membrane. Membranes of 5-10 nm thickness have been studied, the membranes consisting of two parallel sheets of lipids with a hydrophobic environment in the interior of the membrane and the hydrophilic groups directed to the exterior aqueous medium. 

The precise arrangement and degree of ordering of the lipid molecules in the final structure is not known for certain. However, it seems highly probable that the bilayer nature of the assembly is a consequence of the thermodynamics of free-energy changes at the metal-lipid surface and at the lipid-aqueous solution interface [4,13]. Our measurements of the electrical properties of supported lipid bilayers described here are consistent with those of conventional BLMs and closely related systems. 

A number of methods have been developed over the years to study the properties of BLMs such as optical, electrical, mechanical, transport, and permeability. Of these methods, we shall describe only the electrical methods. In the last decade, many new electrochemical methods have been developed and applied to membrane research. Among them, CV turned out to be a very powerful method. The basics of CV consist of cycling the potential of a WE in an unstirred solution and measuring the resulting current. The potential of the... 

Wang et al. reported a simple method to reconstitute membrane receptors into c-BLMs. After reconstitution, the receptor still retains its ligand activity. Furthermore, the relationship between receptor-ligand interactions and electrical properties of reconstituted BLMs... 

For the investigation of the properties of BLMs, electrical methods have been applied at the very beginning. In addition to the CV technique, other methods such as electrical impedance spectroscopy (EIS) have been applied. Shortly after the discovery of the BLM system, Hanai and Hay don reported the thickness measurement of a planar lipid bilayer using the impedance technique [1 - 3]. Their results are in accord with the value obtained on RBC, estimated by Fricke (see Eq. 1). The impedance technique, nowadays also known as EIS, has subsequently used by many others. The basis of the technique is that a small alternating current (AC) of known frequency and amplitude is applied to the system (e.g. a BLM). The resulting amplitude and phase difference that develop across the BLM are monitored. For a BLM of cross-sectional area (A), and thickness the ability of the BLM to conduct and to store electrical charges are described by the following ... 

Generally, either AC (e.g. EIS) or DC is used for investigating BLMs. Recently, a simple setup for measurements of electrical properties of supported planar lipid bilayers (s-BLMs), using a complementary AC/DC method has been reported [3,13]. The results obtained demonstrated the usefulness of such an approach for studying BLMs. The frequency dependence of resistance and capacitance makes it possible to compare different pubKshed data obtained by AC at different frequencies or DC. In some experiments, capacitance... 

The effect of ATP and ATPase on the electrical properties of of reconstituted BLM. (A) Generation of membrane current upon additions of ATPase followed by ATP as a function of time. 

Hitherto, property measurements of BLM have been confined mainly to thickness, water permeability, electrical characteristics, and current-voltage. The bifacial tension (y6) of BLM is believed to be very small, and a value of about 1 dyne per cm. has been estimated (10). Since no detailed investigations of the bifacial tension of BLM have been reported, the immediate purpose of this work was to develop suitable techniques for y6 measurements. The results of measurements on BLM formed from various lipid solutions are given. The general applicability of the apparatus and method described here to studying other interfacial and bifacial phenomena is briefly discussed. 

Efforts to stabilize BLMs by the use of polymerizable lipids have been successful, but the electrochemical properties of these membranes were greatly compromised and ion channel phenomena could not be observed [21]. Microfiltration and polycarbonate filters, polyimide mesh, and hydrated gels have been used successfully as stabilizing supports for the formation of black lipid films [22-25] and these systems were observed to retain their electrical and permeability characteristics [24]. Poly(octadec-l-ene-maleic anhydride) (PA-18) was found to be an excellent intermediate layer for interfacing phospholipids onto solid substrates, and is sufficiently hydrophilic to retain water for unimpeded ion transfer at the electrode-PA-18 interface [26]. Hydrostatic stabilization of solventless BLMs has been achieved by the transfer of two lipid monolayers onto the aperture of a closed cell compartment however, the use of a system for automatic digital control of the transmembrane pressure difference was necessary [27]. 

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