Lipid membrane-electrode interfaces

Design and Electrochemical Characterization of lipid Membrane—Electrode Interfaces... 

NAKASHIMA AND TAGUCHI Lipid Membrane-Electrode Interfaces... 

Redox reactions at bilayer lipid membrane/water interfaces have been studied by many authors [2,7, 38, 39]. Usually the BLM, doped by ubiquinone, tetracyano-p-quinodimethane (TCNQ), or by ferrocene, has the properties of a bipolar electrode [39]. A variety of redox couples in the aqueous phases have been used, including ascorbic acid/dehydroascorbic acid, KI/I2, [PtC ] ", Sn " /Sn, ... 

An equivalent circuit can be derived for the surface-bound membrane formed in this work similar in a manner to the approach taken for porous anodic films and porous electrodes (41-46). An equivalent circuit network, proposed in Figure 8a, corresponds to the model in Figure 7. This network has three RC subnetworks that represent the oxide layer, the surface-bound membrane layer, and the double layer. Cox and Rox are the capacitance and resistance of oxide. and Rdl are the double-layer capacitance and the polarization resistance, known as the charge transfer resistance at the membrane-water interface. For the subnetwork of the surface-bound membrane layer, one branch represents a tightly packed alkylsilane and lipid bilayer in series, and the other branch represents the pores and defects through the bilayer. Calk, Clip and Ralk, Rhp are the capacitances and resistances of... 

At frequencies below 63 Hz, the double-layer capacitance began to dominate the overall impedance of the membrane electrode. The electric potential profile of a bilayer membrane consists of a hydrocarbon core layer and an electrical double layer (49). The dipolar potential, which originates from the lipid bilayer head-group zone and the incorporated protein, partially controls transmembrane ion transport. The model equivalent circuit presented here accounts for the response as a function of frequency of both the hydrocarbon core layer and the double layer at the membrane-water interface. The value of Cdl from the best curve fit for the membrane-coated electrode is lower than that for the bare PtO interface. For the membrane-coated electrode, the model gives a polarization resistance, of 80 kfl compared with 5 kfl for the bare PtO electrode. Formation of the lipid membrane creates a dipolar potential at the interface that results in higher Rdl. The incorporated rhodopsin may also extend the double layer, which makes the layer more diffuse and, therefore, decreases C. ... 

Lipids and phospholipids The study of phospholipid monolayers adsorbed on a mercury electrode and the interaction between phospholipids and proteins has been an active research topic for a number of years. The reason for this is obvious when one considers the currently accepted fluid mosaic model of the bilayer lipid membrane (BLM). In addition to its role as a structural element in cells, etc., the BLM is also important in some foods. Since there is enough phospholipid in milk to form a film on a greatly expanded oil-water interface, this lipid undoubtedly plays an important role in stabilizing dairy and other food products that utilize homogenized milk [123]. 

Another observation should be made with respect to the term elastic in describing interfacial capacitors. It was originally introduced by Crowley [1] for membranes and reflects the compressibility of lipid layers which behave in some respects like an elastic film. Its relation to electrochemical interfaces is less obvious. Consider an interface between a metal electrode and an electrolyte. As we will see in Section III, the effective gap of the interfacial capacitor is the distance between the centers of mass of the electronic, e, and ionic, i, charge density distributions... 

As noted above hpid bilayer films supported on electrodes represent an interesting bioelectrochemical interface where the potentials are of the same order as those of physiological systems, and can be readily modulated. Nevertheless, this area remains less studied except in the case of enzyme complexes such CcO where the hydrophobic environment of the lipids is necessary to maintain the integrity ofthe enzyme when immobilizing it on electrodes [304, 314]. Studies of the interaction of drugs with hpid membranes are important from many points of view, particularly considering the fact that nearly 50% of drug molecules have mem-... 

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]. 

As an example of a membrane model, phospholipid monolayers with negative charge of different density were used. It had already been found ( ) and discussed O) that the physical and biological behavior of phospholipid monolayers at air-water interfaces and of suspensions of liposomes are comparable if the monolayer is in a condensed state. Two complementary methods of surface measurements (using radioactivity and electrochemical measurements), were used to investigate the adsorption and the dynamic properties of the adsorbed prothrombin on the phospholipid monolayers. Two different interfaces, air-water and mercury-water, were examined. In this review, the behavior of prothrombin at these interfaces, in the presence of phospholipid monolayers, is presented as compared with its behavior in the absence of phospholipids. An excess of lipid of different compositions of phos-phatidylserine (PS) and phosphatidylcholine (PC) was spread over an aqueous phase so as to form a condensed monolayer, then the proteins were inject underneath the monolayer in the presence or in the absence of Ca. The adsorption occurs in situ and under static conditions. The excess of lipid ensured a fully compressed monolayer in equilibrium with the collapsed excess lipid layers. The contribution of this excess of lipid to protein adsorption was negligible and there was no effect at all on the electrode measurements. 

Biological membranes are considerably more complex than the models discussed above for electrode/aqueous electrolyte interfaces and there might appear to be few lines of comparison to be drawn between the two cases. Biomembranes possess two-dimensional structure and have a thickness of approximately 100 angstroms. They are formed primarily from amphiphilic phospholipids, which impart the two-dimensional structure to the membrane, and from proteins. In aqueous solutions, the long hydrophobic tails of the lipids are found in the interior of the membrane while the polar head... 

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