Bilayers electrical conductivity

A clue as to why the cationic N-terminal region and the hydrophobic C-terminal portion of SP are required for full histamine-releasing activity comes from studies of the electrical conductivity of black lipid membranes in the presence of peptides. Using SP, these authors [176] concluded that SP probably binds by its N-terminal region to negatively charged sites on membrane lipids, while the C-terminal portion of the molecule penetrates the hydrophobic core of the lipid bilayer, which could induce an increase in membrane permeability or a slight alteration in membrane conformation. 

CNTs and other nano-sized carbon structures are promising materials for bioapplications, which was predicted even previous to their discovery. These nanoparticles have been applied in bioimaging and drag delivery, as implant materials and scaffolds for tissue growth, to modulate neuronal development and for lipid bilayer membranes. Considerable research has been done in the field of biosensors. Novel optical properties of CNTs have made them potential quantum dot sensors, as well as light emitters. Electrical conductance of CNTs has been exploited for field transistor based biosensors. CNTs and other nano-sized carbon structures are considered third generation amperometric biosensors, where direct electron transfer between the enzyme active center and the transducer takes place. Nanoparticle functionalization is required to achieve their full potential in many fields, including bio-applications. 

Artificial bilayer lipid membranes (BLM) have an electrical conductivity of k = 10-14-10-12 S cm-1, less than cell membranes by a factor of 106. The conductivity is increased to physiological levels with the introduction of electron acceptors or proteins in the artificial membranes that form charge transfer complexes with the lipids, as for solid state lipids11. 

Figure 4. Projection of a three-dimensional model of an electrically conducting pore of gramicidin A. To span the full thickness of the lipid bilayer membrane, two molecules, end-to-end, are required. The side chains of the amino acids are not shown. The model was originally proposed by Urry Proc. Nat. Acad. Sci. USA 68, 672 (1971). Reproduced with permission from Ref. 4. Copyright 1983, Springer-Verlag.

The direct measurement of the various important parameters of foam films (thickness, capillary pressure, contact angles, etc.) makes it possible to derive information about the thermodynamic and kinetic properties of films (disjoining pressure isotherms, potential of the diffuse electric layer, molecular characteristics of foam bilayer, such as binding energy of molecules, linear tension, etc.). Along with it certain techniques employed to reveal foam film structure, being of particular importance for black foam films, are also considered here. These are FT-IR Spectroscopy, Fluorescence Recovery after Photobleaching (FRAP), X-ray reflectivity, measurement of the lateral electrical conductivity, measurement of foam film permeability, etc. 

The foam bilayer is the main model system used to obtain experimental results for the stability of bilayers. The proof that the studied foam films are bilayers is based on the experimentally measured h(Cei) dependences and I"I(/i) isotherms. In both cases films with the same thickness are obtained, which corresponds to that of bilayers and does not change with further increase in Cei or IT (e.g. Figs. 3.44, 3.57, 3.62). This leads to the conclusion that the NB foam films do not contain a free aqueous core between its two monolayer of surfactant molecules. A similar conclusion is drawn from the investigatigations of NB foam films by infrared spectra [320,417] and by measuring longitudinal electric conductivity of CB and NB foam films [328,333,418]. 

Another well-defined synthetic membrane is a planar bilayer membrane. This structure can be formed across a 1-mm hole in a partition between two aqueous compartments by dipping a fine paintbmsh into a membrane-forming solution, such as phosphatidyl choline in decane. The tip of the brush is then stroked across a hole (1 mm in diameter) in a partition between two aqueous media. The lipid film across the hole thins spontaneously into a lipid bilayer. The electrical conduction properties of this macroscopic bilayer membrane are readily studied by inserting electrodes into each aqueous compartment (Figure 12.14). For example, its permeability to ions is determined by measuring the current across the membrane as a function of the applied voltage. 

The results of permeability studies of lipid vesicles and electrical-conductance measurements of planar bilayers have shovm that lipid bilayer membranes have a very low permeability for ions and most polar molecules. Water is a conspicuous exception to this generalization it readily traverses such membranes because of its small size, high concentration, and lack of a complete charge. The range of measured permeability coefficients is very wide (Figure 12.15). For example, Na+ and K+ traverse these membranes 10 times as slowly as does H2O. Tryptophan, a zwitterion... 

Figure 12.14. Experimental Arrangement for the Study of Planar Bilayer Membrane. A bilayer membrane is formed across a 1-mm hole in a septum that separates two aqueous compartments. This arrangement permits measurements of the permeability and electrical conductance of lipid bilayers.

After the bottom pole and insulator, a microwinding Cu coil is electrode-posited [121]. The insulator has to be prepared for the electrodeposition of Cu. This preparation involves the deposition of Cr/Cu bilayer by sputtering or evaporation. First, a thin layer (10 nm) of Cr is deposited onto the insulator. The function of the Cr layer is to provide a bonding layer between the insulator and Cu. A thin (50-100 nm) layer of Cu seed layer is then sputter deposited on Cr layer to provide sufficient electrical conductivity for subsequent electrodeposition of Cu. Cu is electrodeposited using deposition-through-mask technique. After electrodeposition of Cu coil, an insulator layer is deposited between the coil and the top pole layer. The top Permalloy pole is electrodeposited in the same way as the bottom pole layer, on thin sputter-deposited Permalloy underlayer (50-100 nm). The top and bottom pole layers are in contact. Finally, Cu interconnect pads, about 25-pm thick, are electrodeposited. The entire structure, poles and coil, is protected by an overcoat, usually sputtered AI2O3. The dimensions... 

In the lipid bilayer systems, since the membrane molecules are arranged in such a way that the charged groups face a water phase and the interior of the membrane is a hydrocarbon phase, the contribution of surface potential to the membrane potential is important. It should be mentioned that the contribution of surface potential to the membrane potential, as discussed above, is generally a transient one in these systems. However, since the electrical conductance due to ion permeation across the lipid bilayer membrane is very low, we can observe the transient potential difference as a quasi-steady state phenomenon. However, if a constant ion distribution is restored by a transport process with a nonelectrical current (active transport) and maintained continuously, the above membrane potential process could become a steady state process. 

The cell membrane is an absolute necessity for life because by it the cell can control its interior by controlling the membrane permeability. If the membrane is destroyed, then the cell dies. The membrane is a layer that separates two solutions and forms two sharp boundaries toward them. The cell membrane consists of phospholipids that form a bilayer lipid membrane (BLM) approximately 7 nm thick (Figure 4.6). Each monolayer has its hydrophobic surface oriented inward and its hydrophilic surface outward toward the intracellular or extracellular fluids. The inside of such a bilayer is hydrophobic and lipophilic. A BLM is a very low electric conductivity membrane and is accordingly in itself closed for ions. It lets lipids pass but not water. However, water molecules can pass specialized membrane channels (cf. Chapter 5). The intrinsic conductance is on the order of 10 S/m, and a possible lipophilic ionic conductivity contribution cannot be excluded. 

The ion permeability of a biological membrane may be approximated by measuring the electric conductivity across a unilamellar lipid bilayer. Such a bilayer immersed in a solution of the desired electrolyte represents a symmetrical situation, i.e., Ci, = c,.2(=C ), T i = T 2 (=E ), and A / j = 0. Applying a small a.c. field (A /, J across the symmetrical bilayer leads to an electric conductivity given by... 

From experiments on planar bilayer membranes (BLM), it was known that lipid bilayers were not able to withstand an increase in the applied voltage above a threshold value. A conductive state followed by a rupture was observed for values of the order of 200 mV. Electropulsation induces a transmembrane potential modulation, bringing a similar membrane instability. Indeed experiments on pure lipid vesicles showed that upon the field pulse the lipid bilayer could become leaky. This was observed on line by the associated increase in conductance of a salt-filled vesicle suspension [26]. But larger molecules could leak out and be directly detected outside the vesicles as observed with radiolabelled sucrose [27] or fluorescent dyes [28]. A very fast detection of the induction of membrane leakage is obtained by electrical conductance and light scattering... 

---