Bilayer, lipidic

In biological systems molecular assemblies connected by non-covalent interactions are as common as biopolymers. Examples arc protein and DNA helices, enzyme-substrate and multienzyme complexes, bilayer lipid membranes (BLMs), and aggregates of biopolymers forming various aqueous gels, e.g, the eye lens. About 50% of the organic substances in humans are accounted for by the membrane structures of cells, which constitute the medium for the vast majority of biochemical reactions. Evidently organic synthesis should also develop tools to mimic the Structure and propertiesof biopolymer, biomembrane, and gel structures in aqueous media. 

As first shown by Hladky and Haydon 7,8), it is possible to observe the current due to a single transmembrane channel by using extensions of the planar lipid hilaver approach of Mueller and Rudin 9). The basic system is shown in Fig. 2 and is commonly referred to as the black lipid membrane (BLM) method. This is because, as the lipid in the hole between the two chambers thins, the areas that have become planar bilayers are seen as black. Additional terms are bilayer lipid membranes or planar lipid bilayer membranes. These lipid bilayer membranes, particularly those which are solvent free, have capacitances which are very close to those of biological membranes. 

The use of Upid bilayers as a relevant model of biological membranes has provided important information on the structure and function of cell membranes. To utilize the function of cell membrane components for practical applications, a stabilization of Upid bilayers is imperative, because free-standing bilayer lipid membranes (BLMs) typically survive for minutes to hours and are very sensitive to vibration and mechanical shocks [156,157]. The following concept introduces S-layer proteins as supporting structures for BLMs (Fig. 15c) with largely retained physical features (e.g., thickness of the bilayer, fluidity). Electrophysical and spectroscopical studies have been performed to assess the appUcation potential of S-layer-supported lipid membranes. The S-layer protein used in aU studies on planar BLMs was isolated fromB. coagulans E38/vl. 

TABLE 8 Electrophysical Parameters of Plain, S-Layer-Supported, and SUM-Supported Bilayer Lipid Membranes... 

A = approximate area of the bilayer lipid membrane G = membrane conductance Gj = specific membrane conductance Cm = membrane capacitance C, = specific membrane capacitance. 

TH Tien, A Ottova-Leitmannova. Membrane Biophysics as Viewed from Experimental Bilayer Lipid Membranes Planar Lipid Bilayers and Spherical Liposomes. New York Elsevier Science, 2000. 

Lahiri, J., Kalal, P., Frutos, A. G., Jonas, S. J. and Schaeffler, R. (2000) Method for fabricating supported bilayer lipid membranes on gold. Langmuir, 16, 7805-7810. 

Selective Ion Transfer of Alkali and Alkaline Earth Metal Ions Facilitated by Naphtho-15-Crown-5 Across Liquid-Liquid Interfaces and a Bilayer Lipid Membrane... 

II. VOLTAMMETRIC ELUCIDATION OF THE CHARGE TRANSPORT PROCESS THROUGH A LIQUID MEMBRANE OR A BILAYER LIPID MEMBRANE IN THE PRESENCE OF SUFFICIENT ELECTROLYTES [10,17,18]... 

D. Voltammograms for Ion Transfer Through a Bilayer Lipid Membrane... 

H. T. Tien, Bilayer Lipid Membranes, Marcel Dekker, New York, 1974, pp. 11-28. 

The oscillations observed with artificial membranes, such as thick liquid membranes, lipid-doped filter, or bilayer lipid membranes indicate that the oscillation can occur even in the absence of the channel protein. The oscillations at artificial membranes are expected to provide fundamental information useful in elucidating the oscillation processes in living membrane systems. Since the oscillations may be attributed to the coupling occurring among interfacial charge transfer, interfacial adsorption, mass transfer, and chemical reactions, the processes are presumed to be simpler than the oscillation in biomembranes. Even in artificial oscillation systems, elementary reactions for the oscillation which have been verified experimentally are very few. 

The voltammetric method and concept are expected to be applicable also to the analysis of the oscillation at a very thin membrane such as a bilayer lipid membrane, since even the ion transfer through a bilayer lipid membrane can be observed as a vol-tammogram and interpreted by the way similar to that for a liquid membrane [20,21]. 

The phase transition of bilayer lipids is related to the highly ordered arrangement of the lipids inside the vesicle. In the ordered gel state below a characteristic temperature, the lipid hydrocarbon chains are in an all-trans configuration. When the temperature is increased, an endothermic phase transition occurs, during which there is a trans-gauche rotational isomerization along the chains which results in a lateral expansion and decrease in thickness of the bilayer. This so-called gel to liquid-crystalline transition has been demonstrated in many different lipid systems and the relationship of the transition to molecular structure and environmental conditions has been studied extensively. 

Antonenko, Y. N. Bulychev, A. A., Measurements of local pH changes near bilayer lipid membrane by means of a pH microelectrode and a protonophore-dependent membrane potential. Comparison of the methods, Biochim. Biophys. Acta 1070, 279-282 (1991). 

Tien, T. H. Ottova, A. L., The lipid bilayer concept and its experimental realization From soap bubbles, kitchen sink, to bilayer lipid membranes, J. Membr. Sci. 189,83-117 (2001). 

Ottova, A. Tien, T. H., The 40th anniversary of bilayer lipid membrane research,... 

Antonenko, Y. N. Denisov, G. A. Pohl, P., Weak acid transport across bilayer lipid membrane in the presence of buffers, Biophys. J. 64, 1701-1710 (1993). 

In electrochemistry similar phenomena are observed, for example, with the formation of insoluble films on electrodes or with ion selective channel formation in bilayer lipid membranes or nerve cell membranes (pages 377 and 458). 

The search for models of biological membranes led to the formation of a separate branch of electrochemistry, i.e. membrane electrochemistry. The most important results obtained in this field include the theory and application of ion-exchanger membranes and the discovery of ion-selective electrodes (including glass electrodes) and bilayer lipid membranes. 

The thickness of the membrane phase can be either macroscopic ( thick )—membranes with a thickness greater than micrometres—or microscopic ( thin ), i.e. with thicknesses comparable to molecular dimensions (biological membranes and their models, bilayer lipid films). Thick membranes are crystalline, glassy or liquid, while thin membranes possess the properties of liquid crystals (fluid) or gels (crystalline). 

Phospholipids are amphiphilic substances i.e. their molecules contain both hydrophilic and hydrophobic groups. Above a certain concentration level, amphiphilic substances with one ionized or polar and one strongly hydrophobic group (e.g. the dodecylsulphate or cetyltrimethylammonium ions) form micelles in solution these are, as a rule, spherical structures with hydrophilic groups on the surface and the inside filled with the hydrophobic parts of the molecules (usually long alkyl chains directed radially into the centre of the sphere). Amphiphilic substances with two hydrophobic groups have a tendency to form bilayer films under suitable conditions, with hydrophobic chains facing one another. Various methods of preparation of these bilayer lipid membranes (BLMs) are demonstrated in Fig. 6.10. 

Fig. 6.10 Methods of preparation of bilayer lipid membranes. (A) A Teflon septum with a window of approximately 1mm2 area divides the solution into two compartments (a). A drop of a lipid-hexane solution is placed on the window (b). By capillary forces the lipid layer is thinned and a bilayer (black in appearance) is formed (c) (P. Mueller, D. O. Rudin, H. Ti Tien and W. D. Wescot). (B) The septum with a window is being immersed into the solution with a lipid monolayer on its surface (a). After immersion of the whole window a bilayer lipid membrane is formed (b) (M. Montal and P. Mueller). (C) A drop of lipid-hexane solution is placed at the orifice of a glass capillary (a). By slight sucking a bubble-formed BLM is shaped (b) (U. Wilmsen, C. Methfessel, W. Hanke and G. Boheim)...

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