Artificial asymmetric membranes

Artificial asymmetric membranes composed of outer membranes of various species of Gram-negative bacteria and an inner leaflet of various phospholipids have been prepared using the Montal-Mueller technique [65]. Such planar bilayers have been used, for example, to study the molecular mechanism of polymyxin B-mem-brane interactions. A direct correlation between surface charge density and self-promoted transport has been found [66]. 

Membranes classified under (c) in the introductory section are of considerable importance. Most natural membranes are macroscopically non-homogeneous. In artificial membranes non-homogeneity may be introduced either deliberately (laminates, asymmetric membranes) or spuriously (e.g. skin layers on films made by extrusion). Variation of S and Dx across the membrane, i.e. in the X direction, is of particular interest non-homogeneity along the plane of the membrane is important in certain special cases, e.g. charged mosaic membranes, which are not of immediate interest here. Also asymmetric membranes prepared for the sole purpose of producing an ultrathin active layer to maximize permeation flux are outside the scope of the present discussion. 

It is by no means necessary to use natural lipids in order to form membranes. Modem bioorganic chemistry rather tends to develop new molecules, which allow production of membrane materials with properties unknown in nature (e.g., ultrathin asymmetrical membranes with different headgroups on the in- and outsides, polymeric membranes, and membranes that can be isolated and stored without water see Sec. 2.5). Table 2.2.5 reproduces a few useful artificial amphiphiles derived from simple fatty acids and fatty alcohols. 

One unique appHcation area for PSF is in membrane separation uses. Asymmetric PSF membranes are used in ultrafiltration, reverse osmosis, and ambulatory hemodialysis (artificial kidney) units. Gas-separation membrane technology was developed in the 1970s based on a polysulfone coating appHed to a hoUow-fiber support. The PRISM (Monsanto) gas-separation system based on this concept has been a significant breakthrough in gas-separation... 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

The core of a micelle and the bilayer of a vesicle are comparable with a liquid-crystalline phase and can influence the stereoregularity of asymmetrically catalyzed reactions. Self-organization and the neighborhood of hydrophilic and hydrophobic regions are close to those of natural systems and we designate this as membrane mimetic or enzyme mimetic chemistry [45]. The large field of artificial enzymes was recently reviewed by Murakami et al. [46]. 

Lipid transfer proteins have proved to be a useful tool for studying artificial and natural membranes (for a recent review see Bloj and Zilver-smit, 1981a). With the ability of phospholipid transfer proteins to replace selectively the phospholipid molecules on the exposed surfaces of membranes, information about the asymmetric distribution of phospholipids across a bilayer and the rate of transbilayer movement of phospholipid... 

Knowledge of the size of the exchangeable pool of lipid in a substrate is necessary to determine the rate constants of the exchange process and establish the specificity of a transfer protein for different classes of phospholipids. The size of the exchangeable pool of each class of phospholipid must be determined individually because phospholipids are often asymmetrically distributed across biological and artificial membranes (Op den Kamp, 1979). 

Solute permeability is also a function of membrane composition. The rate of permeation of doxorubicin measured through artificial lipid bilayers composed of cholesterol, PC, PS, PE, and SM (abbreviations defined in Table 5.1) changes substantially with composition (Figure 5.7). This is an important finding, since cells have distinct membrane compositions. In addition, the composition of the individual leaflets of a membrane can vary (Table 5.1), suggesting the possibility of asymmetric passive transport (i.e., permeabilities that differ depending on the direction of transport, out-to-in or in-to-out). 

ABSTRACT Electrical phenomena in artificial cells are described. The constituent material of the cells, referred to as proteinoid or as thermal protein, have been extensively studied in the context of the origin of life, which led to the finding of excitability as one of the biofunctions. The activities found in proteinoid cells are such as to make them useful models for modern excitable cells as well as for protocells. For example, the proteinoid cells display double membrane, asymmetric permeability, membrane potentials, action potentials, and photoactivity. 

A large number of application fields can be overlaid by this type of electroactive structure endowed with recognition properties namely, (i) ionic and molecular detection (biological and chemical sensors, microsensors) (ii) extraction for recuperation and depollution (water and radioactive wastes, environmental protection) (iii) ionic and molecular transport for separation (artificial membranes) (iv) chemical and electrochemical syntheses, e.g., asymmetric synthesis (Fig. 1). 

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