Membrane extraction characteristics

Though both creative and productive, in retrospect there were certain limitations to this approach. The assumption that the membrane extraction characteristics of the proteins would be conserved was in practice not absolute. Moreover, the inherent problems of working with gel-extracted bands were exemplified by the inability to make identifications for roughly 25% of the gel spots (Dreger et al. 2001). 

Membrane extraction offers attractive alternatives to conventional solvent extraction through the use of dialysis or ultrafiltration procedures (41). The choice of the right membrane depends on a number of parameters such as tlie degree of retention of the analyte, flow rate, some environmental characteristics, and tlie analyte recovery. Many early methods used flat, supported membranes, but recent membrane technology has focused on the use of hollow fibers (42-45). Although most membranes are made of inert polymers, undesired adsorption of analytes onto the membrane surface may be observed, especially in dilute solutions and when certain buffer systems are applied. 

The pertraction efficiency and selectivity can also be increased by adding specific carrier to the membrane phase. Various carrier molecules or ions can be incorporated in the membrane phase to enhance the selectivity and mass transfer. Most carriers used for this were originally developed for solvent extraction 28]. However, many new carriers were designed only for liquid membrane pertraction. Characteristic features of a good carrier for SLM pertraction are ... 

A typical manifold for analytical procedures relying on membrane extraction (Fig. 8.13) is analogous to that used for in-line dialysis (see 8.5.4). Consequently, the main parameters involved, e.g., the geometry of the analytical path, the sample volume introduced, flow rates, timing and membrane characteristics (especially porosity and thickness), have an analogous influence on the analytical figures of merit. 

Peripheral membrane proteins, including lamins, are proteins loosely associated with the membrane. A characteristic of these proteins is their extractability with high salt, urea, and alkaline pH. Extractions of MVs can help identify the nature and function of a variety of peripheral membrane proteins. As mentioned earlier, we have detected B-type lamins in MVs of L. pictus (P. Collas, unpublished). Procedures described below are suitable for extracting lamins from sea urchin egg MVs. Additional unidentified peripheral membrane proteins are also extracted. 

In Figure 25.22, the scheme of experimental setup used for membrane extraction experiments was presented. The setup consisted of membrane contactor with aqueous and organic circuits, two pumps, and the control equipment flowmeters, pressure gauges, and valves. Two phases aqueous and organic solutions circulated countercurrently. Membrane contactor X50 2.5 x 8 Liqui-Cel Extra-Flow, Celgard, was used in the system. The characteristics of the membrane were shown in Table 25.17. The small volume module houses 11,000 capillaries with 1.9 m inner surface area. The module possesses the central baffle, which enables uniform flow inside the shell. 

More recently, another intermediate timescale was explored by NMR relaxom-etry [102]. The spin-lattice relaxation times of water molecules were determined in the range of 20 ns to 20 ps by varying the magnetic field frequency from 10 kHz to 20 MHz and depending on the water content. This technique is well suited for the study of ionomer membranes because of its extreme sensitivity to water-polymer interactions, but it requires a structural and dynamic model to extract characteristic features. The effect of confinement is predominant in polyimides even at high water content (algebraic law with a slope of —0.5 characteristic of porous materials), whereas the diffusion quickly reaches a bulk behavior in Nafion (a plateau is observed at low magnetic fields). 

Nonselective membranes can assist enantioselective processes, providing essential nonchiral separation characteristics and thus making a chiral separation based on enantioselectivity outside the membrane technically and economically feasible. For this purpose several configurations can be applied (i) liquid-liquid extraction based on hollow-fiber membrane fractionation (ii) liquid- membrane fractionation and (iii) micellar-enhanced ultrafiltration (MEUF). 

Membrane Types Key membrane properties include their size rating, selectivity, permeability, mechanical robustness (to allow module fabrication and withstand operating conditions), chemical robustness (to fabrication materials, process fluids, cleaners, and sanitizers), low extractibles, low fouling characteristics, high capacity, low cost, and consistency. 

General Characteristics. Energy addition or extraction from fast fluidized beds are commonly accomplished through vertical heat transfer surfaces in the form of membrane walls or submerged vertical tubes. Horizontal tubes or tube bundles are almost never used due to concern with... 

Fig. 4. a Schematic of porous membrane-based separation of immiscible liquids with different wetting characteristics, b Liquid-liquid extraction device (Kralj et al. 2007)... 

From these overall profiles, it is not easy to extract conformational properties, other than that it will be clear that the lipid molecules are strongly anisotropically oriented in the bilayer. For this, other characteristics are much more appropriate. It is possible to define an order parameter which indicates how much the lipid tails are oriented normal to the membrane ... 

Organic surfactants tend to accumulate in biological membranes due to their lipophilic alkyl chains and their affinity for interfaces. Through this characteristic, organisms are capable of concentrating surfactants. Surfactants of all classes are readily taken up across gills [1]. Therefore, it is important to dispose of techniques that make it possible to extract and quantify the analyte in biological tissues and media. 

A more recent example of this technique has been the study on human absorption characteristics of fexofenadine [109], Fexofenadine has been shown to be a substrate for P-gp in the in vitro cell lines its disposition is altered in knockout mice lacking the gene for MDRla, and co-administration of P-gp inhibitors (e.g. ketoconazole and verapamil) was shown to increase the oral bioavailability of fexofenadine [110-113], Hence, it is suggested that the pharmacokinetics of fexofenadine appears to be determined by P-gp activity. In the human model, the intestinal permeability estimated on the basis of disappearance kinetics from the jejunal segment is low, and the fraction absorbed is estimated to be 2% [114], Co-administration of verapamil/ketoconazole did not affect the intestinal permeability estimates however, an increased extent of absorption (determined by de-convolution) was demonstrated. The increased absorption of fexofenadine was not directly related to inhibition of P-gp-mediated efflux at the apical membrane of intestinal cells as intestinal Peff was unchanged. Furthermore, the effect cannot be explained by inhibition of intestinal based metabolism, as fexofenadine is not metabolised to any major extent. It was suggested that this may reflect modulation of efflux transporters in hepatocyte cells, thereby reducing hepatobiliary extraction of fexofenadine. 

Although mammalian CYPs are attractive candidates for use as commercial biocatalysts, many functional characteristics limit the opportunities to exploit such a system. Association of the enzymes with membranes prevents easy extraction and purification and limits the opportunities to produce useful recombinant enzymes by cloning the relevant genes for expression in microbial systems. All P450s have a porphyrin-haem active site that requires a second protein to reduce the iron component, often cytochrome P450 reductase or... 

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