Polymeric Membrane Models

Figure 3. Possible preparation of polymeric model membranes (X = polymerizable group). (a)-(c) Polymerization preserving head group properties, (d) Polymerization preserving chain mobility (30). Corresponding monomers see Table 1.

All four systems illustrated in Fig. 4 exhibit properties differing from those of cell membranes. Methods a-c have no influence on the head groups and preserve physical properties, such as charge, charge density, etc. The fluidity of the hydrocarbon core, however, is drastically decreased by the polymerization process. In case d, fluidity is not affected, but there is no free choice of head groups. In comparison to biomembranes, all polymerized model membrane systems will show an increase in viscosity and a decrease in lateral mobility of the molecules. 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

Fig. 1 Possible ways to synthesize polymeric model membranes, (x = polymerizable group). a)-c) Polymerization with preservation of headgroup properties d) polymerization with preservation of chain mobility. For examples of corresponding monomers, see table 1.

Johnston, D.S., S. Sanghera, A. Manjon-Rubio and D. Chapman The Formation of Polymeric Model Membranes From Diacetylenic Fatty Acids and Phospholipids. Biochim. Biophys. Acta 602 (1980) 213-216. 

Fig. 27 a and b. Schematic representation of the molecular structure of a side chain polymeric liquid crystals b polymer model membranes studied by 2H NMR... 

Fig. 30. DSC traces showing the phase transition of the model membrane in its monomeric and polymeric form. Note the difference in the enthalpies of the transition monomer AH = 56 J/g, polymer AH = 26 J/g...

The fluidity is one of the most vital properties of biological membranes. It relates to many functions involved in biological system, and effective biomembrane mimetic chemistry depends on the combination of both stability and mobility of the model membranes. However, in the polymerized vesicles the polymer chain interferes with the motion of the side groups and usually causes a decrease or even the loss of the fluid phases inside the polymerized vesicle (72,13). 

The polymerization of the butadiene monomers (3,4) can also be followed spectroscopically by the disappearance of the strong absorption of the monomers at 260 nm, whereas the absorption of the resulting poly-1,4-trans(butadiene)s is too small to be observed in a single monolayer. The polymers from the butadiene and methacryloyl lipids are probably better model membrane systems, because the polymer chains are still mobile and not excessively rigid as the polydiacetylenes. 

Besides polymerization, another type of polyreaction can be used for stabilizing model membrane systems. Recently, Fukuda et al.28) described polyamide formation via polycondensation in monolayers at the gas/water interface (definition of mono-layers see Sect. 3.2). Long-chain esters of glycine and alanine were polycondensed to yield non-oriented polyamide films of polyglycine and polyalanine. 

The results illustrated so far show that polyreactions in oriented planar monolayers are possible and lead to highly ordered and very stable model membranes. This raises the question whether polymerization is also possible in bilayers (like vesicles and BLM) and whether these bilayers exhibit a higher stability than their unpolymerized counterparts. 

So far, it has been shown that the stability of a model membrane can be tremendously increased by polymerization. This increased stability however, is associated with the presence of a polymer chain in the membrane itself or on its surface, bringing about increased viscosity and thus reduced flexibility. How does the reduced membrane mobility affect one of the most vital properties of biomembranes, the phase transition ... 

Polycondensation reactions in oriented monolayers and bilayers proceed without catalysis, and simply occur due to the high packing density of the reactive groups and their orientation in these layers. Bulk condensation of the a-amino acid esters at higher temperatures does not lead to polypeptides but to 2,5-diketopiperazines. No diketopiperazines are found in polycondensed monolayers or liposomes. Polycondensation in monolayers and liposomes leading to oriented polyamides represents a new route for stabilizing model membranes under mild conditions. In addition, polypeptide vesicles may be cleavable by enzymes in the blood vessels. In this case, they would represent the first example of stable but biodegradable polymeric liposomes. 

In biological membranes, integral proteins are amphipatic molecules their hydro-phobic moiety is embedded in the lipid bilayer and their hydrophilic moiety protrudes from the surface of the membrane279. So, it was interesting to prepare polymeric models of such amphipatic proteins. For that purpose, two new classes of block copolymers have been synthetized in Orleans, namely copolymers with a polyvinyl block and a polypeptide block and copolymers with a saccharide and a peptide block. We shall give some information concerning the preparation of these copolymers and then describe their structure. 

Hudecz, F., Nagy, I. B., Koczan, G., Alsina, M. A., and Reig, F. (2001) Carrier design influence of charge on interaction of branched polymeric polypeptides with phospholipid model membranes, in Biomedical Polymers and Polymer Therapeutics (Chiellini, E., Sunamoto, J., Migliaresi, C., Ottenbrite, R. M., and Cohn, D. eds.), Kluwer Academic/Plenum Publishers, New York, pp. 103-120. 

The diacetylenic acids (9,10) have also been widely investigated because of their polymerizability. Here the interesting diacetylenic entity is normally incorporated into the structure of an alkanoic acid to give a compound such as CH3(CH2)ioC=C-C=C(CH2)7COOH. The topochemical polymerization proceeds within the LB layer but results in an array of two-dimensional domains whose size is influenced by material purity. Diacetylenes have also been incorporated into lipidlike molecules and polymerized as model membranes 11, 12). Because of the structural flexibility of LB films, there is likely to be continued interest in polymer LB films research, some of which will involve preformed polymers (13). However, the rigidity of many polymer films prevents this approach being used generally. 

Many questions pertaining to membrane processes in general and ligand/membrane receptor interactions in particular can be addressed by a novel model membrane system, i.e., polymer-supported or polymer-tethered lipid bilayers [12,14], The basic structural unit for this sensor platform is the tethered lipid bilayer membrane [16] displayed in Fig. 2D. The essential architectural elements of this supramolecular assembly include the solid support, e.g., an optical or electrical transducer (device), the polymeric tether system which provides the partial covalent and, hence, very stable attachment of the whole membrane to the substrate surface, and the fluid lipid bilayer, functionalized if needed by embedded proteins. 

Moaddeb and Koros (1997) described the deposition of silica on polymeric MF membranes as non-uniform. This means that cake characterisation is difficult as a cracks could vary the results. Meagher et al (1996) stated that attractive interaction between membranes and particles would cause a flux decline, even if the particles were aggregated. Aggregation reduced the flux decline if there was no attraction between the membranes and colloids. The authors outlined the restrictions of the gel polarisation model, as the porosity of the deposit is not accounted for in the model. It was also suggested that the resistance of the gel layer is more important than the particle-surface interaction (what is often referred to as adsorption). 

Lau, W. J. and Ismail, A. F. 2009. Polymeric nanofiltration membranes for textile dye wastewater treatment F reparation, performance evaluation, transport modelling, and fonling control—A review. Desalination 245 321-348. 

The previous chapter outlined the phenomena and theory associated with gas-separation membranes. The fundamentals of mass transfer and the process design equations that model membranes were also addressed. In this chapter, our attention turns to the industrial application of gas-separation membranes, specifically separations with polymeric membranes. 

We are now preparing and studying membrane models formed by ternary systems amphipatic block copolymer/lipids/water. From the interaction with our polymeric models of lectins (lectins are proteins or glycoproteins specific of different sugar residues] we hope to obtain informations about the respective parts played by the different carbohydrate chains and the polypeptide skeleton of glycoproteins and perhaps help to throw some light on problems as important as cell recognition and cell contact inhibition. 

Herzog and Swarbrick have constructed a polymeric, nonporous, model membrane consisting of 44% ethylcellulose, 44% biological materials including lecithin, cephalin and cholesterol, and 12% mineral oil. When tested in a two-compartment transport ceil using salicylic acid as penetrant, this membrane was found to mimic the functionality of natural barriers. The transport of salicylic acid follows apparent first-order kinetics, with the membrane retaining approximately 2% of the solicylic acid. Both lecithin and mineral oil are found to potentiate transport, apparently due to their contribution to the nonpolar character of the barrier. 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

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