Polymeric biomembrane model

There are two ways different in principle, to approach the problem of creating a polymeric biomembrane model. One can start out from a completely synthetic system and increase the similarity to natural systems by introducing natural lipids and... 

Elbert R, Laschewsky A and Ringsdorf H 1985 Hydrophilic spacer groups in polymerizable lipids— formation of biomembrane models from bulk polymerized lipids J. Am. Ohem. Soc. 107 4134-41... 

Thus far, it could be shown that stable liposomes can be prepared by polymerization of lipids. These vesicle systems, however, are still far away from being a real biomembrane model. As of now, they do not show any typical biological behavior such as surface recognition, enzymatic activities, variable lipid distribution, and the ability to undergo fusion. 

Fig. 28. Schematic presentation of the build up of stable biomembrane models via partial polymerization of the membrane. Key I t>, natural or synthetic lipids Hi, polymerizable lipids >...

Order and Mobility are two basic principles of mother nature. The two extremes are realized in the perfect order of crystals with their lack of mobility and in the high mobility of liquids and their lack of order. Both properties are combined in liquid crystalline phases based on the selforganization of formanisotropic molecules. Their importance became more and more visible during the last years in Material science they are a basis of new materials, in Life science they are important for many structure associated functions of biological systems. The main contribution of Polymer science to thermotropic and lyotropic liquid crystals as well as to biomembrane models consists in the fact that macromolecules can stabilize organized systems and at the same time retain mobility. The synthesis, structure, properties and phototunctionalization of polymeric amphiphiles in monolayers and multilayers will be discussed. 

As mentioned in the beginning, increased membrane stability is not sufficient to build better biomembrane models. The stabilized systems have also to be able to perform biological membrane properties such as selective permeability. Since the polymerized systems discussed so far combine increased stability with significantly decreased permeability, methods have to be found to selectively open up these stabilized membrane systems, A basis for this could be the incorporation of labile components into the polymerizable system which could be destabilized, for instance, by variation of pH (28), photochemical isomerization (29) or enzymatic hydrolysis. 

Dorn, K., Hupfer, B., and Ringsdorf, H. Polymeric Monolayers and Liposomes as Models for Biomembranes How to Bridge the Gap Between Polymer Science and Membrame Biology Vol. 64, pp. 1 —54. 

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). 

Polymeric Monolayers and Liposomes as Models for Biomembranes and Cells... 

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. 

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 ... 

Apart from the possible use of polymerized vesicles as stable models for biomembranes (Sect. 4) there may be a variety of different applications. Polymerized surfactant vesicles have been proposed to act as antitumor agents on a cellular level33 in analogy to the action of the immune system of mammals against tumor cells 85). Polymerized vesicles open the door to chemical membrane dissymmetry 22) which in turn, may lead to enhanced utility in photochemical energy transfer84 (solar energy conversion, artificial photosynthesis). The utilization of unpolymerized lipo-... 

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