Tethered membranes

Sinner E-K, Knoll W (2001) Functional tethered membranes. Curr Opin Chem Biol 5 705-711... 

Wiltschi B, Knoll W, Sinner E-K (2006) Binding assays with artificial tethered membranes using surface plasmon resonance. Methods 39 134-146... 

Sinner EK, Knoll W. Eunctional tethered membranes. Curr. Opin. 

Naumann CA, Knoll W, Frank CW (2001) Hindered diffusion in polymer-tethered membranes a monolayer study at the air-water interface. Biomacromol 2 1097-1103... 

And finally, the very promising application of hydrogels as cushions but also as protective coatings for the tethered membrane architectures, eventually allowing even for operations in air, is presented and discussed. 

Lipo-Polymer Layers as Support for Tethered Membranes. 94... 

Lipo-Glycopolymers as Building Blocks for Tethered Membrane Architectures. . 98... 

Important Basic Properties of Tethered Membranes The Swelling of the... 

Fig. 2 The construction of a polymer-cushioned lipid bilayer membrane. (A) Architecture constructed in a sequential way first, onto the functionalized substrate a polymer layer (cushion) is deposited by adsorption from solution and covalent binding, followed by the (partial) covalent attachment of a lipid monolayer containing some anchor lipids as reactive elements (B) able to couple the whole monolayer to the polymer cushion. (C) Alternatively, a lipopolymer monolayer, organized, e.g., at the water-air interface can be co-spread with regular low-mass amphiphiles and then transferred as a mixed monolayer onto a solid support, prefunctionalized with reactive groups, able to bind covalently to the polymer chains of the lipopolymer molecules, (B). (D) By a fusion step (or a Langmuir Schaefer transfer) the distal lipid monolayer completes the polymer-tethered membrane architecture...

Among the various membrane properties that might need to be optimized for a particular application of the tethered membrane architecture and/or of any incorporated proteins we discuss only two key performance parameters, i.e., (1) the ability of the tethering system to swell by the up-take of a sufficient amount of water into the interstitial space between the lipid bilayer and the solid support, and (2) the high lateral mobility of the individual lipid molecules in the two opposing leaflets of the bilayer membrane. 

The proper function of a membrane is intimately linked to the liquid-crystalline character of its lipid bilayer matrix. Among other factors, this depends crucially on the existence of sufficient water reservoirs on both sides of the bilayer membrane. (The lipid bilayer is a thermotropic and lyotropic smectic liquid-crystal.) While this is naturally given on the distal side of the tethered membrane architecture by the aqueous phase of the flow cell, the coupling of the membrane to the substrate on the proximal layer imposes serious restrictions on the amount and free accessibility, e.g., for ions of the aqueous phase between the bilayer and the solid substrate. In this context, the tethering system not only couples and thus stabilizes mechanically the whole architecture to the support foremost it has to decouple the lipid bilayer from the strong interactions of the headgroups with the polar support. This way ... 

Fig. 11 Time-dependent recovery of the fluorescence after bleaching a hole into the lipopolymer-tethered membranes composed of DMPC and an increasing amount of the lipopolymer DODA-Es5, as indicated. The intensities were obtained by integrating the fluorescence across the hole area (cf. some of the corresponding fluorescence micrographs given in Fig. 10)...

It has been shown that the chemical nature of the polymer cushion can be very flexible with examples ranging from polyelectrolytes to carbohydrate-containing macromolecules and cross-linked hydrogels. So far, the cushion has played only a rather passive role in that it was almost exclusively used as a structural element in the build-up of the tethered membrane architectures. The real potential, however, lies in the fact that these polymer systems could play a crucial functional role for these... 

Flexible, solid membranes, are also of interest. However, they are experimentally much less prevalent and are somewhat more complicated to treat since in addition to the membrane shape one must include the effects of shear. Their curvature energy is discussed in the problems at the end of this chapter. Another type of system that has received much theoretical attention is that of a tethered membrane which may describe polymerized, but not crystalline sheets. While a single fluid membrane that is unconstrained by walls or other membranes is strongly affected by thermal fluctuations ( crumpled ), solid membranes, particularly if self-avoidance of the membrane is included, tend to be more weakly affected by fluctuations and are hence flattef . 

There still are challenges which have come about in extending the Edwards model to tethered membranes D = 2) [3]. The demonstration of the renormalizability of the resulting model is a topic of current research [10]. In this context there does not appear to be an equivalent spin model which describes self-avoidance in such objects. Further extension of these models to membranes and charged species is expected to be an important problem in the general area of soft-condensed-matter physics. A perusal of the literature on these topics is sufficient to appreciate the deep influence of the two landmark papers [4, 8] on polymer physics. 

Recently there has been considerable interest in the phase transition of polymerized (tethered) membranes with attractive interactions [1-4]. In a pioneer work [1], Abraham and Nelson found by molecular dynamics simulations that the introduction of attractive interactions between monomers leads to a collapsed membrane with fractal dimension 3 at a sufficiently low temperature. Subsequently, Abraham and Kardar [2] showed that for open membranes with attractive interactions, as the temperature decreases, there exists a well-defined sequence of folding transitions and then the membrane ends up in the collapsed phase. They also presented a Landau theory of the transition. Grest and Petsche [4] extensively carried out molecular dynamics simulations of closed membranes. They considered flexible membranes the nodes of the membrane are connected by a linear chain of n monomers. For short monomer chains, n = 4, there occurs a first-order transition from the high-temperature flat phase to... 

The study of tethered polymer chains is an area which has received increasing attention in recent years. These are systems in which one or both ends of the chain are constrained in their motion because they are attached to a d dimensional surface. This surface could be a point or small central core (d = 0) as in the case of a many-arm star polymer, a line (d = 1) as in the case of a comb polymer, or a flat surface (d = 2) as in the case of a polymer brush. Polymers attached to themselves to form a polymer network or a tethered membrane are also examples of tethered chain systems. An interesting example of a tethered membrane is the spectrin/actin membrane skeleton of the red blood cell skeleton. A schematic illustration of these four examples of tethered chain is shown in Fig. 9.1. Additional interest in tethered chains is due to their technological applications in colloidal stabilization and lubrication. ... 

In this section, we review some of the recent progress in simulating polymeric fractals, with particular emphasis on tethered membranes made of linear polymer segments connected together to form a two-dimensional surface. After a brief review of the theory, we present results from a munber of groups which show that two-dimensional tethered membranes remain flat and do not crumple. We then consider the effect of changing the solvent quality by adding attractive interactions between nonbonded monomers. While there is clear evidence for a collapsed phase at low T, the nature of the crossover from flat to compact state remains unclear. 

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