Assembled lipid bilayers

In contrast to the conventional BLM system just described, a novel yet extremely simple method for formation of a stable BLM was recently developed in our laboratory (19-23). The technique involves the formation of self-assembled lipid bilayers on solid supports. The supported BLM (s-BLM) has a greatly improved mechanical stability (lasting indefinitely) and has desirable dynamic properties. One of the methods of formation of a s-BLM consists of two distinct steps. In the first step, the tip of a PTFE-coated platinum wire is cut off. To provide the best cut of the platinum wire, we constructed a miniature guillotine (Figure 1) where the sharp knife moves vertically onto the wire placed on the flat base. The cut is performed while the wire is immersed in a drop of lipid solution so that the initial contact of the newly exposed wire surface is with the lipid solution. In the second step, this newly cut lipid-coated PTFE-covered platinum wire is transferred into an aqueous bathing solution. This two-step self-assembled lipid bilayer works because the freshly cut metal surface is hydrophilic and attracts the polar groups of the lipid molecules. Thus, a lipid monolayer is tenaciously formed on the nascent metallic surface. Immersion of the lipid-coated wire into an aqueous solution spontaneously thins the lipid layer to a BLM that is anchored on one side to the solid support and is exposed to water on the other side. Further details of this method are available in the literature (19-23, 41). 

An alternative approach consists in reconstitution of the membrane protein in bilayer-mimicking environments, i.e., self-assembling lipid bilayer nanodiscs (Fig. lb). Nanodiscs consist of a small portion of membrane bilayer that has been solubilized by the addition of two amphipathic proteins, the membrane scaffold proteins (MSP) derived from the apolipoprotein A-1 [8-10]. Details of the preparation can also be found at http //sligarlab.life.uiuc.edu/nanodisc/protocols.html. These proteins wrap around the hydrophobic core of the lipids, effectively creating a soluble portion of membrane. 

IV. SELF-ASSEMBLED LIPID BILAYER-BASED BIOSENSORS... 

In the last decade or so, there have been a number of reports on self-assembled molecules or structures described as advanced materials or smart materials. Without question, the inspiration for this exciting work comes from the biological world, where the lipid bilayer of cell membranes plays a pivotal role. Past and recent achievements in self-assembled lipid bilayers as biosensor will now be described below. 

There has been considerable interest in the simulation of lipid bilayers due to their biological importance. Early calculations on amphiphilic assemblies were limited by the computing power available, and so relatively simple models were employed. One of the most important of these is the mean field approach of Marcelja [Marcelja 1973, 1974], in which the interaction of a single hydrocarbon chain with its neighbours is represented by two additional contributions to the energy function. The energy of a chain in the mean field is given by ... 

VMD is designed for the visualization and analysis of biological systems such as proteins, nucleic acids, and lipid bilayer assemblies. It may be used to view more general molecules, as VMD can read several different structural file formats and display the contained structure. VMD provides a wide variety of methods for rendering and coloring a molecule. VMD can be used to animate and analyze the trajectory of a molecular dynamics (MD) simulation. 

In reconstitution experiments, the self-assembly of the pore-forming protein a-hemolysin of Staphylococcus aureus (aHL) [181-183] was examined in plain and S-layer-supported lipid bilayers. Staphylococcal aHL formed lytic pores when added to the lipid-exposed side of the DPhPC bilayer with or without an attached S-layer from B coagulans E38/vl. The assembly of aHL pores was slower at S-layer-supported compared to unsupported folded membranes. No assembly could be detected upon adding aHL monomers to the S-layer face of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice did not allow passage of aHL monomers through the S-layer pores to the lipid bilayer [142]. 

Lipid bilayers are formed by self-assembly, driven by the hydrophobic effect. When lipid molecules come together in a bilayer, the entropy of the surrounding solvent molecules increases. 

The lipid molecule is the main constituent of biological cell membranes. In aqueous solutions amphiphilic lipid molecules form self-assembled structures such as bilayer vesicles, inverse hexagonal and multi-lamellar patterns, and so on. Among these lipid assemblies, construction of the lipid bilayer on a solid substrate has long attracted much attention due to the many possibilities it presents for scientific and practical applications [4]. Use of an artificial lipid bilayer often gives insight into important aspects ofbiological cell membranes [5-7]. The wealth of functionality of this artificial structure is the result of its own chemical and physical properties, for example, two-dimensional fluidity, bio-compatibility, elasticity, and rich chemical composition. 

Liposomes are formed due to the amphiphilic character of lipids which assemble into bilayers by the force of hydrophobic interaction. Similar assemblies of lipids form microspheres when neutral lipids, such as triglycerides, are dispersed with phospholipids. Liposomes are conventionally classified into three groups by their morphology, i.e., multilamellar vesicle (MLV), small unilamellar vesicle (SUV), and large unilamellar vesicle (LUV). This classification of liposomes is useful when liposomes are used as models for biomembranes. However, when liposomes are used as capsules for drugs, size and homogeneity of the liposomes are more important than the number of lamellars in a liposome. Therefore, "sized" liposomes are preferred. These are prepared by extrusion through a polycarbonate... 

It has been known for some years that gramicidin forms transmembrane ion channels in lipid bilayers and biological membranes and that these channels are assembled from two molecules of the polypeptide 213). The channels are permeable specifically to small monovalent cations [such as H+, Na+, K+, Rb+, Cs+, Tl+, NH4+, CHjNHj, but not (CH3)2NH2+J and small neutral molecules (such as water, but not urea). They do not allow passage of anions or multivalent cations 21 n. 

Cazacu, A., Tong, C., van der Lee, A., Fyles, T.M. and Barboiu, M. (2006) Columnar self-assembled ureidocrown-efhers — an example of ion-channel organization in lipid bilayers. Journal of the American Chemical Society, 128 (29), 9541-9548. 

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