Biomembranes lipid distribution

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. 

The ability to measure the distribution of lipids across natural biomembranes is a potentially powerful capability. Natural biomembranes maintain an asymmetric distribution of lipid components across the membrane with the inner leaflet rich in anionic lipids and both leaflets containing proteins and species specific to each... 

There is independent physical evidence for non-uniform distribution and restriction from transmembrane diffusion of a-Toc in lipid membranes. Differential scanning calorimetry results indicated that it partitioned into the most fluid domains in lipid vesicles. Fluorescence studies showed that a-Toc has a very high lateral diffusion rate in egg lecithin but it does not take part in transbilayer (flip-flop) migration even over many hours . It is not known if this behavior of a-Toc extends to natural biomembranes where actual structures and conditions may dramatically change migration phenomena. 

Perhaps the most important turning point in the development of our current understanding of biomembranes was the fluid mosaic model [17], [25] (Figure 11-2). This model describes the cell membrane as a fluid 2-dimensional lipid bilayer matrix of about 50 A thickness with its associated proteins, and it allows for the lateral diffusion of both lipids and proteins in the plane of the membrane [26]. This model has been further developed. Concerning the distribution of lipids and proteins, it has been assumed that the membrane consists of solid domains coexisting with areas of fluid disordered membrane lipids which... 

The relationship between biomembranes, ion channels, and peripheral proteins needs to be addressed in studies involving the neurotoxicity of pyrethroids. The multiple target systems for pyrethroids that is presented in Table 21 indicate that these lipid soluble insecticides are well distributed in biomembranes and bind to the proteins (i.e., ion channels) associated with them. The extent of this distribution is not well known however, only 1 % of the pyrethroid concentration in nervous tissue... 

The simulation methodologies applied to proteins and nucleic acids are applicable to the biomembrane modeling at the atomic level. The jvidely distributed programs, CHARMM, AMBER, and GROMOS, have been applied for biomembrane studies. The common energy parameters of proteins, nucleic acids, and water are used with minor modifications for lipids. For instance, the AMBER energy function is written as follows ... 

More macroscopic phenoipena of biomembranes are trackable by simplification with abandonment of atomic details in the calculations. In a simulation at oil-water interfaces, water and oil molecules are represented by particles labeled w and o , respectively. An amphiphilic molecule such as a lipid is represented by a chain of two w particles followed by five o particles. A completely repulsive interaction is assumed between o and w, and a Lennard-Jones potential is assumed between particles of the same kind. Thus, this simulation is even simpler than the pioneering simulations of atomic chain models described in Section 2.1. Thus, more macroscopic phenomena can be studied easily. Starting from a i patially random distribution of water, oil, and the amphiphilic molecules, the monolayers and micelles composed of the amphiphilic molecules were spontaneously made at oil-water interfaces as shown in Figure 3. Depletion layers between monolayers and micelles were observed, suggesting the repulsion between biomembranes by the solvation force. 

---