Fluid bilayer

The snapshot from a fluid bilayer simulation shown in Figure 2 reveals that the bilayer/ water interface is quite rough and broad on the scale of the diameter of a water molecule. 

Effect of Cholesterol. Cholesterol inclusion into the lipid bilayers composed of DPPC or DSPC, eliminates apparent Tc and reduces permeability at and above the usual Tc. On the other hand, cholesterol inclusion increases packing of fluid bilayer composed of lipids with unsaturated fatty acyl chains. Since cholesterol rich liposomes are stable in plasma, cholesterol is commonly used as a liposomal component. 

Membranes are asymmetric. Integral membrane proteins can t be washed off. Peripheral membrane proteins can be washed off. Membrane spanning segments and lipid modification (fatty acylation and prenylation), anchor proteins in a fluid bilayer (Singer fluid mosaic model). 

Berger, O., Edholm, O. and Jahnig F. (1997). Molecular dynamics simulations of a fluid bilayer of dipalmitoylphosphatidylcholine at full hydration, constant pressure and constant temperature, Biophys. J., 72, 2002-2013. 

Figure 5 shows small multilamellar vesicles under the electron microscope, visible as concentric spheres. Note the fascinating texture of the spheres - the actually fluid bilayer appears structured, frozen in time at the moment of preparation. 

J. T. Groves, N. Ulman, P. S. Cremer, and S. G. Boxer, Substrate-membrane interactions Mechanisms for imposing patterns on a fluid bilayer membrane, Langmuir 14, 3347-3350 (1998). 

Flip-Flop Diffusion The inner leaflet (monolayer) of the human erythrocyte membrane consists predominantly of phosphatidylethanolamine and phosphatidylserine. The outer leaflet consists predominantly of phosphatidylcholine and sphingomyelin. Although the phospholipid components of the membrane can diffuse in the fluid bilayer, this sidedness is preserved at all times. How ... 

Wiener, M. C. and White, S. H. (1991a). Fluid Bilayer Structure Determination by the Combined Use of X-ray and Neutron Diffraction. I. Fluid Bilayer Models and the limits of Resolution. Biophys. J. 59 162. 

If there is some molecular motion with characteristic times on the order of ICT sec, the NMR spectrum will no longer have the Pake doublet lineshape discussed earlier. For example, in gel-phase bilayers a perdeuterated lipid acyl chain will have a broad, relatively featureless spectmm, as shown in Fig. 3. These spectra do not lend themselves to easy analysis The molecular motion in the membrane is not rapid enough to be axially symmetric (see the description of the fluid bilayer below) on the NMR time scale but is fast enough to influence the average value of the quadrupolar interaction and thus the splittings of the individual labels. 

Cooke IR, Deserno M. Solvent-free model for self-assembling fluid bilayer membranes stabilization of the fluid phase based on broad attractive tail potentials. J. Chem. Phys. 2005 123 224710. 

We note that the elasticity discussed above is only for planar bilayers under compression or tension and does not extend to the bending of bilayers. On the contrary, our analysis has shown that the bending of a bilayer is favoured down to the critical packing radius (assuming that the lipids can freely rearrange by lateral movement and/or flip-flop), and that bending elasticity sets in only for radii smaller than this critical value. The elasticity of a fluid bilayer is therefore seen to be profoundly different from that of a classical elastic plate or shell. 

In contrast to the studies mentioned above, the new approach described here [37] monitors the behavior of the enzyme and/or the changes on the substrate layer during hydrolysis. The substrate used was a supported bilayer of the unsaturated phospholipid l-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Supported bilayers with unsaturated fatty acid chains form more fluid bilayers and are often used for mimicking biological membranes [38]. 

Biological cell membranes are multi-component systems consisting of a fluid bilayer lipid membrane (BLM) and integrated membrane proteins. The main structural features of the BLMs are determined by a wide variety of amphiphilic lipids whose polar head groups are exposed to water while hydrocarbon tails form the nonpolar interior. The BLMs act as the medium for biochemical vectorial membrane processes such as photosynthesis, respiration and active ion transport. However, they do not participate in the corresponding chemical reactions which occur in membrane-dissolved proteins and often need redox-active cofactors. BLMs were therefore mostly investigated by physical chemists who studied their thermodynamics and kinetic behaviour . ... 

Up until 1977, the non-covalent polymeric assemblies found in biological membranes rarely attracted any interest in supramolecular organic chemistry. Pure phospholipids and glycolipids were only synthesized for biophysical chemists who required pure preparations of uniform vesicles, in order to investigate phase transitions, membrane stability and leakiness, and some other physical properties. Only very few attempts were made to deviate from natural membrane lipids and to develop defined artificial membrane systems. In 1977, T. Kunitake published a paper on A Totally Synthetic Bilayer Membrane in which didodecyl dimethylammonium bromide was shown to form stable vesicles. This opened the way to simple and modifiable membrane structures. Since then, organic chemists have prepared numerous monolayer and bilayer membrane structures with hitherto unknown properties and coupled them with redox-active dyes, porous domains and chiral surfaces. Recently, fluid bilayers found in spherical vesicles have also been complemented by crystalline mono-... 

The top panel of Fig. 3.2 presents a selection of results for the lateral organization of some simple one- and two-component lipid bilayers that have been investigated by computer-simulation calculations and atomic force microscopy. A pronounced degree of lateral heterogeneity in terms of lipid domains is found. The lipid domains are either solid hpid patches in fluid bilayers or fluid lipid patches in solid bilayers. In lipid mixtures, the domains may reflect incomplete phase separation. The sizes, the morphology, and the topology of the lipid domain patterns depend on the lipid composition and the thermodynamic conditions. The domains can be enhanced or suppressed by adding further lipid components or solutes [36]. 

Figure 12.11 Space-filling model of a section of phospholipid bilayer membrane. (A) An idealized view showing regular structures. (B) A more realistic view of a fluid bilayer showing more irregular structures of the fatty acid chains.

Finally, Singer and Nicolson produced the fluid mosaic model for membrane structure. This model retained the phospholipid bilayer as the basic structure underlying biological membranes and proposed that the bilayer is fluid. Proteins were considered to be suspended in the fluid bilayer as discrete, individual units. The fluid mosaic model is now accepted as an accurate representation of the fundamental structure of biological membranes. 

The undulation force arises from the configurational confinement related to the bending mode of deformation of two fluid bilayers. This mode consists in undulation of the bilayer at constant bilayer area and thickness (Figure 5.30a). Helfrich et al. doi established that two such bilayers, apart at a mean distance h, experience a repulsive disjoining pressure given by the expression ... 

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