Fluid membranes

The adliesion and fiision mechanisms between bilayers have also been studied with the SEA [M, 100]. Kuhl et al [17] found that solutions of short-chained polymers (PEG) could produce a short-range depletion attraction between lipid bilayers, which clearly depends on the polymer concentration (fignre Bl.20.1 It. This depletion attraction was found to mduce a membrane fusion widiin 10 minutes that was observed, in real-time, using PECO fringes. There has been considerable progress in the preparation of fluid membranes to mimic natural conditions in the SEA [ ], which promises even more exciting discoveries in biologically relevant areas. 

Flelfrioh W 1977 Sterio interaotion of fluid membranes in multilayer systems Z. Naturf. a 33 305... 

For fluid membranes, in which neighbor relations are not maintained, the free energy of a membrane is often written in the form [27,30]... 

As yet, models for fluid membranes have mostly been used to investigate the conformations and shapes of single, isolated membranes, or vesicles [237,239-244], In vesicles, a pressure increment p between the vesicle s interior and exterior is often introduced as an additional relevant variable. An impressive variety of different shapes has been found, including branched polymer-like conformations, inflated vesicles, dumbbell-shaped vesicles, and even stomatocytes. Fig. 15 shows some typical configuration snapshots, and Fig. 16 the phase diagram for vesicles of size N = 247, as calculated by Gompper and Kroll [243]. 

D. C. Morse. Topological instabilities and phase behavior of fluid membranes. Phys Rev E 50 R2419-R2422, 1994. 

L. Golubovie. Passages and droplets in lamellar fluid membrane phases. Phys 50 R2419-R2422, 1994. 

G. Gompper, J. Goos. Fluctuations and phase behavior of passages in a stack of fluid membranes. J Phys II France 5 621-634, 1995. 

Kawasaki et /. (1996) have used a supported membrane catalyst for extraction of erythromycin from its dilute, slightly alkaline aqueous solutions. 1-Decanol was used as an intermediate fluid membrane phase and a buffered acidic aqueous solution was used to strip the organic membrane. 

Proteins are fucntional only in the fluid membrane conditions, Tm of the lipids should be low, most preferebly below 0°C. First of all, the lipids should not inhibit protein functions and induce denaturation. For this we choose sugars as their headgroups, since sugars are empirically known to stabilize protein functions and structures in aqueous solution[29]. Moreover, rich variety of the carbohydrate chemistry can provide powerful means to control their aqueous phase structures. 

The fluidity of membranes primarily depends on their lipid composition and on temperature. At a specific transition temperature, membranes pass from a semicrystalline state to a more fluid state. The double bonds in the alkyl chains of unsaturated acyl residues in the membrane lipids disturb the semicrystalline state. The higher the proportion of unsaturated lipids present, therefore, the lower the transition temperature. The cholesterol content also influences membrane fluidity. While cholesterol increases the fluidity of semicrystalline, closely-packed membranes, it stabilizes fluid membranes that contain a high proportion of unsaturated lipids. 

Fluid membrane hypothesis Anesthetics stabilize, or rather immobilize the cell membrane, hampering membrane fluidity, which produces changes in the ion channel action. 

When dealing with two-dimensional formation of patterns in lipidic-proteic membranes (fluid membranes), not only does the coupling between the chemical reaction in the membrane and the surface diffusion have to be considered (i.e., the Thiele modulus), but one must also consider the coupling with the onset of convection (Navier-Stokes equa-... 

Devaux and McConnell9 took advantage of the fact that in fluid membranes such as egg phosphatidylcholine, the resonance spectra of spin labels such as V and VI depend strongly and monotonically on the label concentration c when c 3= 5 mole %. The normalized paramagnetic resonance spectra S0(H, c) of a series of samples, all of uniform concentration c, were determined experimentally.9 The observed time-dependent spectra are then obtained from the equation... 

Let us now consider a fluid membrane where the lipid haptens can move laterally, and consider the probability that if an IgG molecule is bound to one hapten, it will also be bound to a second hapten. (We assume that there are no steric constants preventing this second binding.) As a very crude estimate, we say that the second IgG binding site is... 

We now consider very briefly the kinetics of antibody binding, leading to two-site attachment. Obviously, for a completely rigid membrane, there will be two-site binding for all antibodies when c c. When c c, the only mechanism for two-site binding is by diffusion from solution to the small proportion of sites that are paired. For a fluid membrane (lipid hapten diffusion constant D — 2 10-8 cm2/s) we expect paired binding whenever condition c 100 k l holds. An appropriate discussion of the kinetics of this process follows. 

Fig. 11. Evidence that a membrane-associated immunochemical reaction (complement fixation) depends on the mobility of the target hapten (IX) in the plane of a model membrane. The extent of the immunochemical reaction, complement fixation, is measured by A Absorbance at 413 nm. Temperature is always 32°C, which is above the chainmelting temperature (23°C) of dimyristoylphosphatidylcholine used for the data given in A and below the chain-melting transition temperature (42°C) of dipalmitoylphosphatidyl-choline used for the data in B. Thus A refers to a fluid membrane and B refers to a solid membrane. The numbers by each curve are equal to c, the mole % of spin-label hapten IX in the plane of the lipid membrane. It will be seen that complement fixation, as measured by A Absorbance at 413 nm is far more effective in the fluid membrane than in the solid membrane at low hapten concentrations (i.e., c 0.3 mo e%). In C the lipid membrane host is a 50 50 mole ratio mixture of cholesterol and dipalmitoylphosphatidylcholine. The immunochemical data suggest that this membrane is in a state of intermediate fluidity. Specific affinity-purified IgG molecules were used in these experiments. (For further details, see Ref. 5.)... 

Liquid/liquid partition constants within pharmaceutical chemistry have been of primary interest because of tlieir correlation with liquid/membrane partitioning behavior. A sufficiently fluid membrane may, in some sense, be regarded as a solvent. With such an outlook, tlie partitioning phenomenon may again be regarded as a liquid/liquid example, amenable to treatment with standard continuum techniques. Of course, accurate continuum solvation models typically rely on the availabihty of solvation free energies or bulk solvent properties in order to develop useful parameterizations, and such data may be sparse or non-existent for membranes. Some success, however, has been demonstrated for predicting such data either by intuitive or statistical analysis (see, for example. Chambers etal. 1999). 

Helfrich, Wolfgang, Bending Elasticity of Fluid Membranes, 6, 51 see also Klosgen, Beate, 6, 243 see also Thimmel, Johannes, 6, 253. 

In the work described above, chemically distinct bilayer arrays were created with common aqueous solutions above them. Another important goal is to address aqueous solutions above an array of planar supported bilayers. In combination with surface specific detection, this strategy would enable the rapid screening of a library of soluble molecules for their efficacy in inhibiting ligand-receptor interactions in a fluid membrane environment that is similar to in vivo conditions.13... 

Dialysis has been applied to the preparation of a wide range of sample types, ranging from foodstuffs to physiological fluids. Membrane-based sample preparations for chromatography have been reviewed by Van de Merbel et al.60 In ordinary dialysis, solutes are transferred from a concentrated to a more dilute solution as a consequence of the concentration gradient. 

Polypeptide antibiotics, such as gramicidin A and polymyxin B, are capable of increasing the permeability of bacterial membranes. As is to be expected, they change the phase transition, much like cholesterol [130]. These substances induce a tightening of fluid membranes and an increase in the fluidity of rigid membranes. It has been shown that polymyxin B produces phase separation and forms a Dimyris-toylphosphatidylcholine (DMPG)-rich phase in DMPG/DMPC membranes [131]. 

Because the lipid components of membranes must be in a fluid state to function as membranes in living cells, it is reasonable to assume that primitive membranes in the first forms of cellular life must also have had this property. Straight-chain hydrocarbons have relatively high melting points due to the ease with which van der Waals interactions can occur along the chains. Any discontinuity in the chains interrupts these interactions and markedly decreases the melting point. As an example, stearic acid contains 18 carbons in its alkane chain and melts at 68 °C, while oleic acid, with a cis-double bond between carbons 9 and 10, has a melting point near 14 °C. If cellular life today requires fluid membranes, it is reasonable to assume that the earliest cell membranes were also composed of amphiphilic molecules in a fluid state. 

E. A. Evans and W. Rawicz, "Entropy-driven tension and bending elasticity in condensed-fluid membranes," Phys. Rev. Lett., 64, 2094-7 (1990) E. A. Evans, "Entropy-driven tension in vesicle membranes and unbinding of adherent vesicles," Langmuir, 7, 1900-8 (1991). 

Helfrich, W. (1978). Steric interaction of fluid membranes in multilayer systems. Z. Natur-forsch., A33, 305-315. 

---