Membrane electrostatic forces

The interest in vesicles as models for cell biomembranes has led to much work on the interactions within and between lipid layers. The primary contributions to vesicle stability and curvature include those familiar to us already, the electrostatic interactions between charged head groups (Chapter V) and the van der Waals interaction between layers (Chapter VI). An additional force due to thermal fluctuations in membranes produces a steric repulsion between membranes known as the Helfrich or undulation interaction. This force has been quantified by Sackmann and co-workers using reflection interference contrast microscopy to monitor vesicles weakly adhering to a solid substrate [78]. Membrane fluctuation forces may influence the interactions between proteins embedded in them [79]. Finally, in balance with these forces, bending elasticity helps determine shape transitions [80], interactions between inclusions [81], aggregation of membrane junctions [82], and unbinding of pinched membranes [83]. Specific interactions between membrane embedded receptors add an additional complication to biomembrane behavior. These have been stud-... 

Figure 7.17 shows the asymmetry ratios of a series of compounds (acids, bases, and neutrals) determined at iso-pH 7.4, under the influence of sink conditions created not by pH, but by anionic surfactant added to the acceptor wells (discuss later in the chapter). The membrane barrier was constructed from 20% soy lecithin in dodecane. All molecules show an upward dependence on lipophilicity, as estimated by octanol-water apparent partition coefficients, log KdaA). The bases are extensively cationic at pH 7.4, as well as being lipophilic, and so display the highest responses to the sink condition. They are driven to interact with the surfactant by both hydrophobic and electrostatic forces. The anionic acids are largely indifferent... 

Solvent-resistant elastomers, 9 560-562 Solvent-resistant membranes, 27 656 Solvent-resistant rubber, 22 583-584 Solvent selection, in liquid-liquid extraction, 70 746-749 Solvent-solute interactions, 26 855,23 91-96 acid/base interactions in, 23 96 dispersion in, 23 92-93 electrostatic forces in, 23 91-92 hydrogen bonding in, 23 94-95 hydrophobic interactions in, 23 95 polarization in, 23 92 repulsion in, 23 93-94 Solvent strength, of pure fluids, 24 3-4 Solvent systems, for acid gas removal, 72 376-377... 

In the unstimulated state electrostatic forces prevent the melting of the vesicle membranes with the outer cell membrane. If the outer cell membrane is depolarized by either an action potential or acetylcholine (in the adrenal medulla) the electrostatic repulsion is neutralized by positively charged... 

It is clear that intermolecular effects strongly influence the properties of materials. The point is that one may make use of them in a controlled fashion to induce specific changes when and where desired. The self-assembly of membranes, molecular layers, films, vesicules, etc. incorporates interactions such as hydrophobic effects, hydrogen bonding, electrostatic forces and surface binding [7.1-7.13, 7.45, 7.87, 9.134-9.141], which may be used to produce specific structural and functional properties. 

Comparative molecular Leld analysis (CoMFA) is another promising approach developed in recent years for QSAR study. CoMFA is a molecular modeling technique forthe determination of molecular stericand electrostatic force Lelds (Tripos, 1992). It has been successfully used in deriving molecular descriptors for prediction of the bioactivity of steroids (Cramer etal., 1988), molecular Lux through a polymer membrane (Liu and Matheson, 1994), and metabolism and cytochrome p450 enzyme activities (Long and V felker, 2003). 

The diffusion barrier. Much attention has been directed toward primitive amphiphile vesicles, inasmuch as they self-assemble from simple components and have an obvious ancestral connection with the more complex membranes that enclose modem cells. A review has been provided by Monnard and Deamer.55 The papers by Segre et al. and Hanczyc et al. contain additional discussion.56 57 Other prominent alternatives that would limit loss by diffusion have been electrostatic forces at mineral surfaces,58 iron sulfide membranes,59 and aerosols at the ocean-atmosphere interface.60 Section 2.7.1 discusses the function of compartmentalization in Earth life today. 

However, if ions move down the concentration gradient (from the inside to the outside of the cell) an electrostatic imbalance will be created, resulting in more positive charges outside of the cell than inside. The resulting electrostatic force will drive the positive potassium ions across the membrane from outside to inside. In thermodynamic equilibrium, the concentration driven potential is exactly balanced by the electrostatic potential, a situation illustrated in Figure 1.3. 

For the values given, m i = 5.7 X 10 kgM = 5.7 X 10" g/cm (570 pg/cm ). If one wishes to reduce this value, one can reduce the membrane thickness, and hence, M. With the device of Figure 3.41, a lower limit on thickness of the composite membrane is set by the minimum thickness of the piezoelectric layer, as extremely thin ZnO layers tend to be randomly oriented and hence have low piezoelectric coupling. Much thinner membranes can be achieved if one dispenses with the piezoelectric and instead drives a very thin membrane with electrostatic forces produced by an opposing interdigital electrode array [62,69]. 

Fibrous or particulate filters are not important anymore because membrane filters are relatively compact and perform very well. For filtration by straining, there is an intermediate air velocity at which filtration efficiency is a minimum because different collection mechanisms predominate at different ranges of velocity. At low velocities, diffusional and electrostatic forces on the particle are important, and increased velocity shortens the time for them to operate. At high velocities, inertial forces that increase with air velocity come into play below a certain air velocity, their effect on collection is zero. Surges or brief power failures could change velocity and collection efficiency... 

The electrostatic force acting between the two membranes can be obtained by integrating the Maxwell stress and the osmotic pressure over an arbitrary surface enclosing any one of the membranes. We may choose two planes x = 00 (in the solution) and x = x at an arbitrary point in the region Q[Pg.314]

FIGURE 16.2 Electrostatic force P between two ion-penetrable membranes, each consisting of two layers, immersed in a monovalent symmetrical electrolyte solution as a function of the electrol3de concentration n (M). The Donnan potentials at n = 0.1 M in the respective... 

The major physical forces, which help the membrane to maintain their structure, consist of hydrophobic and hydrophilic interactions, electrostatic forces, and van der Waals interactions. The main driving force for formation of the bilayer originates from the hydrophobic interactions and van der Waals interaction forces between hydrocarbon chains of the hpid molecules. The hydrophobic forces control the order and packing of hpids and electrostatic interactions between the polar head groups and their interaction with water molecules contribute to bUayer stabUization. The bUayer is continuous and it exhibits semirigid properties. The fluid nature of the membrane is governed by the hpid composition and the namre of the forces that exist between the constituent hpids and proteins. Due to fluid hpid bilayer, the diffusion constant for a phosphohpids is 1 m /s,... 

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