Forces Between Lipid Bilayers

While in the beginning the biological application of the osmotic stress method was mainly used to investigate forces between lipid bilayers (reviewed in Ref [1239]), it has also become a valuable tool to study the hydration forces between biomolecules [1243], for example, DNA [1155], collagen fibers [1244], or polysaccharides [1245]. 

The osmotic stress method has also been applied to colloidal dispersions [1238,1246], emulsions [1247], colloidal crystals [1248], clays [1249, 1250], block copolymers [1251], a mixed nanoparticle/polymer system [1252], and colloids with polyelectrolyte multilayers [1253]. 

To measure the force between lipid layers, a lipid is chosen, which at a certain concentration and temperature range forms an L phase. The L phase is a regularly spaced stack of lamellar fluid bilayers separated by water. From a symmetry point of view, the L phase can be considered a smectic-A (SmA) liquid crystal. The mean repeat distance between lipid bilayers in the L phase is measured by X-ray diffraction versus an applied osmotic pressure. Direct experiments have been carried out with the surface force apparatus. Therefore, the bilayers are formed either by spontaneous vesicle fusion [1254-1256] or by depositing two subsequent monolayers with the Langmuir-Blodgett technique [1255, 1257]. Atomic force microscope experiments, which have been carried out between two bilayers formed by spontaneous vesicle fusion, confirmed earlier results [1258]. 

Different components contribute to the force between lipid bilayers van der Waals attraction, hydration repulsion, steric repulsion, electrostatic forces, and undulation forces. Electrostatic double-layer forces arise if charged lipids are present in the membrane. Surface charges also occur in the presence of divalent ions. Ca and adsorb to lipids, which leads to net positive charge even at concentration of 1 mM [1257]. Double-layer forces are suppressed by adding monovalent salt into the solution. 

The hydration force is the major repulsive component of the force between neutral bilayers [1254, 1257). Phenomenologically, it can be described by an exponentially decaying force up to a distance of 1.0-1.5 nm [1234, 1236,1237,1259]  

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The experimental data reported by Pashley [17] raise two important issues. The first one is why the critical electrolyte concentration at which the hydration force emerges, depends so strongly on the type of electrolyte (6x 10 2 for LiCl, 3x10 / M for KC1). The second issue is why the decay length of the interactions at short separations is about 10 A, about five times largo- than that corresponding to the hydration force between lipid bilayers. 

Lipowsky R, Grotehans S (1993) Hydration versus protrusion forces between lipid bilayers. Europhys Lett 23 599-604... 

R. Lipowsky and S. Grotehans. Hydration vs protrusion forces between lipid bilayers. Biophys. Chem., 49 (1994) 27—37. 

Perera, L., Essmann, U. and Berkowitz, M. L. (1996). Role of water in the hydration force acting between lipid bilayers, Langmuir, 12, 2625-2629. 

Another mechanism for the hydration repulsion between lipid bilayers was more recently proposed by Marcelja.22 It is based on the fact that in water the ions are hydrated and hence occupy a larger volume. The volume exclusion effects ofthe ions are important corrections to the Poisson— Boltzmann equation and modify substantially the doublelayer interaction at low separation distances. The same conclusion was reached earlier by Ruckenstein and Schiby,28 and there is little doubt that the hydration of individual ions modifies the double-layer interaction, providing an excess repulsion force.28 However, while the hydration of ions affects the double-layer interactions, the hydration repulsion is strong even in the absence of an electrolyte, or double-layer repulsion. 

In parallel, another important (although less direct) technique for measuring forces between macromolecules or lipid bilayers was developed, namely, the osmotic stress method [39-41]. A dispersion of vesicles or macromolecules is equilibrated with a reservoir solution containing water and other small solutes, which can freely exchange with the dispersion phase. The reservoir also contains a polymer that cannot diffuse into the dispersion. The polymer concentration determines the osmotic stress acting on the dispersion. The spacing between the macromolecules or vesicles is measured by X-ray diffraction (XRD). In this way, one obtains pressure-versus-distance curves. The osmotic stress method is used to measure interactions between lipid bilayers, DNA, polysaccharides, proteins, and other macromolecules [36]. It was particularly successful in studying the hydration... 

We have seen that hydration forces act between lipid bilayers in aqueous and non-aqueous solvents. The molecular nature of the interaetion is beginning to be understood, and involves solvation effects along with dynamic, and structural properties of the lipid bilayer. The relative importanee of these factors is still a controversial topic. However, because of the complexity of membrane systems, one cannot expect a force, which can be described by a simple mathematical for-... 

Undulation forces not only act between lipid bilayers in the L phase but also prevent vesicles from coagulation and act between membranes and other surfaces [1266]. Emulsions are stabilized by undulation forces [1267]. 

Fig. VI-6. The force between two crossed cylinders coated with mica and carrying adsorbed bilayers of phosphatidylcholine lipids at 22°C. The solid symbols are for 1.2 mM salt while the open circles are for 10.9 roM salt. The solid curves are the DLVO theoretical calculations. The inset shows the effect of the van der Waals force at small separations the Hamaker constant is estimated from this to be 7 1 x 10 erg. In the absence of salt there is no double-layer force and the adhesive force is -1.0 mN/m. (From Ref. 66.)...

Interaction with a lipid bilayer driven by a potential difference and by polar and/or hydrophobic forces between the amino acid side chains of the pardaxin tetramers and the polar membrane lipid head group triggers insertion from a "raft" like structure. 

Liposomes are artificial structures composed of phospholipid bilayers exhibiting amphiphilic properties (chapter 12). In complex liposome morphologies, concentric spheres or sheets of lipid bilayers are usually separated by aqueous regions that are sequestered or compartmentalized from the surrounding solution. The phospholipid constituents of liposomes consist of hydrophobic lipid tails connected to a head constructed of various glycerylphosphate derivatives. The hydrophobic interaction between the fatty acid tails is the primary driving force for creating liposomal bilayers in aqueous solutions. 

Wong et al. [131] measured directly the interaction potential between a tethered ligand and its receptor in aqueous media. Using a surface force apparatus, the interaction force-distance profile was determined between streptavidin immobilized on a lipid bilayer and biotin tethered to the distal end of lipid-PEG. Both lipid bilayers containing streptavidin and biotin were absorbed onto the surface of mica having a specific curvature. Both cationic and anionic polymer grafted... 

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