Polymorphism in Lipid-Water Systems

The preference for a given phase is determined by several factors, including the concentration of the lipids, the temperature, the pressure, the ionic strength of the solvent, and the structural characteristics of the lipid itself. Aggregates are then formed such that the hydrophilic head groups are in contact with water and the hydrophobic acyl chains have minimum or no exposure to water. The thermodynamic and geometric considerations involved in the 

Another aspect of current interest associated with the lipid-water system is the hydration force problem.i -20 When certain lipid bilayers are brought closer than 20-30 A in water or other dipolar solvents, they experience large repulsive forces. This force is called solvation pressure and when the solvent is water, it is called hydration pressure. Experimentally, hydration forces are measured in an osmotic stress (OS) apparatus or surface force apparatus (SFA)2o at different hydration levels. In OS, the water in a multilamellar system is brought to thermodynamic equilibrium with water in a polymer solution of known osmotic pressure. The chemical potential of water in the polymer solution with which the water in the interlamellar water is equilibrated gives the net repulsive pressure between the bilayers. In the SEA, one measures the force between two crossed cylinders of mica coated with lipid bilayers and immersed in solvent. 

The repulsive pressure between bilayers varies exponentially with the distance between the bilayer surfaces. This is given by 

Interaction of small molecules and ions with lipid bilayers is of importance from the point of view of membrane transport and other processes such as aaion of drugs and anesthetics on membranes. This includes a number of antibiotics and fatty acids also. The effect of these perturbations on the lipid bilayer in terms of differences in the structure and dynamics of the lipids close to the perturbative group versus the bulk lipids is also interesting and may 

However, a variety of nonpolar molecules can be transported across biomembranes by simple diffusion. The molecules encounter little or no barrier at the bilayer-water interface and readily partition into the bilayer interior. The permeabilities of a variety of molecules have been found to correlate with their 

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Seddon JM, Templer RH (1995) Polymorphism of lipid-water systems. In Lipowsky R, Sackmann E (eds) Handbook of biological physics. Elsevier Science, Amsterdam, pp 97-160... 

Lipids constitute a diverse and important group of biomolecules. Most lipids can behave as lyotropic liquid crystals. In the presence of water, they self-assemble in a variety of phases with different stmcture and geometry. The lipid polymorphic and mesomorphic behavior, i.e., their ability to form various ordered, crystalline, gel, or liquid-crystalline phases as a function of water content, temperature, and composition, is one of the most intriguing features of lipid-water systems. The mutual transformations between these phases and their physiologic implications are the subject of this article. 

Phase Transitions in Lipid Assemblies. The rich polymorphism of amphiphilic systems, of which the multilamellar and the Hn phases are only two structures, was made evident from the seminal work of Luzzati and co-workers. Since that early work, an immense variety of water-induced phase transitions have been observed and rationalized in terms of an apparently systematic connection between water content and polar group molecular area. Therefore, the recent observation of a double transition—Hn to lamellar back to Hn—from continual hydration of dioleoylphosphatidyl-ethanolamine (40) was a surprise. Furthermore, an estimate of the cost of uncurling the monolayer in the formation of bilayers based on the previously described bending modulus far exceeds the osmotic work that actually produced the transition. Although this transition sequence can successfully be accounted for by simple thermodynamical principles, it, in fact, contains many geometry-dependent free energy contributions that we simply do not yet understand (41). 

Amphiphilic molecules have two parts, one hydrophilic and one hydrophobic, and form liquid crystals the textures of which are of great interest, since they are very close in morphology to biological materials and, in particular, cell membranes [17-19], showing a polymorphism related to that known in water-lipid systems (see Chaps. XV to XVII of Vol. 2 of this Handbook). The lamellar structure displays some usual defects and textures (Fig. 5). The bilayers are more or less separated by water (Fig. 5 a, b) this... 

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