Lipid amphiphilic nature

Since the amphiphilic nature is essential for the phase behaviour, systems of small molecules (e.g., lipid water mixtures) and polymeric systems (e.g., homopolymer copolymer blends) share many connnon features. 

Kozubek, A., Tyman, J. Resorcinolic lipids, the natural non-isoprenoid phenolic amphiphiles and their biological activity. Chemical Reviews, Vol.99, No.l. Qanuary 1999), pp. 1-31, ISSN 0009-2665... 

The most versatile method to prepare such hollow capsules is self-assembly [203-205, 214, 215]. Owing to their amphiphilic nature and molecular geometry, lipid-based amphiphiles can aggregate into spherical closed bilayer structures in water so-called liposomes. It is quite reasonable that the hollow sphere structure of liposomes makes them suitable as precursors for the preparation of more functional capsules via modification of the surfaces with polymers and ligand molecules [205, 216, 217]. Indeed, numerous studies based on liposomes in this context have been performed [205, 209, 213]. 

Figure 22.1 The amphiphilic nature of phospholipids in solution drives the formation of complex structures. Spherical micelles may form in aqueous solution, wherein the hydrophilic head groups all point out toward the surrounding water environment and the hydrophobic tails point inward to the exclusion of water. Larger lipid bilayers may form by similar forces, creating sheets, spheres, and other highly complex morphologies. In non-aqueous solution, inverted micelles may form, wherein the tails all point toward the outer hydrophobic region and the heads point inward forming hexagonal shapes.

If the virus is treated with proteolytic enzymes the fuzzy layer formed by the viral spikes is removed (Osterrieth, 1965 Compans, 1971 Gahm-berg et al, 1972 Sefton and Gaffney, 1974 Utermann and Simons, 1974). Remnants of both El and E2 are left in the bilayer. These have a hydrophobic amino acid composition, and are soluble in lipid solvents such as chloroform-methanol. The amphiphilic nature of the spike protein is also evident from its capacity to bind Triton X-100 (0.6 g/g protein) which binds to the hydrophobic part to form a water-soluble protein-detergent complex (Simons et al., 1973a). The ability of amphiphilic proteins to bind Triton can be used to separate them from hydrophilic proteins using an extraction procedure recendy described... 

An ability to penetrate lipid bilayers is a prerequisite for the absorption of drugs, their entry into cells or cellular organelles, and passage across the blood-brain barrier. Due to their amphiphilic nature, phospholipids form bilayers possessing a hydrophilic surface and a hydrophobic interior (p. 20). Substances may traverse this membrane in three different ways. 

The membrane protein is stably buried in the Hpid bilayer due to the amphiphilic nature of the membrane protein. The surfaces of some parts of the protein have mainly hydrophobic amino acid residues, and hydrophihc residues are located on the other surfaces. The former parts are accommodated in the hydrophobic lipid bilayer and the latter protein regions are exposed to the surface of the water. Membrane proteins work as receptors, channels and so on. Specific interactions between these proteins lead to complex functions such as signal transduction and energy conversion. Many of the functions expressed by the cell membrane can be attributed to the functionality of the membrane proteins. 

Ethanol is entirely miscible in water and reasonably soluble in lipid. The amphiphilic nature of this small, weakly polar molecule allows it to incorporate readily into the structure of water while at the same time partitioning into hydrophobic environments such as the lipid bilayer. At the concentrations needed to produce a cognitive effect, ethanol will distribute fairly evenly throughout the aqueous and hydrophobic regions of the body, easily crossing the gut, blood-brain and placental barriers. 

The primary structural requirement for a molecule to reside in a lipid bilayer is an amphiphilic nature (i. e., part polar, part nonpolar). Molecules are oriented within the lipid bilayer so that their nonpolar portion is inserted into (and sometimes through) the nonpolar core of the bilayer. The polar portion associates with the polar outsides of the bilayer. 

Simple lipids with one straight and saturated hydrocarbon chain linked to a polar head-group, such as a fatty acid, will first be considered. The amphiphilic nature of the molecule is reflected in its solid state structure. Thus the non-polar parts and the polar head-groups form separate regions. This makes possible a very effective interaction between the polar head-groups. A fragment of such a crystal is illus-... 

The basis for the interaction between lipids and proteins is related to their amphiphilic nature and is due to their influence on the water structure, the so-called hydrophobic effect (Tanford, 1980). In general terms four alternative types of phases can occur in lipid-protein-water systems. An aqueous lipid solution can co-exist with a protein in the same solution alternatively a solution of molecular lipid-protein complexes can be formed. It is also possible that the lipid forms a liquid-crystalline phase (or an L2-phase) with water, such a phase can either solubilize a protein or co-exist with a protein solution. The first two interaction alternatives in water solution have been thoroughly discussed (Tanford, 1980). When the lipid concentration is below the CMC there is no interaction beside the eventual association of a few lipid molecules to the protein at certain high-affinity binding sites. At lipid concentrations above the CMC there is a so-called mass co-operative binding of numerous lipid molecules to each protein molecule and this leads to unfolding of the protein structure. 

---