Phospholipids amphiphilic nature

Phospholipids or similar water-insoluble amphiphilic natural substances aggregate in water to form bilayer liquid crystals which rearrange when exposed to ultrasonic waves to give spherical vesicles. Natural product vesicles are also called liposomes. Liposomes, as well as synthetic bilayer vesicles, can entrap substances in the inner aqueous phase, retain them for extended periods, and release them by physical process. 

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.

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. 

These aspects of MDR as well as the presented data underline the statement that permeability properties of compounds - especially their amphiphilic nature - cannot sufficiently be described through their partition coefficient in the octanol-buffer system because of special interactions with the phospholipids constituting the membrane. 

GAP-43 lacks any hydrophobic region found in usual membrane proteins and the palmitoylation which has been implicated in the membrane anchoring is not present in the purified protein. However, the effector domain of basic amphiphilic nature has the ability to bind acidic phospholipids. The domain adopts an a helical conformation when put into hydrophobic environments as shown by the CD and NMR analyses. A growing body of evidence suggests that the basic amphiphilic a-helical domain, which has been initially found as a calmodulin binding motif, serves as a reversible membrane-association signal. 

One interesting aspect of amphiphilic macromolecules is their interactions with phospholipid membranes, natural or artificial. Also amphiphilic in their chemical nature, phospholipid building blocks change their supramolecular ordering by incorporating amphiphilic polymers within their membrane assemblies. Depending on structural and compositional factors, various membrane deformations such as pore or tube formation, or complete disruption have been reported (34-40). In this respect, biological activities of amphiphilic... 

The plasma membranes of cells, mentioned above, are corrstructed of phospholipids. Phospholipids all have a structure that closely resembles the structure of the soaps and detergent surfactants discussed above in that the cortstituerrt molecules have an amphiphilic nature. This rrature arises from the presence of both polar and non-polar regions within the same molecttle. The polar region is hydrophilic (lipophobic) and the non-polar region is hydrophobic (lipophilic). 

The central role of biomembranes in cellular function underlines the importance of membrane research, an area that is by its very nature highly interdisciplinary and ranges from molecular biology to physical chemistry. At its core, however, is an essentially supramolecular interaction, the hydrophobic effect, which causes phospholipid amphiphiles to self-assemble into bilayers and then into complex closed compartments. The elegant self-assembly process that forms these remarkably large structures is an area of keen interest in its own right, but much recent study into self-assembled membranes aims to replicate the functions of biomembranes. Indeed, for cells, phospholipid bilayers are more than just delimiting boundaries they are... 

In aqueous solutions most phospholipids and glycolipids will assemble themselves into double layers in which the polar ends of the molecules are directed outwards and the long fatty acid chains are directed inwards as befits their amphiphilic nature (Figure 8.2b). Bimolecular lipid structures of this type are an essential feature of membrane structure. 

A typical biomembrane consists largely of amphiphilic lipids with small hydrophilic head groups and long hydrophobic fatty acid tails. These amphiphiles are insoluble in water (<10 ° mol L ) and capable of self-organization into uitrathin bilaycr lipid membranes (BLMs). Until 1977 only natural lipids, in particular phospholipids like lecithins, were believed to form spherical and related vesicular membrane structures. Intricate interactions of the head groups were supposed to be necessary for the self-organization of several ten thousands of... 

Vesicles for use as materials can be divided into two categories naturally occurring vesicles, or liposomes, which are composed of natural amphiphiles, usually phospholipids and polymer vesicles, which are generally composed of block copolymers. 

In all three cases amphiphiles orient spontaneously to form structures resembling the phospholipid arrangement in biomembranes. These membrane models allow a variety of investigations of physical membrane properties which could not be conducted with the complex natural systems. 

A decrease in occupied area of the head group results in an increase in packing density of the molecules (45) exhibits only an expanded phase, (46) both a liquid and a solid-like phase, and (47) forms only a condensed film. Monolayer properties of many natural phospholipids and synthetic amphiphiles are described in the literature37 38. Especially the spreading behaviour of diacetylenic phospholipids at the gas-water interface was recently described by Hupfer 120). 

As an example of an asymmetric membrane integrated protein, the ATP synthetase complex (ATPase from Rhodospirillum Rubrum) was incorporated in liposomes of the polymerizable sulfolipid (22)24). The protein consists of a hydrophobic membrane integrated part (F0) and a water soluble moiety (Ft) carrying the catalytic site of the enzyme. The isolated ATP synthetase complex is almost completely inactive. Activity is substantially increased in the presence of a variety of amphiphiles, such as natural phospholipids and detergents. The presence of a bilayer structure is not a necessary condition for enhanced activity. Using soybean lecithin or diacetylenic sulfolipid (22) the maximal enzymatic activity is obtained at 500 lipid molecules/enzyme molecule. With soybean lecithin, the ATPase activity is increased 8-fold compared to a 5-fold increase in the presence of (22). There is a remarkable difference in ATPase activity depending on the liposome preparation technique (Fig. 41). If ATPase is incorporated in-... 

Lysophosphatidilo lipids These are amphiphilic surfactants produced in a natural way from phospholipids by phospholipases. Their mechanism of action as a promoter is not fully understood. It is supposed that, like other surfactants, they can affect intracellular proteins and polar groups of phospholipids in intercellular spaces, which may favor the formation of channels permitting the penetration of water and substances dissolved therein [45]. 

---