Water phospholipid structures formed

When these lipids are dispersed in water, they spontaneously form bilayer membranes (also called lamellae) which are composed of two monolayer sheets of lipid molecules with their hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the structure consists of ... 

Structures formed by phospholipids in aqueous solution. Phospholipids may form a monomolecular layer at the air-water interface, or they may form spherical aggregations surrounded by water. A vesicle consists of a double molecular layer of phospholipids surrounding an internal compartment of water. 

Phospholipids Spontaneously Form Ordered Structures in Water... 

Structures formed by (a) detergents and (b) phospholipids in aqueous solution. Each molecule is depicted schematically as a polar head-group ( ) attached to one or two long, nonpolar chains. Most detergents have one nonpolar chain phospholipids have two. At very low concentrations, detergents or phospholipids form monolayers at the air-water interface. At higher concentrations, when this interface is saturated, further molecules form micelles or bilayer vesicles (liposomes). 

Due to their amphipathic nature, phospholipids spontaneously form ordered structures in water. When phospholipids are agitated in the presence of excess water, they tend to aggregate spontaneously to form bilayers, which strongly resemble the types of structures they form in biological membranes. 

Phospholipids are ideal compounds for making membranes because of their amphipathic nature (see chapter 17). The polar head-groups of phospholipids prefer an aqueous environment, whereas the nonpolar acyl substituents do not. As a result, phospholipids spontaneously form bilayer structures (see fig. 17.6), which are a dominant feature of most membranes. The phospholipid bilayer is the barrier of the cell membrane that prevents the unrestricted transport of most molecules other than water into the cell. Entry of other molecules is allowed if a specific transport protein is present in the cell membrane. Similarly, the phospholipid bilayer prevents leakage of metabolites from the cell. The amphipathic nature of phospholipids has a great influence on the mode of their biosynthesis. Thus, most of the reactions involved in lipid synthesis occur on the surface of membrane structures catalyzed by enzymes that are themselves amphipathic. 

Artificial membranes are used to study the influence of drug structure and of membrane composition on drug-membrane interactions. Artificial membranes that simulate mammalian membranes can easily be prepared because of the readiness of phospholipids to form lipid bilayers spontaneously. They have a strong tendency to self-associate in water. The macroscopic structure of dispersions of phospholipids depends on the type of lipids and on the water content. The structure and properties of self-assembled phospholipids in excess water have been described [74], and the mechanism of vesicle (synonym for liposome) formation has been reviewed [75]. While the individual components of membranes, proteins and lipids, are made up of atoms and covalent bonds, their association with each other to produce membrane structures is governed largely by hydrophobic effects. The hydrophobic effect is derived from the structure of water and the interaction of other components with the water structure. Because of their enormous hydrogen-bonding capacity, water molecules adopt a structure in both the liquid and solid state. 

As phospholipid bilayers form spontaneously when water is added, the important challenge in liposome preparation is not the assembly of simple bilayers (which happens automatically), but in causing the bilayers to form stable vesicles of the desired size, structure and physicochemical properties, with a high drag encapsulation efficiency. There are many different approaches to the preparation of liposomes however, they all have in common that they are based on the hydration of lipids ... 

What properties enable phospholipids to form membranes Membrane formation is a consequence of the amphipathic nature of the molecules. Their polar head groups favor contact with water, whereas their hydrocarbon tails interact with one another, in preference to water. How can molecules with these preferences arrange themselves in aqueous solutions One way is to form a micelle, a globular structure in which polar head groups are surrounded by water and hydrocarbon tails are sequestered inside, interacting with one another (Figure 12,9). 

Phospholipids can form orgaruzed structures, as shown in Figure 1.12, when suspended in water solutions. The small circles represent the ionic watosoluble ends of the phospholipid molecules, containing phosphate and amino groups. The... 

Figure 20.4. Molecular models of cutaway structures formed from the lipid-like peptides with negatively charged heads and glycine tails. Each peptide is c. 2 nm in length. (A, C) Peptide vesicle with an area sliced away. (B, D) Peptide tubes. The glycines are packed inside the bilayer away from water, and the aspartic acids are exposed to water, much like other lipids and surfactants. The modeled dimension is 50-100 nm in diameter. Preliminary experiments suggest that the wall thickness may be c. 4-5 nm, implying that the wall may form a double layer, similar to phospholipids in cell membranes.

When phospholipids are suspended in water, they spontaneously rearrange into ordered structures (Figure 11.6). As these structures form, phospholipid hydrophobic groups are buried in the interior to exclude water. Simultaneously, hydrophilic polar head groups are oriented so that they are exposed to water. When phospholipid molecules are present in sufficient concentration, they form bimolecu-lar layers. This property of phospholipids (and other amphipathic lipid molecules) is the basis of membrane structure (see pp. 353-360). 

When the nonpolar chains of the individual phospholipid molecules are exposed to water, they form a cavity in the water network and order the water molecules around themselves. The ordering of the water molecules requires energy. By associating with one another through hydrophobic interactions, the nonpolar chains of phospholipids release the ordered water by decreasing the total surface area and hence reduce the energy required to order the water. Such coalescence stabilizes the entire system, and membranelike structures form. 

Specifically, phospholipids may form multilamellar structures around the oil-water interface, and presumably these layers will have different spacing depending on the... 

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