Phospholipids liposome formation

Liposome Formation. The pioneering investigations of Bang-ham (5) have shown that thin films of natural phospholipids form bilayer assemblies if they are lyophilized in excess water by simple handshaking above the phase transition temperature. While this procedure results in the formation of large, multibilayered spherical structures, by ultrasonication of such lipid dispersions small unilamellar liposomes are formed (16), which are schematically shown in Figure 10. Additional metTiods for liposome preparation are described in a number of reviews (17,44,45,46). 

Similarly comb-like copolymers of vinyl pyrollidone and vinyl alkyl amines were shown [446] to influence the permeability of negatively charged phospholipid liposomes containing encapsulated carboxyfluorescein. At a pH of approximately 7, the copolymers allowed permeability and solute release due to polymer/liposome complex formation and disruption of the phospholipid membrane. 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

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. 

In this article, some of the fundamental kinetic ideas relating to spontaneous vesicle formation and breakdown by surfactants in aqueous media are described. The work is related to liposome formation and breakdown using phospholipids, and the induced rupture of membranes. 

Liposome Preparation Techniques In most cases, liposomes are named by the preparation method used for their formation, Such as sonicated, dehydrated-rehy-drated vesicle (DRV), reverse-phase evaporation (REV), one step, and extruded. Several reviews have summarized available liposome preparation methods [91,124, 125], Liposome formation happens spontaneously when phospholipids are dispersed in water. However, the preparation of drug-encapsulating liposomes with high drug encapsulation and specific size and lamellarity is not always an easy task. The most important methods are highlighted below. 

Lopez, O., de la Maza, A., Coderch, L., Lopez-Iglesias, C., Wehrli E., and Parra, J. L. (1998), Direct formation of mixed micelles in the solubilization of phospholipid liposomes by Triton X-100, FEBS Lett., 426, 314-318. 

Liposomes are spherical vesioles formed by the aggregation of amphiphilio phospholipid moleoules in a bilayer struoture. Liposome formation ooours when phospholipids are dispersed into an aqueous medium — usually water — as a result of interaotions between phospholipids and water. Thus, liposomes encapsulate part of the aqueous medium in whioh they are suspended. The amphiphilic charaoter of phospholipids allows them to form olosed struotures where hydrophobic and (or) hydrophilio moleoules oan be entrapped or anohored. 

The isolated CFq-CFi has been incorporated into phospholipid liposomes and shown to carry in this form most of the energy-transducing functions which it catalyses within the thylakoid membranes. Thus, the reconstituted ATP synthase carries out ATP-dependent proton translocation resulting in both a 4pH and a developing across the reconstituted liposomes [72,73] an uncoupler-sensitive ATP-Pj exchange reaction [39] and ATP formation driven by artificially imposed 4pH and Ail/ [39,74,75], or by electric field pulses [56]. The ATP synthase proteolipo-somes provide the simplest system available today for the study of electrochemical-gradient-driven phosphorylation. 

The formation of the liposome (single-walled type) was confirmed by NMR measurements. When europium ion (Eu ) is added to a liposome solution, interacts with the choUne groups of the outward facing phospholipid and shifts the NMR signal of choline methyl groups upfield The same shift was observed for DMPC liposome/lipid-heme with the addition of Eu ". This result means the liposome formation for the hposome-embedded heme solution. The liposome formation was also supported by the sharp P-NMR spectrum (0.8 ppm), which.was caused by the spherical geometry of the phosphatidyl group. 

Due to interactions between water molecules and the hydrophobic phosphate groups of the phospholipids, the lipid bilayer closes in on itself. This process of liposome formation is spontaneous because the amphiphilic phospholipids self-associate into bilayers. Inihally liposomes were made of phospholipids from the egg yolk but now with advances in materials science, a variety of synthetic materials are being used to produce liposomes. 

Another important class of surfactants in cosmetics is the phospholipids (e.g., lecithin obtained from egg yolk or soybean), which are used as emulsifiers as well as for the formation of liposomes and vesicles. Liposomes are multi-lamellar bUayers of phospholipids that on sonication produce singular bilayers or vesicles. They are ideal systems for cosmetic applications. They offer a convenient method for solubilizing water insoluble active substances in the hydrocarbon core of the bilayer. They will always form a lamellar liquid crystalline structure on the skin and, therefore, they do not disrupt the structure of the stratum comeum. Phospholipid liposomes may be used as an indicator for studying skin irritation by surfactants. 

J. Y. A, Lehtonen, P. J. K.Kinnunen, Evidence for phospholipid microdomain formation in liquid crystalline liposomes reconstituted with Escherichia coli lactose permease. Biophys. J, 72 (1997) 1247. 

Figure 14-22. Formation of lipid membranes, micelles, emulsions, and liposomes from am-phipathic lipids, eg, phospholipids.

To use liposomes as delivery systems, drug is added during the formation process. Flydrophilic compounds usually reside in the aqueous portion of the vesicle, whereas hydrophobic species tend to remain in the lipid proteins. The physical characteristics and stability of lipsomal preparations depend on pH, ionic strength, the presence of divalent cations, and the nature of the phospholipids and additives used [45 47],... 

Phospholipids are the most important of these liposomal constituents. Being the major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail, and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the vesicles together. 

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