Large unilamellar phospholipidic vesicles

Hydration of Phospholipids with Solutions of Very Low Ionic Strength Very large unilamellar and oligolamellar vesicles can be prepared when a thin lipid film is dispersed in a solution of very low ionic strength (Reeves and Dowben, 1969). The formation of vesicles with diameters up to 300 pm enclosing latex beads with a diameter of 20 pm have been reported (Antanavage et al., 1978). 

Liposomes are formed due to the amphiphilic character of lipids which assemble into bilayers by the force of hydrophobic interaction. Similar assemblies of lipids form microspheres when neutral lipids, such as triglycerides, are dispersed with phospholipids. Liposomes are conventionally classified into three groups by their morphology, i.e., multilamellar vesicle (MLV), small unilamellar vesicle (SUV), and large unilamellar vesicle (LUV). This classification of liposomes is useful when liposomes are used as models for biomembranes. However, when liposomes are used as capsules for drugs, size and homogeneity of the liposomes are more important than the number of lamellars in a liposome. Therefore, "sized" liposomes are preferred. These are prepared by extrusion through a polycarbonate... 

In model systems for bilayers, one typically considers systems which are composed of one type of phospholipid. In these systems, vesicles very often are observed. The size of vesicles may depend on their preparation history, and can vary from approximately 50 nm (small unilamellar vesicles or SUVs) up to many pm (large unilamellar or LUV). Also one may find multilamellar vesicular structures with more, and often many more than, one bilayer separating the inside from the outside. Indeed, usually it is necessary to follow special recipes to obtain unilamellar vesicles. A systematic way to produce such vesicles is to expose the systems to a series of freeze-thaw cycles [20]. In this process, the vesicles are repeatedly broken into fragments when they are deeply frozen to liquid nitrogen temperatures, but reseal to closed vesicles upon thawing. This procedure helps the equilibration process and, because well-defined vesicles form, it is now believed that such vesicles represent (close to) equilibrium structures. If this is the case then we need to understand the physics of thermodynamically stable vesicles. 

Liposomes consist of many phospholipid bilayers of only few, or just one bilayer (Fig. 5). Therefore multilamellar vesicles (MLV), oligolamellar vesicles (OLV), small unilamellar (SUV), and large unilamellar vesicles (LUV) have to be distinguished. Furthermore, multivesicular liposomes (MVL) may be formed. 

Intermediate-sized unilamellarvesicles (lUVs) have diameters of the order of magnitude of 100 nm, and are called large unilamellarvesicles (LUVs) if the size is more than 100 nm and they consist of a single bilayer. For unilamellarvesicles, the phospholipid content is related to the surface area of the vesicles, which is proportional to the square of the radius, while the entrapped volume varies with the cube of the radius. In addition, because of the Lnite thickness of the membrane (ca. 4 nm), as thf vesicles become smaller, their aqueous volume is further reduced since the phospholipids occupy more of the internal space. Consequently, for a given quantity of lipid, large unilamellar liposomes... 

Murata, M., Takahashi, S., Kagiwada, S., Suzuki, A. and Ohnishi, S. (1992) pH-dependent membrane fusion and vesiculation of phospholipid large unilamellar vesicles induced by amphiphilic anionic and cationic peptides. 

Liposomes were first proposed for drug topical administration to the skin more than 25 years ago by Mezei and Gulusekharam [1,2]. The basic components of liposomes are phospholipids (phosphatidylcholine, phophatidylethanolamine, phophatidylserine, dipalmitoyl phosphatidylcholine, and others), cholesterol, and water. Liposomes may vary significantly in terms of size (from tens of nm to microns) and structure. In liposomes, one or more concentric bilayers surround an aqueous core generating small or large unilamellar vesicles (SUV, LUV) or multilamellar vesicles (MLV), respectively [3]. 

Methodology for Liposome Preparation - An informal agreement was reached on the use of a three-letter acronym to designate the type of liposome such as multllamellar vesicles (MLV) or small unilamellar vesicles (SUV) or large unilamellar vesicles (LUV) with the chemical composition in parenthesis after the acronym (Ref. 21, p. 367). The tern liposomes is therefore to be used as a generic name to Include all types of artificial vesicles composed of phospholipids and other amphipathlc lipids. 

Liposomes such as large unilamellar vesicles obtained by extrusion technique (LUVET) are generally accepted as dose models of cell membranes. Figure 6.3 represents a sketch of a veside, where it can be seen that the system consists of two distinct compartments - the aqueous and lipid phases. The phospholipid polar heads face the aqueous internal and external phases, whereas the fatty acid chains form the hydrophobic bilayer of the model membrane. 

The pure phospholipid dispersion was then mixed with purple membrane fragments to a final lipid concentration of 0.4 mg/mL and a protein concentration of about 0.045 mg/mL (a molar ratio of about 300 lipids per bacteriorhodopsin). The final mixture was frozen in liquid nitrogen and thawed. Large unilamellar vesicles ( 0.12-0.3 xm) were obtained after a final short (1 min) sonication. The vesicle suspension was unaffected by centrifugation at speeds at which purple membrane fragments tend to precipitate. 

Micelles, however, differ in important aspects from biological membranes. Micelles have typical diameters of 5 nm and therefore may be too small to mimic organellic membranes. In order to understand the conformational changes that occur upon membrane binding of monomeric ASYN, SDSL EPR was performed with ASYN bound to phospholipid vesicles, e.g., small or large unilamellar vesicles (SUVs or LUVs, respectively). 

Liposomes are vesicles consisting of phospholipid membranes. Among liposomes, one distinguishes multilamellar vesicles (MLVs), small unilamellar vesicles (SUVs), and large unilamellar vesicles (LUVs) (Table 4.1). MLVs consist of many liposomes that are folded around each other like the layers of an onion. They form during contact of phospholipids with watery solutions. 

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