Vesicles unilamellar

Micellar structure has been a subject of much discussion [104]. Early proposals for spherical [159] and lamellar [160] micelles may both have merit. A schematic of a spherical micelle and a unilamellar vesicle is shown in Fig. Xni-11. In addition to the most common spherical micelles, scattering and microscopy experiments have shown the existence of rodlike [161, 162], disklike [163], threadlike [132] and even quadmple-helix [164] structures. Lattice models (see Fig. XIII-12) by Leermakers and Scheutjens have confirmed and characterized the properties of spherical and membrane like micelles [165]. Similar analyses exist for micelles formed by diblock copolymers in a selective solvent [166]. Other shapes proposed include ellipsoidal [167] and a sphere-to-cylinder transition [168]. Fluorescence depolarization and NMR studies both point to a rather fluid micellar core consistent with the disorder implied by Fig. Xm-12. 

Fig. XIII-11. Schematic diagram of a spherical micelle and a unilamellar vesicle. (From Ref. 118.)...

In water, a particle of lecithin exhibits myelin growth, ie, cylindrical sheets that are formed by bdayers and are separated by water which may break up into liposomes (vesicles with a single bilayer of Hpid enclosing an aqueous space). PhosphoHpids more generally form multilamellar vesicles (MLV) (5). These usually are converted to unilamellar vesicles (ULV) upon treatment, eg, sonication. Like other antipolar, surface-active agents, the phosphoHpids are... 

Some techniques to produce small, mainly unilamellar vesicles from MLV (sonication, French pressure cell) are discussed below in separate paragraphs. 

Sonication of MLV dispersions above the Tc of the lipids results in the formation of SUV (Saunders, et al., 1962). Sonication can be performed with a bath sonicator (Papahadjopoulos and Watkins, 1967) or a probe sonicator (Huang, 1969). During sonication the MLV structure is broken down and small unilamellar vesicles with a high radius of curvature are formed. In case of SUV with diameters of about 20 nm (maximum radius of curvature), the outer monolayer can contain over 50% of the phospholipids and in the case of lipid... 

Injection of phospholipid dissolved in ethanol into excess water heated above the Tc of the lipids results in the formation of mainly unilamellar vesicles (Batzri and Korn, 1973). The remaining ethano can be removed by dialysis or by gel filtration (Nordlund et al., 1981). 

A French pressure cell can be used to reduce the size of MLV by extrusion under high pressure. Four extrusions of egg PC-MLV at 4°C resulted in the formation of small unilamellar vesicles 94% of the lipid was found in 31- to 52-nm vesicles (Barenholz et al., 1979). 

Gainesis N., and Hauser, H. (1983). Characterization of small unilamellar vesicles produced in unsonicated phosphatidic acid and phosphatidylcholine-phosphatidic acid dispersions by pH adjustment, Biochim. Biophys. Acta, 731. 31-39. 

Hauser, H., and Gaines, N. (1982). Spontaneous vesiculation of phospholipids a simple and quick method of forming unilamellar vesicles, Proc. Natl. Acad. Sci. USA. 79. 1683-1687. 

Kao, Y. J., and Juliano, R. L. (1981). Interaction of liposomes with the reticuloendothelial system Effects of blockade on the clearance of large unilamellar vesicles, Biochim. Biophys. Acta, 677, 453-461. 

Mayer, L. D., Bally, M. B., Hope, M. J., and Cullis, P. R. (1985b). Uptake of antineoplastic agents into large unilamellar vesicles in response to a membrane potential, Biochim. Biophys. Acta. 816. 294-302. 

Uliana, J. A., Gamble, R. C., and Baldeschwieler, J. D. (1983). Liposomal blockade of the reticuloendothelial system Improved tumor imaging with small unilamellar vesicles. Science. 220. 502-505. 

Wessmann, G. (1978). Comparison of large unilamellar vesicles prepared by a petroleum ether vaporization method with multila-mellar vesicles ESR, diffusion and entrapment analyses, Bio-chim. Biophys. Acta. 542, 137-153. 

In the first step, lipid model membranes have been generated (Fig. 15) on the air/liquid interface, on a glass micropipette (see Section VIII.A.1), and on an aperture that separates two cells filled with subphase (see Section VIII.A.2). Further, amphiphilic lipid molecules have been self-assembled in an aqueous medium surrounding unilamellar vesicles (see Section VIII.A.3). Subsequently, the S-layer protein of B. coagulans E38/vl, B. stearother-mophilus PV72/p2, or B. sphaericus CCM 2177 have been injected into the aqueous subphase (Fig. 15). As on solid supports, crystal growth of S-layer lattices on planar or vesicular lipid films is initiated simultaneously at many randomly distributed nucleation... 

Liposomes — These are synthetic lipid vesicles consisting of one or more phospholipid bilayers they resemble cell membranes and can incorporate various active molecules. Liposomes are spherical, range in size from 0.1 to 500 pm, and are thermodynamically unstable. They are built from hydrated thin lipid films that become fluid and form spontaneously multilameUar vesicles (MLVs). Using soni-cation, freeze-thaw cycles, or mechanical energy (extrusion), MLVs are converted to small unilamellar vesicles (SUVs) with diameters in the range of 15 to 50 nm. ... 

Liposome-water partition potentiometric determinations, 25°C, 0.15 M KCl [8, 71]. Liposomes were made of large (phosphatidylcholine) unilamellar vesicles. 

FIG. 11 Order parameter variation along acyl chains in red cell ghosts ( ), small unilamellar vesicles of egg phosphatidylcholine (V), and paraffin oil (+), as determined by the fluorescence anisotropy decay of the w-anthroyloxy fatty acid probes. (Reprinted by permission from Ref. 12.)... 

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... 

Figure 1. Structure of liposomes and lipid microsphere a), multilamellar vesicle b). unilamellar vesicle c). lipid microsphere. Symbols inside the microsphere indicate di- and tri-acyl glycerol.

LUVs Large unilamellar vesicles 100-1000 nm, large trapped volume ... 

GUVs Giant unilamellar vesicles >1 pm, huge trapped volume, unstable 9,10... 

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