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... 

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Liposomes are members of a family of vesicular structures which can vary widely in their physicochemical properties. Basically, a liposome is built of one or more lipid bilayers surrounding an aqueous core. The backbone of the bilayer consists of phospholipids the major phospholipid is usually phosphatidylcholine (PC), a neutral lipid. Size, number of bilayers, bilayer charge, and bilayer rigidity are critical parameters controlling the fate of liposomes in vitro and in vivo. Dependent on the preparation procedure unilamellar or multilamellar vesicles can be produced. The diameter of these vesicles can range from 25 nm up to 50 ym—a 2000-fold size difference. 

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

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). 

Freeze-thawing If SUV are freeze-thawed in the presence of a high concentration of electrolytes and subsequently dialyzed against a low electrolyte concentration, unilamellar liposomes with a diameter of more than 10 pm are formed. The formation of these giant... 

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). 

Allen, T. M., and Chonn, A. (1987). Large unilamellar liposomes with low uptake into the reticuloendothelial system, FEBS Lett., 223. 42-46. 

Aurora, T. S., Li, W., Cummins, H. Z., and Haines T. H. (1985). Prepai ation and characterization of monodisperse unilamellar phospholipid vesicles with selected diameters of from 300-600 nm, Biochim. Biophys. Acta. 820, 250-258. 

Ellens, H., Morselt, H., and Scherphof, G. (1981). In vivo fate of large unilamellar sphingomyelin-cholesterol Liposomes after intraperitoneal and intravenous injection into rats, Biochim. Biophys. Acta. 674, 10-18. 

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. 

Hauser, H., and Strauss, G. (1987). Stabilization of small unilamellar phospholipid vesicles during spray-drying, Biochim. 

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. 

Choleate lipid cylinders Formation by fusion of unilamellar lipid vesicles, Biochim. Biophys. Acta. 394, 483-491. 

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. 

Rhoden, V., and Goldin, S. M. (1979). Formation of unilamellar lipid vesicles of controllable dimensions by detergent dialysis. Biochemistry, 18, 4173-4176. 

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. 

Senior, J., and Gregoriadis, G. (1982). Stability of small unilamellar liposomes in serum and clearance from the circulation The effect of the phospholipid and cholesterol components. Life Sci., 30. 2123-2136. 

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... 

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