Fluid vesicle dispersions

Figures 9d,e show aqueous dispersions of vesicles. The smaller the vesicles, the less probable is an upcoming cross-fracture. Thus the question whether the vesicle is uni- or multilamellar can hardly be answered. At least for fluid vesicle dispersions it is possible to solve the problem with the help of cryotransmission electron microscopy.

For this purpose it is necessaiy to give sufficient contrast to a thin film of the frozen sample, for example, by use of osmium tetroxide. Then the sample can be viewed directly in the TEM (at — 196°C). The adjustment of the temperature to — 196°C produces a very low vapor pressure, especially of water, so that the examination of the probe is possible by preservation of the microstructure despite the high vacuum. A disadvantage of cryo-TEM is the classification of vesicles according to their size. Due to the fluid property of the vesicle dispersion prior to freezing, the thickness of the sample film varies from the center to the outside. Hence the smaller vesicles stay in the center, where the film is thin, while the larger ones remain at the outside margin in the thicker part of the film. In this outer part, the vesicles evade... 

Vesicular transport occurs when a membrane completely surrounds a compound, particle, or cell and encloses it into a vesicle. When the vesicle fuses with another membrane system, the entrapped compounds are released. Endocytosis refers to vesicular transport into the cell, and exocytosis to transport out of the cell. Endocytosis is further classified as phagocytosis if the vesicle forms around particulate matter (such as whole bacterial cells or metals and dyes from a tattoo), and pinocy-tosis if the vesicle forms around fluid containing dispersed molecules. Receptor-mediated endocytosis is the name given to the formation of clathrin-coated vesicles that mediate the internalization of membrane-bound receptors in vesicles coated on the intracellular side with subunits of the protein clathrin (Eig. 10.14). Potocytosis is the name given to endocytosis that occurs via caveolae (small invaginations or caves ), which are regions of the cell membrane with a unique lipid and protein composition (including the protein caveolin-1). 

The reverse micelles get transformed into a liquid crystalline phase or vesicle dispersion, when it comes in contact with the aqueous body fluids. This reduces the rate of release of the solubilized drugs. ... 

Figure 10 shows the NMR spectrum of sonicated POPC-TTC vesicles at room temperature and selected pressures. Already the one-dimensional NMR spectra exhibit some interesting features. With increasing pressure, the signal intensity of the acyl-chain protons at 0.85 and 1.24 ppm decrease due to the pressure-induced rigidization of the acyl-chains, as it is also observed for pure phospholipid samples. At pressures above the fluid-gel main transition, which is detected at a pressure of about 1200 bar at 20 °C in pure POPC dispersions, the acyl-chain signals of pure lipid samples disappear completely, whereas in the spectra of the POPC-TTC system considerable signal intensities remain even up to pressures of 2800 bar. Furthermore, we observe for the... 

Vesicles, bicelles and L.C. dispersions. The large number of papers devoted to vesicle and bicelle formulations reflects the paramount importance they have in both applied and theoretical field. As for the above paragraph such aggregates has often been used to stabilize membrane protein and peptides to be studied. Furthermore because of their supra-molecular architecture, they represent the most common fluid nanocontainers for drug delivery applications and the most common mimicking systems in biological membranes studies. 

However, usually by the use of phase contrast light microscopy techniques, membrane aspects have been observed that hint towards submicroscopic structures of the fluid membranes. Examples are localized stable wiggles in tubular vesicles [12] and an abundance of tethers in many samples. Conflicting results in the context of the induced adhesion [18,19] suggested a fine superstructure of fluid membranes with an additional hidden area [20,21], superimposed on the well-known undulations [22], The existence of so-called dark bodies [23] in almost any of the lipid systems investigated and their formation processes (see below) implied the assumption that they are developed by the association of very small bilayer particles dispersed in the aqueous volume. Typical examples of dark bodies are shown in Figure 17.1 for DGDG. 

Next, we consider the flexural properties of surfactant adsorption monolayers, which are important for the formation of small droplets, micelles, and vesicles in the fluid dispersions. The contributions of various interactions (van der Waals, electrostatic, steric) into the interfacial bending moment and the curvature elastic moduli are described. The effect of interfacial bending on the interactions between deformable emulsion droplets is discussed. 

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