Dispersion vesicular

The majority of CYP enzymes are located in a hydrophobic environment in the endoplasmic reticulum of cells, although cytosolic enzymes also exist, such as CYP101. In order to mimic the physiological environment of CYP enzymes, a number of groups have used phospholipids to construct biosensors such as DDAB, dimeristoyl-L-a-phosphatidylcholine (DMPC), dilauroylphosphatidylethanolamine (DLPE) and distearoylphosphatidylethanolamine (DSPE). Phospholipid layers form stable vesicular dispersions that bear structural relationship with the phospholipid components of biologically important membranes. By this way a membranous environment is created that facilitates electron transfer between the enzyme s redox center and the electrode. 

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

The point is also made [134] that the very high surface areas and the richly interconnected three-dimensional networks of these micron-sized spaces, coupled with periods of desiccation, could together have produced microenvironments rich in cat-alytically produced complex chemicals and possibly membrane-endosed vesides of bacterial size. These processes would provide the proximate concatenation of lipid vesicular precursors with the complex chemicals that would ultimately produce the autocatalytic and self-replicating chiral systems. A 2.5 km2 granite reef is estimated to contain possibly 1018 catalytic microreactors, open by diffusion to the dynamic reservoir of organic molecules. .. but protected from the dispersive effects of flow and convection [134] as well as protected from the high flux of ultraviolet radiation impinging on the early Earth. [123,137]... 

With some molecules, a high concentration results in a lamellar phase but no additional mesophases are formed if the concentration is reduced. The lamellar phase is dispersed in the form of concentric layered particles in an excess of solvent (water or aqueous solution). This results in a vesicular dispersion. If the mesogenic material eonsists of phospholipids, the vesicular dispersion is called a liposomal dispersion... 

Phospholipids, when dispersed in water, form spherical vesicular structures, an observation first made by Alex Bangham and Robert Home in 1952. An interesting and humorous account of the early work on liposomes has been published by Bangham as an introduction to the book by Ostro (1983) which is recommended reading for the interested student. 

Formulations. In the typical fabric softening formula, the surfactant exists in a vesicular structure and the number of bilayers is a function of the concentration of the cationic surfactant in the solution [48]. The quality of dispersion influences the softening observed on the fabric. A poor dispersion leads to an uneven coating of softener on the fabric and insufficient deposition [46]. Cationic surfactants which do not readily form stable vesicular structures can be made stable by the inclusion of appropriate stabilizers which include ethoxylated... 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

The first self-assembling block copolymers were PS-fe-PMPS- -PS synthesised by Matyjaszewski and Moller. They observed micellar aggregates by ATM after casting dilute dioxane solutions (a solvent selective for the PS block) of the copolymer. The observed micelles were taken to have internal PMPS cores and were measured at 25-30nm in diameter [73], The hrst self-assembling amphiphilic polysilane block copolymers to be investigated was the PMPS-PEO multi-block copolymer with normal distribution PMPS blocks and uniform low polydispersity PEO blocks. After dialysis aqueous dispersions of this copolymer formed micellar as well as vesicular structures [78, 79] as shown in Eig. 19. 

A standard manufacturing procedure of liposomes is the film-forming method. Prior to film formation, the phospholipids are dissolved in an organic solvent. By rotational evaporation of the solvent, a thin multilayered film of phospholipids develops at the inner wall of the vessel. Redispersion of this film in water or aqueous buffer results in the formation of vesicles. The size of the vesicles and the number of bilayers vary. Hence, further manufacturing steps are needed to obtain defined vesicular dispersions with a sufficient shelf-life. 

Alternatively, injection method and reverse-phase dialysis are appropiate procedures for the formation of SUV and LUV. Freeze-thaw procedures enable drug loading of the liposomes and also offer an evaluation of the stability of the vesicular dispersion. For interested readers, general reading is recommended. ... 

In addition to the equilibrium phase structures mentioned above, non-equilibrium surfactant phase structures exist thatare also finding applications in drug delivery. Vesicular forms of surfactants are generally formed by dispersing lamellar phases in an excess of water (or non-aqueous polar solvents such as ethylene glycol or dimethylformamide) or, in the case of reversed vesicles, in an excess of oil. With most surfactants, vesicles are non-equilibrium structures that will eventually re-equilibrate back into the lamellar phases from which they originated. Vesicles are structural analogs of liposomes (discussed later) they are approximately spherical structures and have the ability... 

Figure 4,13 Aqueous dispersions of vesicles embrace one large bulk volume of water (region 1) and 8 vesicular regions with different spatial relationships to region 1 and different polarities. See text.

Several colloidal systems have not been mentioned in this chapter because they are dealt with elsewhere. These include nanoparticle suspensions used in dmg delivery and targeting, and vesicular dispersions (liposomes. 

Final and definite proof that indeed vesicles are formed after the one step method and that they still exist after sonication can be established by observing the morphology of the arsonoliposome dispersions using different types of electron microscopy (EM), as discussed elsewhere (9-12). Additionally, the ability of the vesicles to encapsulate aqueous soluble markers as carboxyfluorescein or calcein (see below) serves as proof that vesicular structures are present in most of the arsonoliposome dispersions prepared. 

Tardi C, Brandi M, Schubert R (1998) Erosion and controlled release properties of semisoHd vesicular phospholipid dispersions. J Control Release (2-3 ) 261-270... 

Recently, reports on the preparations and properties of W/O/W (water-in-oil-in-water) emulsions have come from several laboratories (1-21). Although a variety of phase compositions are employed in different studies, the dispersed globules in the W/O/W emulsions can be characterized by a spherical vesicular structure with single or multiple aqueous compartments (inner aqueous phase), which axe separated from the aqueous suspending fluid (outer aqueous phase) by a layer of the oil phase components. 

Block copolymers can also assemble into vesicular objects when dispersed in an organic solvent selective for one of the blocks. Such systems that form vesicles in nonaqueous media can be exploited for nonbiological applications and will not be an object of discussion here. Information concerning their nature and self-assembly is available in reference [167],... 

Figure lb, d). A rapid sedimentation of insoluble coacervates onto the observation slide was the result of this instability (dark areas in Figure lb). At pH 4.2, vesicular coacervates were present and a number of flocculation/coalescence phenomena between coacervates were visible. As a general trend, BLG/AG dispersions at 2 1 Pr Ps ratio were less stable than at ratio 1 1. For instance, size distribution of coacervates was in the range 1-5 pim at ratio 1 1 and rather in the range 5-10 jxm at ratio 2 1. This may be explained by the better charge neutralization of coacervates in the latter case.14... 

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