Vesicles vesicular phase

It will be clear that encapsulation of the reactant(s) is a prerequisite for vesicle-induced rate effects. In case of unimolecular organic reactions, the change in rate constant relative to the rate constant in bulk aqueous solution, is determined by the change in reaction environment going from water to the reactant binding sites in the vesicle. Several studies have suggested that the reaction environment in the vesicular phase is often less polar than that in micelles. Of course, the kinetic effect is a function of the distribution of the reactant over the aqueous and vesicular pseudophases. If the medium effects on the reaction are understood in some detail, the vesicle-induced rate effects provide information about the nature of the reactant encapsulation process. 

When a solution of E. coli lipid vesicles was incrementally supplemented with a concentrated octylglucoside (OG) solution, exothermic heat pulses were observed, indicative of the integration of the detergent into the vesicular phase. The subsequent steep descent in heat output reported... 

For the quaternary Ci+DMAO/CTAB/hexanol/water system NMR was used to invesh-gate salt-induced changes in the volume fraction occupied by the mulhlamehar vesicles, as well as deformation and shape alterahons. In the same system, FRS yielded informahon on changes in diffusion coefficients upon the transition from rod-like micehes to the vesicular phase, induced by increasing the alcohol concentration. 

Kaler and co-workers [22,23] have done extensive work on the phase diagram of anionic and cationic complexes to determine the vesicular phase region. They have shown that stable single-walled vesicles can form spontaneously from aqueous mixtures of simple, commercially available, single-tailed anionic and cationic surfactants (e.g., sodium dode-... 

The structure of vesicles formed from a given surfactant depends upon the extent of sonication, and over a period of time vesicles fuse and separation of phases occurs. The ease of fusion depends upon vesicular charge and the extent to which it is neutralized by added electrolyte. 

Polymeric phospholipids based on dioctadecyldimethylammonium methacrylate were formed by photopolymerization to give polymer-encased vesicles which retained phase behavior. The polymerized vesicles were more stable than non-polymerized vesicles, and permeability experiments showed that vesicles polymerized above the phase transition temperature have lower permeability than the nonpolymerized ones [447-449]. Kono et al. [450,451] employed a polypeptide based on lysine, 2 aminoisobutyric acid and leucine as the sensitive polymer. In the latter reference the polypeptide adhered to the vesicular lipid bilayer membrane at high pH by assuming an amphiphilic helical conformation, while at low pH the structure was disturbed resulting in release of the encapsulated substances. 

Keenan, 1975 Neville et al., 1981 Watters, 1984 Virk et al., 1985), the presumption is that the formation of casein micelles is orchestrated with the transport of ions, the phosphorylation and glycosy-lation of the caseins, and lactose synthesis, such that the intravesicular ionic environment and casein concentration change continuously during the 20 min or so required for micelle assembly. Patton and Jensen (1975) observed, in electron micrographs, the same density of micellar particles in the alveolus as in mature vesicles, suggesting that, by this stage, the vesicular concentrations are virtually identical to those in the aqueous phase of milk. 

Fig. 8 Phase and fluorescence micrographs of membranous vesicular structures formed from a Murchison meteorite extract (left) compared to vesicles formed by a 20 mM de-canoic acid-decanol mixture [72] (center) and a vesicular structure produced by the photoproduct of an interstellar-ice analog [31]. The vesicles produced by the photochemical ice analog product were allowed to capture pyranine, a fluorescent anionic dye, to demonstrate that a true membrane was present. Scale bars show 20, 10, and 5 pm, from left to right...

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 epithelial membrane of the GI tract consists of a continuous barrier of cells, which allows the transport of low-molecular-weight molecules by simple diffusion or various carrier processes. Macromolecules such as proteins may be absorbed from the intestinal lumen by cellular vesicular processes, through fluid-phase endocytosis (pinocytosis), or by receptor-mediated endocytosis or transcytosis (Fig. 6). In pinocytosis, extracellular fluid is captured within an epithelial membrane vesicle. It begins with the formation of a pocket... 

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... 

The existence of a curved conformation associated with the action potential is supported by the fact that the ion influx at the spike will induce an increased average wedge-shape of the molecules, due to electrostatic screening of the lateral repulsion of phosphatidylserine molecules. Furthermore a conformation associated with the spike would directly relate action potential propagation to the mass-cooperative vesicular fusion, involved in the chemical signal transfer by transmitter molecules at the pre-synaptic membrane. Experimental support for this concept has been recently reported [39]. This well-controlled fusion process of numerous "vesicles" with the presynaptic membrane must take place as a phase transition. The... 

Phospholipids and other surfactants having two hydrophobic chains have CPP values of approximately 1 (see section 6.3.3) and tend to form lamellar phases. When equilibrated with excess water, these lamellar phases may form vesicles that can entrap dmg and these have potential use as dmg carriers. In this section we will consider several types of vesicular stmc-mres Box 6.4 (p. 213) shows some of the amphiphiles that form such stmcmres. 

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