Vesicles spontaneous

Two popular vesicle-forming surfactants are shown in Figure 9.20, fatty acids and palmitoyl-oleoyl-phosphatidylcholine (POPC). In both cases, the hydrophobic parts are emphasized. Oleate, as for most long-chain fatty acids, forms vesicle spontaneously, on simple addition of its concentrated aqueous or methanol solution into water POPC and other lipids also form hposomes spontaneously when added to water from an alcoholic solution, or by first preparing a lipid him from an organic soluhon (by evaporahon), then adding water and stirring so as to induce a vortex. 

Secretory cells often release vesicles spontaneously at a low rate whereas release rates increase when a stimulus arrives at the release site. For example, miniature synaptic potentials represent spontaneous release from the neuronal presynaptic terminal (Katz, 1966 Stevens, 1993). In contrast, sperm have only one secretory vesicle and, as discussed previously, cells that complete the acrosome reaction prior to egg contact are infertile. This problem may be circumvented, in part, by the fact that large numbers of sperm are produced by mammals, thereby minimizing the consequences of premature acrosome reactions in large populations. However,... 

In order to reduce such interferences, successful efforts have been made to isolate the cell membranes, or even their transport-active constituents. One way to achieve this is by preparation of isolated membranes which have a natural tendency to form closed and homogenous vesicles [31,32]. Another approach is by reconstituted systems , i.e. to isolate membrane components involved in specific transport processes, and to incorporate them in artificial lipid membranes, usually liposomes [33,34]. Vesicles have been successfully prepared from various cells and tissues and tested for transport activities. Whenever membranous material has been isolated from other cellular components it tends to form vesicles spontaneously, sometimes with an uniform orientation, right-side-out or inside-out vesicles, respectively. For mixtures of vesicles of the two orientations, methods were developed to separate the two polarities. Furthermore, one can separate vesicles from different cell types or even from different regions of the cell, e.g. brush-border membranes form basal lateral ones [35,36]. 

To evade the interferences due to metabolism or intracellular compartmentalization and sequestration, isolated membrane preparations in the form of vesicles have proved useful, and can be obtained either directly from isolated natural membranes [32,36] or, going one step further, by extracting transport-related proteins and reconstituting them into artificial phospholipid vesicles (liposomes) [34,98]. The preparation of the natural membrane vesicles is aided by the natural tendency of membrane fragments to form closed vesicles spontaneously under suitable conditions. Reconstituted vesicles are more difficult to obtain, for even rather pure preparations of transport components show little tendency to spontaneously integrate themselves in an artificial lipid membrane. Nonetheless, some successful attempts have been described in the literature of such an incorporation. 

Self-Oscillating Vesicles Spontaneous Structural Changes of Synthetic Diblock Copolymers... 

A series of cationic gemini surfactants, alkanediyl-ot,a)-bis[di(2-hydro-xylethyl) dodecylammonium] dibromide (abbreviated as 12-S-12 (OH), with s = 4, 6, 8 and 10 methylenes) have been synthesized, and their aggregation properties in aqueous solution have been studied by surface tension, calorimetry, H NMR, DLS, and TEM. Two critical aggregation concentrations are observed for these surfactants. These surfactants start to form dimers at concentrations well below their critical micelle concentrations (CMC). Above the CMC, these surfactants can form both micelles and vesicles spontaneously with a micelle-to-vesicle transition.The formation and characterization of catanionic vesicles by newly synthesized lysine- and... 

Formation of Hposomal vesicles under controlled conditions of emulsification of Hpids with phosphoHpids has achieved prominence in the development of dmgs and cosmetics (42). Such vesicles are formed not only by phosphoHpids but also by certain nonionic emulsifying agents. Formation is further enhanced by use of specialized agitation equipment known as microfluidizers. The almost spontaneous formation of Hposomal vesicles arises from the self-assembly concepts of surfactant molecules (43). Vesicles of this type are unusual sustained-release disperse systems that have been widely promoted in the dmg and cosmetic industries. 

FIG. 6 Configuration snapshot of a spontaneously formed vesicle from doubletailed amphiphiles in the Larson model (a) entire vesicle (b) vesicle cut in half in order to show its inner side. Black circles represent head particles (+1), gray circles tail particles (—1), white circles the neutral connecting particles (0). (From Bernardes [126].)... 

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. 

Okane et al. measured the CMC values of a-sulfonated fatty acid higher alcohol esters. These molecules can be regarded as double-chain amphiphiles, but the CMC values are about three to six orders of magnitude larger than expected for double-chain amphiphiles that can spontaneously form vesicles in water [60]. 

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. 

Q Studies using v- and t-SNARE ptoteins reconsti-mted into separate lipid bilayer vesicles have indicated that they form SNAREpins, ie, SNARE complexes that hnk two membranes (vesicles). SNAPs and NSF are required for formation of SNAREpins, but once they have formed they can apparently lead to spontaneous fusion of membranes at physiologic temperamre, suggesting that they are the minimal machinery required for membrane fusion. 

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. ... 

Liposomes have been, and continue to be, of considerable interest in drug-delivery systems. A schematic diagram of their production is shown in Fig. 10. Liposomes are normally composed of phospholipids that spontaneously form multilamellar, concentric, bilayer vesicles, with layers of aqueous media separating the lipid layers. These systems, commonly referred to as multilamellar vesicles (MLVs), have diameters in the range of 15 pm. Sonication of MLVs... 

The clay mineral montmorillonite, which is often used in different prebiotic syntheses, is probably now the most important mineral for experiments on prebiotic chemistry. It has shown its abilities in the area of simulation experiments on the formation of primitive cellular compartments montmorillonite accelerates the spontaneous conversion of fatty acid micelles to vesicles. Clay particles are often incorporated into the vesicle, just as is RNA, which is adsorbed at such clay particles. If the vesicles have been formed, they can continue to grow if fatty acids are fed to them via micelles. If the vesicles are pressed through 100 nm pore filters, they divide without dilution of their contents. 

Faure, C., Derre, A. and Neri, W. (2003) Spontaneous formation of silver nanoparticles in multilamellar vesicles. Journal of Physical Chemistry B, 107, 4738-4746. 

Mixtures of phospholipids in aqueous solution will spontaneously associate to form liposomal structures. To prepare liposomes having morphologies useful for bioconjugate or delivery techniques, it is necessary to control this assemblage to create vesicles of the proper size and shape. Many methods are available to accomplish this goal, however all of them have at least several steps in common (1) dissolving the lipid mixture in organic solvent, (2) dispersion in an aqueous phase, and (3) fractionation to isolate the correct liposomal population. 

Phospholipids are the most important of these liposomal constituents. Being the major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail, and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the vesicles together. 

Varoqueaux, F., Sigler, A., Rhee, J. S., Brose, N., Enk, C., Reim, K., and Rosenmund, C. (2002) Total arrest of spontaneous and evoked synaptic transmission but normal synapto-genesis in the absence of Muncl3-mediated vesicle priming. Proc. Natl. Acad. Sci. USA 99, 9037-9042. 

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