Forms vesicle spontaneously

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. 

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]. 

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].)... 

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]. 

With longer chain derivatives, the forces of attraction increase, the curvature decreases and micelles become oblate or form hexagonal (or columnar) phases. When the zero-curvature is reached, the flat oblate micelles can fold and close spontaneously, thus entrapping a volume of water and form vesicles that may contain one or several bilayers of the amphiphile. 

The time course of an actual experiment is shown in Figure 7.17, which shows the hydrolysis of oleic anhydride catalyzed by spontaneously formed oleate vesicles. Note the sigmoid behavior, typical of an autocatalytic process. The lag phase is due to the preliminary formation of vesicles, and in fact the length of the lag phase is shortened when already formed vesicles are pre-added, as shown in the hg-ure. Some mechanistic details of these processes will be discussed in Chapter 10. In this work, an analysis of the number and size distribution of vesicles at the beginning and the end of the reaction was also performed by electron microscopy. 

Morowitz presents an interesting (and controversial) theory of the beginning of cellular life the theory is based on the spontaneous condensation of amphiphilic molecules to form vesicles ( protocells ). 

The important point is that hot and dry conditions were surely existing somewhere on the primitive Earth, for example in ponds and lakes that the Sun or volcanoes had dried up. It is reasonable, therefore, to expect that proteinoids did exist on our planet. But Fox went further, and proved that proteinoids can easily generate higher structures. If a concentrated proteinoid solution is heated to between 120 and 200 °C and then is very slowly cooled down, one can observe that proteinoids spontaneously form vesicles which Fox called... 

While the formation of micelles in the solution was expected, cryo-TEM studies proved that the short-chain PDMS-h-PEO diblock copolymers spontaneously form vesicles at low concentrations in aqueous solutions [4], Cryo-TEM images of the studied systems are shown in Fig. 1. 

In nature, asymmetry is achieved through membrane dissolved proteins. In lipid membrane systems without proteins, only monolayers made of bola-amphiphiles allow a totally asymmetric arrangement of head groups. The simplest asymmetry to be achieved is dependent on the one-sided precipitation of bolaamphiphiles. a,to-Dicarboxylic acids, for example, are often soluble at pH > 8 and spontaneously form vesicles upon acidification to pH 5. At a lower pH, all carboxyl groups become protonated and one usually observes ill-defined precipitates . 

Interactions between oppositely charged micelles in aqueous solutions spontaneously form vesicles. The self-diffusion coefficient of water and 2H relaxation of 2H-labeled dodecyl trimethyl ammonium chloride of the dodecyl trimethyl ammonium chloride-sodium dodecyl benzenesulfonate systems show that in these mixtures there is limited growth of the micelles with changes in composition. The vesicles abruptly begin to form at a characteristic mixing ratio of the two surfactants. The transition is continuous.205 Transformation from micelle to vesicle in dodecyl trimethyl ammonium chloride-sodium perfluoro-nonanoate aqueous solution has been studied by self-diffusion coefficient measurements, and it was found that at a concentration of 35 wt% with a molar ratio of 1 1, the self-diffusion coefficient of the mixed micelles is far smaller than that of the two individual micelles.206 The characteristics of mixed surfactant... 

This self-complexification leads to a major consequence vesicles are the simplest objects in which an inside compartment is formed spontaneously through the interplay of physical factors that we understand at least qualitatively. Such a spontaneous compartmentalization is, of course, an attractive candidate as a prototype of universal cellular organization in living organisms. It defines an inside compartment, separated from the outside medium by a semipermeable fluid membrane. Also, the dimensions of spontaneously formed vesicles are of the same range as those of living cells. But what can we say about the nature of the most primitive amphiphiles ... 

Figure 9. The hypothetical "hydrophobic start" in the origin of life. The hydrophobic, spontaneously formed vesicles can undergo self-reproduction if they bind the corresponding precursor they can scavenge hydrophobic peptides and condense them into longer chain once a hydrophobic condensing agent is also present and they can also bind water-soluble peptide catalyst (or any other potential hydrophobic catalyst) and induce an enzyme-like turnover. The catalyst can eventually be internalized, thus giving rise to a protocellular structure capable of a primitive metabolism.

Vesicles form when a surfactant bilayer encapsulates an aqueous core (Fig. 1). The microstructure resembles that of a biological cell in which the plasma membrane has been replaced by a surfactant bilayer. These structures can form either spontaneously or as a result of shear or other processing of lamellar liquid crys-... 

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. 

Ghoroghchian PP, Li GZ, Levine DH, Davis KP, Bates FS, Hammer DA, Therien MJ (2006) Bioresorbable vesicles formed through spontaneous self-assembly of amphiphilic polyethyleneoxide-block-polycaprolactone. Macromolecules 39 1673-1675 Du J, Armes SP (2005) pH-responsive vesicles based on a hydrolytically self-cross-tinkable copolymer. J Am Chem Soc 127 12800-12801... 

The story is different for large solutes. Large molecules, particularly the hydrophilic ones, find it too diflBcult to pass through the lipid bilayer. To circumvent this difficulty, nature invokes a novel mechanism that keeps the solvation of these hydrophilic moieties nearly intact but transports them to the inside of the cell, within a sack formed by spontaneous fluctuation, to be metaphorical. In reality, the molecule to be transported is enclosed by a part of the membrane which then forms a vesicle that can move through a part of the bilayer and then opens up inside the cell. A reverse process is used to transport large molecules from the inside of the ceU to the exterior of the cell. 

Glycosylated block copolymers, prepared by a thiol-ene radical photoaddition reaction of 2,3,4,6-fefra-O-acetyl-l-thio-fc-D-glucopyranose onto l,2-polybutadiene-( -poly(ethylene oxide), have been demonstrated to self-assemble in dilute aqueous solution and spontaneously form vesicles (glycosomes) with sugar-coated asymmetric membranes (Fig. 5.20) [45]. 

---