Vesicles self-forming

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. 

After more than 20 years, Walde et al. (1994) returned in a way to coacervate experiments, although using other methods. Walde (from the Luisi group) repeated nucleotide polymerisation of ADP to give polyadenylic acid, catalysed by polynucleotide phosphorylase (PNPase). But instead of Oparin s coacervates, the Zurich group used micelles and self-forming vesicles. They were able to demonstrate that enzyme-catalysed reactions can take place in these molecular structures, which can thus serve as protocell models. Two different supramolecular systems were used ... 

Other surface-active compounds self-assemble into bilayer structures (schematically illustrated in Fig. 10b), which normally spherilize into structures termed vesicles. When vesicles are formed from phospholipids, the term liposome is used to identify the structures, which also provide useful drug delivery systems [71]. Solutes may be dispersed into the lipid bilayer or into the aqueous interior, to be subsequently delivered through a variety of mechanisms. Liposomes have shown particular promise in their ability to act as modifiers for sustained or controlled release. 

The autocatalytic hypothesis was backed by the reaction s pH sensitivity. Addition of CO2 to the micelles increased acidity resulting in vesicles as the pH dropped below 7. Overall the experiment showed that vesicles could form from a reaction that generates amphiphilic molecules that in turn form autocatalytic micelles. Upon addition of a gas, common in planetary atmospheres, the micelles form vesicles that could function as protocells. The protocells are self-replicating so this process can be considered to be autopoietic, from the Greek for self-forming, and therefore fulfils an essential step in the chemical evolution of life. 

Summary PDMS-6-PEO short-chain diblock copolymers were prepared via anionic ring-opening polymerization of cyclosiloxanes. Applying this method, various well-defined block copolymers with different compositions were synthesized and their phase behavior was investigated. The polymers predominantly showed lamellar phases in aqueous solutions. At small surfactant concentrations, vesicles were formed, as observed via cryogenic TEM. The aggregates of the diblock copolymers were used for the formation of lamellar thin films, applying the evaporation-induced self-assembly approach. 

Bozic, B. and Svetina, S. A relationship between membrane properties forms the basis of a selectivity mechanism for vesicle self-reproduction. European Biophysics Journal, 33 (2004), 565-71. 

FIGURE 2.1.7 (a) Schematic illustration of the self-reproducing giant vesicles (i) locked precursor A is incorporated into a vesicle composed of V and catalyst C and is unlocked to produce reactive precursor A (ii) A reacts with lipophilic precursor B inside the vesicle to form vesicular molecule V (iii) new vesicles are generated as V is produced (iv) generated vesicles are extruded through the membrane to the bulk water, (b) Self-reproducing system of multilamellar vesicle [105,106],... 

In some cases, vesicles can also be obtained simply by dissolving and mixing the required components in aqueous solution, that is, purely by self-assembly. For example, the so-called catanionic vesicles self-assemble when cationic and anionic amphiphiles (each in the form of a micellar solution) are mixed in an equimolar ratio. Also supramolecular amphiphiles (Section 3.3) form vesicles by self-assembly when two (or more) components are mixed in the appropriate molar ratio. 

Vesicle-forming amphiphiles must not be held together by covalent interactions exclusively it is easily conceivable that an amphiphile is formed by noncovalent interaction of two (or more) components. Hence, although the individual components cannot form vesicles, vesicles self-assemble upon mixing of the components in the appropriate molar ratio. In this respect, the first example of... 

Aqueous solutions of didodecyldimethylammonium bromide (DDAB) exhibit an interesting self-assembly phenomenon." Each amphiphilic molecule of DDAB has two hydrophobic tails, so that DDAB molecules spontaneously form inverted micelles in hydrocarbons and nano-size spherical vesicles in dilute aqueous solutions. However, in aqueous solutions, at the DDAB concentration of about lO moldm , the spherical vesicles self-assemble into a multilayer onion-like structure shown in Figure 7.3. It is also notable that at the same DDAB concentration, the interfacial tension between water and a hydrocarbon (octane) almost vanishes, becoming less than 0.1 mNm and suggesting that the formation of the multilayer structure may be accompanied by water-oil interface instability. 

Block copolymer self-assembled nanoparticles [13-15] form in selective solvents, that is, solvents for only some of the blocks and precipitant for the others. In the simplest case of a diblock copolymer, core/shell nanoparticles such as micelles or vesicles are formed, the core of which consists of collapsed or weakly swollen solvophobic blocks and the shell (also referred to as the corona) of strongly swollen solvophilic blocks. In the case of amphiphilic micelles in aqueous solutions, water is too strong a precipitant for core blocks, and such polymers are not directly soluble in water (unless the hydrophobic block is very short), and the aqueous solutions have to be prepared indirectly using a cosolvent (a solvent for the core block miscible with water) which can be removed from the solution by dialysis or distillation after the dissolution of the copolymer [16]. 

The main supramolecular self-assembled species involved in analytical chemistry are micelles (direct and reversed), microemulsions (oil/water and water/oil), liposomes, and vesicles, Langmuir-Blodgett films composed of diphilic surfactant molecules or ions. They can form in aqueous, nonaqueous liquid media and on the surface. The other species involved in supramolecular analytical chemistry are molecules-receptors such as calixarenes, cyclodextrins, cyclophanes, cyclopeptides, crown ethers etc. Furthermore, new supramolecular host-guest systems arise due to analytical reaction or process. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Amphipilic polypeptides that are synthesized with appropriate ratios of hydrophilic to hydrophobic blocks can form ordered vesicular shapes. Although many polypeptides can self-assemble into vesicles when simply dissolved in the correct solvent, others require more processing steps. This section provides an overview of the techniques that have been developed to process various polypeptide and polypeptide hybrid systems into vesicles. 

Many polypeptide-based materials are able to self-assemble into vesicles when directly dissolved into the appropriate solvent. In fact, this method was used to form some of the earliest polypeptide vesicles in the literature. Lecommandoux and... 

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