Polymerization in vesicles

In this paper two new polymerized vesicle systems have been presented. The first lipid can be polymerized in vesicle through UV irradiation. Because the second lipid contains a flexible spacer group it can be prepolymerized in benzene and then converted to vesicles by ultrasonication in water. The polymerization improves the stabilities of the synthetic liposomes. Since there is a acetal linkage between the... 

Polymerization in Vesicle Membranes and Interactions with Polymers... 

Polymerization offers an approach to making vesicle formulations suitable for appUcations. The maj or benefits of polymerization include increasing the chemical-mechanical strength of the vesicle architecture, and the potential for performing subsequently a variety of reactions to create a highly functionalized surface. The most common approach to polymerization in vesicles is to use polymerizable surfactants (Fig. 2a). The use of polymerizable surfactants is best described as the polymerization of vesicles or fixation of vesicles, and so is a synergistic template synthesis. Typically, unsaturated biological surfactants have been specificaUy synthesized for these types of polymerizations, and there are a number of excellent reviews of this subject [3-6]. 

An alternative to the polymerization o/vesicles with polymerizable surfactants is the polymerization in vesicles, or vesicle templating, in which a hydrophobic monomer swells the surfactant bilayer and is subsequently polymerized (Fig. 2b). In this case the vesicle is used like a mold that directs the polymerization process, and the surfactant molecules themselves are not incorporated into the final polymer. This is a transcriptive synthesis. The polymeric... 

At least five groups have attempted polymerization in vesicles [7-11]. Pertinent experimental conditions, results, and appropriate references are summarized in Table 1. The polymerized morphologies observed are hollow polymer shells, parachutes , matrioshka structures, and necklaces as shown in Fig. 3. ( Matrioshka refers to the concentric sphere morphology that resembles the nesting Russian dolls bearing the same name.) Structures of the surfactants and monomers used in the experiments are shown in Fig. 4. 

German et al. have reported the most complete study of polymerization in vesicles. They have systematically investigated polymerization in and o/vesicles. This group has carefully characterized the morphologies resulting from the polymerization of a variety of monomers in several surfactant systems. With the exception of their first publication in this area [8], their characterization results clearly demonstrate that phase separation occurs for nearly every set of polymerization conditions attempted. The authors conclude from these observations that phase separation in one form or another is a general phenomenon of polymerization in vesicles. 

A number of other aspects of polymerization in vesicles studied systematically by this group include initiation chemistry, cross-linking, monomer-surfactant compatibility, polymerization kinetics, and incorporation of polymerizable surfactants. Although phase separation was observed under all conditions, the degree of the phase separation appears to depend heavily on each of these polymerization conditions in a surprisingly intuitive way. 

All the observed morphologies (including the phase separated parachutes and necklaces) are potentially useful in a variety of applications. German et al. have proposed that necklaces composed of a variety of different polymeric beads could be useful as controlled release materials [20]. Most importantly, the implications of polymerization in vesicles can reveal more about the fundamental properties of vesicles as well as provide information about polymerization reactions in confined media. 

Type III system. DNA synthesis inside fatty acid vesicles. DNA is today the cellular macromolecule where the information about the proteins sequence is stored as ordered nucleobases sequence. DNA is thought to appear later than RNA in the molecular evolution, mainly with the function of storage. Its minor reactivity (due to loss of 2 -OH group) favour its chemical stability. The use of DNA polymerization in vesicle-based cell models aims to demonstrate how nucleic acids can be synthesized inside compartments, again in an origin-of-life perspective. However, due to the recent developments of DNA-based biotechnologies, it is possible that DNA-reactions in micro- or submicro-compartments... 

Poly(A) synthesis also occurred in the second system, but the product remained within the vesicles. Walde also determined the increase of the vesicle concentration, which corresponds to that expected for an autocatalytic process. In this experiment, the enzyme PNPase is first captured by the vesicle envelope, and in the second step, ADP and oleic anhydride are added the anhydride is hydrolysed to the acid. ADP passes through the vesicle double layer and is polymerized in the interior of the vesicle by PNPase to give poly(A). Hydrolysis of the anhydride causes a constant additional delivery of vesicle-forming material, so that the amount of vesicle present increases during the poly(A) synthesis. These experiments demonstrated a model for a minimal cell. Autocatalytically synthesised giant vesicles could be prepared under similar conditions and observed under a microscope (Wik et al., 1995). 

Pt-catalysed reduction of methylene blue and 10-methyl-5-deazoisoalloxazine -3-propanesulfonic acid Polymerized surfactant vesicles. Catalytic efficiencies high in these systems Kurihara and Fendler, 1983... 

Polymerization in Bilayers. Upon irradiation with UV light the monomer vesicles are transferred to polymer vesicles (Figure 12.). In the case of the diyne monomers (2,5-9,12,13,14) the polyreaction can again be followed by the color change via blue to red except phospholipids (5,6), which turn red without going through the blue intermediate as observed in monolayers. The VIS spectra of these polymer vesicle dispersions are qualitatively identical to those of the polymer monolayers (Figure 13.). 

Fig. 8. Proposed formation of polymeric surfactant vesicles in which the outer redox-active viologen groups have been removed by nucleophilic cleavage of the ester bond...

Fig. 12. Phospholipase A2 mediated release of self-quenched carboxyfluorescein from unpolymerized and polymerized mixed vesicles of 15 and DPPC (1 1) at 30 °C. (A) Monomeric and polymerized vesicles in the absence of enzyme. (B) Polymerized vesicles after the addition of phospholipase A2 (30 pg/ml). (C) Unpolymerized vesicles after the addition of phospholipase A2 (30 pg/ml).

In the artificial system Figure 4b, a polymerized surfactant vesicle is substituted for the thylakoid membrane. Energy is harvested by semiconductors, rather than by PSI and PSII. Electron transfer is rather simple. Water (rather than C02) is reduced in the reduction half cycle to hydrogen, at the expense of benzyl alcohol. In spite of these differences, the basic principles in plant and mimetic photosyntheses are similar. Components of both are compartmentalized. The sequence of events is identical in both systems energy harvesting, vectorial charge separation, and reduction. 

The need for increased stabilities and for controllable permeabilities and morphologies led to the development of polymerized surfactant vesicles [55, 158-161]. Vesicle-forming surfactants haw been functionalized by vinyl, methacrylate, diacetylene, isocyano, and styrene groups in their hydrocarbon chains or headgroups. Accordingly, SUVs could be polymerized in their bilayers or across their headgroups. In the latter case, either the outer or both the outer and inner surfaces could be polymerized separately (Fig. 38). Photopolymerization links both surfaces selective polymerization of the external SUV surface is accomplished by the addition of a water-soluble initiator (potassium persulfate, for example) to the vesicle solution. 

Dissymmetrical SUVs can be formed by limiting reactions to the outer surfaces of polymerized surfactant vesicles (Fig. 39). Chemical dissymmetry has been created, for example, in polymerized vesicles prepared from surfactants containing ester-linked viologen moieties in their headgroups. Cleavage of the... 

Apart from the possible use of polymerized vesicles as stable models for biomembranes (Sect. 4) there may be a variety of different applications. Polymerized surfactant vesicles have been proposed to act as antitumor agents on a cellular level33 in analogy to the action of the immune system of mammals against tumor cells 85). Polymerized vesicles open the door to chemical membrane dissymmetry 22) which in turn, may lead to enhanced utility in photochemical energy transfer84 (solar energy conversion, artificial photosynthesis). The utilization of unpolymerized lipo-... 

Reviews have already been published by J. H. Fendler on Polymerized Surfactant Vesicles 91 92,931 which refer to Novel Membrane Mimetic Systems , synthetic strategies leading to them and their characterization and potential utilization in various areas such as solar energy conversion and reactivity control. It is the intend of this appendix to bring the reader up to date on the state of the art of polymerized liposomes. 

A polymeric stack of macrocycles has been synthesized [6.72] and a cyclodextrin-based model of a half-channel has been reported [6.73]. Channel-type conduction of Na+ ions has been reported for a tris-macrocyclic ligand [6.74]. A derivative of the acyclic polyether ionophore monensin forms lithium channels in vesicles [6.75a], which may be sealed by diammonium salts [6.75b]. 

Colloidal semiconductor particles were in situ generated and coated by catalysts in reversed micelles, surfactant vesicles and polymerized surfactant vesicles. 

CdS fluorescence was also observed in cationic DODAC and 1 vesicles, albeit with much less intensity than that found in DHP vesicles. The preparation pH influenced the CdS fluorescence in all vesicles. Interestingly, CdS fluorescence was somewhat stronger in unpolymerized than polymerized vesicles prepared from 1. This was not a consequence of the uv irradiation applied to polymerize tfie vesicles, since such irradiation had no effect on CdS fluorescence in nonpolymerizable surfactant vesicles (such as DHP or DODAC). However, this effect correlated with the ability of EDTA to act as a sacrificial electron donor in hydrogen production experiments (vide infra). 

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