Vesicle Systems

An observation of vectorial PET across the membranes of lipid vesicles was first reported in 1976 by Mangel [41]. Since then numerous systems for vectorial PET across the membranes of vesicles have been reported (see Table 1, part 1). In spite of the differences between the systems of Table 1, all of them have one important common feature. This feature is the asymmetry of the content of aqueous phases inside and outside the vesicle which is required to provide vectorial PET across the membrane in any system. Note also that each vesicle system of Table 1, may have an analog with reversed topology (i.e. with the reversed contents of the inner and outer aqueous phases). 

Regarding the design of PET across membranes, the systems studied can be divided into two large groups (i) — the systems with the photosensitizer embedded into vesicle membrane and (ii) - the systems with the photosensitizer located outside the membrane. These two types of membranes can be named as photoactive and photopassive, respectively. 

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Scattering experiments can be performed to help determine the size and shape of the vesicles without the need for the extensive sample preparation required for electron microscopy and AFM. Dynamic (DLS) and static light scattering (SLS) are widely used to determine the size and possible shape of vesicle systems [40,42,48,49,51,... 

Fig.3. Mechanism of PET across a lipid membrane, coupled with H2 evolution in EDTA-Ru(bipy) j -V -Pd / lipid vesicle system.

The membrane-bound catalyst for water oxidation to O2 can be prepared via oxidation of Mn(Il) and Co(ll) salts to Mn(IV) and Co(Ill) hydroxides, respectively, in the presence of lipid vesicles. Using these catalysts and photogenerated Ru(bipy)j complex as an oxidant, it is possible to oxidize water to O2 in vesicle systems. One of such systems for O2 evolution is schematically represented in Fig. 4. 

Fig.4. Photocatalytic S O -Ru(bipy) -- (OH)x -Co0(3 .,y2 lipid vesicle system for O2 evolution.

Although several allelochemicals (primarily phenolic acids and flavonoids) have been shown to inhibit mineral absorption, only the phenolic acids have been studied at the physiological and biochemical levels to attempt to determine if mineral transport across cellular membranes can be affected directly rather than indirectly. Similar and even more definitive experiments need to be conducted with other allelochemicals that are suspected of inhibiting mineral absorption. Membrane vesicles isolated from plant cells are now being used to elucidate the mechanism of mineral transport across the plasma membrane and tonoplast (67, 68). Such vesicle systems actively transport mineral ions and thus can serve as simplified systems to directly test the ability of allelochemicals to inhibit mineral absorption by plant cells. 

The polymerized vesicles showed much greater stability over the unpolymerized ones. On standing at room temperature, the unpolymerized vesicles remained stable only for about 2 days, after which time precipitation of some of the lipid was noted substantial precipitation of the lipid was observed after 6 days of standing. By comparison, there was only a trace of precipitation in the polymerized vesicle system after standing 6 days at room temperature, and no substantial precipitation was observed after a 2-week period. The electron microscope pictures of Figure 4 showed that most of the polymerized vesicles retained their initial sizes and and shapes after 6 days. 

Figure 5. UV absorption at 238 nm of the vesicle systems of Lipid 1 ( polymerized vesicles o nonpolymerized 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... 

Mucosal brush border membrane vesicles and basolateral membrane vesicles can be isolated to study solute uptake across specific enterocyte boundaries. These more isolated vesicle systems allow for investigation of solute transport across a particular membrane barrier and permit separation of membrane trans-... 

Bally MB, Mayer LD, Loughrey H, et al. Dopamine accumulation in large unilamellar vesicle systems induced by transmembrane ion gradients. Chem Phys Lipids 1988 47 97. 

Vesicles are capsules in which the shells are composed of amphiphilic small molecules or polymers. Generally, the shell is an amphiphilic bilayer with an aqueous interior. These differ fundamentally from capsules generated in a water-in-oil emulsion because the oil phase in the vesicle system is only in the shells, which are surrounded by an outer aqueous phase. 

When alamethicin is added to a ternary vesicle system comprising PDA, phospholipid, and lipopolysaccharide (LPS), the addition of polymyxin, an LPS-binding antibiotic, sensitizes the vesicles to alamethicin (Katz et al. 2003). Cholesterol-containing PDA liposomes have been used to colorimetrically detect streptolysin O, a cholesterol-dependent pore-forming toxin (Ma and Cheng 2005). 

Katz M, Tsubery H, Kolusheva S, Shames A, Fridkin M, Jelinek R. Lipid binding and membrane penetration of polymyxin B derivatives studied in a biomimetic vesicle system. Biochem J 2003 375 405-413. 

Yu WL, Pei J, Huang W, Zhao GX (1997) Formation of CdS nanoparticles in mixed cationic-anionic surfactant vesicle system. Mater Chem Phys 49 87-92... 

In the various reviews on self-reproduction in recent years, practically no mention is made of such micelles or vesicle systems. The reason lies most prohahly in the bias of classic hiochemical literature, according to which self-replication is tantamount to nucleic acid systems lacking this are therefore deemed not to he relevant. In this particular regard, it is argued that self-reproduction of micelles and vesicles proceeds without transmission of information. 

Figure 9.23 Partial equilibrium in vesicle systems, with an irreversible step leading to water-insoluble large multilamellar aggregates, see Luisi, 2001.

Simultaneous oleic anhydride hydrolysis resulting in a self-reproducing vesicle system. 

In the meantime, the intense study of the simpler vesicle systems has unravelled novel, unsuspected physicochemical aspects - for example growth, fusion and fission, the matrix effect, self-reproduction, the effect of osmotic pressure, competition, encapsulation of enzymes, and complex biochemical reactions, as will be seen in the next chapter. Of course the fact that vesicles are viewed under the perspective of biological cell models renders these findings of great interest. In particular, one tends immediately to ask the question, whether and to what extent they might be relevant for the origin of life and the development of the early cells. In fact, the basic studies outlined in this chapter can be seen as the prelude to the use of vesicles as cell models, an aspect that we will considered in more detail in the next chapter. 

In such vesicle systems, the electrons are transported through the membrane. Electron carriers such as quinones or alloxazines in the vesicle wall enhance remarkably the rate of photoinduced charge separation. The vesicle system shown in Fig. 6 contains the surfactant Zn-porphyrine complex (ZnC12TPyP) in the wall 23). 

Thus far, it could be shown that stable liposomes can be prepared by polymerization of lipids. These vesicle systems, however, are still far away from being a real biomembrane model. As of now, they do not show any typical biological behavior such as surface recognition, enzymatic activities, variable lipid distribution, and the ability to undergo fusion. 

The sterols that were chosen as substrates contained two double bonds, one at various positions in the side chain and A5 in the steroid nucleus. Whereas the latter double bond was never touched in reactions with the Fe(III) porphyrin vesicle system 183 in the presence of PhIO, the side chain double bonds of desmosterol 186 and fucosterol 187 were epoxidized to 188 and 189 in 32% and 22% yield, respectively (Fig. 31). In contrast, stigmasterol 190 was not reactive, since the double bond cannot approach the reactive iron-oxo intermediate. 

The analysis of the autocorrelation function data by the Coulter Model N4 is carried out by the Size Distribution Program (SDP), which gives the particle size distribution in the form of various output displays (see Section 10.4). The SDP analysis utilizes the computer program CONTIN developed by S.W. Provencher (ref. 467-470 see also Section 10.2). (This program has been tested on computer-generated data, monomodal polystyrene samples, and a vesicle system (ref. 466-468,471).) Since the SDP does not fit to any specific distribution type, it offers the ability to detect multimodal and very broad distributions. 

The high chloride affinity and selectivity afforded by cholapods such as 4.97 suggests that they may be of use in the treatment of cystic fibrosis (Section 4.1) by acting as artificial chloride transporters. Chloride transport has been measured in an artificial vesicle system in which the fluorescent dye lucigenin is incorporated into the inner aqueous pore of the vesicle in the presence of a background... 

Among the numerous catalysts of dihydrogen evolution known at present (see e.g. the reviews [253-257]) the most promising for application in the vesicle systems... 

Thus the data presented in this Section prove that vectorial PET across membranes in vesicle systems indeed can be conjugated with the catalytic process of water reduction to dihydrogen. 

To employ the catalysts for 02 evolution in the vesicle systems, it was essential to check whether their selectivity towards evolution of 02 remains high enough after immobilization on the lipid membrane. Shilov, Shafirovich and co-workers prepared [268-271] a membrane-bound catalyst for water oxidation by oxidation of Mn(II) salts in the presence of lipid vesicles. The Mn(IV) hydroxide catalyst... 

The photoinduction ion flux derives from the similarity of vesicle systems to the proton flux in halobacterium halobium cell envelopes in the bacteriorhodopsin photocycle [126]. Liposome permeability to glucose can similarly be induced by photoexdtation in vesicles containing polyacetylene or thiophene as ion mediators [127]. As in planar bilayers, the surface charge [128] of the vesicle and the chain length of the component surfactant [129] influence assodation between the donor-acceptor pairs, and hence the distance of separation of components inside and outside the vesicle walls. 

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