Liposomes monomeric

In order to enhance the stability of hposomes and to provide a biocompatible outermost surface shucture for controlled immobihzation (see Section IV), isolated monomeric and oligomeric S-layer protein from B. coagulans E38/vl [118,123,143], B. sphaericus CCM 2177, and the SbsB from B. stearothermophilus PV72/p2 [119] have been crystallized into the respective lattice type on positively charged liposomes composed of DPPC, HD A, and cholesterol. Such S-layer-coated hposomes are spherical biomimetic structures (Fig. 18) that resemble archaeal ceUs (Fig. 14) or virus envelopes. The crystallization of S-... 

In the 1980s, polymerization was introduced to overcome the limited stability of synthetic vesicles (2-4). It was found that the stability of the polymerized vesicles was improved dramatically compared to the unpolymerized vesicle and that entrapped substances are released to a much smaller extent from polymerized liposomes than from monomeric ones. 

Figure 11. Electron micrograph of monomeric liposomes from (9), negatively stained with uranylacelate...

One factor determining toxicity of AmB formulations is the form in which the antibiotic is released monomeric or aggregated because only self-associated AmB can complex cholesterol in eukaryote membranes (25). The differential toxicity of the lipid formulations toward macrophages could be related to their stability in the culture medium. For example, the Ampho-liposome formulation, which is destabilized in the presence of serum (24), has... 

Figure 1 Schematic structures of micelle and liposome, their formation and loading with a contrast agent, (a) A micelle is formed spontaneously in aqueous media from an amphiphilic compound (1) that consists of distinct hydrophilic (2) and hydrophobic (3) moieties. Hydrophobic moieties form the micelle core (4). Contrast agent (asterisk gamma- or MR-active metal-loaded chelating group, or heavy element, such as iodine or bromine) can be directly coupled to the hydrophobic moiety within the micelle core (5), or incorporated into the micelle as an individual monomeric (6) or polymeric (7) amphiphilic unit, (b) A liposome can be prepared from individual phospholipid molecules (1) that consists of a bilayered membrane (2) and internal aqueous compartment (3). Contrast agent (asterisk) can be entrapped in the inner water space of the liposome as a soluble entity (4) or incorporated into the liposome membrane as a part of monomeric (5) or polymeric (6) amphiphilic unit (similar to that in case of micelle). Additionally, liposomes can be sterically protected by amphiphilic derivatization with PEG or PEG-like polymer (7) [1].

Figure 12.11 Strategies for immobilization of diacetylene liposomes on micropattemed glass substrates (a) immobilization after polymerization of monomeric liposomes and (b) polymerization after immobilization of monomeric liposomes. Reprinted from Shim et al. (2004). Copyright 2004 Elsevier Science. (See color insert.)...

Aqueous dispersions of polymerizable lipids and surfactants can be polymerized by UV irradiation (Fig. 18). In the case of diacetylenic lipids the transition from monomeric to polymeric bilayers can be observed visually and spectroscopically. This was first discussed by Hub, 9) and Chapman 20). As in monomolecular layers (3.2.2) short irradiation brings about the blue conformation of the poly(diacetylene) chain. In contrast, upon prolonged irradiation or upon heating blue vesicles above the phase transition temperature of the monomeric hydrated lipid the red form of the polymer is formed 23,120). The visible spectra of the red form in monolayers and liposomes are qualitatively identical (Fig. 19). 

An even more striking effect is observed by addition of the surfactant sodiumdodecyl sulfate (SDS) to vesicles (Fig. 23). While monomeric vesicles of (19) and dipalmitoylle-cithin are destroyed by low SDS concentrations, the polymerized vesicles are stable up to 2 - 10 3 mol/1 SDS25). Polymerized vesicle dispersions can be diluted with ethanol without precipitation.231 Polymeric liposomes of (20) are stable in 80% ethanol for weeks. This could also be shown by Regen et al. for polymerized vesicles of the methacryloylic lipids (4) and (6)13141 (Fig. 24) by monitoring the turbidity (absor-... 

Fig. 22. Release of entrapped 6-carboxyfluorescein from liposomes of monomeric (O) and polymeric ( ) (19) 33,25). For comparison dipalmitoylphos-phatidylcholine vesicles ( )25)...

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

In contrast, the phase transition of polymeric liposomes is retained if the polymer chain is more flexible or located on the surface of the vesicles instead within the hydrophobic core. Polymerized vesicles of methacrylamide (29) show a phase transition temperature which is slightly lower than the one for the corresponding monomeric vesicles (Fig. 26). This can be explained by a disordering influence of the polymer chain on the head group packing 15). 

Fig. 41. ATPase activity of ATP synthetase incorporated into liposomes (-O-) incubation of the enzyme with polymerized liposomes (-+-) incubation of the enzyme with monomeric liposomes of (22) followed by polymerization (- ) activity of ATP synthetase in soybean lecithin liposomes24. ATPase activity was measured in 100 mM Tris-HCl (pH 8.0) in the presence of 1 mM ATP at 37 °C by determining the liberated orthophosphate...

K. Dorn 105 > polymerized dialkylammonium lipids with the polymerizable methacryloyl moiety either in the head group (29) or at the end of one of the hydrophobic chains (5). GPC revealed Mw 1.9 x 106, Mn 3.5 x 105, Mw/Mn 5.4 for (29) and Mw 1.9 xl06,Mn 3.9 x 105, Mw/Mn 2.4 for (5). It was also found that Mw varies inversely with the time of sonication, i.e. in smaller liposomes lower-molecular-weight polymers are formed. In a following paper, K. Dorn 108 present data for the permeability of monomeric and polymeric vesicles from (29). 

The incorporation of a membrane protein into a polymerizable liposome from (22) was demonstrated by R. Pabst n9). The chromoprotein bacteriorhodopsin — a light-driven proton pump from halophilic bacteria — was incorporated into monomeric sulfolipid liposomes by ultrasonication. The resulting proteoliposomes were poly-... 

Another membrane property physically affected by flavonoids is the surface potential. Working with liposomes composed of PC and PS, we found that (—)-epicatechin and certain procyanidins (dimer to hexamer) decreased liposome surface potential. This effect relied on both, procyanidin concentration and number of monomeric units [Verstraeten et al., 2003]. On the other hand, when liposomes were composed exclusively of PC, it was found that (—)-epi-catechin dimers Al and B2, and the trimers A and C2 increased liposome surface potential in a concentration-dependent manner [Verstraeten et al.,... 

Isele, U., van Hoogevest, P., Hil ker, R., Capraro, H., Schieweck, K., and Leuenberger, H. (1994). Large-scale production of liposomes containing monomeric zinc phthalocyanine by controlled dilution of organic solventsJ. Pharm. Sci., 83, 1608-1616. 

As is clearly discussed in a recent review of polymerized liposomes (15). a distinction must be drawn between polymerized and polymeric surfactant microstmctures. In polymeric microstmctures, the polymoization is carried out before the preparation of the phase, whereas the term polymerized means that the microstmcture is formed first, and then the polymerization reaction performed with the aim of fixating the microstmcture as formed by the monomeric components. Although this chapter deals mainly with polymerized microstmctures, polymeric cubic phases are discussed in a separate section at the end. 

Neutron diffraction of samples with added deuterated water [130], as well as hydrogen exchange studies [118], suggest that no aqueous channels exist between the protein molecules in purple membranes, or between the helices, i.e., the lipids completely fill the spaces. As expected from the crystalline structure and the relatively low lipid/protein ratio, the packing of protein in purple membrane is quite rigid, and the mobility of the protein in the lattice appears low, as determined by flash dichroism [131,132]. On the other hand, bacteriorhodopsin incorporated into liposomes at high lipid/protein ratios exists as a monomeric molecule [133], and shows rapid rotation by this criterion (relaxation time 15 /us) above the phase... 

Many different methods [340] are available for the reconstitution of bacteriorhodopsin into liposomes (a) sonication of purple membrane with dried, dispersed lipids [341] (b) co-precipitation of lipids and bacteriorhodopsin from organic solvents, such as dimethylsulfoxide [340] (c) dispersion of purple membrane sheets or monomeric bacteriorhodopsin and lipids in detergents, such as Triton X-100 [340], cholate [17,325], deoxycholate [170], or octyl-glucoside [327,342], followed by dialysis or removal of the detergent with Bio-Beads. 

Lipid/DNA particles represent a nonliposomal but lipid-based delivery system for gene transfer. Monomeric or micellar lipids are allowed to interact with DNA in the presence of detergent or some other surface-active agent that is then removed by dialysis. As the surface-active agent diffuses out, solid, condensed particles of lipid and DNA form (17). These can be prepared such that they are smaller and more homogeneous than liposome/DNA complexes yet transfect cells equally well (F. Wong, unpublished observations). 

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