Adsorbed lipid vesicles

Visualization of Adsorbed Lipid Vesicles and Bilayers Sample Preparation Vesicle Fusion... 

Figure 5.1 shows the typical shifts in Af and AD as a function of time for the adsorption of lipid vesicles onto the resonator surfaces. In Fig. 5.1a, A/decreases and AD increases rapidly in the initial stage and then gradually level off, indicating the saturation of lipid vesicles on the gold-coated surface. The monotonic changes in Af and AD demonstrate that the lipid vesicles absorbed on the gold surface are intact. In contrast, the adsorption of lipid vesicles onto the Si02-coated resonator surface looks quite different (Fig. 5.1b). A/first decreases and then increases with a minimum, while AD exhibits an opposite behavior with a maximum in the adsorption isotherm. The changes of A/and AD in Fig. 5.1b are indicative of a vesicle-to-bilayer transition [13, 14]. Specifically, the initial decrease in A/and increase in AD indicate that the intact vesicles are adsorbed onto the resonator surface. The following increase in Af and decrease in AD reflect that the vesicles rupture and fuse into a bilayer. In other words, the adsorbed lipid vesicles form a continuously solid-supported lipid bilayer (s-SLB) on the Si02-coated resonator surface.

Vesicle-mediated hydrophobic photolabeling (VMHL). The extremely hydrophobic photolabel, [ I]3-trifluoromethyl-3-(m-iodophenyl)diazirine, [ I]TID, was introduced by Brunner and Semenza for the study of proteins in biological membranes [13]. By carefully observing certain limitations of the method it is possible to characterize hydrophobic contacts between peptides and lipid vesicles (liposomes), and to distinguish them from the labeling caused by hydrophobic peptide-[ I]TID aggregates in solution and on the vesicle surfaces [14-16]. Hydrophilic peptides or peptide segments that are only adsorbed to the vesicle surface (e.g. by electrostatic interactions) are very weakly labeled. 

Figure 5.4 shows the shifts in Af and AD as a function of time for the adsorption of PEG-OH and PEG-CH3 onto the layer formed by lipid vesicles at a concentration of 0.05 mg/mL. The introduction of PEG-OH only leads to a slight change in Af, indicating that PEG-OH chains only slightly adsorb on lipid vesicle surface. The relatively large shift in AD might be attributed to the formation of loops or tails of a few PEG-OH chains on the vesicle surface, which has a marked effect on... 

Fig. 3 Vesicle stabilization by PLL covering (a, b), followed by separation of well-covered single vesicles from excess of nonbound PLL (b, c). Native vesicles are ruptured upon adsorption on a (PLL/HA)i2/PLL film, forming a lipidic bilayer (a, d). Free non-bound PLL is preferably adsorbed on a (PLL/HA)i2 film rather than on PLL-covered vesicles (b, e). Liposome-containing film (PLL/HA)i2/Lip-PLL/HA/PLL/HAis formed by adsorption of PLL-covered liposomes (Lip-PLL) on a (PLL/HA)i2 film, followed by additional coating with HA/PLL/HA layers (c, f). Reproduced from [82]...

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

To gain insight into the effect of physical state and/or molecular organization on lipid oxidation, a variety of model systems have been used. These include dispersions, liposomes or vesicles (37,38), monolayers adsorbed on silica (39,40,41), and red blood cell ghosts (42). In most of these studies, oxidation was conducted at relatively low temperatures, i.e., 20 - 40°C. Very little information is available on the effects of physical state on high temperature oxidative reactions or interactions of lipids. 

In membranes, the motional anisotropies in the lateral plane of the membrane are sufficiently different from diffusion in the transverse plane that the two are separately measured and reported [4b, 20d,e]. Membrane ffip-ffop and transmembrane diffusion of molecules and ions across the bilayer were considered in a previous section. The lateral motion of surfactants and additives inserted into the lipid bilayer can be characterized by the two-dimensional diffusion coefficient (/)/). Lateral diffusion of molecules in the bilayer membrane is often an obligatory step in membrane electron-transfer reactions, e.g., when both reactants are adsorbed at the interface, that can be rate-limiting [41]. Values of D/ have been determined for surfactant monomers and probe molecules dissolved in the membrane bilayer typical values are given in Table 2. In general, lateral diffusion coefficients of molecules in vesicle... 

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