Mixed bilayer vesicles

In mixed bilayer vesicles diacetylenic and natural lipids exhibit the same miscibility behavior as in monomolecular films. This can be demonstrated using differential scanning calorimetry (DSC). The neutral lipid (23) is immiscible with DSPC or DOPC as indicated by the two phase transitions of the mixed liposomes which occur at the same temperatures as those of the pure components (Fig. 33 a). 

A similar study by O Brien and coworkers utilized bilayers composed of a shorter chain diacetylenicPC (9) and DSPC or DOPC [37]. Phase separation was demonstrated in bilayers by calorimetry and photopolymerization behavior. DSC of the 9/DSPC (1 1) bilayers exhibited transitions at 40 °C and 55 °C, which were attributed to domains of the individual lipids. Polymerization at 20 °C proceeded at similar rates in the mixed bilayers and pure 9 bilayers. A dramatic hysteresis effect was observed for this system, if the bilayers were first incubated at T > 55 °C then cooled back to 20 °C, the DSC peak for the diacetylenicPC at 40 °C disappeared and the bilayers could no longer be photopolymerized. The phase transition and polymerizability of the vesicles could be restored simply by cooling to ca. 10 °C. A similar hysteretic behavior was also observed for pure diacetylenicPC bilayers. Mixtures of 9 and DOPC exhibited phase transitions for both lipids (T = — 18 °C and 39 °C) plus a small peak at intermediate temperatures. Photopolymerization at 20 °C initially proceeded at a similar rate as observed for pure 9 but slowed after 10% conversion. These results were attributed to the presence of mixed lipid domains... 

A liquid crystal is a general term used to describe a variety of anisotropic structures formed by amphiphilic molecules, typically but not exclusively at high concentrations. Hexagonal, lamellar, and cubic phases are all examples of liquid crystalline phases. These phases have been examined as drug delivery systems because of their stability, broad solubilization potential, ability to delay the release of encapsulated drug, and, in the case of lamellar phases, their ability to form closed, spherical bilayer structures known as vesicles, which can entrap both hydrophobic and hydrophilic drug. This section will review SANS studies performed on all liquid crystalline phases, except vesicles, which will be considered separately. Vesicles will be considered separately because, with a few exceptions, generally mixed systems, vesicles (unlike the other liquid crystalline phases mentioned) do not form spontaneously upon dispersal of the surfactant in water and because there have been many more SANS studies performed on these systems. 

The interaction between bacterial lipopolysaccharides (EPS) and phospholipid cell membranes was studied by various physical methods of deep rough mutant EPS (ReEPS) of Escherichia coH incorporated in phospholipid bilayers as simple models of cell membranes. SS P-NMR spectroscopic analysis suggested that a substantial part of ReEPS is incorporated into l,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers when mixed multilamellar vesicles were prepared. Furthermore the lipid lateral diffusion coefficients measurements at various molar ratios of ReEPS/egg-PC/POPG indicated that the incorporated ReEPS reduces the diffusion coefficients of the phospholipids in the membrane. EUV formed by the ReEPS from Salmonella enterica, eventually in mixture with dilauroyl phosphatidylcholine (DEPC), have been prepared and characterized by DES, SANS and EPR. PFGSE NMR measurements have shown that water permeability through the lipid bilayer is low at room temperature. However, above a transition temperature centered at 30-35 °C, the water permeability increases. ... 

Polymerization studies were performed on multilamellar dispersions prepared at low concentration, 2 mg/ml. The dispersions were gel filtered on Sephadex G 50-150 column, so that any of the short chain lipid component that might be present in micelle form would be separated from the mixed lipid vesicles. The vesicle preparations from the 1 1, 1 2 and 2 1 DC 8,9 PC mixtures were used in the polymerization study. The UV irradiated samples showed no evidence of participation by the acetylenic lipid in the polymerization reaction, as indicated by measuring the phosphorus content after separation of monomer by TLC. Diacetylenes polymerize when they are aligned, - and in bilayer systems this situation arises only below the lipid phase transition temperature. Thus, temperature may be expected to play an important role in polymerization. Indeed, after UV irradiation at room temperature brown or yellow dispersions resulted which contained a substantial fraction of unreacted monomer. However, polymerization of the mixtures for 1 minute at -5 C resulted in extensive polymerization (more than 90% elimination of monomer). Figure 4 shows that visible spectra of both the samples at 1 minute irradiation contained two major peaks at 532 and 494 nm. In contrast, pure DC 8,9 PC dispersion, prepared and irradiated under identical conditions, showed no clear signature of a polymer spectrum (defined peak around 500 nm) and very little monomer participation. Schoen and Yager, on the other hand, have demonstrated that the diacetylene lipid in tubular morphology shows a defined spectrum, but that the participation of monomers in polymerization process is low. ... 

Addition of surfactant molecules to lipid vesicles first leads to a partitioning of the surfactant into the lipid bilayers, an equilibrium is established, the partition coefficient depending on the nature of the lipids in the membrane and the chemical stmcture of the surfactant. Further increase of the total surfactant concentration leads finally to a saturation of the bilayers with surfactant molecules and first mixed micelles of a different surfactant to lipid ratio appear. Over a certain concentration range mixed bilayers and mixed micelles coexist until finally all bilayers have been transformed into micelles. 

When surfactants are added to lipid vesicles or natural membranes, they are incorporated into the membranes and, when the surfactant concentration is high enough, the membranes are disrupted or solubilized and mixed micelles of surfactant and membrane lipids are fonned. These processes can easily be studied by ITC, provided a temperature is chosen, where the transfer enthalpies are not zero. The solubilization of lipid bilayer vesicles by surfactants has been commonly described by a model, in which three different stages occur and this is depicted in Figure 40 [134-137]. 

The 31P n.m.r. of phospholipids has been the subject of a number of papers.62-89 These have been primarily aimed at investigating the conformation and motion of phospholipids in bilayers, but information has also been obtained on gel-to-liquid crystal transformations of phospholipids.65-67 A P31 1H nuclear Overhauser effect indicates that there is little tendency for mixed phosphatidylcholine/phosphatidyl-ethanolamine vesicles to segregate in separate domains.68 69 A phosphonium analogue (23) of choline chloride has been prepared and converted chemically into... 

The method of introduction of the fluorophore into the membrane is also important. Many probes are introduced into preexisting vesicles, natural membranes, or whole cells by the injection of a small volume of organic solvent containing the fluorophore. For DPH, tetrahydrofuran is commonly used, while methanol is often employed for other probes. The amount of solvent used should be the absolute minimum possible to avoid perturbation of the lipids, since the solvent will also partition into the membrane. With lipid vesicles this potential problem can be avoided by mixing the lipids and fluorophore followed by evaporation of the solvent and codispersing in buffer. For fluorophores attached to phospholipids, this is the only way to get the fluorophore into the bilayer with natural membranes, phospholipid exchange proteins or other techniques may have to be employed. 

A liposome is a spherical vesicle with a membrane composed of a phospholipid and cholesterol (less than 50%) bilayer. Liposomes can be composed of naturally derived phospholipids with mixed lipid chains (such as egg phosphatidylethanolamine) or... 

The very peculiar molar ratio 0.4 DDAB to 0.6 oleate, which gives rise to the narrow size distribution, is really noteworthy. This molar ratio corresponds closely to electroneutrality (this is not at 50 50 molarity, due to the relatively high pK of oleate carboxylate in the bilayer) and suggests that small mixed vesicles with an approximately equal number of positive and negative charges may enjoy particular stability. More detailed studies are needed, and this indicates the richness of the unexplored in the field of vesicles. This is shown in its fullness in the next section on the matrix effect, which is also an unexpected phenomenon and one that may have implications for the origin of early cell. 

Having described the four complexes that make up the respiratory chain, we now turn to the question of how electrons move between the complexes. To address this question, purified complexes were recombined so that they reconstituted longer stretches of the respiratory chain. First, the complexes are mixed in the presence of phospholipids and a detergent. When the detergent is removed by dilution, the phospholipids assemble into vesicles and the proteins are incorporated into the phospholipid bilayer. If UQ is added, vesicles that contain complex I and complex III catalyze electron transfer from NADH to cytochrome c. In the presence of cytochrome c, vesicles containing complex III and complex IV transfer electrons from UQH2 to 02. 

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