Multilamellar phases

Golubovic, L. and Golubovic, M. (1998) Fluctuations of quasi-two-dimensional smectic intercalated between membrane in multilamellar phases of DNA-cationic lipid complexes. Phys. Rev. Lett., 80,4341—4344. 

Liposomes (multilamellar bilayers) are produced by dispersion of phospholipids, e.g. lecithin, in water by simple agitation. When these multilamellar phases are... 

In previous publications the shear modulus for the multilamellar phases was considered to be the result of the interactions of hard sphere particles [46-48]. In this picture each charged multilamellar vesicle is treated as a hard sphere. The theoretical treatment of the samples would then be similar to latex systems. The modulus of the systems depends on the chainlength of the surfactants that are used for the preparation of the systems if all other parameters like charge density, salinity, and concentration of surfactants and cosurfactants are kept constant. It can be argued that the differences of the moduli result from a change of the particle density of the vesicles. But these values are not known exactly. Systems with different chainlengths have similar conductivities which suggests that the particle density is also similar and therefore not responsible for the different shear moduli. 

FIGURE 3.9 Diagrams demonstrating three basic multilamellar phase geometries flat, cylindrical, and spherical. In these images, the sheets indicate the arrangement of the surfactant bilayers in a solvent medium. (Images... 

Figure 2 Snapshot from an MD simulation of a multilamellar liquid crystalline phase DPPC bilayer. Water molecules are colored white, lipid polar groups gray, and lipid hydrocarbon chains black. The central simulation cell containing 64 DPPC and 1792 water molecules, outlined m the upper left portion of the figure, is shown along with seven replicas generated by the periodic boundary conditions. (From Ref. 55.)...

Phospholipids e.g. form spontaneously multilamellar concentric bilayer vesicles73 > if they are suspended e.g. by a mixer in an excess of aqueous solution. In the multilamellar vesicles lipid bilayers are separated by layers of the aqueous medium 74-78) which are involved in stabilizing the liposomes. By sonification they are dispersed to unilamellar liposomes with an outer diameter of 250-300 A and an internal one of 150-200 A. Therefore the aqueous phase within the liposome is separated by a bimolecular lipid layer with a thickness of 50 A. Liposomes are used as models for biological membranes and as drug carriers. 

Vesicles [10, 11] these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase. 

The binding of carotenoids within the lipid membranes has two important aspects the incorporation rate into the lipid phase and the carotenoid-lipid miscibility or rather pigment solubility in the lipid matrix. The actual incorporation rates of carotenoids into model lipid membranes depend on several factors, such as, the kind of lipid used to form the membranes, the identity of the carotenoid to be incorporated, initial carotenoid concentration, temperature of the experiment, and to a lesser extent, the technique applied to form model lipid membranes (planar lipid bilayers, liposomes obtained by vortexing, sonication, or extrusion, etc.). For example, the presence of 5 mol% of carotenoid with respect to DPPC, during the formation of multilamellar liposomes, resulted in incorporation of only 72% of the pigment, in the case of zeaxanthin, and 52% in the case of (1-carotene (Socaciu et al., 2000). A decrease in the fluidity of the liposome membranes, by addition of other... 

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

The mixing of nematogenic compounds with chiral solutes has been shown to lead to cholesteric phases without any chemical interactions.147 Milhaud and Michels describe the interactions of multilamellar vesicles formed from dilauryl-phosphotidylcholine (DLPC) with chiral polyene antibiotics amphotericin B (amB) and nystatin (Ny).148 Even at low concentrations of antibiotic (molar ratio of DLPC to antibiotic >130) twisted ribbons are seen to form just as the CD signals start to strengthen. The results support the concept that chiral solutes can induce chiral order in these lyotropic liquid crystalline systems and are consistent with the observations for thermotropic liquid crystal systems. Clearly the lipid membrane can be chirally influenced by the addition of appropriate solutes. 

The quality of alignment in the oriented membrane samples was examined by P-NMR. Figure 4A shows that the lipids are well oriented at all temperatures above and below the lipid phase transition at Tm 23 °C. Any potential disturbance of the bilayer by the peptide was also examined by P-NMR, using a very high peptide lipid ratio of 1 10 in multilamellar vesicles. The charac-... 

Sustained release from disperse systems such as emulsions and suspensions can be achieved by the adsorption of appropriate mesogenic molecules at the interface. The drug substance, which forms the inner phase or is included in the dispersed phase, cannot pass the liquid ciystals at the interface easily and thus diffuses slowly into the continuous phase and from there into the organism via the site of application. This sustained drug release is especially pronounced in the case of multilamellar liquid crystals at the interface. 

Liposomes can be created by shaking or sonicating phospholipids in water. Low shear rates create multilamellar liposomes, which have many layers like an onion. Continued high-shear sonication tends to form smaller unilamellar liposomes. In this technique, the liposome contents are the same as the contents of the aqueous phase. Sonication is generally considered a gross method of preparation, and newer methods such as extrusion are employed to produce materials for human use. 

Closed bilayer aggregates, formed from phospholipids (liposomes) or from surfactants (vesicles), represent one of the most sophisticated models of the biological membrane [55-58, 69, 72, 293]. Swelling of thin lipid (or surfactant) films in water results in the formation of onion-like, 1000- to 8000-A-diameter multilamellar vesicles (MLVs). Sonication of MLVs above the temperature at which they are transformed from a gel into a liquid (phase-transition temperature) leads to the formation of fairly uniform, small (300- to 600-A-diameter) unilamellar vesicles (SUVs Fig. 34). Surfactant vesicles can be considered to be spherical bags with diameters of a few hundred A and thickness of about 50 A. Typically, each vesicle contains 80,000-100,000 surfactant molecules. 

Multilamellar bilayers in the fluid phase are also ordered in the sense that they are smectic liquid crystals. Of great interest is the range of molecular order this is long-range in the sense that the molecules are confined to two dimensions there is also some kind of short-range order in molecular orientations and conformations, but the range of this latter ordering is not known at present. 

Liquid crystals, liposomes, and artificial membranes. Phospholipids dissolve in water to form true solutions only at very low concentrations ( 10-10 M for distearoyl phosphatidylcholine). At higher concentrations they exist in liquid crystalline phases in which the molecules are partially oriented. Phosphatidylcholines (lecithins) exist almost exclusively in a lamellar (smectic) phase in which the molecules form bilayers. In a warm phosphatidylcholine-water mixture containing at least 30% water by weight the phospholipid forms multilamellar vesicles, one lipid bilayer surrounding another in an "onion skin" structure. When such vesicles are subjected to ultrasonic vibration they break up, forming some very small vesicles of diameter down to 25 nm which are surrounded by a single bilayer. These unilamellar vesicles are often used for study of the properties of bilayers. Vesicles of both types are often called liposomes.75-77... 

Experiments by Muller et al. [17] on the lamellar phase of a lyotropic system (an LMW surfactant) under shear suggest that multilamellar vesicles develop via an intermediate state for which one finds a distribution of director orientations in the plane perpendicular to the flow direction. These results are compatible with an undulation instability of the type proposed here, since undulations lead to such a distribution of director orientations. Furthermore, Noirez [25] found in shear experiment on a smectic A liquid crystalline polymer in a cone-plate geometry that the layer thickness reduces slightly with increasing shear. This result is compatible with the model presented here as well. 

The ability of the above mentioned substances to self-organize into bilayer membranes is caused by their amphiphility. During the formation of the vesicles the amphiphilic molecules orient themselves in such a way that their polar heads contact aqueous phases outside and inside the vesicle, while their nonpolar tails are directed towards the interior of the bilayer as shown in Fig. 2c. Vesicles can be classified in multilamellar, small unilamellar (d = 200-500 A) and large unilamellar (d = 1000-5000 A) ones. Since these are small unilamellar vesicles that are typically used for studying PET, in further discussion the term vesicle will always refer to the vesicles of this type, unless otherwise specified. 

The structural dependence of phospholipid solutions on water content is called lysotropic polymorphism. At a water content of up to 30% dipalmitoylphosphatidyl-choline (DPPC) forms lamellar phases consisting of superimposed bilayers. Increasing the water content results in heterogeneous dispersions formed by multilamellar structures, the so-called liposomes (see also Section 1.3.1). 

A thermodynamic treatment, similar to that used for microemulsions, as well as an approximate statistical mechanical one, are developed to explain the phase transition in monolayers of insoluble surfactants [3.8], A similar thermodynamic approach is applied to multilamellar liquid crystals, and it is shown that, for a given set of interactions and bending moduli, only narrow ranges of the thicknesses of the water and oil layers are allowed [3.9]. 

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