Multi-lamellar vesicles

Curve B pyranine trapped in the thin water layer (30 A thickness) of multi-lamellar vesicles made of dipalmitoylphosphatidyl choline (pH 5.5) ... 

One of the several shapes that micelles can take is laminar. Since the ends of such micelles have their lyophobic portions exposed to the surrounding solvent, they can curve upwards to form spherical structures called vesicles. Vesicles are spherical and have one or more surfactant bilayers surrounding an internal pocket of liquid. Multi-lamellar vesicles have concentric spheres of uni-lamellar vesicles, each separated from one another by a layer of solvent [193,876] (Figure 14.1). The bilayers are quite thin (-10 nm) and are stabilized by molecules such as phospholipids, cholesterol, or other surfactants (Figure 14.2). Vesicles made from phospholipid bi-layers are called liposomes. Liposomes can be made by dispersing phospholipids (such as lecithin) into water and then agitating with ultrasound. 

A droplet characterized by the presence at its surface of a lipid bi-molecular film (bi-layer) or series of concentric bi-layers. A vesicle can be single or multi-lamellar and stabilized by natural or synthetic surfactants. Multi-lamellar vesicles are also termed liposomes. See also Bi-molecular Film. 

The lamellar phases on the surface of the emulsion particles are mainly composed of monoacylglycerides, lyso-phospholipids and ionized fatty acids. When the phases have reached a certain size, they will desorb from the emulsion surface and form multi-lamellar vesicles, which are transformed into uni-lamellar vesicles upon increased incorporation of bile salts (Rigler et al., 1986). Upon further incorporation of bile salts, the ratio of lipid amphiphiles to bile salts will decrease to 1 or lower, whereby the uni-lamellar vesicles are transformed to mixed micelles (Staggers et al., 1990). These events are presented in Figure 4. 

Chaikov and Dong et al. [39,40] described symmetric triblock copolymers with a glyco methacrylate middle-block and two outer poly(L-alanine) (or poly(y-benzyl L-glutamate)) blocks. The aggregates formed in dilute aqueous solution were spherical in shape and were 200-700 nm in diameter (TEM). TEM further revealed a compact structure of the aggregates like for multi-lamellar vesicles. The dimension of the particles, however, was found to decrease with increasing concentration of the copolymer. 

As a consequence of this unacceptable situation, the interest in model systems suitable for the construction and study of complex lipid/protein membrane architectures increased steadily over the years [7], The classical portfolio of model membranes used for biophysical and interfacial studies of lipid (bi)layers and lipid/protein composites includes Langmuir monolayers assembled at the water/air interface, (uni- and multi-lamellar) vesicles in a bulk (liposomal) dispersion, the bi-molecular lipid membrane (BLMs), and various types of solid supported membranes [8], All these have their specific advantages but also suffer from serious drawbacks. 

It must be recalled that the ordering of the water next to the surface is limited to a few water molecules (4-7) per head-group, hardly enough to cover the surface with a continuous layer. Thus, the innermost solvation layer can exhibit lateral inhomogeneity, where ordered water forms patches over the surface of the membrane. Under such conditions, the most efficient trajectory for proton transfer between two sites on the surface will follow through the less ordered water molecules. This pathway may be longer, yet the overall passage time may be shorter. Indeed, direct measurements of proton dissociation in the ultra-thin water layers, only 8-11A thick, that are interspaced between the phospholipids layers in multi-lamellar vesicles, yielded values of 8-9 X 10 cm s [45]. 

DiCorleto and Zilversmit (1977) reported that the phospholipid exchange proteins from either beef heart or beef liver were not able to catalyze the exchange of phosphatidylcholine between phosphatidylcholine multi-lamellar vesicles and small unilamellar vesicles. However, by adding acidic phospholipids to the multilamellar vesicles, protein-enhanced exchange was observed. This assay is versatile and can be easily performed. Both substrates are well-defined particles in which the composition can be readily manipulated. The separation of acceptor and donor membranes by centrifugation is rapid and nearly complete with 98—99% of the multilamellar vesicles being sedimented and with 90% of the small unilamellar vesicles remaining in the supernatant. 

Derre A, Faure C, Neri W. 2003. Spontaneous formation of silver nanoparticles in multi-lamellar vesicles. J Phys Chem B 707 4738-4746. 

Further developments Szostak et al generating multi-lamellar vesicles dividing long threadlike vesicles protoceUs with native and synthesized ribozymes. Mann S... 

Based on these dimensions, the high lamellar phase volume and the microscopic observations, a schematic model of the vesicles was constructed and is shown in Figure 1. The 1/2 (w/w) SDS/Ci OH (3 wt. %) composition consists predominately of multi-lamellar vesicles of 1 fim diameter composed of 3-4 alternating bilayers and water layers surronding a large central water cavity. A FFTEM image of a sample of this composition is shown in Figures 2A and 3A. 

HPTS fluorescence has been employed in the study of proton diffusion within inverted micelles [24], in liposomes [4a], in apomyoglobin and the inter-membranal hydration layers of multi-lamellar vesicles [4b]. The simplest case for analysis involves the HPTS molecule in the center of a sphere (inverted micelle, liposome) whose walls are impermeable to protons on the timescale of the experiment. This outer wall is therefore described by an additional reflective boundary condition. Inside such a sphere, even a single proton/anion pair ultimately reaches an equilibrium situation The long-time tail approaches a plateau, rather than decaying to zero. The smaller the radius of the sphere, the higher the expected asymptotic plateau. 

---