Multilamellar vesicle

In water, a particle of lecithin exhibits myelin growth, ie, cylindrical sheets that are formed by bdayers and are separated by water which may break up into liposomes (vesicles with a single bilayer of Hpid enclosing an aqueous space). PhosphoHpids more generally form multilamellar vesicles (MLV) (5). These usually are converted to unilamellar vesicles (ULV) upon treatment, eg, sonication. Like other antipolar, surface-active agents, the phosphoHpids are... 

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. 

Liposomes are members of a family of vesicular structures which can vary widely in their physicochemical properties. Basically, a liposome is built of one or more lipid bilayers surrounding an aqueous core. The backbone of the bilayer consists of phospholipids the major phospholipid is usually phosphatidylcholine (PC), a neutral lipid. Size, number of bilayers, bilayer charge, and bilayer rigidity are critical parameters controlling the fate of liposomes in vitro and in vivo. Dependent on the preparation procedure unilamellar or multilamellar vesicles can be produced. The diameter of these vesicles can range from 25 nm up to 50 ym—a 2000-fold size difference. 

Subcutaneous injection of insulin encapsulated in liposomes in rats resulted in prolonged hypoglycemic effects compared to a solution of free insulin this study also indicated that a substantial fraction of hand-shaken multilamellar vesicles could enter the circulation in intact form after subcutaneous injection (Stevenson et al., 1982). The neutral liposomes used in this study were cleared more slowly from the injection site than the negatively charged liposomes. 

H. E., and Crommelin, D. J. A., Characterization of liposomes (1987). The influence of extrusion of multilamellar vesicles through polycarbonate membranes on particle size, particle size distribution and number of bilayers, Int. J. Pharm.. 35, 263-274. 

Mayer, L. D., Hope, M. J., Cullis, P. R., and Janoff, A. S. (1985a). Solute distributions and trapping efficiencies observed in freeze-thawed multilamellar vesicles, Biochim. Biophys. Acta, 817, 193-196. 

Perez-Soler, R., Khokhar, A. R., Hacker, M. P., and Lopez-Berestein, G. (1986). Toxicity and antitumor activity of cis-bis-cyclopentenecarboxylato-1,2-diaminocyclohexane platinum(II) encapsulated in multilamellar vesicles. Cancer Res., 46. 6269-6273. 

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. 

Liposomes are formed due to the amphiphilic character of lipids which assemble into bilayers by the force of hydrophobic interaction. Similar assemblies of lipids form microspheres when neutral lipids, such as triglycerides, are dispersed with phospholipids. Liposomes are conventionally classified into three groups by their morphology, i.e., multilamellar vesicle (MLV), small unilamellar vesicle (SUV), and large unilamellar vesicle (LUV). This classification of liposomes is useful when liposomes are used as models for biomembranes. However, when liposomes are used as capsules for drugs, size and homogeneity of the liposomes are more important than the number of lamellars in a liposome. Therefore, "sized" liposomes are preferred. These are prepared by extrusion through a polycarbonate... 

Figure 1. Structure of liposomes and lipid microsphere a), multilamellar vesicle b). unilamellar vesicle c). lipid microsphere. Symbols inside the microsphere indicate di- and tri-acyl glycerol.

Liposomes have been, and continue to be, of considerable interest in drug-delivery systems. A schematic diagram of their production is shown in Fig. 10. Liposomes are normally composed of phospholipids that spontaneously form multilamellar, concentric, bilayer vesicles, with layers of aqueous media separating the lipid layers. These systems, commonly referred to as multilamellar vesicles (MLVs), have diameters in the range of 15 pm. Sonication of MLVs... 

A method where phospholipids are entrapped in the pores of resin beads, in the forms of multilamellar vesicles, has been described [313-319,376]. In some ways, the idea is similar to that of IAM chromatography, even though the resin is modified differently. The retention indices correlate very well with the partition coefficients measured in liposome-water systems (described below). 

Since soy lecithin ( 20% extract from Avanti) was selected as a basis for absorption modeling, and since 37 % of its content is unspecified, it is important to at least establish that there are no titratable substituents near physiological pH. Asymmetric triglycerides, the suspected unspecified components, are not expected to ionize. Suspensions of multilamellar vesicles of soy lecithin were prepared and titrated across the physiological pH range, in both directions. The versatile Bjerrum plots (Chapter 3) were used to display the titration data in Fig. 7.33. (Please note the extremely expanded scale for %.) It is clear that there are no ionizable groups... 

Figure 7.46 Fluorescence quenching of doxorubicin by DNA [597] (a) doxorubicin in aqueous solution, quenched immediately on addition of DNA (b) doxorubicin fluorescence not affected by vesicles (c) Doxorubicin preequihbrated with vesicles, and then subjected to DNA. The fraction bound to the outer membrane leaflet is immediately quenched by the DNA. (d) Same as (c), but multilamellar vesicles used. The left arrow represents a 5-min interval and applies to the first three cases the right arrow represents 30-min interval and applies to (d) only. [Reprinted from Ronit Regev and Gera D. Eylan, Biochemical Pharmacology, vol. 54, 1997, pp. 1151-1158. With permission from Elsevier Science.]...

Kolev, V.D. and D.N. Kafalieva. 1986. Miscibihty of beta-carotene and zeaxanthin with dipalmitoylphos-phatidylcholine in multilamellar vesicles A calorimetric and spectroscopic study. Photobiochem. Photobiophys. 11 257-267. 

The aforementioned results refer to unilamellar membrane models but essentially similar results are obtained in multilamellar vesicles, though the kinetics are more complex in such systems. The numerical values observed in these model membranes simply show that one or more of the aforementioned factors arise however, in the in vivo situation, the preeminent effect is unknown but may well be the proximity of the hydroxyl group to the water interface. 

Liposomes, also known as lipid vesicles, are aqueous compartments enclosed by lipid bilayer membranes [56,57]. Figure 10.11 shows how lipid bilayers are arranged in the liposome and the lipid structures in large unilamellar vesicles and multilamellar vesicles. Lipids consist of two components ... 

Figure 10.11 Liposome structures, including multilamellar vesicles (MLV) and large unilamellar...

Faure, C., Derre, A. and Neri, W. (2003) Spontaneous formation of silver nanoparticles in multilamellar vesicles. Journal of Physical Chemistry B, 107, 4738-4746. 

Olea, D. and Faure, C. (2003) Quantitative study of the encapsulation of glucose oxidase into multilamellar vesicles and its effect on enzyme activity. Journal of Chemical Physics, 119, 6111-6118. 

Meso- and (+ )-azobis[6-(6-cyanododecanoic acid)] were synthesized by Porter et al. (1983) as an amphipathic free radical initiator that could deliver the radical center to a bilayer structure controllably for the study of free radical processes in membranes. The decomposition pathways of the diazenes are illustrated in Fig. 36. When the initiator was decomposed in a DPPC multilamellar vesicle matrix, the diazenes showed stereo-retention yielding unprecedented diastereomeric excesses, as high as 70%, in the recombination of the radicals to form meso- and (+ )-succinodinitriles (Brittain et al., 1984). When the methyl esters of the diazene surfactants were decomposed in a chlorobenzene solution, poor diastereoselectivity was observed, diastereomeric excesses of 2.6% and 7.4% for meso- and ( )-isomers respectively, which is typical of free radical processes in isotropic media (Greene et al, 1970). 

---