Interfaces liposome

A study by Bames and co-workers of the equilibrium spreading behavior of dimyristol phosphatidylcholine (DMPC) reconciles the differences between spreading of bulk solids and dispersions of liposomes [41]. This study shows the formation of multibilayers below the monolayer at the air-water interface. An incipient phase separation, undetectable by microscopy, in DMPC-cholesterol... 

Liposomes have been widely used as model membranes and their physicochemical properties have therefore been studied extensively. More recently, they have become important tools for the study of membrane-mediated processes (e.g., membrane fusion), catalysis of reactions occurring at interfaces, and energy conversion. Besides, liposomes are currently under investigation as carrier systems for drugs and as antigen-presenting systems to be used as vaccines. 

An alternative approach is the use of pH-sensitive fluorophores (Lichtenberg and Barenholz, lOSS). These probes are located at the lipid-water interface and their fluorescence behavior reflects the local surface pH, which is a function of the surface potential at the interface. This indirect approach allows the use of vesicles independent of their particle size. Recently, techniques to measure the C potential of Liposome dispersions on the basis of dynamic light scattering became commercially available (Muller et al., 1986). 

In this chapter we describe the basic principles involved in the controlled production and modification of two-dimensional protein crystals. These are synthesized in nature as the outermost cell surface layer (S-layer) of prokaryotic organisms and have been successfully applied as basic building blocks in a biomolecular construction kit. Most importantly, the constituent subunits of the S-layer lattices have the capability to recrystallize into iso-porous closed monolayers in suspension, at liquid-surface interfaces, on lipid films, on liposomes, and on solid supports (e.g., silicon wafers, metals, and polymers). The self-assembled monomolecular lattices have been utilized for the immobilization of functional biomolecules in an ordered fashion and for their controlled confinement in defined areas of nanometer dimension. Thus, S-layers fulfill key requirements for the development of new supramolecular materials and enable the design of a broad spectrum of nanoscale devices, as required in molecular nanotechnology, nanobiotechnology, and biomimetics [1-3]. 

A similar technique, the so-called spontaneous emulsification solvent diffusion method, is derived from the solvent injection method to prepare liposomes [161]. Kawashima et al. [162] used a mixed-solvent system of methylene chloride and acetone to prepare PLGA nanoparticles. The addition of the water-miscible solvent acetone results in nanoparticles in the submicrometer range this is not possible with only the water-immiscible organic solvent. The addition of acetone decreases the interfacial tension between the organic and the aqueous phase and, in addition, results in the perturbation of the droplet interface because of the rapid diffusion of acetone into the aqueous phase. 

The next level of sophistication involved studies of systems where separate donors and acceptors interact across an interface. Chlorophyll-quinone photochemical studies have been conducted using liposomes (23-25) and acetate films (26). Calvin and his coworkers (27) have conducted a variety of experiments... 

Models for biological membranes have either been realized as planar lipid monolayers at the gas-water interface (3) or as bi-molecular lipid membranes (BLM) (4) and spherical liposomes (vesicles), respectively (5 6) (Figure 2.). All these models that are only composed of lipid molecules exhibit a diminished stability compared to natural cell membranes. Obviously the protein part besides being functionally important plays a role in terms of stability of biomembranes. This is the case not only for the integral but especially for the boundary proteins ( 7). 

Figure 2. Model membrane systems, (a) Monolayer at the gas-water interface, (b) Planar bimolecular lipid membrane (BLM). (c) Liposomes.

Although monolayers at the gas-water interface are useful to study adsorption phenomena of e.g. proteins at membranes they are not a very good model, since they represent only one half of a biological membrane. Attempts have therefore been made to extend this concept of polymer monolayers to bilayers and particularly to liposomes. It was to prove, whether the monomers (Table I.) could form bilayers and whether a polyreaction within these bi-layers was possible under retention of the structure and the orientation of the molecules. 

In the previous chapters it has been shown that stable cell membrane models can be realized via polymerization of appropriate lipids in planar monolayers at the gas-water interface as well as in spherical vesicles. Moreover, initial experiments demonstrate that polymeric liposomes carrying sugar moieties on their surface can be recognized by lectins, which is a first approach for a successful targeting of stabilized vesicles being one of the preconditions of their use as specific drug carriers in vivo. 

Barenholz Y. Liposome application problems and prospects. Curr Opin Colloid Interface Sci 2001 6 66-77. 

K Kawakami, Y Nishihara, K Hirano. Compositional homogeneity of liposomal membranes investigated by capillary electrophoresis. J Coll Interface Sci 206 177-180, 1998. 

At the most fundamental level, monolayers of surfactants at an air-liquid interface serve as model systems to examine condensed matter phenomena. As we see briefly in Section 7.4, a rich variety of phases and structures occurs in such films, and phenomena such as nucleation, dendritic growth, and crystallization can be studied by a number of methods. Moreover, monolayers and bilayers of lipids can be used to model biological membranes and to produce vesicles and liposomes for potential applications in artificial blood substitutes and drug delivery systems (see, for example, Vignette 1.3 on liposomes in Chapter 1). 

Fig. 5a—c. Orientation of amphiphilic compounds in model membranes33 a) monolayer at the gas/water interface b) bimolecular lipid membrane (BLM) c) liposomes. Between b and c a cross section through the bilayer of BLM or liposomes is shown... 

Fig. 52a-c. Scheme of the fusion process of giant liposomes and the formation of small unilamellar vesicles (SUV) at the interface, a) lipid bilayers in contact b) pores generated by electric breakdown and lipid reorientation forming SUVs c) reconstitution of lipid membranes formation of a fused giant liposome and SUVs . 

Structures formed by (a) detergents and (b) phospholipids in aqueous solution. Each molecule is depicted schematically as a polar head-group ( ) attached to one or two long, nonpolar chains. Most detergents have one nonpolar chain phospholipids have two. At very low concentrations, detergents or phospholipids form monolayers at the air-water interface. At higher concentrations, when this interface is saturated, further molecules form micelles or bilayer vesicles (liposomes). 

Drug is soluble in neither oil nor water however, it can be retained at the interface of an emulsion. Thus, if a liposomal preparation can be made in which the drug resides in the lipid bilayer, or if it can be solubilized into micelles by an appropriate detergent, an emulsion can probably be made wherein the drug resides at the interface. 

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