Bilayers closed structures

Phospholipids are a major component of living cell membranes. Physical and chemical properties of bilayer structure composed of phospholipids have been well studied, (ij.2) One of the intriguing properties of phospholipids is that they form a closed structure hereafter referred to as vesicles. Vesicles have attracted much attention since they are considered to mimic biocelIs. 

There is a close resemblance between fatty-acid salts and phospholipids (p. 790) in that both possess long hydrocarbon tails and a polar head. Phospholipids also aggregate in a polar medium to form micelles and continuous bilayer structures such as shown in Figure 18-5. The bilayer lipid structure is very important to the self-sealing function of membranes and their impermeability to very polar molecules. 

CO oxidation [22, 23]. A similar conclusion was drawn from scanning tunneling microscopy (STM) measurements on the Au/Ti02 system, where again bilayer high deposits turned out to be the most active ones [24, 25]. Naturally, a close structure-reactivity relationship is not restricted to Au particles but can be found for other metal species in a multitude of chemical reactions [26]. 

Membranes naturally tend to form closed structures, to avoid exposing the hydrophobic ends of lipid bilayers to the solvent. Synthetic closed structures, called liposomes, can be made with membrane fragments or synthetic phospholipids. Liposomes can be made to contain compounds buried in the membrane or totally enclosed and are a... 

Liposomes are spherical self-closed structures, composed of curved lipid bilayers, which enclose part of the surrounding solvent into their interior (Figure 51.15)d The size of a liposome ranges from some 20 nm up to several micrometers and they may be composed of one or several concentric membranes, each with a thickness of about 4 nm. Liposomes possess unique properties owing to the amphiphilic character of the lipids, which make them suitable for drug delivery. ... 

The actual structure also depends on the surfactant molecular structure. For instance, dual-tail amphiphiles such as sulfosuccinate surfactants are more likely to produce W/O-type miniemulsions and microemulsions with water core islands. If too much water is present, because of the inability of this surfactant to accommodate its branched double tails in an oily core, it would result in more complex structures such as vesicles, in which a surfactant bilayer closes on itself, as shown in Fig. 4. 

FIGURE 15.13. Surfactants that form closed bilayer aggregate structures such as vesicles usually produce multilayered systems such as (a). Smaller, single bilayer vesicles such as b) can be formed by disruption of the multilayer systems. 

A class of self-assembled structures that deserves special attention is the bilayer. This is a lamellar structure composed of two molecular layers of amphiphilic molecules. Amphiphiles having a J/v close to unity usually assemble into bilayers in which (in aqueous media) the apolar parts of the molecules are directed toward each other. Free-floating bilayers do not exist it is too unfavorable to expose the hydro-phobic edges to water. The bilayer closes into a spherical geometry, the so-called vesicle, or its edges are embedded in a nonaqueous environment. See Figure 11.13. 

For a lecithin with chain containing 18 carbon atoms, a value of d = 70 A is found, which is nearly equal to two times the length of a fully extended lecithin molecule. This and other findings support the conclusion that the membrane consists essentially of a bimolecular layer (bilayer) of oriented lipid molecules. The polar head groups of the lipid molecules point toward the aqueous medium, whereas the fatty acid chain forms the interior of the membrane. Some hydrocarbon solvent remains dissolved in the film, but otherwise the structure of the artificial bilayer closely resembles the arrangement of the lipid molecules in biological membranes. Experiments with artificial lipid membranes have indicated some of the basic mechanisms by which ions may cross biological membranes [323]. 

Figure 1 Representation of the types of sample preparations used for studying membrane lipids and proteins. Micelles and bicelles are usually small (diameters 20-50 nm) structures and bilayers can be sonicated into small (20-50 nm diameter) vesicles, or produced as extended (diameters 100nm) multi-bi-layered or single bilayered closed or open structures, depending upon the method of preparation. Natural membranes are usually as large bilayer fragments or closed structures containing a complex and heterogeneous mixture of lipids and proteins and possibly carbohydrates.

In Chapter 6 we described how many membrane lipids, when isolated, could form natural bilayers in aqueous systems. To prevent any contact of the hydrophobic acyl chains with the aqueous medium, such bilayers close to form vesicles. Usually, under the experimental conditions used to rehydrate lipid samples and with the commonly used membrane lipids (such as phosphatidylcholine) stable multi-layered structures are formed. These structures, which have been compared to the multi-layered appearance of onions, were termed liposomes by Bangham. He realized when studying... 

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

The chain arrangement of this morphology was schematically proposed as in Fig. 10. The cell of the microsphere has a hexagonal surface, and the AB diblock copolymers form a bilayer between the microspheres. From this schematic arrangement, the optimal blend ratio of the AB block copolymer in this system was calculated as 0.46. This value was very close to the blend ratio of the AB type block copolymer 0.5 at which the blend showed the hexagonal packed honeycomb-like structure. 

According to the depth profile of lithium passivated in LiAsF6 / dimethoxyethane (DME), the SEI has a bilayer structure containing lithium methoxide, LiOH, Li20, and LiF [21]. The oxide-hydroxide layer is close to the lithium surface and there are solvent-reduction species in the outer part of the film. The thickness of the surface film formed on lithium freshly immersed in LiAsF /DME solutions is of the order of 100 A. 

Upon the spontaneous rearrangement of anhydrous phospholipids in the presence of water into a hydrated bilayer structure, a portion of the aqueous phase is entrapped within a continuous, closed bilayer structure. By this process water-soluble compounds are passively entrapped in liposomes. The efficiency of encapsulation varies and depends, for example, on the method of preparation of liposomes and the phospholipid concentration during preparation. Different parameters can be used to describe the encapsulation efficiency ... 

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