Conventional vesicles

Other interesting and potentially useful physical characteristics of conventional vesicles include their activity as osmotic membranes, their ability to undergo phase transitions from liquid crystalline to a more fluid state, and their permeability to many small molecules and ions, especially protons and hydroxide. Because of their similarity to natural biological membranes, vesicles also have great potential as models for naturally occurring analogues that may be difficult to manipulate directly. 

Major barriers to the use of conventional vesicles in many applications include (1) the inherent long-term instability of the systems, (2) their potential for interaction with enzymes and blood lipoproteins, and (3) their susceptibility to the actions of other surface-active materials. For such critical applications as controlled-release drug delivery, even the most stable systems with a lifetime of several months do not begin to approach the shelf life requirements. 

The general approach used to attain such structures has been the synthesis of conventional vesicle-forming amphiphilic materials containing polymerizable functionalities in the molecule, vesicle formation, and subsequent polymerization, preferably by some nonintrusive means such as irradiation. In principle, the polymerizable functionality can be located at the end of the hydrophobic tail, centrally within the tail, or in association with the ionic or polar head group (Fig. 15.14). The choice of a preferred structure will probably be determined by the final needs of the system and the synthetic availability of the desired materials. 

The mechanical and chemical stability of the cross-linked vesicles were analyzed by negative staining TEM after exposure to protic and aprotic organic solvents. The solubilizing effects of organic solvents are known to destroy the structural integrity of conventional vesicles. We have shown that the cross-... 

Table 1 Comparison of typical single phospholipid giant vesicles (diameters 5 or 50 pm) with typical conventional vesicle preparations (diameters 50 or 500 nm). The calculations are made for POPC as phospholipid, using a constant mean head group area of 0.72 nm and a bilayer thickness of 3.7nm [18]. (lfemtoliter= 1 fl= 1 x 10 1)...

In contrast, when using giant vesicles, one single vesicle is used at a time (see Table 1). One of the consequences of this is that processes with conventional vesicles in which intervesicular interactions are involved may not occur in the case of a single giant vesicle (see the concluding remarks). 

Concerning the results obtained with phospholipase A2, a recent cryotransmission electron microscopy study using a venom phospholipase hj and conventional vesicles made from saturated phosphatidylcholines has shown that the vesicles are destabilized upon (exovesicular) enzyme addition, again after a starting lag phase [44]. This is in good agreement with the observations of Section 3.3 [16]. 

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