Preparation and Structure of Vesicles

2 Membrane Systems Providing Vectorial PET 2.1 Preparation and Structure of Vesicles 

The ability of the above mentioned substances to self-organize into bilayer membranes is caused by their amphiphility. During the formation of the vesicles the amphiphilic molecules orient themselves in such a way that their polar heads contact aqueous phases outside and inside the vesicle, while their nonpolar tails are directed towards the interior of the bilayer as shown in Fig. 2c. Vesicles can be classified in multilamellar, small unilamellar (d = 200-500 A) and large unilamellar (d = 1000-5000 A) ones. Since these are small unilamellar vesicles that are typically used for studying PET, in further discussion the term vesicle will always refer to the vesicles of this type, unless otherwise specified. 

One of the most popular methods of vesicle preparation was suggested by Huang [33]. It is based on sonification of the lipid or surfactant in water (see Fig. 2 a). Another widely used method of vesicles preparation is a fast injection of an alcohol or ether solution of a lipid into an aqueous solution. Numerous studies concerning the structure and properties of vesicles obtained by these procedures are analyzed in Refs. [19, 20, 34-36]. 

Components of a photosystem can be inserted selectively into the lipid wall or the inner cavity of the vesicle. For this purpose the lipid and components insoluble in water are dispersed together in aqueous solution by sonification. This leads to an occlusion of water insoluble components within the lipid bilayer. The vesicle membrane is sufficiently stable and impermeable for a number of ions. This allows one to prepare, by gel-filtering, the media of different ionic composition inside and outside the vesicle as shown in Fig. 2b. Such asymmetry of chemical content can be preserved for a rather long time (from several hours to several days). Recently the surfactant molecules with double bonds, which can be polymerized after vesicle preparation, were used for further enhancement of vesicle stability [37-39]. Such polymerized vesicles are stable for several months. 

One of the peculiarities of reactions in suspensions of vesicles is the possibility of obtaining a much higher local concentration of reagents located in membranes or the inner cavity than their average concentration in suspension. For instance, the presence of one molecule in the inner cavity of a DPL vesicle (diameter 220 A, membrane thickness 50 A [40]) corresponds to the local concentration of about 3 x 10 3 mol/1. These values correspond to only about 10 6 mol/1 average concentration of the same species in the suspension containing 3 x 10 3 mol/1 of the DPL lipid, since the overall volume of the vesicle cavities in such suspensions 

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