Oleate vesicle formation

Consider the experiment illustrated in Figure 10.19, which shows oleate vesicle formation when an aliquot of concentrated surfactant is added to water compared to the situation in which the same amount of surfactant is added to a solution containing pre-formed vesicles. In the second case, the formation of vesicles is remarkably accelerated, as if in the presence of a strong catalytic effect whereas over one hour is needed to reach the turbidity plateau for oleate addition to water, the plateau is reached in less than ten minutes, curve (b), in the second case. 

Rasi, S., Mavelli, F., and Luisi, P. L. (2003). Cooperative micelle binding and matrix effect in oleate vesicle formation. J. Phys. Chem. B, 107, 14068-76. 

The time course of an actual experiment is shown in Figure 7.17, which shows the hydrolysis of oleic anhydride catalyzed by spontaneously formed oleate vesicles. Note the sigmoid behavior, typical of an autocatalytic process. The lag phase is due to the preliminary formation of vesicles, and in fact the length of the lag phase is shortened when already formed vesicles are pre-added, as shown in the hg-ure. Some mechanistic details of these processes will be discussed in Chapter 10. In this work, an analysis of the number and size distribution of vesicles at the beginning and the end of the reaction was also performed by electron microscopy. 

Figure 10.11 The use of ferritin as a label for the mechanism of growth of vesicles (adapted from Berclaz et al, 2001a b). Schematic representation of the possible vesicle formation and transformation processes when oleate, and oleic acid, are added to pre-formed vesicles which have been labelled, (a) The situation if only de novo vesicle formation occurs, (b) Growth in size of the pre-formed and labeled vesicles which may lead to division, either yielding vesicles that all contain marker molecules (case i, a statistical redistribution of the ferritin molecules) or also yielding vesicles that do not contain markers (case ii). Compare all this with Figure 10.9.

Figure 10.19 Effect of pre-added vesicles on the formation of oleic acid/oleate vesicles. Turbidity measured at 500 nm (1 cm path length) is plotted as a function of time, T=2°C. (a) 62 p,l of 80 mM aqueous sodium oleate was added to 2.438 ml of0.2Mbicine buffer, pH 8.8 ([oleic acid/oleate] = 2 mM). (b)62 p,l of 80mM aqueous sodium oleate was added to 2.438 ml of a 2 mM oleic acid/oleate 100 nm vesicle suspension (0.2 M bicine buffer, pH 8.8 ([oleic acid/oleate] = 4 mM). (c) Turbidity of 2 mM oleic acid/oleate 100 nm vesicles , (d) the same as (b), but using a 50 nm vesicle suspension, (e) Turbidity of 2 mM oleic acid/oleate 50 nm vesicles . (Modified from Blochiger etal, 1998.)...

Experiments have confirmed the idea that micelles as well as vesicles could grow autocatalytically (see [41] for a good overview). In a landmark paper Bachmann et al. [42] observed the formation of autocatalytically replicating micelles from sodium caprylate. The micelles could be converted into more stable vesiscles by pH change. Oleic acid/oleate vesicles can also mul-... 

In contrast with phospholipid vesicles, the oleic acid-oleate vesicle system is characterized by a relatively high monomer solubility. The critical sodium oleate concentration for micelle formation (cmc) at about pH 10.5 is in the range 0.7-1.4mM [10] it has been reported that the monomer solubility at pH 7.4 is around 10-20 pM [11]. (For comparison, the monomer concentration of 1,2-dipalmitoyl-5n-glycero-3-phosphocholine (DPPC) is known to be aroimd 10 M [12].) Due to the relatively high monomer concentration, several physicochemical properties of oleic acid-oleate vesicles are different from vesicles made from phospholipids, such as phosphatidylcholines. The kinetics of lipid exchange is expected to be faster in the... 

Lonchin et al. (27) have also studied the matrix effect of phospholipid vesicles. These authors investigated the formation of mixed phospholipid/fatty acid vesicles by using a combination of kinetic and structural methods to study the process of spontaneous formation of vesicles on addition of oleate micelles to preexisting vesicles of l-palmitoyl-2-oleoyl-.yyn-glycero-3-phosphocholine (POPC). 

Evidence was found for a POPC matrix effect, in that the pre-added vesicles influenced the size distribution of the newly produced vesicles. The matrix effect leads to rapid formation of vesicle aggregates and control over the final size distribution, such that the mixed systems are more monodisperse than in the absence of the matrix, where the oleic acid vesicles grow in an uncontrolled way. This effect was more obvious at lower concentrations of oleic acid/oleate than at higher concentrations. 

In this example of spontaneous vesiculation, a total of about 10 oleic acid-oleate molecules are involved in the formation of one vesicle. It is self-assembly and self-organization in the highest form, and unprecedented in chemistry-unless we consider crystallization. It is useful to remember that this creation of order operates under thermodynamic control ordered structures are built by a process which is... 

One is the problem of reproducibility. Events witnessed with one giant vesicle are sometimes not easily observed again. For example, self-reproduction, budding and growth of oleic acid-oleate giant vesicles in one long scries of experiments [1], could be video recorded in real time, but it was not possible to repeat such behavior subsequently. A second drawback, already mentioned, is that the formation of giant vesicles seems to be restricted to a small class of surfactants. 

Dilution-induced formation of oleic acid-oleate giant vesicles... 

---