Oleate micelle

As apparent from Figure 9.30, there is a gradual transition from the oleate micelle region to the vesicle region, and in fact a common way to obtain vesicles is to inject a few microliters of a high-alkaline-pH sodium oleate solution into a pH... 

Let us consider the mechanism. When monomeric oleate or oleate micelles are added to a solution containing oleate vesicles, two limiting situations may occur. 

Matrix effect in oleat-micelles-vesicles transformations. Orig. Life Evol. 

The diameter of the micelles increases on solubilization for example, according to X-ray measurements on styrene in potassium oleate micelles, the diameter increases from 4.3 to 5.5 nm. As polymerization proceeds, however, the diameter of the micelles decreases first because of the contraction. Polymerization does not take place in all the micelles, since, for example, in the styrene/potassium dodecanoate system, only 1/700 of all the micelles are converted to latex particles. 

Fig, 15. Structure of the Na oleate micelle (Hess, Kiessig, and Philippoff, 1941). 

Fig. 37. With a caproate micelle the layer of water between the polar groups will be much less thick than with an oleate micelle because the oleate micelle is much more flattened.

A novel analytieal tool for the selective detection of local water inside soft mol. assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water has been presented. Through the use of dynamic nuclear polarization (DNP), the NMR signal of water is ampUfied, as it interacts with stable radicals that possess about 658 times higher spin polarization. Stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures have been used, allowing to study the local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assembhes. ... 

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). 

In 1977, Kellogg and Fridovich [28] showed that superoxide produced by the XO-acetaldehyde system initiated the oxidation of liposomes and hemolysis of erythrocytes. Lipid peroxidation was inhibited by SOD and catalase but not the hydroxyl radical scavenger mannitol. Gutteridge et al. [29] showed that the superoxide-generating system (aldehyde-XO) oxidized lipid micelles and decomposed deoxyribose. Superoxide and iron ions are apparently involved in the NADPH-dependent lipid peroxidation in human placental mitochondria [30], Ohyashiki and Nunomura [31] have found that the ferric ion-dependent lipid peroxidation of phospholipid liposomes was enhanced under acidic conditions (from pH 7.4 to 5.5). This reaction was inhibited by SOD, catalase, and hydroxyl radical scavengers. Ohyashiki and Nunomura suggested that superoxide, hydrogen peroxide, and hydroxyl radicals participate in the initiation of liposome oxidation. It has also been shown [32] that SOD inhibited the chain oxidation of methyl linoleate (but not methyl oleate) in phosphate buffer. 

The value of h the number of palmitate ions in the micelle is at least ten. The examination of a solution of potassium oleate at a temperature of 0T8°C. by these methods has resulted in the following values for the concentrations of the various substances present. 

Molality Neutral colloid Neutral salt Ionic micelle Oleate anion Alkali ion... 

Of all mentioned prebiotic membranogenic molecules, the ones that have gained more attention in the literature are long-chain fatty acids. In addition to their prebiotic relevance, these compounds are relatively simple from the structural point of view, and most of them are easily available. We will see in the next chapter that these vesicles have acquired a particular importance in the held of the origin of life. In fact, the hrst inveshgations on self-reproducing aqueous micelles and vesicles were carried out with caprylate (Bachmann et al, 1992) and most of the recent studies on vesicles involve vesicles from oleic acid/oleate (for simplicity we will refer to them as oleate vesicles). In this section, I would like to illustrate some of the basic properties of these surfactant aggregates. 

The cac (critical aggregate concentration) values for oleate are in the millimolar range, which means that at the operational concentration of 10-50 mM there will be a signihcant concentration of monomer in equilibrium with the aggregate. This consideration allows us to go back to the question of whether vesicles are chemical equilibrium systems. Oleate vesicles cannot be considered proper chemical equilibrium systems, however they behave in a mixed way, with some features that are typical of micelles in equilibrium (Luisi, 2001). 

Figure 9.30 The behavior of oleate surfactants as a function of pH equilibrium titration curve of sodium oleate at 25 °C. Note the micelles at higher pH, and the vesicles at lower pH. The chemical name of oleic acid is ctT-9-octadecenoic acid, with 18 carbon atoms. (Modified from Cistola et al, 1988.)...

In contrast to this is the addition of oleate surfactant - in the form of micelles or free monomer - to oleate or to POPC vesicles. In this case, the ratio of the two competitive rates is such that a considerable binding of the added fresh surfactant to the pre-existing vesicles takes place. The efficient uptake of oleate molecules by POPC liposomes (Lonchin et al., 1999) as well as to oleate vesicles (Blochiger et al., 1998) is well documented in the literature. 

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. 

A nonpolar solubilizate such as hexane penetrates deeply into such a micelle, and is held in the nonpolar interior hydrocarbon environment, while a solubilizate such as an alcohol, which has both polar and nonpolar ends, usually penetrates less, with its polar end at or near the polar surface of the micelle. The vapor pressure of hexane in aqueous solution is diminished by the presence of sodium oleate m a manner analogous to that cited above for systems in nonpolar solvents. A 5% aqueous solution of potassium oleate dissolves more than twice the volume of propylene at a given pressure than does pure water. Dnnethylaminoazobenzene, a water-insoluble dye, is solubilized to the extent of 125 mg per liter by a 0.05 M aqueous solution of potassium myristate. Bile salts solubilize fatty acids, and this fact is considered important physiologically. Cetyl pyridinium chloride, a cationic salt, is also a solubilizing agent, and 100 ml of its A/10 solution solubilizes about 1 g of methyl ethyl-butyl either m aqueous solution. 

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-... 

The o c experimental results for the water/pentanol/potas-slum oleate system (18,19) show that a pentanol/potasslum oleate molecular ratio of 5.5 and lower should give a premlcellar aggregate/lamellar liquid crystal transition instead of the pre-micellar aggregate/Inverse micelle transition at high alcohol/ soap ratios. 

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