Replicating micelles

Self-replicating micelles aqueous micelles and enzymatically driven reactions in reverse micelles. J. Am. Chem. Soc., 113, 8204-9. 

Luisi, P. L. and Varela, F. J. (1990). Self-replicating micelles - a chemical version of minimal autopoietic systems. Orig. Life Evol. Biosph., 19, 633 3. 

Replicating micelles have been realized by producing within the micelle, through a chemical (or enzymatic) reaction, the same surfactant as that constituting the micelle, which therefore grows and redistributes the surfactant by dividing into new micelles [9.207]. 

Bachmann, P.A., Luisi, P.L., Lang, J. (1992). Autocatalytic self-replicating micelles as models for prebiotic structures. Nature (London) 357, 57-59. 

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-... 

Bachmann, P. A., Walde, P, Luisi, P. L. and Long, J. (1991). Self-replicating micelles aqueous micelles and enzymatically driven reactions in reverse micelles. Journal of the American Chemical Society, 113, 8204-9. 

In a recent example, the group of Giuseppone described self-replicating micelles that were coupled to a dynamic combinatorial approach, thus associating for the first time a self-repUcatmg assembly with a selection process (Figure 15). ... 

A particularly interesting type of micellar catalysis is the autocatalytic self-replication of micelles [58]. Various examples have been described, but a particularly interesting case is the biphasic self-reproduction of aqueous caprylate micelles [59]. In this system ethyl caprylate undergoes hydroxyl catalysed hydrolysis to produce the free carboxylate anion, caprylate. Caprylate micelles then fonn. As these micelles fonn, they solubilize ethylcaprylate and catalyse further production of caprylate anion and caprylate micelles. 

The encapsulation results in a chance collection of molecules that then form an autocatalytic cycle and a primitive metabolism but intrinsically only an isolated system of chemical reactions. There is no requirement for the reactions to reach equilibrium because they are no longer under standard conditions and the extent of reaction, f, will be composition limited (Section 8.2). Suddenly, a protocell looks promising but the encapsulation process poses lots of questions. How many molecules are required to form an organism How big does the micelle or liposome have to be How are molecules transported from outside to inside Can the system replicate Consider a simple spherical protocell of diameter 100 nm with an enclosed volume of a mere 125 fL. There is room within the cell for something like 5 billion molecules, assuming that they all have a density similar to that of water. This is a surprisingly small number and is a reasonable first guess for the number of molecules within a bacterium. 

Micelles are capable of self-replication if an appropriate chemical reaction occurs within the micelle itself that produces more of the same amphiphile that forms the micelle. Such self-replication has been demonstrated for both ordinary micelles in an aqueous medium [139] as well as for reverse micelles, [140] which are spherules of water stabilized by an amphiphile in an organic solvent. Some of the prebiotic potentialities of replicating membranous vesicles have been investigated, [141] and they have been characterized as "minimum protocells. [142]... 

So far, only the self-replication mechanisms of linear molecules have been described it is now time to consider closed spherical structures, such as micelles and vesicles. Here, the term self-reproduction will be used rather than self-replication, because, as it will be seen, the population increase is generally based on statistical processes. The subject of micelles and vesicles self-reproduction is dealt with in other chapters in this book a certain degree of repetition and/or mis-match is unavoidable. 

In the various reviews on self-reproduction in recent years, practically no mention is made of such micelles or vesicle systems. The reason lies most prohahly in the bias of classic hiochemical literature, according to which self-replication is tantamount to nucleic acid systems lacking this are therefore deemed not to he relevant. In this particular regard, it is argued that self-reproduction of micelles and vesicles proceeds without transmission of information. 

We have also learned that self-replication is not a prerogative only of nucleic acids, but it can be shared by different kinds of chemical families see the formose reaction, the self-replicating peptides, and the self-reproducing micelles and vesicles. The list should include the cellular automata and the corresponding devices of artificial life. Self-reproduction of vesicles and liposomes is important because it represents a model for cell reproduction. 

Bdhler, C., Bannwarth, W., and Luisi, R. L. (1993). Self-replication of oligonucleotides in reverse micelles. Helv. Chim. Acta, 76, 2313-20. 

A self-replicating system based on the catalytic action of reversed micelles has been presented in Chapter 1. Other cases of micellar catalysis have been discussed in Ref. 119. The use of semi crystalline fibers to immobilize catalysts and substrates was also proposed [120]. Another approach to the enhancement... 

Interestingly, self replication has teen demonstrated in reversed micelles [291, 292]. Reaction between cetylbromide and trimethylamine in an organic solvent gave hexadecyltrimethyl-amine which formed, in the presence of small amounts of water, reversed micelles. This system was considered to self replicate since an endogenous growth of the micelles corresponded to the in situ appearance of surfactants [291]. 

Vesicle and micelles are considered to be useful models for minimum protocells that had emerged in prebiotic times [200]. One of their properties should have been to sequester other molecules, including macromolecules, for self-replication. A central enigma to be addressed is related to various routes by which the enantiopure homochiral biopolymers were formed within such architectures. Polymerization of NCA of natural hydrophobic amino acids in water in the presence of phospholipids by Luisi et al. [201] has demonstrated that the hydrophobic environment enhances their rate of polymerization. 

In an attempt to design a protocell, a Los Alamos group proposed a system essentially composed of non-enzymatic template replication coupled to micelle growth [55,56]. The micelle aggregate is assumed to incorporate from the medium precursors of lipids and template building blocks (monomers or oligomers). The authors assume that for this particular construct PNAs [57] would serve better because of their hydrophobic nature. It is assumed that single-stranded molecules face the hydrophilic anterior whereas double-stranded molecules immerse into the hydrophobic interior of the micelles. Alternation between these two states is assumed to facilitate replication. 

The autocatalytic hypothesis was backed by the reaction s pH sensitivity. Addition of CO2 to the micelles increased acidity resulting in vesicles as the pH dropped below 7. Overall the experiment showed that vesicles could form from a reaction that generates amphiphilic molecules that in turn form autocatalytic micelles. Upon addition of a gas, common in planetary atmospheres, the micelles form vesicles that could function as protocells. The protocells are self-replicating so this process can be considered to be autopoietic, from the Greek for self-forming, and therefore fulfils an essential step in the chemical evolution of life. 

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