Micelles self-replication

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. 

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. 

Bachmann, P. A., Luisi, P. L., and Lang, J. (1992). Autocatalytic self-replication of micelles as models for prebiotic structures. Nature, 357, 57-9. 

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

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

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. 

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. 

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. 

Bachman PA, Luisi PL, Lang J (1992) Autocatalytic self-replicating micelles as models for prebiotic structures. Nature 357 57-59... 

Figure 6 The experiment of Luisi and coworkers with ethyl caprylate leading to micelles of sodium caprylate by hydrolysis. Because of the continuous supply of ethyl caprylate, the micellar system self-replicates...

We have mentioned before the possibility of combining chemical evolution with self-replication. In principle, chemical evolution can be associated to self-reproducing micelles or vesicles. There are in principle two ways to conceive this in this case on the one hand, the surfactants of the self-reproducing vesicles could be chemically transformed during their reproduction cycles into compounds which may give rise to more efficient cell-like compartments. This possibility has been discussed theoretically some time ago. On the other hand, the supramolecular structure can help and determine the evolution of internalized compounds—i.e. permitting certain reactions and avoiding others thanks to the semipermeable character of the membrane. As already mentioned, studies of this type with vesicles still remain to be initiated. 

Another important observation in this context is that micelles are capable of self-reproduction, that is, the amphiphiles (fatty acids) within the micelles are capable of catalyzing the hydrolysis of fatty acid esters, causing the micelle to grow and eventually split into two new ones. The term reproduction is used to distinguish it from self-replication as discussed above, as it is not limited to discrete (linear) molecular structures but rather is confined in geometry. Because each new micelle... 

One particular asset of structured self-assemblies is their ability to create nano- to microsized domains, snch as cavities, that could be exploited for chemical synthesis and catalysis. Many kinds of organized self-assemblies have been proved to act as efficient nanoreactors, and several chapters of this book discnss some of them such as small discrete supramolecular vessels (Chapter Reactivity In Nanoscale Vessels, Supramolecular Reactivity), dendrimers (Chapter Supramolecular Dendrlmer Chemistry, Soft Matter), or protein cages and virus capsids (Chapter Viruses as Self-Assembled Templates, Self-Processes). In this chapter, we focus on larger and softer self-assembled structures such as micelles, vesicles, liquid crystals (LCs), or gels, which are made of surfactants, block copolymers, or amphiphilic peptides. In addition, only the systems that present a high kinetic lability (i.e., dynamic) of their aggregated building blocks are considered more static objects such as most of polymersomes and molecularly imprinted polymers are discussed elsewhere (Chapters Assembly of Block Copolymers and Molecularly Imprinted Polymers, Soft Matter, respectively). Finally, for each of these dynamic systems, we describe their functional properties with respect to their potential for the promotion and catalysis of molecular and biomolecu-lar transformations, polymerization, self-replication, metal colloid formation, and mineralization processes. 

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). ... 

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