Micelles self-reproduction

Reverse micelles are the first compartment structures for which the phenomenon of micelle self-reproduction has been described (Bachman et al., 1990 1991). This experimental work was a follow up of a theoretical study by Varela and Luisi (Luisi and Varela, 1990), and is it this that eventually brought to light the self-reproduction of aqueous micelles and vesicles. This has been covered already in Chapter 7, on the chemistry of self-reproduction. 

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. 

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. 

The work was experimentally initiated in my laboratory in Zurich in the 1980s, at a time in which I was very much involved with reverse micelles - and in fact reverse micelles were the first spherically closed system that underwent self-reproduction. 

Figure 7.13 The first self-reproduction scheme conceived for reverse micelles, (a) A reverse micelle, (b) Two reagents, A and B, penetrate inside the water pool and react with each other inside the boundary, yielding the very surfactant S that makes the micelles. The S thus produced migrates to the boundary and induces growth and eventually multiplication of the micelle. (Adapted from Luisi and Varela, 1990).

Figure 7.15 Autocatylatic self-reproduction of aqueous caprylate micelles. (Adapted, with some modifications, from Bachmann et al., 1992.)...

In general, the mechanism of self-reproduction of micelles and vesicles can be considered an autopoietic mechanism, since growth and eventually division comes from within the structure itself. This point will be considered again in Chapter 8, on autopoiesis, where the mechanism of the self-reproduction process will also be discussed. 

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. 

The term chemical autopoiesis indicates the experimental implementation of autopoiesis in the chemistry laboratory. The most well known of these processes is the self-reproduction of micelles and vesicles. This has been discussed in the previous chapter, where the original idea of Francisco Varela and myself was to work with bounded systems that would produce their own components due to an internal reaction, respecting the scheme illustrated in Figure 8.3. We came up with the idea of using reverse micelles (refer back to Figure 7.13) with two reagents. 

Micellar catalysis is a broad field (Fendler and Fendler, 1975 Rathman, 1996 Rispens and Engberts, 2001), and caution is needed when using this term. In fact, whereas the broad term catalysis is justihed when referring to an increase of the velocity of reachon, this does not always mean that the velocity constant is increased (namely that there is a decrease of the specific activation energy). Rather, the velocity effect can be due to a concentration effect operated by the surface of the micelles. This is also the case for the autocatalytic self-reproduction of micelles discussed in the previous chapter, where the lipophilic precursor of the surfactant is concentrated on the hydrophobic surface of the fatty acid micelles (Bachmann et al., 1992), a feature that has given rise to some controversy (Mavelli and Luisi, 1996 Buhse etal, 1991 1998 Mavelli, 2004). 

Self-reproduction of micelles and vesicles models for the mechanisms of life from the perspective of compartmented chemistry. Adv. Chem. Phys., 92,425-38. 

Figure 2. Micellar autocatalysis. The biphasic system with the autocatalytic self-reproduction of aqueous caprylate micelles. Hydrolysis of supernatant ethylcaprylate (EC) takes place first at the microscopic interphase of the biphasic system and is very slow, until the cmc is reached. Then the process becomes autocatalytic (see text).

The main point of this chapter is to show that supramolecular aggregates can play an important role in the early history of the origin of life. The main driving force in all processes we have illustrated is the hydrophobic interaction—this is responsible for the self-assembly of vesicles, for the binding of hydrophobic substances to the vesicle membranes and the corresponding autocatalytic self-reproduction of micelles and vesicles, as well as for the corresponding chemical events of polymerization. Since the hydrophobic forces take place generally spontaneously and with... 

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... 

Fig. 4 Self-reproduction of a micelle according to Bachmann et al. (Ref [26]). The ester EC is hydrolyzed slowly to C outside the micelles (indicated with C ) but much quicker inside, and as the micelles grow, they generate new ones.

The first supramolecular self-reproduction experiments involved reverse micelles. Bachman et aL (22) have demonstrated that a reverse micellar system or water-in-oil microemulsion is endowed with the property of self-reproduction. The starting point is a microemulsion stabilized by octanol and octanoic acid. To this system, the fatty acid ester octyl octanoate was added. This is hydrolysed by LiOH to give the fatty acid and alcohol. As the reaction proceeds, more droplets of a similar structure/composition to those present initially are produced. This is because the reaction is localized within the boundary of the structure and so the reaction leads to the production of components of the boundary. 

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. 

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