Amphiphiles prebiotic

Hazen and Deamer looked at the chemical and physical properties of the end products of hypothetical prebiotic reactions carried out under extreme conditions of pressure and temperature, for example in CCh-rich regions of hydrothermal vents. The results of laboratory experiments indicate that prebiotic syntheses leading to a variety of products could have occurred in hydrothermal systems some of these have amphiphilic properties, and would have been capable of self-organisation processes. 

The formation of relatively stable vesicles did not require the presence of pure compounds mixtures of components could also have done the job. However, whether the concentrations of the compounds isolated from the Murchison meteorite would have been sufficient for the formation of prebiotic protocells or vesicles is unclear, even if concentration effects are assumed. Sequences in which the technical Fischer-Tropsch synthesis is the role model have been proposed as possible sources of amphiphilic building blocks. 

D. W. Deamer and J. P. Dworkin have reported in detail on the contribution of chemistry and physics to the formation of the first primitive membranes during the emergence of precursors to life the authors discussion ranges from sources of amphiphilic compounds, growth processes in protocells, self-organisation mechanisms in mixtures of prebiotic organic compounds (e.g., from extracts of the Murchison meteorite) all the way to model systems for primitive cells (Deamer and Dworkin, 2005). 

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]... 

The question as to the potential availability of the requisite amphiphilic precursors in the prebiotic environment has been addressed experimentally by Deamer and coworkers, [143,145] who looked into the uncontaminated Murchison chondrite for the presence of such amphiphilic constituents. Samples of the meteorite were extracted with chloroform-methanol and the extracts were fractionated by thin-layer chromatography, with the finding that some of the fractions afforded components that formed monomolecular films at air-water interfaces, and that were also able to self-assemble into membranous vesicles able to encapsulate polar solutes. These observations dearly demonstrated that amphiphiles plausibly available on the primitive Earth by meteoritic infall have the ability to self-assemble into the membranous vesides of minimum protocells. ... 

The conclusion is that membranous vesicles readily form a variety of amphiphilic molecules that would have been available in the early Earth environment, along with hundreds of other organic species. It is likely that during the chemical evolution leading to the first catalytic and replicating molecules, the ancestors of today s proteins and nucleic acids, membranous vesicles were available in the prebiotic environment, and ready to provide a home for the first forms of cellular life. 

The other lipid commonly present in eukaryotic membranes is cholesterol, a polycyclic structure produced from isoprene by a complex biosynthetic pathway. It is interesting to ask whether it is conceivable that prebiotically plausible reactions might also produce complex amphiphiles. The earliest investigations aiming to answer this question were carried out by Hargreaves et al. [36], Oro and coworkers [37,38], and, more recently, Ourisson et al. [39] and Conde-Frieboes and Blochliger [40]. In all such reactions,... 

Although it is clear that complex lipids can be synthesized under laboratory simulations using pure reagents, the list of required ingredients does not seem plausible under prebiotic conditions. Therefore, it is unlikely that early membranes were composed of complex lipids such as phospholipids and cholesterol. Instead, there must have been a source of simpler amphiphilic molecules capable of self-assembly into membranes. One possibility is lipidlike fatty acids and fatty alcohols, which are products of FTT simulations of prebiotic geochemistry [12] and are also present in carbonaceous meteorites. Furthermore, as will be discussed later, these compounds form reasonably stable lipid bilayer membranes by self-assembly from mixtures (Fig. 4a). 

Various comprehensive studies on the polymerization of enantiopure and racemic esters of a-amino acids performed at the air/water interface to yield peptides have been reported over the years [189,190]. Recent reinvestigations of the products of these reactions by MALDI-TOF MS have demonstrated, however, that they are not longer than dipeptides [191]. For this reason, such esters cannot be regarded as realistic prebiotic model systems for the formation of long oligopeptides. On the other hand, amphiphilic Na-carboxyanhydrides [192] and thio-esters [193] of a-amino acids yield longer oligopeptides. 

It is unlikely that under prebiotic conditions the complex and sophisticated biomacromolecules commonplace in modem biochemistry would have existed. Thus, research into the origin of life is intimately associated with the search for plausible systems that are much simpler than those we see today. However, it is also plausible that these simple building blocks of life might have been amphiphilic molecules in which water could have had an enormous influence on their prebiotic molecular selection and evolution, because water can either form clathrate stmctures or drive these simplest molecules together (Ball, 2001). 

Figure 20.10. Amphiphilic ionic self-complementary peptides. This class of peptides has 16 amino acids, c. 5 nm in size, with an alternating polar and non-polar pattern. They form stable (3-strand and 3-sheet structures thus, the side chains partition into two sides, one polar and the other non-polar. They undergo self-assembly to form nanofibers with the non-polar residues inside positively and negatively charged residues form complementary ionic interactions, like a checkerboard. These nanofibers form interwoven matrices that further form a scaffold hydrogel with a very high water content ( 99.5%). The simplest peptide scaffold may form compartments to separate molecules into localized places where they can not only have high concentration, but also form a molecular gradient, one of the key prerequisites for prebiotic molecular evolution.

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