Selfassembly processes in prebiotic organic mixtures

We can now return to considering process that are likely to have occurred on the prebiotic Earth, ultimately leading to the beginning of life and initiating Darwinian evolution. We will first ask what physical properties are required if a molecule is to become incorporated into a stable cellular compartment. As discussed earlier, all membrane-forming molecules are amphiphiles, with a hydrophilic 'head' and a hydrophobic 'tail' on the same molecule. If amphiphilic molecules were present in the mixture of organic compounds available on the early Earth, it is not difficult to imagine that their self-assembly into molecular aggregates was a common process. Is this a plausible premise? In order to approach this question, we can assume that the mixture of organic compounds in carbonaceous meteorites such as the Murchison meteorite resembles components available on the early Earth through extraterrestrial infall. A series of organic acids represents the most abundant water-soluble fraction in carbonaceous meteorites (Cronin et al., 1988). Samples of the Murchison meteorite have been extracted in an organic solvent commonly used to extract membrane lipids from biological sources (Deamer andPashley, 1989). When this material was allowed to interact with aqueous phases, one class of compounds with acidic properties was clearly capable of forming membrane-bounded vesicles (Figure 5.4).

Fig. 5.4. Membranous compartments are produced by self-assembly of amphiphilic molecules extracted from the Murchison meteorite. The larger vesicles have a diameter of2030 ^m. The image on the right shows a fluorescent dye (1 mM pyranine) encapsulated within the vesicular volume. The fact that anionic dye molecules can be trapped shows that the vesicles are bounded by true membranes representing a permeability barrier. Bars show 20 ^m.

Fig. 5.5. Membranous compartments composed of very simple amphiphiles are capable of encapsulating macromolecules in stable vesicles. In the image above, DNA molecules have been trapped in decanoic acid vesicles. The DNA is stained with acridine orange so that it can be visualized by fluorescence microscopy. Bar shows 20 ^m.

Fig. 5.5. Membranous compartments composed of very simple amphiphiles are capable of encapsulating macromolecules in stable vesicles. In the image above, DNA molecules have been trapped in decanoic acid vesicles. The DNA is stained with acridine orange so that it can be visualized by fluorescence microscopy. Bar shows 20 ^m.

From these results, it is reasonable to conclude that a variety of simpler amphiphilic molecules were present on the early Earth that could participate in the formation of primitive membrane structures. Even if membranous vesicles were commonplace on the early Earth and had sufficient permeability to permit nutrient transport to occur, these structures would be virtually impermeable to larger polymeric molecules that were necessarily incorporated into molecular systems on the pathway to cellular life. The encapsulation of macromolecules in lipid vesicles has been demonstrated by hydration-dehydration cycles that simulate an evaporating lagoon (Shew and Deamer, 1985). Molecules as large as DNA can be captured by such processes. For instance, when a dispersion of DNA and fatty acid vesicles is dried, the vesicles fuse to form a multilamellar sandwich structure with DNA trapped between the layers. Upon rehydration, vesicles reform that contain highly concentrated DNA, a process that can be visualized by staining with a fluorescent dye (Figure 5.5).

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