Abstract
This review summarizes our recent findings on the role of mineral salts in prebiotic RNA synthesis, which is catalyzed by montmorillonite clay minerals. The clay minerals not only catalyze the synthesis of RNA but also facilitate homochiral selection. Preliminary data of these findings have been presented at the “Horizontal Gene Transfer and the Last Universal Common Ancestor (LUCA)” conference at the Open University, Milton Keynes, UK, 5–6 September 2013. The objective of this meeting was to recognize the significance of RNA in LUCA. We believe that the prebiotic RNA synthesis from its monomers must have been a simple process. As a first step, it may have required activation of the 5'-end of the mononucleotide with a leaving group, e.g., imidazole in our model reaction (Figure 1). Wide ranges of activating groups are produced from HCN under plausible prebiotic Earth conditions. The final step is clay mineral catalysis in the presence of mineral salts to facilitate selective production of functional RNA. Both the clay minerals and mineral salts would have been abundant on early Earth. We have demonstrated that while montmorillonite (pH 7) produced only dimers from its monomers in water, addition of sodium chloride (1 M) enhanced the chain length multifold, as detected by HPLC. The effect of monovalent cations on RNA synthesis was of the following order: Li+ > Na+ > K+. A similar effect was observed with the anions, enhancing catalysis in the following order: Cl− > Br− > I−. The montmorillonite-catalyzed RNA synthesis was not affected by hydrophobic or hydrophilic interactions. We thus show that prebiotic synthesis of RNA from its monomers was a simple process requiring only clay minerals and a small amount of salt.
Highlights
RNA is proposed as an important biopolymer in early life on the Earth where it would have provided both catalysis and acted as a repository of genetic information [1,2,3,4]
It is noted that a partial incorporation of sodium ions in the bilayer of montmorillonite is essential to the catalytic activity of clay minerals in our model system for RNA synthesis using phosphorimidazolides of the nucleosides [19]
This investigation was carried out to examine whether the Na+-montmorillonite-catalyzed reactions of activated mononucleotides are able to generate RNA oligomers and if the same system may facilitate chiral selection in a racemic mixture
Summary
RNA is proposed as an important biopolymer in early life on the Earth where it would have provided both catalysis and acted as a repository of genetic information [1,2,3,4]. Ferris and coworkers [12,13] have shown that the oligomerization of activated mononucleotides can be achieved by the use of a montmorillonite catalyst, which occurs naturally on Earth. The mononucleotides, formed by prebiotic processes on the primitive Earth, are likely to be present in racemic mixtures even though only D-ribose is present in naturally occurring RNA. It is noted that a partial incorporation of sodium ions in the bilayer of montmorillonite is essential to the catalytic activity of clay minerals in our model system for RNA synthesis using phosphorimidazolides of the nucleosides [19]. This review provides a comprehensive summary of one of the authors (PCJ) presentation on the significance of montmorillonite clay and mineral salts in prebiotic RNA synthesis at the “Horizontal.
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