9 Prebiotic RNA synthesis: significance of mineral salts in montmorillonite-catalyzed reactions

Abstract

The dual properties of RNA as an enzyme catalyst and its ability to store genetic information suggest that early life was based on RNA, and DNA and protein evolved from it. Our lab has demonstrated synthesis of long RNA oligomers by Na+-montmorillonite-catalyzed reactions of 5′-end-activated mononucleotides (Joshi et al., 2009). The Na+-montmorillonite not only catalyzes the prebiotic synthesis of RNA but also facilitates homochiral selection (Joshi et al., 2011, 2013). The montmorillonite-catalyzed reactions of 5′-phosphorimidazolide of adenosine were further investigated to study the effect of salts. These reactions were found to be dependent on the nature of mineral salts present. While montmorillonite (pH 7) produced only dimers in water, addition of sodium chloride (1 M) enhanced the chain length of oligomers to 10-mers as detected by HPLC. Magnesium chloride produced a similar effect but the presence of both sodium chloride and magnesium chloride did not produce any difference in the oligomer chain length. The effect of monovalant cations in 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−. Inorganic salts that tend to salt out organic compounds from water and salts which show salt-in effects had no effect in the oligomerization process, indicating that the montmorillonite-catalyzed RNA synthesis is not affected by hydrophobic or hydrophilic interactions. A 2.3-fold decrease in the yield of cyclic dimer was observed upon increasing the sodium chloride concentration from 0.2 M to 2.0 M. Inhibition of cyclic dimer formation is essential for increasing the yield of linear dimers as well as the overall chain length. The results of this study show that the presence of salts is essential in prebiotic RNA synthesis catalyzed by clay minerals.