Significance of Mineral Salts in Prebiotic RNA Synthesis Catalyzed by Montmorillonite

The montmorillonite-catalyzed reactions of the 5′-phosphorimidazolide of adenosine used as a model generated RNA type oligomers. These reactions were found to be dependent on the presence of mineral salts. Whereas montmorillonite (pH 7) produced only dimers and traces of trimer in water, addition of sodium chloride (0.1–2.0 M) enhanced the chain length of oligomers to 10-mers as detected by HPLC. Maximum catalytic activity was observed with sodium chloride at a concentration between 0.8 and 1.2 M. This concentration of sodium chloride resembled its abundance in the ancient oceans (0.9–1.2 M). Magnesium chloride produced a similar effect but its joint action with sodium chloride did not produce any difference in the oligomer chain length. Therefore, Mg²⁺ was not deemed necessary for generating longer oligomers. The effect of monovalent cations upon RNA chain length was: Li⁺ > Na⁺ > K⁺. A similar effect was observed with the anions with enhanced oligomer length in the following order: Cl^? > Br^? > I^?. Thus, the smaller ions facilitated the formation of the longest oligomers. Inorganic salts that tend to salt out organic compounds from water and salts which show salt-in effects had no influence on the oligomerization process indicating that the montmorillonite-catalyzed RNA synthesis is not affected by either of these 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 to 2.0 M. Inhibition of cyclic dimer formation is vital for increasing the yield of linear dimers and longer oligomers. In summary, sodium chloride is likely to have played an essential role in any clay mineral-catalyzed prebiotic RNA synthesis.