THE ROLE OF MONTMORILLONITE CLAY MINERALS IN THE CATALYSIS AND CHIRAL SELECTION OF RNA SYNTHESIS

The “RNA World” hypothesis for origin of life proposes that RNA formed first and the DNA-Protein world evolved from it. We have proposed a montmorillonite-catalyzed reaction of the 5´-phosphorimidazolide of nucleosides to probe prebiotic synthesis of RNA. This model generates RNA oligomers despite deoxy-mononucleotides adsorbing equally well onto the montmorillonite. Optimum catalytic activity was observed over a narrow range of pH (6-9) and salinity. Whereas montmorillonite (pH 7) produced only dimers and trimers in water, NaCl (0.1-1.2 M) enhanced the chain length to 12-mers as detected by HPLC. Similar sample analyzed by MALDI mass spectra and PAGE detected 29-33mers and 50-mers, respectively. Maximum catalytic activity was observed between 0.8 and 1.2 M NaCl, which resembled its abundance in the ancient oceans. When the weathering steps of early Earth that generated catalytic montmorillonite were simulated to meet Martian soil conditions, the catalytic activity remained intact. The effect of monovalant cations upon increasing RNA chain length was in Li⁺>Na⁺>K⁺ order suggesting that smaller ions facilitated formation of longer oligomers. The RNA synthesis was independent of hydrophobic or hydrophilic interactions. For studying homochiral selectivity, the reactions of racemic mixtures of ImpA and ImpU were carried out on Na⁺-montmorillonite. Twelve linear and three cyclic dimers were isolated and characterized with <64% homochirality. Out of the sixteen trimers isolated, ten were homochiral with <74% homochirality. The tetramers and pentamers were separated into 24 and 20 isomers with 93% and 97% homochirality, respectively. Thus Na⁺-montmorillonite not only catalyzes the RNA synthesis but also facilitates homochiral selection. Research was supported by NASA grant NNA09DA80A.