The enzyme-free regioselective phosphorylation of ribonucleosides is promoted by metal ions

Elucidating the pathway by which RNA efficiently emerged and persisted under prebiotic conditions remains a fundamental challenge in origin-of-life research. Molecules capable of bridging the gap between prebiotic precursors (such as ribonucleosides and inorganic phosphates) and the spontaneous formation of RNA chains within the thermodynamic barriers of a plausible early-Earth environment still remain unclear. Here, we demonstrate that metal ions play a crucial role in promoting the regioselective formation of 2',3'-cyclic ribonucleotides, which are often considered as competent components for RNA chain growth. By adding Ni²⁺ or Co²⁺ to a mixture of ribonucleosides, water-soluble inorganic phosphate, urea, and ammonium formate and exposing this to wet-dry cycles, we observed a four-fold increase in the formation of 2',3'-cyclic ribonucleotides compared to assays without metal ions. Dinucleotide formation was also detected in the same assay solutions. The formation of different regioisomers of mononucleotides was primarily confirmed by HPLC-based analysis, along with MALDI-ToF mass spectrometry and ³¹P NMR spectroscopy. Our current findings could help in bridging the gap between how reactive RNA precursors could have formed and how those precursors eventually emerged as the RNA chains needed for the origin of life.