One-pot synthesis of non-canonical ribonucleosides and their precursors from aldehydes and ammonia under prebiotic Earth conditions

The formation of polymers that can hold gene information and work as catalysts is a crucial step for the origin of life. The discovery of catalytic RNA (i.e., ribozyme) supports the hypothesis that RNA might have served these functions at the early stage of life on the Earth. Given this, the spontaneous formation of RNA monomers (i.e., ribonucleotides) and their polymerization on Hadean Earth are essential steps for the origin of life. Previous experiments have investigated the chemical reactions that allow the formation of ribonucleotides and their components. These works have revealed the required molecules to form biological ribonucleotides (i.e., canonical ribonucleotides). Based on geochemical perspectives, abundantly available reactive molecules spontaneously react with each other to provide abundant products. Aldehydes and ammonia are reactive molecules assumed to have been present in considerable amounts on Hadean Earth. However, little is understood about whether or not nucleotides and their components were formed from these molecules under prebiotic conditions. We investigated the incubation products of alkaline aqueous solutions of aldehydes and ammonia. The product solution contained sugars (including ribose), various imidazole derivatives, and ribosyl imidazole (i.e., imidazole ribonucleoside). Ribosyl imidazole is formed via ribosyl amine, which reveals a new reaction pathway for prebiotic ribonucleoside synthesis. The imidazole ribonucleoside was then phosphorylated to imidazole ribonucleotide via a simple dry-down reaction with phosphate. Borate ion improved the reaction yields of these nucleosides and nucleotides. Because all the reactants were available on prebiotic Earth and the reactions progressed spontaneously, imidazole ribonucleotides could have accumulated in prebiotic environments. The experimental simplicity of the present reaction suggests that imidazoles were more abundant than canonical nucleobases on the prebiotic Earth. This further implies that prebiotic oligonucleotides contained imidazole bases in addition to the canonical nucleobases. The improvement of the reaction yields by borate indicates that borate-rich environments were conducive places for the formation and accumulation of non-canonical nucleosides and nucleotides. Such environments could have facilitated the formation of primordial ribonucleic acids on Hadean Earth.