Spontaneous formation and base pairing of plausible prebiotic nucleotides in water.

The RNA World hypothesis presupposes that abiotic reactions originally produced nucleotides, the monomers of RNA and universal constituents of metabolism. However, compatible prebiotic reactions for the synthesis of complementary (that is, base pairing) nucleotides and mechanisms for their mutual selection within a complex chemical environment have not been reported. Here we show that two plausible prebiotic heterocycles, melamine and barbituric acid, form glycosidic linkages with ribose and ribose-5-phosphate in water to produce nucleosides and nucleotides in good yields. Even without purification, these nucleotides base pair in aqueous solution to create linear supramolecular assemblies containing thousands of ordered nucleotides. Nucleotide anomerization and supramolecular assemblies favour the biologically relevant ß-anomer form of these ribonucleotides, revealing abiotic mechanisms by which nucleotide structure and configuration could have been originally favoured. These findings indicate that nucleotide formation and selection may have been robust processes on the prebiotic Earth, if other nucleobases preceded those of extant life.

Spontaneous prebiotic formation of a ß-ribofuranoside that self-assembles with a complementary heterocycle.

The RNA World hypothesis is central to many current theories regarding the origin and early evolution of life. However, the formation of RNA by plausible prebiotic reactions remains problematic. Formidable challenges include glycosidic bond formation between ribose and the canonical nucleobases, as well as the inability of nucleosides to mutually select their pairing partners from a complex mixture of other molecules prior to polymerization. Here we report a one-pot model prebiotic reaction between a pyrimidine nucleobase (2,4,6-triaminopyrimidine, TAP) and ribose, which produces TAP-ribose conjugates in high yield (60-90%). When cyanuric acid (CA), a plausible ancestral nucleobase, is mixed with a crude TAP+ribose reaction mixture, micrometer-length supramolecular, noncovalent assemblies are formed. A major product of the TAP+ribose reaction is a ß-ribofuranoside of TAP, which we term TARC. This nucleoside is also shown to efficiently form supramolecular assemblies in water by pairing and stacking with CA. These results provide a proof-of-concept system demonstrating that several challenges associated with the prebiotic emergence of RNA, or pre-RNA polymers, may not be as problematic as widely believed.