227 Views of RNA: Is RNA Unique in Its Chemical Isomer Space?

Ribonucleic Acid (RNA) is one of the two nucleic acids used by extant biochemistry, and plays a central role as the intermediary carrier of genet-ic information in transcription and translation. If RNA was involved in the origin of life, it should have a facile prebiotic synthesis. A wide variety of such syntheses have been explored. However, to date no one-pot reaction has been shown capable of yielding RNA monomers from likely prebiotically-abundant starting materials, though this does not rule out the possibility that simpler, more easily prebiotically-accessible nucleic acids may have preceded RNA. Given structural constraints, such as the ability to form complementary base-pairs and a linear covalent polymer, a variety of structural isomers of RNA could potentially function as genetic platforms. Using structure generation software, all of the potential structural isomers of the ribosides (BC5H9O4, where B is nucleobase), as well as a set of simpler minimal analogues derived from them, that can potentially serve as mono-meric building blocks of nucleic acid-like molecules were enumerated. Molecules were selected based on their likely stability under biochemically relevant conditions (e.g. moderate pH and temperature), and the presence of at least two functional groups allowing the monomers to be incorporated into linear polymers. The resulting structures were then evaluated using molecular descriptors typically applied in quantitative structure–property relationship (QSPR) studies and predicted physicochemical properties. Several databases were queried to see if any of the computed isomers had been synthesized previously. Very few of the molecules that emerged from this structure set have been previously described. We conclude that ribonucleosides may have competed with a multitude of alternative structures whose potential proto-biochemical roles and abiotic syntheses remain to be explored