Glyoxylate as a backbone linkage for a prebiotic ancestor of RNA

The origin of the first RNA polymers is central to most current theories for the origin of life. Difficulties associated with the prebiotic formation of RNA have lead to the general consensus that a simpler polymer preceded RNA. However, polymers proposed as possible ancestors to RNA are not much easier to synthesize than RNA itself. One particular problem with the prebiotic synthesis of RNA is the formation of phosphoester bonds in the absence of chemical activation. Here we demonstrate that glyoxylate (the ionized form of glyoxylic acid), a plausible prebiotic molecule, represents a possible ancestor of the phosphate group in modern RNA. Although in low yields (similar to 1%), acetals are formed from glyoxylate and nucleosides under neutral conditions, provided that metal ions are present (e.g., Mg2+), and provided that water is removed by evaporation at moderate temperatures (e.g., 65 degrees C), i.e. under "drying conditions". Such acetals are termed ga-dinucleotides and possess a linkage that is analogous to the backbone in RNA in both structure and electrostatic charge. Additionally, an energy-minimized model of a gaRNA duplex predicts a helical structure similar to that of A-form RNA. We propose that glyoxylate-acetal linkages would have had certain advantages over phosphate linkages for early self-replicating polymers, but that the distinct functional properties of phosphoester and phosphodiester bonds would have eventually lead to the replacement of glyoxylate by phosphate.