Double Hydrogen Bonding between Side Chain Carboxyl Groups in Aqueous Solutions of Poly (β-L-Malic Acid): Implication for the Evolutionary Origin of Nucleic Acids

The RNA world hypothesis holds that in the evolutionary events that led to the emergence of life RNA preceded proteins and DNA and is supported by the ability of RNA to act as both a genetic polymer and a catalyst. On the other hand, biosynthesis of nucleic acids requires a large number of enzymes and chemical synthesis of RNA under presumed prebiotic conditions is complicated and requires many sequential steps. These observations suggest that biosynthesis of RNA is the end product of a long evolutionary process. If so, what was the original polymer from which RNA and DNA evolved? In most syntheses of simpler RNA or DNA analogs, the D-ribose phosphate polymer backbone is altered and the purine and pyrimidine bases are retained for hydrogen bonding between complementary base pairs. However, the bases are themselves products of complex biosynthetic pathways and hence they too may have evolved from simpler polymer side chains that had the ability to form hydrogen bonds. We hypothesize that the earliest evolutionary predecessor of nucleic acids was the simple linear polyester, poly (β-D-malic acid), for which the carboxyl side chains could form double hydrogen bonds. In this study, we show that in accord with this hypothesis a closely related polyester, poly (β-L-malic acid), uses carboxyl side chains to form robust intramolecular double hydrogen bonds in moderately acidic solution.