The early Earth was likely exposed to very high levels of UV radiation due to the absence of an ozone layer and the radiation characteristics of the young sun. UV sunlight is an attractive energy source for creating the organic molecules needed to start life on the early Earth. However, these same molecules are vulnerable to photolysis because UV photons have energies comparable to those of chemical bonds. Possible implications include photochemical selection of molecular building blocks of life.Recent experiments have shown that following UV light exposure, excited states of the canonical RNA and DNA bases (A, G, C, T, and U) generally decay orders of magnitude faster than in other heterocyclic compounds, endowing the nucleobases with high intrinsic photostability. These properties would have been highly advantageous for the first self-replicating molecules in prebiotic times before modern enzymatic repair and before the formation of the ozone layer that later attenuated the high levels of UV radiation on the early Earth. The remarkable photophysical properties of the bases thus appear to confirm the hypothesis, first proposed by Sagan, that UV light played a major role in early chemical evolution.We hypothesize that the remarkably efficient dissipation of excess electronic energy by the RNA and DNA bases is functionality preserved from the prebiotic world, that the natural bases were selected in part due to their high intrinsic photostability or UV hardiness, and that this is a vital property of the set of prebiotic molecules that could give rise to life on Earth or elsewhere in the universe. We study the photoproperties of RNA bases in great detail by laser spectroscopy of nucleobases, isolated in a molecular beam and will report on results and potential implications for modeling prebiotic chemistry.

Understanding UV hardiness in prebiotic nucleic acids

248th National Meeting (2014)

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