Formamide as a prebiotic precursor of DNA nucleobases: The free radical routes

Abstract

In this review we probe the reaction mechanisms for formation of DNA nucleobases from formamide (FM) following the free radical pathways that have been identified by quantum chemical computations. Formamide is known as a precursor of nucleobases in the prebiotic chemistry of the Early Earth or in the nitrogen-rich atmosphere such as Titan, regardless of whether water is present or absent. Laboratory simulation experiments of the extra-terrestrial conditions are not trivial to setup because of the limited knowledge of the unseen environments. However, increasing synthetic and spectral evidence for organic compounds have been found in the organic haze of Titan’s atmosphere. Formation of complex molecules under such environments can be expected to proceed in part through free radical pathways. The free radicals constitute promising routes for the nonthermal construction of biomolecules. They further promote our understanding in the prebiotic synthesis of heterocyclic organic compounds. All of the suggested mechanisms employ simple radical reactions such as H rearrangements (shifts), ●H/●OH/●NH₂ radical losses, and most importantly intramolecular radical 1, n-cyclization. The advantages of free radical mechanisms are the inherently low energy barriers that are observed in most reaction steps and the highly exergonic nature of the whole reaction chains. The channels lead to more than one product at the end, which brings in the concept of diversity in biomolecules formed from a single prebiotic route.