Radical Pathways for the Prebiotic Formation of Pyrimidine Bases from Formamide

The prebiotic formation of nucleobases, the building blocks of RNA/DNA, is of current interest. Highly reactive radical species present in the atmosphere under irradiation have been suggested to be involved in the prebiotic synthesis of nucleobases from formamide (FM). We studied several free radical reaction pathways for the synthesis of pyrimidine bases (cytosine, uracil, and thymine) from FM under cold conditions. These pathways are theoretically determined using density functional theory (DFT) computations to examine their kinetic and thermodynamic feasibilities. These free radical reaction pathways share some common reaction types such as H-rearrangement, (•)H/(•)OH/(•)NH2 radical loss, and intramolecular radical cyclization. The rate-determining steps in these pathways are characterized with low energy barriers. The energy barriers of the ring formation steps are in the range of 3-7 kcal/mol. Although DFT methods are known to significantly underestimate the barriers for addition of (•)H radical to neutral species, many of these reactions are highly exergonic with energy release of -15 to -52 kcal/mol and are thus favorable. Among the suggested pathways for formation of cytosine (main route, routes 7a and 1a), uracil (main route, routes 7b and 1b), and thymine (main route and route 26a), the main routes are in general thermodynamically more exergonic and more kinetically favored than other alternative routes with lower overall energy barriers. The reaction energies released following formation of cytosine, uracil, and thymine from FM via the main radical routes amount to -59, -81, and -104 kcal/mol, respectively. Increasing temperature induces unfavorable changes in both kinetic and thermodynamic aspects of the suggested routes. However, the main routes are still more favored than the alternative pathways at the temperature up to the boiling point of FM.