Synthesis of RNA nucleobases from formamide is one of the recurring topics of prebiotic chemistry research. Earlier reports suggest that thymine, the substitute for uracil in DNA, may also be synthesized from formamide in the presence of catalysts enabling conversion of formamide to formaldehyde. In the current paper, we show that to a lesser extent conversion of uracil to thymine may occur even in the absence of catalysts. This is enabled by the presence of formic acid in the reaction mixture that forms as the hydrolysis product of formamide. Under the reaction conditions of our study, the disproportionation of formic acid may produce formaldehyde that hydroxymethylates uracil in the first step of the conversion process. The experiments are supplemented by quantum chemical modeling of the reaction pathway, supporting the plausibility of the mechanism suggested by Saladino and coworkers.

Central European Institute of Technology Brno University of Technology Purkynova 123 CZ 61200 Brno Czech Republic

Department of Chemistry and Biochemistry Mendel University in Brno Zemědělská 1 CZ 61300 Brno Czech Republic

Institute for Chemical Physical Processes Italian National Research Council Viale Ferdinando Stagno d'Alcontres 37 98158 Messina Italy

Institute of Biophysics of the Czech Academy of Sciences Královopolská 135 CZ 61265 Brno Czech Republic

J Heyrovský Institute of Physical Chemistry Czech Academy of Sciences Dolejškova 3 CZ 18223 Prague 8 Czech Republic

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