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Reaction Routes for Experimentally Observed Intermediates in the Prebiotic Formation of Nucleobases under High-Temperature Conditions
The
prebiotic synthesis of nucleobases is of particular interest,
given the experimental evidence that indicated formation of the nucleobases
under abiotic conditions on the Early Earth under high-temperature
conditions. Biomolecules have been formed under meteoritic impact
scenarios that lead to high temperature and the generation of high
energy. Free radical pathways for the formation of biomolecules are
appropriate under these conditions. Density functional theory computations
were used to study the free radical routes for the formation of nucleobases
at the UB3LYP/6-311G(d,p) level. We have found that both 5-aminoimidazole-4-carboxamide
(AICA) and 5-(formylamino)imidazole-4-carboxamide (fAICA) are formed
first from formamide then the nucleobases are formed. Calculated results
show the radical reaction routes of AICA as a precursor for guanine.
Both hypoxanthine and xanthine are formed from radical pathways of
fAICA. In addition, generation of imino-AICA and imino-fAICA has been
shown for the first time to be needed for the production of adenine,
purine, and isoguanine. Formation of hypoxanthine and adenine/purine
from fAICA and imino-fAICA, respectively, is consistent with experiments
performed nearly seven decades ago