12. Hints from Computational Chemistry: Mechanisms of Transformations of Simple Species into Purine and Adenine by Feasible Abiotic Processes

The chemical evolution of biomolecules such as nucleobases and their analogues from simple, one carbon containing molecules under abiotic conditions is a puzzle closely connected to the origin of life. Theoretical elucidation of the abiotic reaction routes leading from basic molecules cyanide acid (HCN) and formamide (H₂NCHO) to the formation of purine and adenine is reviewed here. The mechanism of three pathways: from formamide dimer via pyrimidine to purine, from AICN (4-aminoimidazole-5-carboxamidine) to adenine, and from formamide to purine and adenine, are discussed. Based on the comparison of step-by-step mechanism of the reaction pathways, in the addition reaction formamide is suggested to be more reactive than HCN. Beside its simplicity, the formamide self-catalyzed mechanism is energetically more viable than either water-catalyzed mechanism or non-catalyzed process. Moreover, this self-catalyzed mechanism is able to explain the ratio of purine to adenine observed in experiments. The formamide self-catalyzed mechanism for the route leading from formamide to purine and/or adenine is most likely for the formation of adenine (and purine) in the formamide solutions in the early stage of the earth.