How Does Adenine Form from Hydrogen Cyanide?

The abiotic formation of adenine from hydrogen cyanide (HCN) has long been suspected to be a key step in the origin of life. However, the inherent complexity of HCN's self-reaction chemistry has challenged researchers for decades, obscuring the detailed mechanistic pathway to adenine. In this study, we employ quantum chemistry and microkinetic modeling to predict and compare four interwoven base-catalyzed pathways to adenine in liquid HCN. Our analysis incorporates both previously proposed aminomalononitrile (AMN) and diaminomaleonitrile (DAMN) intermediates and reveals previously unknown reaction steps, including one in which polyimine can serve as an oxidizing agent. Our modeling offers compelling evidence of a complex, nonequilibrium interplay between these pathways and confirms DAMN as a necessary intermediate. This work establishes a foundational reference for the exploration of abiotic nucleobase formation and highlights how rigorous testing of origin-of-life chemistry pushes the boundaries of state-of-the-art computational chemistry.