Novel Chemical Pathways for the Formation of Nucleobase Precursors via Benzene {\pi}-Bond Addition to HCN

TL;DR

This study introduces a computationally supported pathway for forming nucleobase precursors from benzene and HCN, relevant to prebiotic chemistry on early Earth and Mars, highlighting potential routes for organic synthesis in extraterrestrial environments.

Contribution

It proposes a novel nitrogen insertion mechanism into benzene via HCN cycloaddition and C2H2 fragmentation, supported by quantum chemistry calculations, expanding understanding of prebiotic organic synthesis pathways.

Findings

Pathway enables pyrimidine formation from benzene and HCN.

Photochemistry at ocean surfaces or impact events could facilitate organics synthesis.

Model suggests organics could be transported and concentrated in aqueous environments on Mars.

Abstract

We propose a simple and efficient pathway for the formation of precursors to core nucleobases in DNA and RNA using a suite of computational chemistry methods. Benzene, which is thermochemically stable in N2- or CO2-dominated atmospheres, could have formed via upper-atmospheric photochemistry or surface lightning and accumulated on the early Earth or Mars. However, nitrogen insertion into the benzene ring to form pyrimidine and purine is widely considered to be challenging. We propose that nitrogen incorporation occurred through HCN 1,4-cycloaddition to benzene's {\pi}-system, followed by a C2H2 fragmentation mechanism, as confirmed by quantum chemistry calculations. This pathway, potentially facilitated by photochemistry at the ocean surface or episodic impact events on local reservoirs, can lead to pyrimidine formation, which can further react with NH3 and HCN to produce purine. Extending this pathway to early Mars, our photochemical model simulates heterocyclic compound formation under cold, dry surface conditions that favor high benzene and HCN concentrations but lack liquid water. We thus propose that organics formed during dry phases may have later dissolved into surface waters during wet phases and become concentrated as ocean sediments. This result supports Mars Sample Return efforts focused on ancient aqueous environments likely to retain prebiotic signatures.