Nonequilibrium Formation of Pyridine (C5H5N) in Interstellar Ice Analogs: Implications for Biorelevant Nitrogen Heterocycles in Ryugu and Bennu

Abstract Biorelevant nitrogen heterocycles, including the five canonical nucleobases found in RNA and DNA, have recently been identified in samples returned from the carbonaceous asteroids Ryugu and Bennu, supporting their potential exogenous delivery to the early Earth. Although pyridine (C5H5N), the simplest aromatic nitrogen heterocycle, has long been proposed as a key precursor to more complex nitrogen heterocycles, including nucleobases, its formation under astrophysical conditions remains poorly understood. Here, we report the first synthesis of pyridine in low-temperature acetylene–hydrogen cyanide (C2H2–HCN) interstellar ice analogs exposed to energetic electrons as proxies for galactic cosmic rays. Pyridine was identified isomer-selectively in the gas phase by vacuum ultraviolet photoionization reflectron time-of-flight mass spectrometry (ReToF-MS) and resonance-enhanced multiphoton ionization spectroscopy (REMPI). These results fundamentally change our understanding of how aromatic nitrogen heterocycles can emerge from simple interstellar ice constituents under galactic cosmic ray irradiation. As a plausible precursor to more complex prebiotic nitrogen heterocycles, pyridine provides a direct chemical link between interstellar ice chemistry and the molecular inventory detected in Ryugu and Bennu, offering new insight into the chemical evolution of prebiotic organic matter in interstellar environments and its inheritance by primitive Solar System bodies.