Pathways to Glycine and Other Amino Acids in Ultraviolet-irradiated Astrophysical Ices Determined via Quantum Chemical Modeling

A recent experimental study reported that glycine and other amino acids were formed when cryogenic H₂O ice containing small amounts of CH₃OH, NH₃, and HCN was subjected to ultraviolet (UV) irradiation. Quantum chemical calculations were employed to evaluate the viability of various pathways to the formation of glycine, alanine, and serine in dilute H₂O ice containing CH₃OH and HCN. Under the experimental processing conditions of deposition and UV irradiation at 15 K followed by heating to room temperature, amino acids can form by recombining radicals produced by dehydrogenating H₂O and CH₃OH and subsequently hydrogenating HCN. The study indicates that isotopic substitution experiments would identify CH₃OH as the source of the C atom in the COOH carboxylic acid group of the amino acids observed in the irradiation experiments, with the CO+OH reaction playing an important role. The remaining C and N atoms in glycine are predicted to originate from HCN via sequential hydrogenation to yield CH₂NH₂. Formation pathways for alanine and serine are also discussed.