2018MNRAS.474.2796Q

Peptide bonds (N-C = O) play a key role in metabolic processes since they link amino acids into peptide chains or proteins. Recently, several molecules containing peptide-like bonds have been detected across multiple environments in the interstellar medium, growing the need to fully understand their chemistry and their role in forming larger pre-biotic molecules. We present a comprehensive study of the chemistry of three molecules containing peptide-like bonds: HNCO, NH₂CHO, and CH₃NCO. We also included other CHNO isomers (HCNO, HOCN) and C₂H₃NO isomers (CH₃OCN, CH₃CNO) to the study. We have used the UCLCHEM gas-grain chemical code and included in our chemical network all possible formation/destruction pathways of these peptide-like molecules recently investigated either by theoretical calculations or in laboratory experiments. Our predictions are compared to observations obtained towards the proto-star IRAS 16293-2422 and the L1544 pre-stellar core. Our results show that some key reactions involving the CHNO and C₂H₃NO isomers need to be modified to match the observations. Consistently with recent laboratory findings, hydrogenation is unlikely to produce NH₂CHO on grain surfaces, while a combination of radical-radical surface reactions and gas-phase reactions is a better alternative. In addition, better results are obtained for NH₂CHO when a slightly higher activation energy of 25 K is considered for the gas-phase reaction NH₂ + H₂CO - NH₂CHO + H. Finally, our modelling shows that the observed correlation between NH₂CHO and HNCO in star-forming regions may come from the fact that HNCO and NH₂CHO react to temperature in the same manner rather than from a direct chemical link between the two species.