2021MNRAS.508.3009Y

As an important class of carbon reservoirs in the interstellar medium (ISM), polycyclic aromatic hydrocarbon (PAH) molecules play an important role in the evolutionary network of prebiotic molecules. Here, the experimental evidence of astronomically relevant amino-acid derivatives - PAH/amino-acid clusters - is provided, and we study their ion-molecular collision reactions in the gas phase. With the initial molecular precursors dicoronylene (DC, C₄₈H₂₀)/alanine (Ala, C₃H₇NO₂) and DC/isoleucine (Ile, C₆H₁₃NO₂), the experiments indicate that PAH-amino-acid cluster cations (e.g. (Ala)C₄₈H_(0–19) ⁺ and (Ile)C₄₈H_(0–19) ⁺) and graphene-amino-acid cluster cations (e.g. (Ala)_nC₄₈⁺ and (Ile)_nC₄₈⁺, n = 1, 2, 3, 4) are efficiently formed in a strong interstellar radiation field. In addition, the structure of clusters and the binding energy of their formation pathways are studied by a quantum chemistry calculation method: gas-phase reactions (ion-molecule reactions) between PAH cations with amino acids readily occur (exothermic energy around 2.0-4.7 eV), and these newly formed clusters have a complex molecular configuration (C-O and C-N bond type). These laboratory studies provide a cluster growth pathway (through an ion-molecule reaction) towards the formation of amino-acid derivatives in a bottom-up process and insight into their chemical-evolution behaviour, opening up aromatic-based chemistry that is available to the species (dehydrogenated PAHs or graphene molecules) that formed from the photofragmentation process of PAHs in interstellar environments. When conditions are suitable (e.g. have similar molecular abundance spatial distributions in the ISM), amino-acid derivatives can form efficiently, and newly built large PAH/amino-acid clusters may be widespread in space.