Formation and elongation of polyglycine via unimolecular reaction in the gas phase
Résumé
Peptide chains of amino acids play a crucial role in the emergence of living organisms. Understanding the growth of peptide chains under abiotic conditions is an important question, especially considering the increasing amount of complex molecules detected in various astrophysical environments [1]. Gas-phase collisions between isolated molecules or cosmic ray- induced reactions in seeded ice and dust grains are presently the main pathways considered for polymer synthesis under astrophysical conditions. Recently, we found that the growth of peptide chains can occur under gas-phase conditions via a dehydration reaction in a protonated molecular amino acid dimer ion [2]. Combining experimental results and quantum chemistry calculations, we show that polymerization reaction leading to the peptide of glycine (diglycine) is taking place after excitation of a protonated glycine dimer in a single high velocity collision, and we provide evidence the further growth of the peptide chain. The formation of a protonated mixed dimer consisting of a diglycine and a glycine molecule is dominant and its further excitation can lead to the formation of a tripeptide. The various relaxation pathways of the excited protonated dimers and the unimolecular reaction dynamics is investigated by velocity measurements of the water molecules eliminated in the polymerization reactions. The proton facilitates the dehydration reaction in a pure molecular dimer and chaperons the further growth of the peptide-chain via further dehydration reactions in mixed dimer and cluster ions. These results obtained with glycine, the most abundant amino acid detected in extraterrestrial objects, shows that the detected intra-dimer dehydration reaction could be a conceivable abiotic pathway towards the possible existence of peptide chains in Space
