Abstract
Discovering the chemical pathways by which biological molecules, such as amino acids, can spontaneously form is important for understanding the origin of life on Earth. Samples recovered from asteroids, meteorites and comets have been shown to contain most proteinogenic amino acids. However, no confirmed detections of interstellar amino acids have been made from radioastronomy. Different regions of interstellar space have different reservoirs of molecular species for reactions, as well as surfaces on which these reactions may occur. For biological molecules to be found inside meteorites, they must be produced after the primordial diffuse cloud stage, where their contents are degraded by harsh, high-energy cosmic rays, but before the meteorite bodies are formed from the aggregation of grains and pebbles during the early solar system. Between these periods of the stellar evolution cycle are dense molecular clouds and protostars, regions dominated by water ice and mineral grain surfaces, respectively.
This study investigated whether olivine ((Mg,Fe)2SiO4, a common interstellar mineral) or water ice surfaces are more effective at producing amino acids, specifically glycine (HCOO-CH2-NH2), the simplest amino acid. The reactants that were selected to form glycine were CHxNHy (x=0-3, y=0-2) and (H)COO(H). These motifs were selected as they represent the hydrogenation products from the glycine backbone fragments CN and CO2 after C-C bond rupture.
This study was performed using both computational and experimental methods. Once methodological parameters were validated, the computational work began by investigating the adsorption of the different species using density functional theory (DFT). The adsorption studies identified that the olivine surfaces prefer to bind adsorbates through strong covalent bonds (chemisorption), while the ice surfaces tended to have longer adsorption distances and weaker binding that were more representative of intermolecular bonds (physisorption).
The hydrogenation of the species from CN to CH3NH2 and CO2 to HCOOH were then studied to understand which specific species would be most likely to be present on the surface and available as precursors for surface chemistry leading to glycine. It was found that hydrogenation of the CN motif on olivine was facile for the first few hydrogenation steps, but had significant activation barriers that allowed for both radical and open-shell species on the hydrogenation pathway (CH2NH, CH2NH2, CH3NH, and CH3NH2) to remain on the surface to combine with COO-containing precursors. CN hydrogenation on water ice, however, was more likely to progress all the way to CH3NH2 without build-up of intermediate species.
Reactions between the COO- and CN-containing species to form glycine and unsaturated glycine derivatives were then investigated. Viable reactions involving HCOOH and CH3NH2 could not be found on either surface, likely due to the kinetic barriers in removing a hydrogen from the CN carbon atom in order for C-C coupling to occur. Three reactions were able to proceed on olivine (CH2NH, CH2NH2, CH3NH + CO2), all exothermic, with the highest barrier found being at +0.4 eV. Excluding HCOOH and CH3NH2, all reactions investigated were found to be able to be catalysed by olivine, demonstrating its viability for producing interstellar glycine. In contrast, only one reaction on the ice surface was located with surmountable barriers (CH2NH2 + CO2), but was endothermic with a low barrier to breaking the newly formed bond.
Laboratory simulations of these glycine formation pathways were performed only on water ice, as olivine surfaces would not allow for sufficient transmission of IR beams for spectroscopic monitoring. While no glycine signals were identified from coadsorbing CO2 with CH3NH2 with surface chemistry initiated via electron beam irradiation (a proxy for cosmic ray driven processes), there were C≡N signals present. This indicates that water ice efficiently cycles CN species back to the unsaturated components HCN and HNC (i.e. H2O facilitates H abstraction), while the computational studies indicate that the olivine surface is more suitable for producing glycine (or its derivatives).