Synthesis of glycine-containing complexes in impacts of comets on early Earth

Journal name:
Nature Chemistry
Volume:
2,
Pages:
949–954
Year published:
DOI:
doi:10.1038/nchem.827
Received
Accepted
Published online

Abstract

Delivery of prebiotic compounds to early Earth from an impacting comet is thought to be an unlikely mechanism for the origins of life because of unfavourable chemical conditions on the planet and the high heat from impact. In contrast, we find that impact-induced shock compression of cometary ices followed by expansion to ambient conditions can produce complexes that resemble the amino acid glycine. Our ab initio molecular dynamics simulations show that shock waves drive the synthesis of transient C–N bonded oligomers at extreme pressures and temperatures. On post impact quenching to lower pressures, the oligomers break apart to form a metastable glycine-containing complex. We show that impact from cometary ice could possibly yield amino acids by a synthetic route independent of the pre-existing atmospheric conditions and materials on the planet.

At a glance

Figures

  1. The cumulative probability of impact for our simulated shock velocities.
    Figure 1: The cumulative probability of impact for our simulated shock velocities.

    The computed probability corresponds to the chance of impact from zero up to a given particle velocity, which is related to the shock velocity as shown in the Methods section. A comet that travels at a velocity of 29 km s−1 would have to impact a planet at an angle of 8° to achieve a shock velocity of 5 km s−1, and at 24° for 10 km s−1. For a shock velocity of 10 km s−1, the impact angle distribution model of Shoemaker35 gives a cumulative probability of 17%. Shock velocities from previous studies13, 20 correspond to probabilities of impact of about 0.5% and 2%, respectively. Inset: computed impact angles for our simulated shock velocities.

  2. Snapshots of the computational cell.
    Figure 2: Snapshots of the computational cell.

    a,b At the initial conditions (a) and during shock compression at 9 km s−1 (47 GPa) (b). For all of our simulation snapshots, H2O molecules are coloured green, NH3 black, CO light blue, CO2 purple and CH3OH orange. For the atoms, oxygen is red, hydrogen is white, carbon is light blue and nitrogen is dark blue. In (b) the smaller size of the simulation cell results from the shock compression. For clarity we show only atomic sites for C–N bonded species (opaque) formed during shock compression. All other species (transparent) are shown by their bonds only, excluding H+ ions, which are shown as yellow spheres. After shock compression, all of the CH3OH and CO2 are consumed. A small number of H2O and NH3 molecules are still observed, as well as a single CO molecule. Shock compression caused several exotic species to form, including the large carbon chain-like oligomer with several C–N bonds shown in the middle of the snapshot.

  3. Simulated mass spectra of the C–N bonded molecules found during our simulations.
    Figure 3: Simulated mass spectra of the C–N bonded molecules found during our simulations.

    a, During shock compression to 47 GPa (9 km s−1). Chemical formulae are provided for selected peaks, and graphics are shown for the more exotic species. At 47 GPa, numerous C–N oligomers with mass peaks over 300 AMU had lifetimes less than our lifetime criterion (50 fs) and are not included in the plot. b, After quenching both temperature and pressure, we observed significant quantities of a glycineCO2 complex at a mass peak of 118 AMU.

  4. Mechanism for glycine–CO2 complex synthesis on expansion and cooling.
    Figure 4: Mechanism for glycine–CO2 complex synthesis on expansion and cooling.

    a, The high pressures and temperatures from shock compression (47 GPa (9 km s−1)) caused a large C–N bonded oligomer to form. In this species, we observed a sequence of carbon and nitrogen atoms that corresponded to that of glycine (only the atomic sites that eventually form the complex OCO–NH–CH2–COOH are shown here and in (b) and (c)). b, During isentropic expansion (16.6 GPa, 2,673 K), the large C–N bonded chain in (a) broke apart to form several fragments. The C–N sequence that corresponds to glycine remained intact during the expansion. A significant quantity of H+ ions remained in the system, although we also observed an increase in the number of H2O molecules. c, On cooling to the initial conditions (0.5 GPa, 300 K), we observed the formation of several stable and metastable C–N bonded species, including HCN, NH2–COOH and a more complex molecule with several C–N bonds. A single CO2 molecule was observed also. Smaller moieties, such as H and OH, are eliminated from the C–N backbone that forms OCO–NH–CH2–COOH. This species can react with a proton source, such as H3O+, to form glycine.

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Affiliations

  1. Physical and Life Sciences Directorate, Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, California 94550, USA

    • Nir Goldman,
    • Evan J. Reed,
    • Laurence E. Fried,
    • I.-F. William Kuo &
    • Amitesh Maiti
  2. Present address: Department of Materials Science and Engineering, Stanford University, Stanford, California 94304, USA

    • Evan J. Reed

Contributions

N.G. originated the central idea for this work and performed and analysed the simulations. E.J.R. is lead author for the MSST algorithm and helped write the paper. L.E.F. co-created the MSST algorithm, wrote the molecular analyser code and helped write this paper. I.W.K. helped with the initial equilibration simulations. A.M. performed the Gibbs free energy of reaction calculations.

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