Experimental and Theoretical Constraints on Amino Acid Formation from PAHs in Asteroidal Settings

Abstract

Amino acids and polycyclic aromatic hydrocarbons (PAHs) belong to the range of organic compounds detected in meteorites. In this study, we tested empirically and theoretically if PAHs are precursors for amino acids in carbonaceous chondrites, as previously suggested. We conducted experiments to synthesize amino acids from fluoranthene (PAH), with ammonium bicarbonate as a source for ammonia and carbon dioxide under mimicked asteroidal conditions. In our thermodynamic calculations, we extended our analysis to additional PAH-amino acid combinations. We explored 36 reactions involving the PAHs naphthalene, anthracene, fluoranthene, pyrene, triphenylene, and coronene and the amino acids glycine, alanine, valine, leucine, phenylalanine, and tyrosine. Our experiments do not show the formation of amino acids, whereas our theoretical results hint that PAHs could be precursors of amino acids in carbonaceous chondrites at low temperatures.

Figure 1

Calculated CO₂–NH₃–H₂O system for the initial concentration of ammonium bicarbonate of 3.79 × 10^–2 M at 20 °C. With subsequent increasing temperature, pH and dissolved gas concentrations evolve.

Figure 2

Gibbs free energy of reaction ΔG_R as a function of temperature for the reaction forming glycine (GLY) and alanine (ALA) formation by fluoranthene (FLUO). The gray lines represent amino acid formation (Strecker synthesis) with formaldehyde. (a) glycine formation; (b) alanine formation, (c) valine formation, (d) leucine formation, (e) phenylalanine formation, and (f) tyrosine formation (see reactions and ΔG_R values in Table S4). The black dashed line indicates the transition to an exergonic reaction (ΔG_R < 0).

Figure 3

Calculated amino acid concentrations per species for all 36 reactions (Table 4) at 150 °C, sorted by amino acid molecule size. The observed trends are similar from 25 to 150 °C. Only reactions leading to the formation of alanine result in a concentration above the analytical detection limit of 1 × 10^–8 M (dashed line). NAPH = naphthalene, ANTH = anthracene, FLUO = fluoranthene, PYR = pyrene, TRI = triphenylene, CORO = coronene; GLY = glycine, ALA = alanine, VAL = valine, LEU = leucine, PHE = phenylalanine, TYR = tyrosine

Figure 4

Amino acid concentrations as a function of temperature. The concentrations are calculated with 15 ppm of PAH, 106 ppm of CO₂, and 19 ppm of NH₃ as representative values for carbonaceous chondrites. The indicated detection limit is from the AccQ•Tag method used and serves as a guide as to what would be possible to measure. GLY = glycine, ALA = alanine, VAL = valine, LEU = leucine, PHE = phenylalanine, and TYR = tyrosine.