Formation of nucleobases in a Miller-Urey reducing atmosphere

Abstract

The Miller-Urey experiments pioneered modern research on the molecular origins of life, but their actual relevance in this field was later questioned because the gas mixture used in their research is considered too reducing with respect to the most accepted hypotheses for the conditions on primordial Earth. In particular, the production of only amino acids has been taken as evidence of the limited relevance of the results. Here, we report an experimental work, combined with state-of-the-art computational methods, in which both electric discharge and laser-driven plasma impact simulations were carried out in a reducing atmosphere containing NH₃ + CO. We show that RNA nucleobases are synthesized in these experiments, strongly supporting the possibility of the emergence of biologically relevant molecules in a reducing atmosphere. The reconstructed synthetic pathways indicate that small radicals and formamide play a crucial role, in agreement with a number of recent experimental and theoretical results.

Keywords: asteroid impact; life origins; reducing atmosphere.

Fig. 1.

Data published in ref. on the oxidation state of trace elements in early zircons (part A, blue circles), which are compared with the timescale of impact flux on early Earth (part B, violet curve; EHB and LHB are Early and Late Heavy Bombardment periods), and the age of carbon inclusions exhibiting ¹³C deficiency consistent with their biogenic origin (part C, red points). Data from refs. , , , and , and references therein.

Fig. 2.

Emission spectrum of a reducing atmosphere (NH₃, CO, H₂O) discharge plasma. Among others, carbon monoxide and the ·CN radical in a wide range of energetic states dominate the spectra.

Fig. 3.

Absorption spectra of a reducing atmosphere (NH₃, CO, H₂O) discharge plasma.

Fig. S1.

A portion of absorption spectrum in the region of a carbonyl vibration shows the absorption v₄ band of formamide covered with very strong series of water lines. The formamide reference spectrum is shown in red.

Fig. S2.

Absorption spectra of ammonia and carbon monoxide with treatment by an LIDB plasma shock wave. A shows the blank before the irradiation, B supplies the spectra recorded after 15 pulses of the laser without catalytic material, C depicts the spectra after 15 laser pulses in presence of montmorillonite, and D shows spectra of products of nucleobase treatment in discharge plasma.

Fig. S3.

GC-MS detection of nucleic bases in all of the experiments. All samples were analyzed after derivatization by with MTBSTFA. A shows the chromatogram of a blank of the canonical nucleic bases, B shows the chromatogram of the experiment with irradiation of the mixture of NH₃ + CO, C depicts chromatogram of the experiment with NH₃ + CO + H₂O in presence of clay, and D shows the discharge decomposition of NH₃ + CO + H₂O. E and F depict, respectively, chromatograms of adenine and cytosine glow discharge decomposition upon nitrogen atmosphere in presence of water vapor.

Fig. S4.

The typical mass spectra received using GC-MS for nucleic bases and glycine after derivatization. All the mass spectra are observed exlusively in typical retention time of each particular molecule as shown in Fig. S3 [for nucleobases; Fig. S3A for standards and Fig. S3 B–F for experimental samples) and in the Fig. S5 (for glycine; Fig. S5A for standard and Fig. S5 B–F for experimental samples). A depicts mass spectrum of uracil standard with the typical fragment m/z = 283 compared with uracil mass spectra discovered in the experimental sample; B shows comparison of experimental spectra with adenine-1 standard with the typical fragment m/z = 192; C carries out the similar comparison for cytosine with the typical fragment m/z = 282; D depicts this comparison for the adenine-2 mass spectrum with typical m/z = 306; E depicts this comparison for guanine with typical fragment m/z = 322; and F shows glycine with fragments m/z = 147, 226, and 246. The corresponding structures of typical mass fragments are depicted in Fig. S6.

Fig. S5.

GC-MS detection of the simplest amino acid glycine in all of the experiments. All samples were analyzed after derivatization by with MTBSTFA. A shows the chromatogram of a blank of the glycine standard, B shows the chromatogram of the experiment with irradiation of the mixture of NH₃ + CO, C depicts chromatogram of the experiment with NH₃ + CO + H₂O in presence of clay, and D shows the discharge decomposition of NH₃ + CO + H₂O. E and F depict, respectively, chromatograms of adenine and cytosine glow discharge decomposition upon nitrogen atmosphere in presence of water vapor.

Fig. S6.

Structure and typical masses of main fragments of the canonical nucleic bases and glycine after derivatization. Corresponding chromatograms and mass spectra are presented in the Figs. S3–S5.

Fig. S7.

Chromatograms of blank measurements, all of them with a silylation agent, together with a chromatogram of NH₃ + CO + H₂O discharge products for comparison depicted in A. Nucleobases are observed on the parts per million level in this experiment. B and C show a chromatographic record of a cell for laser shock wave experiment not exposed to plasma and discharge cell, respectively, both washed after 1 d with water; D shows chromatogram of a cell touched with finger without gloves; and E shows a record of water after washing of montmorillonite catalyst. Numbers 1–9 mark manifold products of mutual reactions in derivatization agent.

Fig. 4.

Reaction mechanisms and free-energy profiles (at 650 K in blue and at 4500 K in red; values in kilocalories per mole) from metadynamics and umbrella sampling simulations, for binary reactions between simple Miller-like molecules, leading to formamide (A) and formic acid (B).

Fig. 5.

Reaction mechanisms and free-energy profiles (at 650 K in blue and at 4500 K in red; values in kilocalories per mole) from metadynamics and umbrella sampling simulations, starting from formamide (A) or from formic acid and ammonia (B).

Fig. 6.

Summary of the chemistry in our model reducing atmosphere (NH₃, CO). A reducing atmosphere is generated by impact delivery and degassing as proposed in ref. . Upon exposure to shock waves and discharges, formamide is synthesized as a reactive intermediate (A). In subsequent chemistry described in our previous work (19, 66), reactive molecules are produced by formamide decomposition (B), and nucleobases are synthesized (C) in subsequent steps from 2,3-diaminofumaronitrile (DAFN) and 4-amino-5-cyanoimidazole (AlCN).