Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment

Abstract

Archived samples from a previously unreported 1958 Stanley Miller electric discharge experiment containing hydrogen sulfide (H(2)S) were recently discovered and analyzed using high-performance liquid chromatography and time-of-flight mass spectrometry. We report here the detection and quantification of primary amine-containing compounds in the original sample residues, which were produced via spark discharge using a gaseous mixture of H(2)S, CH(4), NH(3), and CO(2). A total of 23 amino acids and 4 amines, including 7 organosulfur compounds, were detected in these samples. The major amino acids with chiral centers are racemic within the accuracy of the measurements, indicating that they are not contaminants introduced during sample storage. This experiment marks the first synthesis of sulfur amino acids from spark discharge experiments designed to imitate primordial environments. The relative yield of some amino acids, in particular the isomers of aminobutyric acid, are the highest ever found in a spark discharge experiment. The simulated primordial conditions used by Miller may serve as a model for early volcanic plume chemistry and provide insight to the possible roles such plumes may have played in abiotic organic synthesis. Additionally, the overall abundances of the synthesized amino acids in the presence of H(2)S are very similar to the abundances found in some carbonaceous meteorites, suggesting that H(2)S may have played an important role in prebiotic reactions in early solar system environments.

Fig. 1.

Five- to 33-min HPLC-UVFD chromatograms of 1-min OPA/NAC derivatized aliquots of: (A) original H₂S spark discharge experimental samples from Stanley Miller’s archived collection, (B) a composite amino acid and amine standard trace, and (C) a reagent blank. Amino acids were identified based on retention times compared to standard runs. Peak identifications: 1 = D,L-Asp; 2 = L,D-Glu; 3 = D,L-Ser; 4 = D,L-Isoser; 5 = Gly; 6 = β-Ala; 7 = γ-ABA; 8 = β-AIB; 9 = D-Ala; 10 = L-Ala + D-β-ABA; 11 = L-β-ABA; 12 = α-AIB; 13 = ethanolamine; 14 = D,L-α-ABA; 15 = D,L-Isoval; 16 = L,D-Met; 17 = methylamine; 18 = ethylamine. The chromatogram displayed in A is a composite of several sample chromatograms of varying dilutions and is intended to demonstrate the diversity of products that were detected in these spark discharge residues. Thus, peak height/areas are not indicative of relative abundances. The composite trace (A) was created from seven separate runs labeled on the figure with their attenuations given (relative to glycine) and correspond to Miller’s labeled residues as follows: (i) 85–91, (ii) 63–67, (iii) 73–78, (iv) 98–105, (v) 79–84, (vi) 107–111, and (vii) 56–61. Those peaks in trace A that have been attenuated are marked with an asterisk above the number of the peak in question. The standard composite trace B was created from four standard runs as shown below the trace. A procedural blank for comparison was not found in the sample set saved by Miller, so trace C is a laboratory analytical solvent blank.

Fig. 2.

Molar ratios (relative to glycine = 1) of amino acids and amines detected using UPLC-UVFD/ToF-MS. Sulfur-containing organic compounds are marked with an asterisk.

Fig. 3.

Comparison of amino acid and amine molar ratios (relative to glycine = 1) found in Miller’s classic, volcanic, silent, and H₂S electric discharge experiments. The H₂S electric discharge data were obtained using UPLC-UVFD/ToF-MS. Data from the classic, volcanic, and silent discharge experiments were reported in ref. . Sulfur-containing compounds are not compared here because the classic, volcanic, and silent discharge experiments did not contain sulfur. Asterisks indicate compounds that are at least one order of magnitude more abundant relative to glycine in the H₂S experimental samples than in all three other experiments. α-ABA was not detected in the silent discharge experiment. ETN, ethanolamine.

Fig. 4.

Comparison of amino acid molar ratios (relative to glycine = 1) found in Miller’s H₂S and classic spark discharge experiments, and the Murchison USNM 5453, Murchison USNM 6650, and the LON 94102 meteorites. Data from the two Murchison meteorite samples were averaged and are presented here as “Murchison Average”. The H₂S spark discharge experimental data were obtained in this study as reported above. The classic spark discharge experiment data were reported in ref.  and used similar analytical techniques as those used in this study. Literature data were used for the Murchison USNM 5453 (33), Murchison USNM 6650 (34, 35), and LON 94102 (33, 35) CC samples. Sulfur amino acids are not compared here because they were not detected in the CCs used for comparison. β-AIB, ethanolamine, and serine are not included in the comparison because serine was not detected in the Murchison USNM 5453 meteorite sample, and because both ethanolamine and β-AIB were not detected in any of the meteorites used in the comparison. The yield of β-AIB in the discharge experiments is lower than that of α-AIB and the α-, β-, and γ- isomers of aminobutyric acid (Fig. 2), which is consistent with its absence in the meteorite samples.