Prebiotic synthesis of methionine and other sulfur-containing organic compounds on the primitive Earth: a contemporary reassessment based on an unpublished 1958 Stanley Miller experiment

Abstract

Original extracts from an unpublished 1958 experiment conducted by the late Stanley L. Miller were recently found and analyzed using modern state-of-the-art analytical methods. The extracts were produced by the action of an electric discharge on a mixture of methane (CH(4)), hydrogen sulfide (H(2)S), ammonia (NH(3)), and carbon dioxide (CO(2)). Racemic methionine was formed in significant yields, together with other sulfur-bearing organic compounds. The formation of methionine and other compounds from a model prebiotic atmosphere that contained H(2)S suggests that this type of synthesis is robust under reducing conditions, which may have existed either in the global primitive atmosphere or in localized volcanic environments on the early Earth. The presence of a wide array of sulfur-containing organic compounds produced by the decomposition of methionine and cysteine indicates that in addition to abiotic synthetic processes, degradation of organic compounds on the primordial Earth could have been important in diversifying the inventory of molecules of biochemical significance not readily formed from other abiotic reactions, or derived from extraterrestrial delivery.

Fig. 1

Sample chromatogram and mass spectra traces for 6 sulfur compounds detected in Miller’s original sample extracts. All chromatogram traces displayed resulted from selective ion monitoring analysis except the methionine chromatogram trace, which was produced by HPLC-UV analysis. The chromatograms obtained by selective ion monitoring are plotted as signal intensity versus time, whereas the methionine chromatogram obtained by HPLC-UV is plotted as fluorescence sensitivity versus time. In each chromatogram, the asterisk demarcates the detection of the species in question. The mass spectra traces that accompany each chromatogram were obtained using ToF-MS analysis and are plotted as spectral intensity versus mass. Mass spectra traces were used to verify the sulfur distribution of the organosulfur species identified during selective ion monitoring. In all cases, the bottom mass spectra trace is the standard trace and the top mass spectra trace is the experimental trace. In each mass spectra trace, the underlined mass is the parent mass in question. Note: RT is retention time and MA is methylamine

Fig. 2

Moles (relative to glycine = 1) of the various sulfur compounds detected in vials of dried residues obtained from the sparking of a CH₄, H₂S, NH₃ and CO₂ gas mixture

Fig. 3

Prebiotic synthesis of methionine, methionine sulfoxide, methionine sulfone, ethionine, and homocysteic acid in the presence of acrolein, which is based in part on the scheme proposed by Van Trump and Miller (1972). Asterisks denote species that were detected in this study

Fig. 4

Two possible mechanisms for the prebiotic synthesis of cysteine from glycine via serine or serine hydantoin, which would form dehydroalanine or its hydantoin. Reaction of the latter intermediates with H₂S would yield cysteine derivatives. Asterisks represent sulfur-containing compounds detected in this study