Elemental sulfur, hydrogen sulfide, and carbon monoxide-driven dimerization of glycine at sub-millimolar concentrations: Implications for the origin of life

The life’s origin in submarine hydrothermal systems is a long-standing scenario supported by diverse scientific disciplines, while verification of its chemical plausibility remains a major bottleneck. Most scenarios of hydrothermal origin postulate peptides as key players in prebiotic chemistry; nevertheless, it remains unknown how peptides were formed from geochemically available amino acids that must be low in concentration. Here we show that a simple mixing with elemental sulfur (S0), hydrogen sulfide (HS–), and carbon monoxide (CO) enables an effective dimerization of glycine (Gly) at micro to several millimolar concentrations. Incubation of 1 mM Gly with the three inorganic compounds in a warm alkaline solution (pH 9.3 and 35 or 50 °C) led up to 18% conversion of Gly to glycylglycine (GlyGly). Dimerization of 0.01 mM Gly was also discerned in the yield far exceeding the thermodynamic equilibrium (0.4% yield). In this reaction, CO is oxidized to CO2 concomitant with the reduction of S0 to polysulfides, serving as the driving force for the Gly-to-GlyGly conversion. Although the formation of CO2 as a byproduct suppressed the formation of GlyGly, its adverse effect was mitigated by carbonate precipitation upon adding Mg2+ and Ca2+. To our knowledge, 0.01 mM is the lowest concentration of amino acid oligomerized experimentally in the prebiotic context. Given the ubiquity of S0, HS–, and CO in primordial ocean hydrothermal systems, and the abundance of Mg2+ and Ca2+ in seawater, our demonstrated favorable condition for amino acid dimerization is likely to have occurred on the Hadean seafloor. This possibility supports an abiotic appearance of peptides and thereby facilitates the hydrothermal origin of life.