[U-08] Biogeochemistry of CO world

[U08-P08] Examination of electrochemical carbon dioxide reduction activity of metal sulfide mineral (FeNi₂S₄) in the presence of amino acids

*Akira Yamaguchi^1,2 (1.Tokyo Institute of Technology, 2.RIKEN)

One possible scenario of the starting point for origin of life is that carbon fixation was catalyzed by metal sulfide minerals to produce organic compounds at the hydrothermal vent in deep sea floors. The carbon sources are considered to be carbon dioxide (CO₂)/carbon monoxide (CO), and a candidate of metal sulfides which contributed to CO₂ reduction is nickel (Ni)-doped greigite (Ni-Fe₃S₄) because of its structural similarity to the active center of extant CO₂ reduction enzyme (Carbon Monoxide Dehydrogenaze, CODH). In the conventional scenario, the electron source to reduce CO₂ was hydrogen (H₂), but recently, an alternative reducing power: electricity generated around hydrothermal vent, has been proposed (Nakamura et al., Angew. Chem. Int. Ed., 49, 7692, 2010). Thus, in this work, we investigated electrochemical CO₂ reduction activity of greigite to get insight concerning electricity-driven carbon fixation toward origin of life.
Greigite sample was synthesized with hydrothermal method using iron sulfate and L-cystine as precursors. Electrochemical CO₂ reduction experiment using greigite-deposited carbon paper as a working electrode demonstrated that Ni doping and amine modification (triethylamine or polyallylamine) improved the electrochemical CO₂ reduction activity of greigite to produce CO and methane (CH₄), while bare greigite was almost inactive (Yamaguchi et al., Electrochim. Acta, 141, 311, 2014). Based on X-ray diffraction pattern, the Ni-doped greigite sample was confirmed to be a mixture of nickel sulfide (NiS) and violarite (FeNi₂S₄). Indeed, violarite shares the similar structure with the active center of Acetyl-CoA Synthase (ACS), which is responsible to synthesizing acetyl-CoA with CO produced by CODH via CO₂ reduction. Based on this observation, we synthesized pure violarite and evaluated its electrochemical CO₂ reduction activity. The synthesis of violarite was conducted by a solvothermal decomposition at 230 °C of Fe- and Ni-dithiocarbamate complex in oleylamine as solvent and capping agent. The electrochemical results showed that violarite actually had CO₂ reduction activity and produced CO, CH₄ and formate (HCOOH). It should be noted that the CO₂ reduction activity of violarite was higher than that of pure greigite and NiS, indicating concerted effect between Fe and Ni contributed to the activity.
Furthermore, we investigated the effect of amino acid, especially histidine (C₆H₉N₃O₂), on CO₂ reduction activity of violarite inspired by the active center of CODH, where Fe₄NiS₅ cluster are surrounded by histidine residue. Actually, histidine addition to the electrolyte enhanced CO₂ reduction activity of violarite. Further study using various amino acids and spectroscopic method such as Infra-red and Raman spectroscopy revealed that combination of carboxyl (COOH) and imidazolyl groups of histidine contributed to CO and CH₄ formation, while sole imidazolyl group enhanced HCOOH generation.
The results above bought us insight concerning how metal sulfide minerals had functionalized to CO₂ reduction catalysts as extant enzymes, and contributed to the birth of primitive life.