Investigating Prebiotic Reductive Amination in a Fe / NOx Redox System

Iron minerals are highly reactive catalysts for prebiotic chemistry, and in the presence of ammonia or other reduced nitrogen compounds, have been shown to drive amino acid and alpha-hydroxy acid synthesis from organic precursors. However, on the early Earth or Mars, nitrogen may also have been present in the form of oxidized nitrogen species (NO_x) such as nitrate and/or nitrite, both of which can be reduced by Fe(II) to form various species including N₂O, N₂, and/or ammonia. The possible production of ammonia from Fe(II)-driven nitrate/nitrite reduction may be able to feed in to prebiotic organic reactions including reductive amination. In this study, we explored the reactivity of keto-acids glyoxylate and pyruvate with ferrous / ferric iron hydroxides and oxidized nitrogen species nitrate and nitrite. We observed that nitrate and nitrite were both highly reactive with Fe²⁺ in solution and Fe(II)-bearing minerals, resulting in immediate iron oxidation during mineral precipitation and eventually forming oxidized mineral phases after several days. Despite various indicators of nitrate / nitrite reduction in these experiments, we did not observe any evidence for ammonia formation, though it cannot be ruled out. The organic precursors pyruvate and glyoxylate reacted in NO_x-containing systems to form reduced organic products (from the Fe(II) as reductant). Amino acid formation was not observed in any experiments containing nitrate or nitrite, even with the addition of ammonia. However, we did observe unknown organic products in experiments with pyruvate and nitrite. The fate of ammonia in these experiments is uncertain; it is possible that amino acids cannot form under these conditions, or, that ammonia was undergoing reactions with other nitrogen species and not reacting with the organics at all. Our results show that the presence of nitrate or nitrite with Fe(II)-hydroxide minerals affects organic chemistry in these systems: through iron oxidation and thus affecting the reactivity of mineral phases, and, the presence of NO_x species affects the expected organic reactions with ammonia. Further study is needed to identify nitrogen redox pathways under these conditions and to determine which early Earth or Mars-relevant mineral compositions would best facilitate abiotic / prebiotic organic chemistry.