The prebiotic geochemistry of Mn and cyanide on early Earth

Prebiotic chemists have examined and passionately debated the geochemical environments that could have promoted primitive metabolic pathways, or permitted the accumulation of building block molecules. Recent advances in molecular biochemistry have identified a UV-driven reaction network involving hydrogen cyanide (HCN) homologation that can produce essential building block molecules with minimum by-products. However, we have a poor understanding of geochemical processes that could have facilitated or hindered the accumulation and processing of major feedstock molecules (i.e., HCN) on the early Earth. Such a gap in our knowledge leaves us unable to constrain the likely processes and/or environments that were critical to the origins of life.

Under the CO2-rich, anoxic surface conditions on the early Earth, HCN from transient sources (e.g., lightning and impact) would dissolve in Fe2+-rich surface water and produce stable ferrocyanide anion (Fe(CN)64-). The transience of HCN has motivated suggestions to concentrate it within sediments, specifically through the crystallisation of ferrocyanide salts with common cations. These sedimentary ferrocyanide salts are thought to release simple cyanide back into the water column upon decomposition and rehydration, serving as a feedstock for prebiotic chemistry. Which, then are the most suitable ferrocyanide salt(s) for processing HCN in the surface water on the early Earth?

The present work experimentally investigate the precipitation kinetics of sparingly soluble Mn-ferrocyanide (Mn2Fe(CN)6). Considering the plausible abundance of Mn2+ in the anoxic surface water, the formation of Mn2Fe(CN)6 could have been an effective pathway of concentrating HCN near the transient sources. Whether the products of Mn2Fe(CN)6 decomposition/alteration are suitable for prebiotic synthesis holds the key to our understanding of the environmental factors that could have restricted prebiotic HCN chemistry.