Ferrocyanide survival under near ultraviolet (300–400 nm) irradiation on early Earth

Abstract

Prebiotic lake environments containing ferrocyanide could have fostered origins of life chemistry on early Earth. Ferrocyanide can act to concentrate hydrogen cyanide (HCN), a prebiotically important molecule. Additionally, ferrocyanide, coupled with sulfite, can participate in an ultraviolet (UV)-driven photoredox cycle to generate solvated electrons, which can reduce cyanide to form all four major building blocks of life: sugars, amino acids, nucleotides, and lipid precursors. However, longer wavelength UV light (∼300–400 nm) causes photoaquation of ferrocyanide into pentacyanoaquaferrate, Fe(CN)5H2O. This species can either regain cyanide to reform ferrocyanide or ultimately lose cyanide ligands, which removes ferrocyanide from solution. Here, we investigate this near ultraviolet (300–400 nm) UV-driven loss of ferrocyanide. In addition to determining the wavelength dependence of the loss and the implications from the UV environment on early Earth, we also study the effects of pH, temperature, and concentration. We find that in dilute, slightly alkaline solutions, ferrocyanide would degrade significantly on the order of minutes under the near UV radiation expected on early Earth. We further determine that the lifetime of ferrocyanide is extended at more alkaline pH, lower temperatures, and higher concentrations. Under a reasonable set of planetary conditions, we find that ferrocyanide lifetimes in irradiated environments range from minutes to hours, or longer. Our results can help to determine the constraints implied by the UV-driven loss of ferrocyanide in prebiotic environments. We assess the potential environmental limits and circumstances that would allow for successful retention of significant amounts of ferrocyanide in prebiotic lakes; we further evaluate how ferrocyanide photoaquation may fit in to the larger network of reactions potentially occurring throughout prebiotic chemistry. For example, our experiments show that CN- can be released from ferrocyanide evaporite salts by aqueous dissolution and UV light, making it potentially available for prebiotic reactions. These results can aid in the construction of consistent and plausible circumstances for prebiotic chemistry on early Earth.