Favorable Environments for the Formation of Ferrocyanide, a Potentially Critical Reagent for Origins of Life

Abstract

Cyanide and its derivatives play important roles in prebiotic chemistry through a variety of possible mechanisms. In particular, cyanide has been shown to allow for the synthesis of ribonucleotides and amino acids. Although dissolved hydrogen cyanide can be lost as a gas or undergo hydrolysis reactions, cyanide can also potentially be stored and stockpiled as ferrocyanide (Fe(CN) 6 −4), which is more stable. Furthermore, ferrocyanide aids in some prebiotic synthetic reactions. Here, we investigate the formation rates and yields of ferrocyanide as a function of various environmental parameters, such as the pH, temperature, and concentration. We find that ferrocyanide formation rates and yields are optimal at slightly alkaline conditions (pH 8−9) and moderate temperatures (≈20−30 °C). Given the wide range of possible lake environments likely available on early Earth, our results help to constrain the environmental conditions that would favor cyanide-and ferrocyanide-based prebiotic chemistries. We construct lake box models and find that ferrocyanide may be able to form and reach significant concentrations for prebiotic chemistry on the time scale of years under favorable conditions.

Releasing Cyanide from Ferrocyanide through Carbon Monoxide Ligand Exchange in Alkaline Aqueous Environments

“…While this may be possible, ferrocyanide is more likely to form and is stabilized at higher pH conditions . In addition, the p K a of cyanide is 9.2, and therefore the formation of the ferrocyanide complex is favored by alkaline conditions, which has been demonstrated experimentally . Finally, while some cyanide may be lost to hydrolysis, under the temperature and pH conditions of these experiments, this process is expected to occur over 2–4 days .…”

“…If there were facile pathways to liberate [CN – ] from ferrocyanide complexes, this would provide critical information about plausible environmental conditions that promote cyanide-dependent organic syntheses. On early Earth, most of the crust would have been mafic or ultramafic. − Regardless of whether landmasses and surface lakes were available, or all rock was suboceanic, waters that were buffered by mafic and ultramafic rocks would have been alkaline to hyperalkaline depending upon the exact mineralogy and extent of reaction. , Alkaline conditions are also significant because appreciable ferrocyanide formation requires pH > 8. ,,, In addition, the fluids buffered by these rocks would contain aqueous Fe 2+ . Thus, these mafic and ultramafic rock buffered systems, with their alkaline pH and aqueous Fe 2+ availability, are ideal for ferrocyanide complex formation.…”