Shadow of a Shadow Ferrocyanide and Nitroprusside as a Sunscreen for Photosensitive Prebiotic Molecules

Abstract

Stellar irradiation is thought to be a significant contributor to the origin of life. Ultraviolet (UV) light interacting with iron cyanide complexes may play an important role in prebiotic chemistry. The UV-Visible (UV-Vis) spectra of these iron cyanide complexes can be measured by the same source that drives the chemistry, providing a real-time \textit{in-situ} quantitative analysis of prebiotically relevant UV-driven photochemistry. We measure the UV-Vis absorbance of ferrocyanide and nitroprusside, and relate this absorbance to known concentrations. We show that these absorbances can be combined to accurately predict the concentrations of ferrocyanide-nitroprusside mixtures that could be generated from ferrocyanide and nitroxyl salts irradiated by ultraviolet light. The ferrocyanide molar attenuation coefficients were found to be: epsilon_{ferrocyanide}(340 nm) = (2.2 +/- 0.4) x 10^3 dm^2 mol^{-1}. Nitroprusside peaks show the following values: epsilon_{nitroprusside}(340 nm) = (4.1 +/- 0.3) x 10^2 dm^2 mol^{-1}, epsilon_{nitroprusside}(400 nm) = (1.71 +/- 0.05) x 10^2 dm^2 mol^{-1}, and epsilon_{nitroprusside}(500 nm) = 62.1 +/- 1.7 dm^2 mol^{-1}. With the help of our measured absorbances, we consider ferrocyanide and nitroprusside as sunscreens. In the absence of extra ferrocyanide sources, UV-sensitive compounds could be protected on timescales of months. This would allow for compounds like nicotinamide adenine dinucleotide, NADH, to survive for over a year at depths of 5 m, compared to a lifetime of 6 months when unprotected. Our toy model constrains the photochemical survival of compounds of interest to the origin of life community across a comprehensive spectral range and can be used to constrain the survival using different exoplanetary irradiative conditions; thus exploring the UV environment with the presence of ferrocyanide and nitroprusside and contributing to the wider discussion surrounding the prevalence of the origin of life in the Universe.