Timescales for Prebiotic Photochemistry Under Realistic Surface Ultraviolet Conditions

Abstract

Ultraviolet (UV) light has long been invoked as a source of energy for prebiotic chemical synthesis, but experimental support does not involve sources of UV light that look like the young Sun. Here we experimentally investigate whether the UV flux available on the surface of early Earth, given a favorable atmosphere, can facilitate a variety of prebiotic chemical syntheses. We construct a solar simulator for the UV light of the faint young Sun on the surface of early Earth, called StarLab. We then attempt a series of reactions testing different aspects of a prebiotic chemical scenario involving hydrogen cyanide (HCN), sulfites, and sulfides under the UV light of StarLab, including hypophosphite oxidation by UV light and hydrogen sulfide, photoreduction of HCN with bisulfite, the photoanomerization of a-thiocytidine, the production of a chemical precursor of a potentially prebiotic activating agent (nitroprusside), the photoreduction of thioanhydrouridine and thioanhydroadenosine, and the oxidation of ethanol (EtOH) by photochemically generated hydroxyl radicals. We compare the output of StarLab to the light of the faint young Sun to constrain the timescales over which these reactions would occur on the surface of early Earth. We predict that hypophosphite oxidation, HCN reduction, and photoproduction of nitroprusside would all operate on the surface of early Earth in a matter of days to weeks. The photoanomerization of a-thiocytidine would take months to complete, and the production of oxidation products from hydroxyl radicals would take years. The photoreduction of thioanhydrouridine with hydrogen sulfide did not succeed even after a long period of irradiation, providing a lower limit on the timescale of several years. The photoreduction of thioanhydroadenosine with bisulfite produced 2¢-deoxyriboadenosine (dA) on the timescale of days. This suggests the plausibility of the photoproduction of purine deoxyribonucleotides, such as the photoproduction of simple sugars, proceeds more efficiently in the presence of bisulfite.

UV Transmission in Natural Waters on Prebiotic Earth

“…However, follow-up studies ( e.g. , characterization of action spectra) are required to verify whether pathways discovered with such sources could have functioned in realistic natural environments on prebiotic Earth (Ranjan and Sasselov, 2016 ; Todd et al , 2018 ; Rimmer et al , 2021 ).…”

A scenario for the origin of life: Volume regulation by bacteriorhodopsin required extremely voltage sensitive Na‐channels and very selective K‐channels

“…The cell contains nucleic acids and negatively charged metabolites in its interior, and a negative transmembrane electrical potential is established, thereby generating an energy source Young Sun suggests that the photoproduction of simple sugars and deoxyribonucleotides, in the presence of bisulfite or HCN, could have occurred on the timescale of hours or days. [30,31] In addition, surface daily wet-dry cycles could have produced phospholipidic vesicles. Furthermore, solar radiation affecting only superficial waters could provide convective chemical mixing due to differential warming and evaporation of deep and shallow layers.…”

UV Transmission in Natural Waters on Prebiotic Earth

“…UV light in the ∼ 240 − 300 nm wavelength range was not significantly attenuated by the geologically-derived solutes considered in this study in most terrestrial prebiotic waters, meaning that low-pressure mercury lamps with primary emission at 254 nm remain reasonable proxies for UV flux in initial simulations of prebiotic chemistry. However, follow-up studies (e.g., characterization of action spectra) are required to verify that pathways discovered with such sources could have functioned in realistic natural environments on prebiotic Earth (Ranjan & Sasselov, 2016;Todd et al, 2018;Rimmer et al, 2021).…”