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 α-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 α-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.
Highlights
The starting points for origins of life research are diverse and researchers do not agree about the best place to begin
StarLab has the same spectral shape as the faint young Sun between 200 and 400 nm, it does not have the same intensity as that expected on the surface of early Earth, and so, the timescale in the laboratory does not directly represent but predicts the timescale on early Earth
We performed a series of seven experiments with StarLab exploring the various stages of a prebiotic chemical scenario with UVdriven aqueous chemistry starting with cyanide, bisulfite, and sulfides, with a focus on the role of the UV light
Summary
The starting points for origins of life research are diverse and researchers do not agree about the best place to begin. Others start with chemistry and find synthetic reactions and pathways that result in greater chemical complexity that synthesize the building blocks of the macromolecules that constitute life, such as DNA, RNA, proteins, and lipids. Others start with geology or exogeology, considering the mineral surfaces, volcanic degassing, atmospheric conditions, astrochemistry, and cometary chemistry and delivery of these building blocks. We explore a particular scenario that started with chemical synthesis and fruitfully underwent refinements using inputs from biochemistry and geochemistry (Sasselov et al, 2020). Starting with hydrogen cyanide (HCN), hydrogen sulfide, cyanamide, cyanoacetylene, phosphates (or reduced phosphorus species, see Ritson et al, 2020) in liquid water and exposing these mixtures to ultraviolet (UV) light and combining them in particular sequences result in high
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