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Prebiotic Metabolic Systems Driven by Colloidal Semiconductor Photocatalysis
PI: Martin, Scot
The proposed work will test the hypothesis that important reactions of prebiotic metabolic systems can be driven by photocatalysis on colloidal semiconductor particles. How prebiotic metabolic cycles could have developed and been driven in the absence of catalytic networks is an important unanswered question about the origins of life because such prebiotic cycles are considered as necessary platforms in the development of more advanced self-replicating biotic systems. The prototypical model system we propose to employ to evaluate these possibilities is the reductive (reverse) citric acid cycle. This cycle has received much attention in the literature of prebiotic evolution because repeated cycling provides a core mechanism for the synthesis of useful biomolecules from CO 2. Colloidal photochemistry, which has received limited attention in the Exobiology community, opens new reaction pathways because of the interactions of excited-state species, and these reaction rates can be very rapid with high yields.
The key questions to be addressed in the proposed work are: (1) What are the rates and the yields of the photoreduction reactions? (2) What is the effect of reaction conditions, including the wavelength of irradiation, pH, temperature, and chemical competitors, on enhancing or constraining the reaction rates and yields? (3) How and why do interactions of aqueous reactants & products with the surface of the solid semiconductor photocatalyst affect the rates and the yields of the reactions? Specifically, we propose to determine the capabilities and limitations of colloidal semiconductor particles to photocatalyze at solar wavelengths the reduction reactions of oxaloacetate to malate, fumarate to succinate, succinate/CO 2 to oxoglutarate, oxoglutarate/CO 2 to oxalosuccinate, and oxalosuccinate to isocitrate. Photocatalysis provides the free energy for these reactions and, in addition, could promote rapid reactions rates because of high overpotentials. We will initially focus on Fe-doped ZnS colloidal particles, prepared by hydrothermal methods, because of their probable high occurrence in marine environments of the prebiotic Earth and their proven high performance towards CO 2 photoreduction.
The impacts of the proposed study, defining the role of colloidal photochemistry in driving prebiotic metabolic cycles, will be forward steps in achieving (1) NASA’s Exobiology Program goal of understanding what chemical systems could have served as precursors of metabolic and replicating systems on Earth and elsewhere (Astrobiology Roadmap goal 3, objectives 3.1 and 3.2) and (2) NASA’s Strategic Objectives of providing a deeper understanding of how early life could have originated.
