Pathways for the Photoreduction of Fumarate on ZnS

Semiconductor mineral particles can act as photocatalysts for organic redox reactions that occur enzymatically in modern biological metabolic pathways. Semiconductor mineral-mediated photocatalysis may have contributed to the prebiotic synthesis of organic acids on the early Earth, but assessing the plausibility of this hypothesis is impeded by the lack of knowledge about the mechanisms for light-driven organic redox reactions on mineral surfaces. We selected one step in the reverse tricarboxylic acid (rTCA) cycle, the reduction of fumarate to succinate, that has been shown to be photocatalyzed by zinc sulfide (ZnS). Using static and time-resolved optical emission and absorption spectroscopy, we studied the adsorption of fumarate and the rates and pathways for charge transfer. We find that ZnS transfers photoexcited electrons to bound and dissolved fumarate on a wide range of time scales but not to succinate, supporting the concept that ZnS mediated photoreduction of fumarate could have operated in oceans of the early Earth. Optical transient absorption (TA) spectroscopy identified a signature tentatively attributed to the fumarate radical anion that is stable for at least 8 ns, providing evidence that fumarate photoreduction under solar illumination levels occurs by successive photoelectron transfer. The model for electronic excitation, relaxation, and interfacial charge-transfer processes in ZnS provided here will inform all future studies of the photochemical reactions of this mineral.