The mechanism of fumarate photoreduction on zinc sulfide nanoparticles

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(1) Technical Abstract Light-driven redox interactions between semiconducting sulfide minerals and organic molecules may have initiated the abiotic carbon chemistry needed for the appearance of life on Earth. In particular, the tricarboxylic acid (TCA) cycle has been proposed as a primordial metabolism for emerging organisms. Testing this proposal requires knowledge of the products and mechanisms of candidate mineral-organic reactions. We will determine the rates, mechanism, and intermediate species in one light-driven reaction from the TCA cycle. Specifically, we will study the two-electron reduction of fumarate to succinate on colloidal zinc sulfide (ZnS) surfaces, using time-resolved transient optical and infrared absorption spectroscopy and Fourier transform electron paramagnetic resonance spectroscopy to probe reaction progress from the subnanosecond to the microsecond timescales. These complementary techniques will allow correlating the temporal evolution of the charge distribution in the semiconductor and the breaking and formation of bonds in the organic reactant. This research will contribute new constraints on the geochemical conditions of the early Earth that could have permitted the establishment of a prebiotic carbon cycle driven by solar energy.(2) Broader significance and importanceThe absorption of light by metal sulfide minerals can initiate photochemical reactions between the mineral surface and adsorbed organic molecules. This kind of semiconductor photochemistry must have been occurring under the conditions known to exist in the early Earth and it is speculated that they played an important role in synthesis of complex organic molecules required for the development of life. One particular reaction that transforms one small organic acid molecule, fumarate, into another, succinate, occurs readily on the surface of zinc sulfide (ZnS) particles under ultraviolet (UV) illumination. In this process, the surface transfers two electrons to fumarate, which also acquires two protons from water, to form two new carbon-hydrogen bonds. The precise pathway of the reaction, including the ordering of electron and proton transfer steps, is currently unknown. In particular, we hypothesize that the binding of fumarate to the ZnS surface stabilizes the intermediate chemical species and enables this relatively complex redox reaction to proceed without forming side products. In our proposed research, we plan on using time-resolved spectroscopic techniques to capture chemical signatures of the different intermediate species and thereby determine the reaction pathway. By subsequently investigating how the reaction pathway and rate changes in response to changing ZnS or solution chemistry, we expect to be able to determine the role of the sulfide surface in influencing this reaction. This work will put constraints on the geo chemical conditions of the early Earth that could have permitted solar-driven, abiotic organic reactions. This work has implications not only for understanding the origins of life, but also the development of materials for solar energy applications based upon Earth-abundant elements. This work also addresses fundamental science at the basis of mineral-biomolecule interactions, which may provide further insights into topics in medical geology.