The mechanism of fumarate photoreduction on zinc sulfide nanoparticles

半导体硫化物矿物与有机分子之间的光驱动氧化还原相互作用可能启动了地球上生命出现所需的非生物碳化学反应。特别是，三羧酸（TCA）循环被认为是新兴生物的原始代谢。测试这一建议需要了解候选矿物-有机反应的产物和机制。我们将确定从TCA循环的一个光驱动反应的速率，机制和中间物质。具体来说，我们将研究富马酸盐在胶体硫化锌（ZnS）表面上的双电子还原为琥珀酸盐，使用时间分辨瞬态光学和红外吸收光谱以及傅里叶变换电子顺磁共振光谱来探测从亚纳秒到微秒时间尺度的反应过程。这些互补技术将允许将半导体中电荷分布的时间演变与有机反应物中键的断裂和形成联系起来。这项研究将为早期地球的地球化学条件提供新的限制，这些条件可能允许建立由太阳能驱动的益生元碳循环。(2)更广泛的意义和重要性金属硫化物矿物对光的吸收可以引发矿物表面与被吸附的有机分子之间的光化学反应。这种半导体光化学一定是在已知的地球早期存在的条件下发生的，据推测，它们在生命发展所需的复杂有机分子的合成中起着重要作用。在紫外线照射下，硫化锌（ZnS）颗粒表面很容易发生一种特殊的反应，将一种小的有机酸分子富马酸盐转化为另一种有机酸分子琥珀酸盐。在这个过程中，表面将两个电子转移给富马酸盐，富马酸盐也从水中获得两个质子，形成两个新的碳氢键。反应的确切途径，包括电子和质子转移步骤的顺序，目前尚不清楚。特别是，我们假设富马酸盐与ZnS表面的结合稳定了中间化学物质，并使这种相对复杂的氧化还原反应能够在不形成副产物的情况下进行。在我们提出的研究中，我们计划使用时间分辨光谱技术来捕获不同中间物质的化学特征，从而确定反应途径。通过随后研究反应途径和速率如何随着ZnS或溶液化学的变化而变化，我们希望能够确定硫化物表面在影响该反应中的作用。这项工作将限制早期地球的地球化学条件，这些条件可能允许太阳能驱动的非生物有机反应。这项工作不仅对了解生命的起源有意义，而且对基于地球丰富元素的太阳能应用材料的开发也有意义。这项工作还涉及矿物-生物分子相互作用的基础科学，这可能为医学地质学的主题提供进一步的见解。

(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.