Iron catalyzed formation of methyl radicals as a common source of environmentally important volatile organic compounds

含一个碳原子（C1）的有机和无机挥发性化合物，如二氧化碳、甲烷、甲醇、甲醛、一氧化碳和氯甲烷等，在环境中普遍存在，它们作为温室气体，破坏平流层和对流层臭氧，控制大气氧化能力，在大气物理和化学中起着重要作用。此外，这些化合物在全球碳循环中起着重要作用。到目前为止，环境中的C1化合物大多与生物体内复杂的代谢和酶促途径或有机物的燃烧过程有关。到目前为止，人们还没有认识到，地球生物圈中的许多C1化合物可能也有一个共同的起源，来自甲基取代底物的甲基，这些甲基被铁催化形成的甲基自由基所切割。在这个跨学科的提案中，我们的目标是结合我们两个研究小组的专业知识，这两个研究小组在环境领域和实验室工作以及实验和计算/理论机制工作方面相互补充，以研究铁催化甲基自由基形成作为环境重要C1化合物的共同来源，并确定这些过程背后的精确化学。根据我们的初步结果，我们假设在各种杂原子甲基化底物的环境中，甲基自由基的形成是普遍存在的，非生物过程也可能是生化过程。我们提出，通过生成甲基自由基，可以形成一整套碳氧化态为-IV到+IV的C1化合物，也可以形成短链正构烷烃和烯烃。形成的单个C1物种的相对数量可能取决于氧化还原环境和生物地球化学条件，如甲基自由基供体的可用性、铁物种、氧、pH和可能的一系列其他参数。在这个项目中，我们将在实验室中系统地研究C1化合物的形成，使用不同的铁种类和生物地球化学相关的甲基取代底物，并应用稳定同位素技术。此外，为了深入了解这些过程背后的化学作用，我们将确定甲基自由基和C1形成的机理步骤，并利用基于密度泛函数理论和从头算量子化学研究的计算模型，结合反应动力学、中间体光谱和产物分布，验证机理情景。最后，我们将把我们的知识运用到实践中，研究这些反应在水和土壤样品中从现场收集。我们项目的结果将有助于提高我们对地球生物圈中C1化合物形成过程的认识，并有助于设计未来的研究，在类似野外的条件下调查这些过程，包括应用生物和地球化学工具。

Organic and inorganic volatile compounds containing one carbon atom (C1), such as carbon dioxide, methane, methanol, formaldehyde, carbon monoxide and chloromethane are ubiquitous in the environment and play an important role in atmospheric physics and chemistry as they act as greenhouse gases, destroy stratospheric and tropospheric ozone and control the atmospheric oxidation capacity. Furthermore, these compounds play an important role in global carbon cycling. Up to now, most C1 compounds in the environment were associated to complex metabolic and enzymatic pathways in organisms or combustion processes of organic matter. So far, it was not recognized that many C1 compounds in the geobiosphere might also have a common origin in methyl groups from methyl-substituted substrates that are cleaved by the iron-catalyzed formation of methyl radicals. In this interdisciplinary proposal, we aim to combine the expertise of our two research groups that ideally complement each other in the environmental field and laboratory work, and experimental as well as computational/theoretical mechanistic work, to investigate the iron catalyzed formation of methyl radicals as a common source of environmentally important C1 compounds, and to determine the precise chemistry behind these processes. Based on our preliminary results we hypothesize that formation of methyl radicals by abiotic and possibly also by biochemical processes is ubiquitous in the environment with various heteroatom-methylated substrates. We propose that by generating methyl radicals formation of the entire set of C1 compounds with carbon oxidation states of -IV to +IV but also formation of short chain n-alkanes and alkenes is possible. The relative amounts of the formed individual C1 species might depend on the redox milieu and biogeochemical conditions such as the availability of methyl radical donors, iron species, oxygen, pH and possibly a range of other parameters. In this project, we will investigate systematically the formation of C1 compounds in the laboratory, using different iron species and biogeochemically relevant methyl-substituted substrates and applying stable isotope techniques. Furthermore, to thoroughly understand the chemistry behind these processes, we will identify the mechanistic steps of methyl radical and C1 formation and verify mechanistic scenarios using computational modeling based on density functional theory and ab-initio quantum-chemical studies, also in combination with reaction kinetics, spectroscopy of intermediates and product distributions. Finally, we will put our assembled knowledge into practice and study these reactions in water and soil samples collected from the field. The results from our project will help to improve our knowledge about formation processes of C1 compounds in the geobiosphere and help to design future studies investigating these processes under field-like conditions involving the application of both biological and geochemical tools.