Deciphering the function of Fe/S cofactors with alternative cluster ligands: model studies using synthetic analogues

铁硫（Fe/S）簇是最广泛和最古老的生物辅助因子之一，几乎存在于生命的所有领域。它们最突出的作用是多种途径的电子传递（ET），而一系列不同核数的Fe/S团簇可用于此任务。它们大多含有硫化物桥接的铁簇核，由蛋白质末端的半胱氨酸（Cys）硫代酸盐连接。化学合成Fe/S蛋白活性位点的类似物对我们了解生物辅助因子的电子结构和内在特性有重要贡献。近年来，越来越多的蛋白质被发现含有Fe/S簇和非半胱氨酸配体，称为替代簇配体。与此同时，除了电子转移之外，生物Fe/S辅助因子的功能也在迅速增加，包括它们参与小分子的感知，如NO、DNA修复、自由基反应和催化。沿着SPP第一个资助期的路线，并在迄今为止取得的成果的基础上，该项目的最终目标是通过合成模拟方法破译Fe/S辅助因子与替代簇配体的功能。具体目标是：(A)阐明Fe/S团簇（主要是具有his末端配体的[2Fe-2S]团簇）上质子耦合电子转移（PCET）反应的热力学和动力学参数的内在趋势，以证明his连接在Rieske中心以外的潜在PCET作用。(B)揭示了特定设计的[2Fe-2S]簇的内在配体重排和交换过程，这与理解生物Fe/S簇成熟和转移过程中的配体交换有关；具体来说，将研究固体硫铁酸盐（MFeS2）的聚合物结构产生[2Fe-2S]单元和更大的碎片。(C)表征混合S/N连接[2Fe-2S]簇与一氧化氮反应形成的物质，以揭示簇氧化态和His质子化态对NO反应途径的影响，以及对NO、H2S和其他小信号分子间潜在串扰的影响。(D)模拟自由基SAM酶中辅助Fe/S辅因子的自由基反应性，并阐明控制Fe/S簇向底物的硫捐赠的因子，如生物素合成酶（BioB）或脂酰合成酶（LipA）。(E)为SPP网络中的其他组提供合适的化学合成Fe/S模型系统，作为新的光谱或分析方法的基准系统。特别是，合成策略将朝着在簇核内包含碳化物(C)或碳炔单元（CR）的第一Fe/S簇进行。

Iron-sulfur (Fe/S) clusters are among the most widespread and ancient biological cofactors, occurring in virtually all domains of life. Their most prominent role is electron transfer (ET) in diverse pathway, and a range of Fe/S clusters of different nuclearity is available for that task. They mostly contain a sulfide-bridged iron cluster core that is ligated by terminal cysteine (Cys) thiolates from the protein. Chemically synthesized analogues of the active sites of Fe/S proteins have contributed significantly to our understanding of the electronic structures and intrinsic properties of the biological cofactors. In recent years, an increasing number of proteins containing Fe/S clusters with non-cysteine ligands, called alternative cluster ligands, is being discovered. This goes along with a rapidly increasing number of functions beyond electron transfer that are recognized for biological Fe/S cofactors, including their involvement in sensing of small molecules such as NO, DNA repair, radical reactions and catalysis.Continuing along the lines of the first SPP funding period and building upon the results achieved so far, this projects ultimately aims at deciphering the function of Fe/S cofactors with alternative cluster ligands in a synthetic analogue approach. Specific objectives are: (A) Elucidating intrinsic trends for the thermodynamic and kinetic parameters of proton coupled electron transfer (PCET) reactions at Fe/S clusters, mainly [2Fe-2S] clusters with His-like terminal ligands, to demonstrate a potential PCET role of His-ligation beyond the Rieske centers. (B) Unravelling intrinsic ligand rearrangement and exchange processes at specifically designed [2Fe-2S] clusters, which is relevant for understanding ligand exchange during biological Fe/S cluster maturation and transfer; specifically, the generation of [2Fe-2S] units and larger fragments from the polymeric structure of solid thioferrates (MFeS2) will be investigated. (C) Characterizing the species that form upon reaction of mixed S/N ligated [2Fe-2S] clusters with nitric oxide in order to enlighten the effect of cluster oxidation state and His protonation state on NO reactivity pathways, and on potential crosstalk between NO, H2S and other small signaling molecules. (D) Emulating radical reactivity at auxiliary Fe/S cofactors in radical SAM enzymes and elucidating factors that control sulfur donation from the Fe/S cluster to a substrate, as seen in biotin synthase (BioB) or lipoyl synthase (LipA). (E) Providing suitable chemically synthesized Fe/S model systems, which serve as benchmark systems for new spectroscopic or analytical methods, for other groups in the SPP network. In particular, synthetic strategies will be pursued towards a first Fe/S cluster that contains a carbide (C) or carbyne unit (CR) within the cluster core.