A Functional Model for the Rieske Center: Full Characterization of a Biomimetic N-Ligated [2Fe-2S] Cluster in Different Protonation States
A Functional Model for the Rieske Center: Full Characterization of a Biomimetic N-Ligated [2Fe-2S] Cluster in Different Protonation States
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DOI：
10.1002/chem.201301760
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发表时间：
2013-07-29
影响因子：
4.3
通讯作者：
Meyer, Franc
中科院分区：
文献类型：
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作者：
Albers, Antonia;Bayer, Thomas;Meyer, Franc
Rieske蛋白含有一个独特的[2Fe-2S]簇，其特征是异构性末端连接，涉及一个Fe的两个半胱氨酸，但另一个铁的两个组氨酸。[1]这些氧化还原活性Rieske簇在各种生物电子转移反应中发挥着重要作用。在细菌加氧酶中，它们介导芳香族化合物的氧化羟基化，它们存在于呼吸链和光合链的喹酚氧化复合体(细胞色素bc1和细胞色素b6f复合体)中。[2]后者是BC型蛋白质中的Rieske铁硫簇，具有异常高的氧化还原电位和pH相关的氧化还原电位，这与常见的铁氧还素型[2Fe-2S]簇不同。[3]这归因于Rieske蛋白S的氧化还原电位与铁结合的咪唑环的质子化状态的偶联。虽然两个组氨酸在还原(混价)形式下都有很高的pKa值(约12.5)，但在二铁形式中，其中一个的pKa约为7.5，接近生理pH(另一个的pKa约为9.5)。[4，5]从机械上讲，这导致二铁Rieske簇与对苯二酚底物反应时电子和质子转移的耦合。对仿生模型络合物的研究为铁-硫辅因子的性质和电子结构提供了有价值的见解，[6]但第一个模拟Rieske簇异构性配位环境的合成类似物仅在几年前才被报道，即[2Fe-2S]络合物12±(图1)。[7]尽管12±很好地复制了氧化二铁状态下的Rieske中心，而13±是混合价FeIIFeIII形式的很好的光谱模拟，后者被证明是非常不稳定的，并且避免了分离。此外，12中缺乏外围N原子，阻碍了对BC型Rieske蛋白功能关键的质子和电子转移耦合的研究。对于相关的双(苯并咪唑)连接的簇合物，有可能首次以晶体的形式分离出混合价状态的合成的[2Fe-2S]类似物23±，这使得对这种FeIIFeIII物种进行详细的光谱研究成为可能。[8，9]对于这个体系，甚至可以分离出超还原的FeIIFeII簇合物24±。[10]在这里，我们报道了22±和23±的质子化对其电子性质和氧化还原性质的影响，并观察到了类似于生物Rieske簇的质子耦合电子转移(PCET)。萨乌玛·埃塔尔。最近的交流表明，相关的N-连接的[2Fe-2S]络合物经历了协同的质子耦合电子转移，他们在PCET方形方案中建立了相互转化的热化学。[11]可以两边质子化的同质团簇的方形方案如图2所示。目前的工作提供了各种物种的完整光谱特征，包括[a]A.Albers，T.Bayer，S.Demeshko博士，S.Dechert博士，Dr.F.Meyer教授
Rieske proteins contain a unique [2Fe-2S] cluster that features heteroleptic terminal ligation involving two cysteines at one Fe, but two histidines at the other.[1] These redoxactive Rieske clusters play an important role in various biological electron-transfer reactions. In bacterial oxygenases, they mediate oxidative hydroxylation of aromatic compounds and they were found in the quinol-oxidizing complexes (cytochrome bc1 and cytochrome b6f complexes) of the respiratory and photosynthetic chains.[2] Rieske iron–sulfur clusters in the latter, the bc-type proteins, have unusually high and pH-dependent redox potentials, which distinguishes them from common ferredoxin-type [2Fe-2S] clusters.[3] This is attributed to coupling the Rieske protein s redox potential to the protonation states of the Fe-bound imidazole rings. Although both histidines have high pKa values (ca. 12.5) in the reduced (mixed-valent) form, in the diferric form one of them has a pKa of approximately 7.5, which is near physiological pH (the other has a pKa of about 9.5).[4, 5] Mechanistically this results in a coupling of electron and proton transfer upon reaction of the diferric Rieske cluster with the hydroquinone substrates. The investigation of biomimetic model complexes has provided valuable insight into the properties and electronic structures of iron–sulfur cofactors,[6] but a first synthetic analogue that emulates the heteroleptic coordination environment of the Rieske cluster has been reported only few years ago, namely, the [2Fe-2S] complex 12À (Figure 1).[7] Although 12À replicates well the Rieske center in the oxidized diferric state, and 13À is a good spectroscopic mimic for the mixedvalent FeIIFeIII form, the latter proved quite unstable and eluded isolation. Furthermore, the lack of peripheral N atoms in 12À precluded the investigation of the coupling of proton and electron transfer that is key to the function of bc-type Rieske proteins. For related bis (benzimidazolate) ligated clusters, it was then possible to isolate for the first time, in crystalline form, a synthetic [2Fe-2S] analogue 23À in the mixed-valent state, which allowed for a detailed spectroscopic investigation of this FeIIFeIII species.[8, 9] Even a super-reduced FeIIFeII cluster 24À could be isolated for this system.[10] Herein, we report the effect of protonation of 22À and 23À on its electronic and redox properties and the observation of proton-coupled electron transfer (PCET) akin to the biological Rieske cluster. Saouma etal. most recently communicated that a related N-ligated [2Fe-2S] complex undergoes concerted proton-coupled electron transfer, and they established the thermochemistry of interconversions in the PCET square scheme.[11] The square scheme for a homoleptic cluster that can be protonated on both sides is shown in Figure 2. The present work now presents a full spectroscopic characterization of the various species, including an [a] A. Albers, T. Bayer, Dr. S. Demeshko, Dr. S. Dechert, Prof. Dr. F. Meyer