Krewald Vera, Retegan Marius, Neese Frank, Lubitz Wolfgang, Pantazis Dimitrios A, Cox Nicholas
Max Planck Institute for Chemical Energy Conversion , Stiftstr. 34-36, Mülheim an der Ruhr 45470, Germany.
Inorg Chem. 2016 Jan 19;55(2):488-501. doi: 10.1021/acs.inorgchem.5b02578. Epub 2015 Dec 23.
In transition-metal complexes, the geometric structure is intimately connected with the spin state arising from magnetic coupling between the paramagnetic ions. The tetramanganese-calcium cofactor that catalyzes biological water oxidation in photosystem II cycles through five catalytic intermediates, each of which adopts a specific geometric and electronic structure and is thus characterized by a specific spin state. Here, we review spin-structure correlations in Nature's water-splitting catalyst. The catalytic cycle of the Mn4O5Ca cofactor can be described in terms of spin-dependent reactivity. The lower "inactive" S states of the catalyst, S0 and S1, are characterized by low-spin ground states, SGS = 1/2 and SGS = 0. This is connected to the "open cubane" topology of the inorganic core in these states. The S2 state exhibits structural and spin heterogeneity in the form of two interconvertible isomers and is identified as the spin-switching point of the catalytic cycle. The first S2 state form is an open cubane structure with a low-spin SGS = 1/2 ground state, whereas the other represents the first appearance of a closed cubane topology in the catalytic cycle that is associated with a higher-spin ground state of SGS = 5/2. It is only this higher-spin form of the S2 state that progresses to the "activated" S3 state of the catalyst. The structure of this final metastable catalytic state was resolved in a recent report, showing that all manganese ions are six-coordinate. The magnetic coupling is dominantly ferromagnetic, leading to a high-spin ground state of SGS = 3. The ability of the Mn4O5Ca cofactor to adopt two distinct structural and spin-state forms in the S2 state is critical for water binding in the S3 state, allowing spin-state crossing from the inactive, low-spin configuration of the catalyst to the activated, high-spin configuration. Here we describe how an understanding of the magnetic properties of the catalyst in all S states has allowed conclusions on the catalyst function to be reached. A summary of recent literature results is provided that constrains the sequence of molecular level events: catalyst/substrate deprotonation, manganese oxidation, and water molecule insertion.
在过渡金属配合物中，几何结构与顺磁性离子之间磁耦合产生的自旋态密切相关。在光系统II中催化生物水氧化的四锰 - 钙辅因子会经历五个催化中间体的循环，每个中间体都采用特定的几何和电子结构，因此具有特定的自旋态。在此，我们综述了自然界中析水催化剂的自旋 - 结构相关性。Mn₄O₅Ca辅因子的催化循环可以用自旋依赖性反应性来描述。催化剂的较低“非活性”S态，即S₀和S₁，其特征是低自旋基态，SGS = 1/2和SGS = 0。这与这些状态下无机核心的“开放立方烷”拓扑结构有关。S₂态以两种可相互转化的异构体形式表现出结构和自旋异质性，并被确定为催化循环的自旋转换点。第一种S₂态形式是具有低自旋SGS = 1/2基态的开放立方烷结构，而另一种则代表催化循环中首次出现的与较高自旋基态SGS = 5/2相关的封闭立方烷拓扑结构。只有这种较高自旋形式的S₂态会进展到催化剂的“活化”S₃态。最近的一份报告解析了这个最终亚稳催化态的结构，表明所有锰离子都是六配位的。磁耦合主要是铁磁性的，导致高自旋基态SGS = 3。Mn₄O₅Ca辅因子在S₂态采用两种不同结构和自旋态形式的能力对于S₃态中的水结合至关重要，它允许自旋态从催化剂的非活性、低自旋构型转变为活化的、高自旋构型。在此我们描述了对所有S态下催化剂磁性性质的理解如何得出关于催化剂功能的结论。提供了近期文献结果的总结，这些结果限制了分子水平事件的顺序：催化剂/底物去质子化、锰氧化和水分子插入。
