Spin State as a Marker for the Structural Evolution of Nature’s Water-Splitting Catalyst

详细信息    查看全文

• 作者：Vera Krewald ; Marius Retegan ; Frank Neese ; Wolfgang Lubitz ; Dimitrios A. Pantazis ; Nicholas Cox
• 刊名：Inorganic Chemistry
• 出版年：2016
• 出版时间：January 19, 2016
• 年：2016
• 卷：55
• 期：2
• 页码：488-501
• 全文大小：916K
• ISSN：1520-510X

文摘

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, S_GS = ¹/₂ and S_GS = 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 S_GS = 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 S_GS = 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 S_GS = 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.