Li Xichen, Siegbahn Per E M
College of Chemistry, Beijing Normal University, 100875, Beijing, China.
Phys Chem Chem Phys. 2015 May 14;17(18):12168-74. doi: 10.1039/c5cp00138b.
In a previous detailed study of all the steps of water oxidation in photosystem II, it was surprisingly found that O2 release is as critical for the rate as O-O bond formation. A new mechanism for O2 release has now been found, which can be described as an opening followed by a closing of the interior of the oxygen evolving complex. A transition state for peroxide rotation forming a superoxide radical, missed in the previous study, and a structural change around the outside manganese are two key steps in the new mechanism. However, O2 release may still remain rate-limiting. Additionally, for the step forming the O-O bond, an alternative, experimentally suggested, mechanism was investigated. The new model calculations can rule out the precise use of that mechanism. However, a variant with a rotation of the ligands around the outer manganese by about 30° will give a low barrier, competitive with the old DFT mechanism. Both these mechanisms use an oxyl-oxo mechanism for O-O bond formation involving the same two manganese atoms and the central oxo group (O5).
在之前对光系统II中水分子氧化所有步骤的详细研究中，令人惊讶地发现，氧气释放对反应速率的关键程度与氧 - 氧键形成相当。现在已经发现了一种新的氧气释放机制，其可描述为放氧复合体内部先打开然后关闭。先前研究中遗漏的过氧化物旋转形成超氧自由基的过渡态以及外部锰周围的结构变化是新机制中的两个关键步骤。然而，氧气释放可能仍然是限速步骤。此外，对于形成氧 - 氧键的步骤，研究了一种实验提出的替代机制。新的模型计算可以排除该机制的精确应用。然而，配体围绕外部锰旋转约30°的变体将产生一个低势垒，可与旧的密度泛函理论机制相竞争。这两种机制都使用氧代 - 氧机制形成氧 - 氧键，涉及相同的两个锰原子和中心氧代基团（O5）。
