Song Yu-Tian, Li Xi-Chen, Siegbahn Per E M
College of Chemistry, Beijing Normal University, Beijing 100875, China.
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
J Phys Chem B. 2023 Aug 3;127(30):6643-6647. doi: 10.1021/acs.jpcb.3c03029. Epub 2023 Jul 19.
The leading mechanism for the formation of O in photosystem II (PSII) has, during the past decade, been established as the so-called oxyl-oxo mechanism. In that mechanism, O is formed from a binding between an oxygen radical (oxyl) and a bridging oxo group. For the case of higher plants, that mechanism has recently been criticized. Instead, a nucleophilic attack of an oxo group on a five-coordinated Mn(V)═O group forming O has been suggested in a so-called water-unbound (WU) mechanism. In the present study, the WU mechanism has been investigated. It is found that the WU mechanism is just a variant of a previously suggested mechanism but with a reactant and a transition state that have much higher energies. The addition of a water molecule on the empty site of the Mn(V)═O center is very exergonic and leads back to the previously suggested oxyl-oxo mechanism.
在过去十年中，光系统II（PSII）中氧气形成的主要机制已被确立为所谓的氧自由基-氧桥机制。在该机制中，氧气由一个氧自由基（oxyl）和一个桥连氧基团之间的结合形成。对于高等植物的情况，该机制最近受到了批评。相反，在所谓的水非结合（WU）机制中，有人提出一个氧基团对五配位的Mn(V)═O基团进行亲核攻击形成氧气。在本研究中，对WU机制进行了研究。发现WU机制只是先前提出的一种机制的变体，但反应物和过渡态的能量要高得多。在Mn(V)═O中心的空位上添加一个水分子是非常放能的，并导致回到先前提出的氧自由基-氧桥机制。
