Trapping of metalloradical intermediates of the S-states at liquid helium temperatures. Overview of the phenomenology and mechanistic implications.
Trapping of metalloradical intermediates of the S-states at liquid helium temperatures. Overview of the phenomenology and mechanistic implications.
复制标题
在液氦温度下捕获 S 态金属自由基中间体。
作者：
V. Petrouleas;D. Koulougliotis;N. Ioannidis
光系统II（PSII）的放氧复合物（OEC）由Mn簇（被认为是四核的）和酪氨酸（Tyr Z或Y（Z））组成。在PSII连续吸收四个光子的过程中，OEC经历四个氧化转变，S（0）到S（1），.，S（3）到（S（4））S（0）。氧在S（3）到S（0）的转变期间析出（S（4）是瞬态）。捕获S-状态转变的中间体，特别是涉及酪氨酰基自由基的中间体，一直是最终重要的目标，因为这可以严格测试采用Tyr Z在质子（除了电子）转移中的作用的模型，并且还提供关于水氧化机制的重要线索。然而，直到最近，关键的实验信息仍然缺乏。我们回顾和评估最近的观测捕获的金属自由基中间体的S-状态转换，在液氦温度。这些瞬变被分配到Tyr Z（*）与Mn簇的磁性相互作用。除了捕获这种独特催化机制的中间体的重要性之外，液氦温度还提供了额外的优势，即质子运动（与电子转移不同）被阻止，除非可能通过强氢键。本文总结了最近的观察，并讨论了现象学施加的限制。
The oxygen-evolving complex (OEC) of photosystem II (PSII) consists of a Mn cluster (believed to be tetranuclear) and a tyrosine (Tyr Z or Y(Z)). During the sequential absorption of four photons by PSII, the OEC undergoes four oxidative transitions, S(0) to S(1), ..., S(3) to (S(4))S(0). Oxygen evolves during the S(3) to S(0) transition (S(4) being a transient state). Trapping of intermediates of the S-state transitions, particularly those involving the tyrosyl radical, has been a goal of ultimate importance, as that can test critically models employing a role of Tyr Z in proton (in addition to electron) transfer, and also provide important clues about the mechanism of water oxidation. Until very recently, however, critical experimental information was lacking. We review and evaluate recent observations on the trapping of metalloradical intermediates of the S-state transitions, at liquid helium temperatures. These transients are assigned to Tyr Z(*) magnetically interacting with the Mn cluster. Besides the importance of trapping intermediates of this unique catalytic mechanism, liquid helium temperatures offer the additional advantage that proton motions (unlike electron transfer) are blocked except perhaps across strong hydrogen bonds. This paper summarizes the recent observations and discusses the constraints that the phenomenology imposes.