Molecular Biomimetics, Department of Photochemistry and Molecular Science, Ångström Laboratory, Box 523, Uppsala University, SE-751 20 Uppsala, Sweden.
Biochemistry. 2012 Jan 10;51(1):138-48. doi: 10.1021/bi200627j. Epub 2011 Dec 6.
The stability of the S(3) and S(2) states of the oxygen evolving complex in photosystem II (PSII) was directly probed by EPR spectroscopy in PSII membrane preparations from spinach in the presence of the exogenous electron acceptor PpBQ at 1, 10, and 20 °C. The decay of the S(3) state was followed in samples exposed to two flashes by measuring the split S(3) EPR signal induced by near-infrared illumination at 5 K. The decay of the S(2) state was followed in samples exposed to one flash by measuring the S(2) state multiline EPR signal. During the decay of the S(3) state, the S(2) state multiline EPR signal first increased and then decreased in amplitude. This shows that the decay of the S(3) state to the S(1) state occurs via the S(2) state. The decay of the S(3) state was biexponential with a fast kinetic phase with a few seconds decay half-time. This occurred in 10-20% of the PSII centers. The slow kinetic phase ranged from a decay half-time of 700 s (at 1 °C) to ~100 s (at 20 °C) in the remaining 80-90% of the centers. The decay of the S(2) state was also biphasic and showed quite similar kinetics to the decay of the S(3) state. Our experiments show that the auxiliary electron donor Y(D) was oxidized during the entire experiment. Thus, the reduced form of Y(D) does not participate to the fast decay of the S(2) and S(3) states we describe here. Instead, we suggest that the decay of the S(3) and S(2) states reflects electron transfer from the acceptor side of PSII to the donor side of PSII starting in the corresponding S state. It is proposed that this exists in equilibrium with Y(Z) according to S(3)Y(Z) ⇔ S(2)Y(Z)(•) in the case of the S(3) state decay and S(2)Y(Z) ⇔ S(1)Y(Z)(•) in the case of the S(2) state decay. Two kinetic models are discussed, both developed with the assumption that the slow decay of the S(3) and S(2) states occurs in PSII centers where Y(Z) is also a fast donor to P(680)(+) working in the nanosecond time regime and that the fast decay of the S(3) and S(2) states occurs in centers where Y(Z) reduces P(680)(+) with slower microsecond kinetics. Our measurements also demonstrate that the split S(3) EPR signal can be used as a direct probe to the S(3) state and that it can provide important information about the redox properties of the S(3) state.
在菠菜 PSII 膜制剂中，通过外加电子受体 PpBQ 在 1、10 和 20°C 下直接探测 PSII 中氧释放复合物（OEC）的 S(3)和 S(2)态的稳定性。通过在 5 K 下用近红外光照射测量分裂的 S(3)EPR 信号，跟踪暴露于两个闪光后的样品中 S(3)态的衰减。通过测量 S(2)态多线 EPR 信号，跟踪暴露于一个闪光后的样品中 S(2)态的衰减。在 S(3)态的衰减过程中，S(2)态多线 EPR 信号的幅度先增加后减小。这表明 S(3)态向 S(1)态的衰减是通过 S(2)态发生的。S(3)态的衰减是双指数的，具有几秒钟的快速动力学半衰期。这种情况发生在 10-20%的 PSII 中心。在剩余的 80-90%的中心中，慢速动力学半衰期范围从 700 s（在 1°C 时）到~100 s（在 20°C 时）。S(2)态的衰减也是双相的，表现出与 S(3)态衰减非常相似的动力学。我们的实验表明，辅助电子供体 Y(D)在整个实验过程中被氧化。因此，Y(D)的还原形式不会参与我们在这里描述的 S(2)和 S(3)态的快速衰减。相反，我们建议 S(3)和 S(2)态的衰减反映了从 PSII 的受体侧到 PSII 的供体侧的电子转移，从相应的 S 态开始。提出在 S(3)Y(Z) ⇔ S(2)Y(Z)(•)的情况下，S(3)和 S(2)态的衰减存在与 Y(Z)平衡，在 S(2)Y(Z) ⇔ S(1)Y(Z)(•)的情况下，S(2)和 S(1)态的衰减存在与 Y(Z)平衡。讨论了两种动力学模型，都是基于以下假设开发的：S(3)和 S(2)态的缓慢衰减发生在 Y(Z)也是快速供体的 PSII 中心，P(680)(+)在纳秒时间范围内工作，并且 Y(Z)以较慢的微秒动力学还原 P(680)(+)的 S(3)和 S(2)态的快速衰减发生在中心。我们的测量还表明，分裂的 S(3)EPR 信号可用作 S(3)态的直接探针，并且它可以提供有关 S(3)态氧化还原性质的重要信息。
