The tetranuclear manganese cluster in photosystem II: location and magnetic properties of the S2 state as determined by saturation-recovery EPR spectroscopy.
The tetranuclear manganese cluster in photosystem II: location and magnetic properties of the S2 state as determined by saturation-recovery EPR spectroscopy.
复制标题
光系统 II 中的四核锰簇：由饱和恢复 EPR 光谱确定的 S2 态的位置和磁性。
DOI：
10.1021/bi970326t
复制
发表时间：
1997
期刊：
影响因子：
--
通讯作者：
Brudvig,GW
中科院分区：
文献类型：
--
作者：
Koulougliotis,D;Schweitzer,RH;Brudvig,GW
利用饱和-恢复EPR谱测量了光系统II（PSII）放氧复合物（OEC）的S2态对暗稳定酪氨酸自由基YD·的自旋-晶格弛豫增强作用。两种形式的S2状态进行了比较：多线EPR信号物种在未经处理的PSII和改变多线EPR信号物种在NH3处理的PSII。先前的工作表明，在S2态PSII中YD·的非单指数自旋晶格弛豫动力学是由于与OEC的Mn 4团簇的偶极-偶极相互作用。通过考虑OEC的S2态多线EPR信号形式的有效磁矩的温度变化，我们提供了YD·的自旋晶格弛豫随温度的增强的定量分析。在不同温度范围内，Mn 4团簇S2态的不同自旋态对该效应的贡献不同：当T ≤ 10 K时，它是基态（S=1/2）;当T ≥ 30 K时，它是第一激发自旋态;在中温时，两种自旋态的贡献相当. YD·的弛豫增强对于所检查的两种形式的S2态多线EPR信号是等效的，这表明对于未处理的PSII和NH3处理的PSII，Mn 4团簇在S2态的磁性非常相似。EPR渐进式微波功率饱和也被用来评估Mn 4团簇的自旋晶格弛豫特性，在PSII的NH3衍生物中给出了改变的S2态多线EPR信号。Orbach机制被证明提供了主要的弛豫途径;基态和第一激发自旋态之间的能量差估计为30 ± 2 cm-1，这与未处理的PSII中S2态多线EPR信号物种的值非常相似。低于4K时，S2态多线EPR信号物种作为YD·自旋弛豫增强剂的有效性急剧下降。这被解释为是因为温度依赖的55 Mn核自旋晶格弛豫导致在YD·的自旋晶格弛豫的时间尺度期间S2态多线EPR信号物种的有效拉莫尔频率的平均;由于S2态多线EPR信号的线形由各向同性55 Mn核超精细分裂支配，这种核弛豫过程允许接近YD·的共振频率，从而产生更大的弛豫增强。通过使用包括YD·和S2态多线EPR信号的线形状的偶极模型，分析了YD·的自旋晶格弛豫增强，得到YD·和OEC之间距离的下限为22 μ m。连同最近的研究表明，紧密接近的Mn 4簇YZ·，这些结果提供了进一步的支持，相对于PSII中的两个氧化还原活性酪氨酸的Mn 4簇的不对称位置。
The spin−lattice relaxation enhancement of the dark-stable tyrosine radical, YD•, by the S2state of the O2-evolving complex (OEC) of photosystem II (PSII) has been measured by using saturation−recovery EPR spectroscopy. Two forms of the S2state have been compared: the multiline EPR signal species in untreated PSII and the altered multiline EPR signal species in NH3-treated PSII. Previous work has shown that the non-single-exponential spin−lattice relaxation kinetics of YD•in S2-state PSII result from a dipole−dipole interaction with the Mn4cluster of the OEC. By taking into account the temperature variation of the effective magnetic moment of the S2-state multiline EPR signal form of the OEC, we provide a quantitative analysis of its temperature-dependent enhancement of the spin−lattice relaxation of YD•. Different spin states of the Mn4cluster in the S2state are responsible for the effect at different temperature regimes: forT≤ 10 K, it is the ground spin state (S=1/2); forT≥ 30 K, it is the first excited spin state; and at intermediate temperatures, the contributions of the two spin states are comparable. The relaxation enhancement of YD•is equivalent for both forms of the S2-state multiline EPR signal examined, indicating that the magnetic properties of the Mn4cluster are very similar in the S2state for both untreated and NH3-treated PSII. EPR progressive microwave-power saturation has also been used to assess the spin−lattice relaxation properties of the Mn4cluster giving the altered S2-state multiline EPR signal in the NH3derivative of PSII. The Orbach mechanism is shown to provide the dominant relaxation pathway; the energy difference between the ground and first excited spin states is estimated to be 30 ± 2 cm-1, which is very similar to the value found for the S2-state multiline EPR signal species in untreated PSII. Below 4 K, the effectiveness of the S2-state multiline EPR signal species as a spin relaxation enhancer of YD•drops dramatically. This is interpreted to occur because of temperature-dependent55Mn nuclear spin−lattice relaxation which causes averaging of the effective Larmor frequency of the S2-state multiline EPR signal species during the time scale for spin−lattice relaxation of YD•; because the line shape of the S2-state multiline EPR signal is dominated by isotropic55Mn nuclear hyperfine splittings, such nuclear relaxation processes allow frequencies in near resonance with that of YD•to be accessed, thereby producing a greater relaxation enhancement. By using a dipolar model that includes the line shapes of both the YD•and S2-state multiline EPR signals, the spin−lattice relaxation enhancement of YD•is analyzed to obtain a lower limit of 22 Å for the distance between YD•and the OEC. Together with recent studies showing a close proximity of the Mn4cluster to YZ•, these results provide further support for an asymmetric location of the Mn4cluster with respect to the two redox-active tyrosines in PSII.