Haumann M, Müller C, Liebisch P, Iuzzolino L, Dittmer J, Grabolle M, Neisius T, Meyer-Klaucke W, Dau H
FB Physik, Freie Universität Berlin, Arnimallee 14, D-14195 Berlin, Germany.
Biochemistry. 2005 Feb 15;44(6):1894-908. doi: 10.1021/bi048697e.
Structural and electronic changes (oxidation states) of the Mn(4)Ca complex of photosystem II (PSII) in the water oxidation cycle are of prime interest. For all four transitions between semistable S-states (S(0) --> S(1), S(1) --> S(2), S(2) --> S(3), and S(3),(4) --> S(0)), oxidation state and structural changes of the Mn complex were investigated by X-ray absorption spectroscopy (XAS) not only at 20 K but also at room temperature (RT) where water oxidation is functional. Three distinct experimental approaches were used: (1) illumination-freeze approach (XAS at 20 K), (2) flash-and-rapid-scan approach (RT), and (3) a novel time scan/sampling-XAS method (RT) facilitating particularly direct monitoring of the spectral changes in the S-state cycle. The rate of X-ray photoreduction was quantitatively assessed, and it was thus verified that the Mn ions remained in their initial oxidation state throughout the data collection period (>90%, at 20 K and at RT, for all S-states). Analysis of the complete XANES and EXAFS data sets (20 K and RT data, S(0)-S(3), XANES and EXAFS) obtained by the three approaches leads to the following conclusions. (i) In all S-states, the gross structural and electronic features of the Mn complex are similar at 20 K and room temperature. There are no indications for significant temperature-dependent variations in structure, protonation state, or charge localization. (ii) Mn-centered oxidation likely occurs on each of the three S-state transitions, leading to the S(3) state. (iii) Significant structural changes are coupled to the S(0) --> S(1) and the S(2) --> S(3) transitions which are identified as changes in the Mn-Mn bridging mode. We propose that in the S(2) --> S(3) transition a third Mn-(mu-O)(2)-Mn unit is formed, whereas the S(0) --> S(1) transition involves deprotonation of a mu-hydroxo bridge. In light of these results, the mechanism of accumulation of four oxidation equivalents by the Mn complex and possible implications for formation of the O-O bond are considered.
光系统II（PSII）的水氧化循环中，锰（4）钙复合物的结构和电子变化（氧化态）是人们最为关注的。对于半稳定S态之间的所有四个转变（S(0)→S(1)、S(1)→S(2)、S(2)→S(3)以及S(3),(4)→S(0)），不仅在20K温度下，而且在水氧化功能正常的室温（RT）下，通过X射线吸收光谱（XAS）研究了锰复合物的氧化态和结构变化。采用了三种不同的实验方法：（1）光照-冷冻法（20K下的XAS），（2）闪光-快速扫描法（室温），以及（3）一种新颖的时间扫描/采样-XAS方法（室温），该方法特别有助于直接监测S态循环中的光谱变化。对X射线光还原速率进行了定量评估，从而证实了在整个数据收集期间（20K和室温下，所有S态均>90%），锰离子保持其初始氧化态。对通过这三种方法获得的完整XANES和EXAFS数据集（20K和室温数据，S(0)-S(3)，XANES和EXAFS）进行分析，得出以下结论。（i）在所有S态中，锰复合物的总体结构和电子特征在20K和室温下相似。没有迹象表明结构、质子化状态或电荷定位存在显著的温度依赖性变化。（ii）以锰为中心的氧化可能发生在三个S态转变中的每一个上，从而导致S(3)态。（iii）显著的结构变化与S(0)→S(1)和S(2)→S(3)转变相关，这两个转变被确定为锰-锰桥连模式的变化。我们提出，在S(2)→S(3)转变中形成了第三个Mn-(μ-O)(2)-Mn单元，而S(0)→S(1)转变涉及μ-羟基桥的去质子化。根据这些结果，考虑了锰复合物积累四个氧化当量的机制以及对O-O键形成的可能影响。
