Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
J Am Chem Soc. 2011 Mar 16;133(10):3635-48. doi: 10.1021/ja110145v. Epub 2011 Feb 22.
The electronic structures of the native Mn(4)O(x)Ca cluster and the biosynthetically substituted Mn(4)O(x)Sr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S(2) state, were studied by X- and Q-band CW-EPR and by pulsed Q-band (55)Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl(2) or SrCl(2) containing media were measured. The obtained CW-EPR spectra of the S(2) state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809-22819]. In sharp contrast, the manganese ((55)Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and (55)Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the (55)Mn-ENDOR spectra; (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca(2+) and Sr(2+) containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca(2+)/Sr(2+) substitution; (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca(2+) site may modulate the fine structure tensor of the Mn(III) ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S(2) state the coordination of the Mn(III) ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788-6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate Mn(III) is a potential site for substrate 'water' (H(2)O, OH(-)) binding. Its location within the cuboidal structural unit, as opposed to the external 'dangler' position, may have important consequences for the mechanism of O-O bond formation.
从 Thermosynechococcus elongatus 中分离出的光合作用系统 II (PSII) 核心复合物中的天然 Mn(4)O(x)Ca 簇和生物合成取代的 Mn(4)O(x)Sr 簇，处于 S(2)状态，其电子结构通过 X 波段和 Q 波段连续波 CW-EPR 和脉冲 Q 波段 (55)Mn-ENDOR 光谱进行了研究。在 CaCl(2)或 SrCl(2) 存在的培养基中，通过光自养方式生长的野生型和酪氨酸 D 缺失突变体都进行了测量。S(2)状态下获得的 CW-EPR 光谱显示出多线 EPR 信号的超精细图案明显的差异 [Boussac 等人。J. Biol. Chem.2004, 279, 22809-22819]。相比之下，Ca 和 Sr 形式的 OEC 的锰 ((55)Mn) ENDOR 光谱非常相似。使用自旋哈密顿形式主义对 X 波段和 Q 波段 CW-EPR 和 (55)Mn-脉冲 ENDOR 光谱进行多频模拟，以研究这一令人惊讶的结果。结果表明：(i) 所有四个锰离子都对 (55)Mn-ENDOR 光谱有贡献；(ii) 对于含有 Ca(2+)和 Sr(2+)的 OEC，拟合各向同性超精细值仅略有变化，表明 Ca(2+)/Sr(2+)取代后整体自旋分布（电子耦合方案）没有变化；(iii) CW-EPR 超精细图案的变化可以用至少两个超精细张量的各向异性的微小减小来解释。据提议，Ca(2+) 位点的修饰可能会调节 Mn(III)离子的精细结构张量。DFT 计算支持上述结论。我们的数据分析也为以下观点提供了强有力的支持，即在 S(2)状态下，Mn(III)离子的配位是四方锥（5 配位）或八面体（6 配位），具有正方拉长。此外，结果表明，目前发表的 OEC 模型之一，Siegbahn 结构 [Siegbahn，P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. 等人。物理化学化学物理。2009, 11, 6788-6798]，与这里呈现的所有数据一致。这些结果为 OEC 的结构和水分解机制提供了重要信息。特别是，5 配位的 Mn(III)是潜在的底物“水”（H(2)O，OH(-)）结合位点。与外部“悬挂”位置相比，其在立方结构单元内的位置可能对 O-O 键形成机制有重要影响。
