Orbital Configuration of the Valence Electrons, Ligand Field Symmetry, and Manganese Oxidation States of the Photosynthetic Water Oxidizing Complex: Analysis of the S(2) State Multiline EPR Signals.
Orbital Configuration of the Valence Electrons, Ligand Field Symmetry, and Manganese Oxidation States of the Photosynthetic Water Oxidizing Complex: Analysis of the S(2) State Multiline EPR Signals.
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DOI：
10.1021/ic9512340
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发表时间：
1996-05
影响因子：
4.6
通讯作者：
M. Zheng;G. Dismukes
中科院分区：
文献类型：
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作者：
M. Zheng;G. Dismukes
本文提出了一个理论框架，用于分析光合作用水氧化络合物S(2)氧化态四锰(Mn(4))团簇产生的三个“多线”电子顺磁共振谱(MLS)。精确的模拟包括g和(4)(55)Mn超精细张量的各向异性，这些超精细张量是根据先前使用无偏最小二乘谱拟合法获得的(55)Mn(III)和(55)Mn(IV)超精细张量的数据库选择的。考虑到所有络合物中的Mn(III)超精细张量都有很大的(30%)各向异性，以前假定各向同性超精细常数的MLS模拟需要物理上不现实的参数。找到了一个简单的模型，它既能很好地模拟天然的“19-21线”MLS，也能很好地模拟“26线”的NH(3)束缚形式。用二聚体模型和扭曲三角磁模型描述了Mn(4)团簇中Heisenberg交换作用的对称性，从而确定了该团簇的初始电子基态。显式考虑了菱形对称扭曲的影响。这两种磁性模型都对应于独立于Mn EXAFS研究提出的几种可能的Mn(4)团簇结构模型之一。对于两种活性氧化模型(3Mn(III)-1Mn(IV)或3Mn(IV)-1Mn(III))，模拟了WOC中Mn(4)团簇的8个(或7个)双重态中的每一个，并使用了大范围的轴向Mn超精细张量，具有同轴或垂直张量排列。我们用3Mn(III)-1Mn(IV)氧化模型进行了精确的模拟。在二聚体耦合模型中，S(12)，S(34)，S(T)|(7)/(2)，4，(1)/(2)和|(7)/(2)，3，(1)/(2)和|(7)/(2)和|(7)/(2)和|(7)/(2)>；和|(7)/(2)，3，(1)/(2)和gt；之间的自旋态转换解释了从天然分子轨道到NH(3)束缚分子轨道的超精细分裂中观察到的大的(25%)收缩。这种激发态作为新基态的稳定化是由锰间交换耦合的变化引起的，而本征超精细张量没有明显的变化。缺乏对Ca(2+)耗竭的MLS的良好模拟表明，Ca(2+)耗竭改变了锰的连接和锰间交换耦合。3Mn(IV)-1Mn(III)氧化模型被认为是不利的，因为对于自然的MLS只能找到近似的模拟，而与NH(3)结合的MLS没有得到一致的结果。锰(III)和锰(IV)离子的超精细张量的标量部分与二锰(III，IV)过氧化氢酶的标量部分接近(+/-5%)，这表明总体配体类型相似。然而，Mn(III)超精细相互作用的大的各向异性部分(30%)与所有四方扩展的六坐标Mn(III)离子的各向异性部分在符号上相反(即通常的Jahn-Teller分裂)。每个Mn(III)离子的高自旋d(4)电子组态的自旋密度分布对应于一个扁平(扁平)椭球。这种电子分布在具有三角压缩双锥几何构型的五坐标配位场中是有利的，但原则上它也可能出现在具有四角压缩几何构型的应变六坐标配位场中。[Mn(2)(-O)](4+)(反向Jahn-Teller失真)。所得到的价电子组态分别被描述为e‘(2)e“(2)和(d(Pi))(3)(d(X)()2(-)(Y)()2)(1)，而不是(d(Pi))(3)(d(Z)()2)(1)构型，如在几个合成二聚体和过氧化氢酶中的[Mn(2)(-O)(2)](3+)核心中发现的)。前两种几何构型都预测了强氧化的Mn(III)离子，从而暗示了WOC中Mn(4)团簇的氧化反应的结构基础。解释MLS所需的磁模型与从EXAFS对WOC的研究中推导出的最简单的结构和电子模型并不容易协调。
A theoretical framework is presented for analysis of all three "multiline" EPR spectra (MLS) arising from the tetramanganese (Mn(4)) cluster in the S(2) oxidation state of the photosynthetic water oxidizing complex (WOC). Accurate simulations are presented which include anisotropy of the g and (four) (55)Mn hyperfine tensors, chosen according to a database of (55)Mn(III) and (55)Mn(IV) hyperfine tensors obtained previously using unbiased least-squares spectral fitting routines. In view of the large (30%) anisotropy common to Mn(III) hyperfine tensors in all complexes, previous MLS simulations which have assumed isotropic hyperfine constants have required physically unrealistic parameters. A simple model is found which offers good simulations of both the native "19-21-line" MLS and the "26-line" NH(3)-bound form of the MLS. Both a dimer-of-dimers and distorted-trigonal magnetic models are examined to describe the symmetry of the Heisenberg exchange interactions within the Mn(4) cluster and thus define the initial electronic basis states of the cluster. The effect of rhombic symmetry distortions is explicitly considered. Both magnetic models correspond to one of several possible structural models for the Mn(4) cluster proposed independently from Mn EXAFS studies. Simulated MLS were constructed for each of the eight (or seven) doublet states of the Mn(4) cluster in the WOC for the two viable oxidation models (3Mn(III)-1Mn(IV) or 3Mn(IV)-1Mn(III)), and using a wide range of axial Mn hyperfine tensors, with either coaxial or orthogonal tensor alignments. We find accurate simulations using the 3Mn(III)-1Mn(IV) oxidation model. In the dimer-of-dimers coupling model, the spin state conversion between two doublet states |S(12),S(34),S(T)|(7)/(2),4,(1)/(2)> and |(7)/(2),3,(1)/(2)> is found to explain the large (25%) contraction in the hyperfine splitting observed upon conversion from the native MLS to the NH(3)-bound MLS. Stabilization of this excited state as the new ground state is caused by change in the intermanganese exchange coupling, without appreciable change in the intrinsic hyperfine tensors. The lack of good simulations of the Ca(2+)-depleted MLS suggests that Ca(2+)-depletion changes both Mn ligation and intermanganese exchange coupling. The 3Mn(IV)-1Mn(III) oxidation model is disfavored because only approximate simulations could be found for the native MLS and no agreement with the NH(3)-bound MLS was obtained. The scalar part of the hyperfine tensors for both Mn(III) and Mn(IV) ions were found to approximate (+/-5%) the values for the dimanganese(III,IV) catalase enzyme, suggesting similar overall ligand types. However, the large (30%) anisotropic part of the Mn(III) hyperfine interaction is opposite in sign to that found in all tetragonally extended six-coordinate Mn(III) ions (i.e., the usual Jahn-Teller splitting). The distribution of spin density from the high-spin d(4) electron configuration of each Mn(III) ion corresponds to a flattened (oblate) ellipsoid. This electronic distribution is favored in five-coordinate ligand fields having trigonally compressed bipyramidal geometry, but it could also arise, in principle, in strained six-coordinate ligand fields having tetragonally compressed geometry, i.e. [Mn(2)(-O)](4+) (reverse Jahn-Teller distortion). The resulting valence electronic configurations are described as e'(2)e"(2) and (d(pi))(3)(d(x)()()2(-)(y)()()2)(1), respectively, in contrast to the (d(pi))(3)(d(z)()()2)(1) configuration common to unstrained six-coordinate tetragonally-extended Mn(III) ions, such as found in the [Mn(2)(-O)(2)](3+) core in several synthetic dimers and catalase. Both of the former geometries predict strongly oxidizing Mn(III) ions, thereby suggesting a structural basis for the oxidative reactivity of the Mn(4) cluster in the WOC. The magnetic model needed to explain the MLS is not readily reconciled with the simplest structural and electronic models deduced from EXAFS studies of the WOC.