Magnetic circular dichroism spectroscopic studies of mononuclear non-heme ferrous model complexes. Correlation of excited- and ground-state electronic structure with geometry
Magnetic circular dichroism spectroscopic studies of mononuclear non-heme ferrous model complexes. Correlation of excited- and ground-state electronic structure with geometry
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DOI：
10.1021/ja973735l
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发表时间：
1998-04-29
影响因子：
15
通讯作者：
Solomon, EI
中科院分区：
文献类型：
--
作者：
Pavel, EG;Kitajima, N;Solomon, EI
非血红素铁酶催化多种生物学上重要的含氧反应，然而，非血红素铁活性位点一直难以用大多数光谱方法研究。近红外（NIR）磁性圆二色性（MCD）和变温，变场（VTVH）MCD光谱的组合已被应用到24个结构定义的单核非血红素亚铁模型配合物，严格关联的光谱数据与几何和电子结构。虽然激发态分裂的一般趋势已经预测配位场理论，这些预测现在进行评估，通过系统地研究一系列的高自旋（S = 2）亚铁模型的近红外MCD光谱的范围广泛的配位数和几何形状。VTVH MCD光谱是用来探测基态电子结构，和一个完整的MCD强度表达式的非Kramers系统，包括z-偏振，B-条款，并引发状态已被推导出来。这个表达式已被应用到这些模型复合物，以确定零场分裂的迹象，并获得基态自旋哈密顿参数，这可以与基态配体场分裂。这些实验的基态数据被用来开发的信息内容可从VTVH MCD，特别是探测特定的金属配体键合相互作用的能力，为不同的配位环境。激发态配体场数据被用来构建一组光谱准则，结合基态信息，允许一个明确地确定一个未知的亚铁中心的配位数和几何形状，只有少数模棱两可的情况下除外。此外，MCD数据提供了深入了解MCD G-项强度和低对称亚铁中心符号的起源。通过这些模型研究获得的结果，现在提供的基础上调查亚铁活性位点的非血红素铁酶探测的几何和电子结构的网站相对于氧的反应性和了解如何在结构上的差异与反应性的差异。
Mononuclear non-heme iron enzymes catalyze a variety of biologically important reactions involving dioxygen, and yet, the non-heme ferrous active sites have been difficult to study by most spectroscopic methods. A combination of near-infrared (NIR) magnetic circular dichroism (MCD) and variable-temperature, variable-field (VTVH) MCD spectroscopies has been applied to 24 structurally defined mononuclear non-heme ferrous model complexes to rigorously correlate spectral data with geometric and electronic structure. While general trends for the excited-state splittings have been predicted by ligand field theory, these predictions are now evaluated by systematically studying the NIR MCD spectra of a series of high-spin (S = 2) ferrous models with a wide range of coordination numbers and geometries. VTVH MCD spectroscopy is used to probe groundstate electronic structure, and a complete MCD intensity expression for non-Kramers systems that includes z-polarization, B-terms, and elicited states has been derived. This expression has been applied to these model complexes to determine signs of the zero-field splitting and to obtain ground-state spin-Hamiltonian parameters, which can be related to ground-state ligand field splittings. These experimental ground-state data are used to develop the information content available from VTVH MCD, in particular the ability to probe specific metal-ligand bonding interactions for different coordination environments. The excited-state ligand field data are used to construct a set of spectroscopic guidelines which, combined with the ground-state information, allow one to clearly determine the coordination number and geometry of an unknown ferrous center, with the exception of only a few ambiguous cases. Additionally, the MCD data provide insight into the origin of the MCD G-term intensities and signs for low-symmetry ferrous centers. The results obtained through these model studies now provide the basis for investigating ferrous active sites of non-heme iron enzymes to probe the geometric and electronic structure of a site with respect to oxygen reactivity and understanding how differences in structure correlate with differences in reactivity.