Models for Cytochromes c′: Observation of an Extremely Labile Spin State in Monoimidazole Complexes of Saddle-Shaped Iron(III) Porphyrinates
Models for Cytochromes c′: Observation of an Extremely Labile Spin State in Monoimidazole Complexes of Saddle-Shaped Iron(III) Porphyrinates
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DOI：
10.1002/anie.200902224
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Nakamura, Mikio
中科院分区：
文献类型：
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作者：
Ikezaki, Akira;Takahashi, Masashi;Nakamura, Mikio
细胞色素cо具有非常独特的结构，因为它们与作为第五配体的组氨酰咪唑基团五配位。在氧化状态下，这些蛋白质表现出独特的自旋状态，即混合的中间自旋 (S= 3/2) 和高自旋 (S= 5/2) 状态，范围从主要为 S= 5/2 到主要为 S= 3/2 状态，具体取决于细菌来源和用于测量的光谱方法。 [1]为了揭示这些蛋白质的模糊自旋状态，有必要对合成的单咪唑复合物进行结构和光谱研究。尽管我们已经报道了多种[Fe(TMP)L]+模型配合物的自旋态，但我们无法获得良好的晶体用于结构研究。[2, 3]事实上，由于稳定的双加合物的形成，单咪唑配合物的分离相当困难。迄今为止，Scheidt 等人报道的唯一具有结构特征的配合物是 [Fe (OEP)(2-MeIm)] ClO4。[4]该复合体保持S= 5/2状态，至少在60K以上。在本文中，我们报道了单咪唑配合物 [Fe (oetpp) L] ClO4，其中 L 为 HIm (1)、2-MeIm (2) 或 2-MeBzIm (3)，根据 1H NMR、13CNMR、EPR、穆斯堡尔谱和 X 射线晶体学，表现出极其不稳定的自旋态。配合物 1-3 是通过分别向 [Fe (oetpp)(thf) 2] ClO4 的 CH2Cl2 溶液中添加 HIm、2-MeIm 或 2-MeBzIm（1.0 当量）来制备。将由此获得的黑色晶体从CHCl 3 /环己烷中重结晶。 1和2的分子结构分别在123和120 K下通过X射线晶体学测定。图1为2的分子结构； 1 的分子结构在支持信息中给出。 1和2都表现出高度鞍状的卟啉核心。 1 和 2 的平均 Fe-Np 键长分别为 1.969 (3) 和 1.966 (2)；这些值非常接近 S= 3/2 [Fe (oetpp)] ClO4 中的平均 Fe-Np 键长，即 1.963 (7),[5]，并且比 S= 5/2 [Fe (OEP)(2-MeIm)] ClO4 中的平均 Fe-Np 键长短得多，即 2.038 (6)。[4]相反，2中的Fe-N轴键长比1中的长0.013。轴向 2-MeIm 配体沿 N-Fe-N 轴排列。FeIII 离子与 4N 平均平面的偏差相当小 (0.26)，这又非常接近 [Fe (oetpp)] ClO4 中的偏差 (0.25)。[5]表1列出了1和2的结构参数，以及上述一些类似配合物的结构参数。[4-7]1和2的结构参数与[Fe(oetpp)] ClO4的结构参数相似，表明1和2采用S= 3/2状态。
Cytochromes cо have a quite unique structure in the sense that they are pentacoordinate with a histidyl imidazole group as the fifth ligand. In the oxidized state, these proteins exhibit a unique spin state, a mixed intermediate-(S= 3/2) and highspin (S= 5/2) state that ranges from a mainly S= 5/2 to a mainly S= 3/2 state depending on the bacterial sources and spectroscopic methods applied for the measurements.[1] To reveal the ambiguous spin state of these proteins, both structural and spectroscopic studies with synthetic monoimidazole complexes are necessary. Although we have reported the spin states of a wide variety of [Fe (TMP) L]+ model complexes, we were unable to obtain good crystals for structural studies.[2, 3] In fact, the isolation of monoimidazole complexes is quite difficult because of the formation of stable bisadducts. To date, the only structurally characterized complex is [Fe (OEP)(2-MeIm)] ClO4, reported by Scheidt et al.[4] This complex maintains the S= 5/2 state, at least above 60K. In this paper, we report that the monoimidazole complexes [Fe (oetpp) L] ClO4, in which L is HIm (1), 2-MeIm (2), or 2-MeBzIm (3), exhibit extremely labile spin states, on the basis of 1H NMR, 13CNMR, EPR, and Mössbauer spectroscopy and X-ray crystallography. Complexes 1–3 were prepared by the addition of HIm, 2-MeIm, or 2-MeBzIm (1.0 equiv), respectively, to CH2Cl2 solutions of [Fe (oetpp)(thf) 2] ClO4. The black crystal thus obtained was recrystallized from CHCl3/cyclohexane. The molecular structures of 1 and 2 were determined by X-ray crystallography at 123 and 120 K, respectively. Figure 1 shows the molecular structure of 2; the molecular structure of 1 is given in the Supporting Information. Both 1 and 2 exhibit a highly saddled porphyrin core. The average FeÀNp bond lengths are 1.969 (3) and 1.966 (2) for 1 and 2, respectively; these values are quite close to the average FeÀNp bond length in S= 3/2 [Fe (oetpp)] ClO4, that is, 1.963 (7),[5] and are much shorter than that in S= 5/2 [Fe (OEP)(2-MeIm)] ClO4, that is, 2.038 (6).[4] In contrast, the FeÀNaxial bond length in 2 is longer than that in 1 by 0.013. The axial 2-MeIm ligand is aligned along the NÀFeÀN axis.The deviation of the FeIII ion from the 4N mean plane is rather small (0.26), which is again quite close to that in [Fe (oetpp)] ClO4 (0.25).[5] Table 1 lists the structural parameters of 1 and 2, together with those of some analogous complexes mentioned above.[4–7] The similarity of the structural parameters of 1 and 2 to those of [Fe (oetpp)] ClO4 suggests that 1 and 2 adopt the S= 3/2 state.