Characterization of the Bridged Hyponitrite Complex {[Fe(OEP)]2(渭-N2O2)}: Reactivity of Hyponitrite Complexes and Biological Relevance

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• 作者：Timothy C. Berto ; Nan Xu ; Se Ryeon Lee ; Anne J. McNeil ; E. Ercan Alp ; Jiyong Zhao ; George B. Richter-Addo ; Nicolai Lehnert
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：July 7, 2014
• 年：2014
• 卷：53
• 期：13
• 页码：6398-6414
• 全文大小：704K
• ISSN：1520-510X

文摘

The detoxification of nitric oxide (NO) by bacterial NO reductase (NorBC) represents a paradigm of how NO can be detoxified anaerobically in cells. In order to elucidate the mechanism of this enzyme, model complexes provide a convenient means to assess potential reaction intermediates. In particular, there have been many proposed mechanisms that invoke the formation of a hyponitrite bridge between the heme b₃ and nonheme iron (Fe_B) centers within the NorBC active site. However, the reactivity of bridged iron hyponitrite complexes has not been investigated much in the literature. The model complex {[Fe(OEP)]₂(渭-N₂O₂)} offers a unique opportunity to study the electronic structure and reactivity of such a hyponitrite-bridged complex. Here we report the detailed characterization of {[Fe(OEP)]₂(渭-N₂O₂)} using a combination of IR, nuclear resonance vibrational spectroscopy, electron paramagnetic resonance, and magnetic circular dichroism spectroscopy along with SQUID magnetometry. These results show that the ground-state electronic structure of this complex is best described as having two intermediate-spin (S = ³/₂) iron centers that are weakly antiferromagnetically coupled across the N₂O₂^{2鈥?/sup> bridge. The analogous complex {[Fe(PPDME)]₂(渭-N₂O₂)} shows overall similar properties. Finally, we report the unexpected reaction of {[Fe(OEP)]₂(渭-N₂O₂)} in the presence and absence of 1-methylimidizole to yield [Fe(OEP)(NO)]. Density functional theory calculations are used to rationalize why {[Fe(OEP)]₂(渭-N₂O₂)} cannot be formed directly by dimerization of [Fe(OEP)(NO)] and why only the reverse reaction is observed experimentally. These results thus provide insight into the general reactivity of hyponitrite-bridged iron complexes with general relevance for the N鈥揘 bond-forming step in NorBC.}