Modeling the Signatures of Hydrides in Metalloenzymes: ENDOR Analysis of a Di-iron Fe(渭-NH)(渭-H)Fe Core

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• 作者：R. Adam Kinney ; Caroline T. Saouma ; Jonas C. Peters ; Brian M. Hoffman
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：August 1, 2012
• 年：2012
• 卷：134
• 期：30
• 页码：12637-12647
• 全文大小：533K
• 年卷期：v.134,no.30(August 1, 2012)
• ISSN：1520-5126

文摘

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L<sub>3sub>Fe(渭-NH)(渭-H)FeL<sub>3sub> (L<sub>3sub> = [PhB(CH<sub>2sub>PPh<sub>2sub>)<sub>3sub>]<sup>鈭?/sup>) complex, 3, is a novel S = <sup>1sup>/<sub>2sub> type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. <sup>1,2sup>H and <sup>14,15sup>N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (渭-H) and the proton of the (渭-NH) of the crystallographically characterized 3 show the proposed signature of a 鈥榖ridging鈥?hydride that is essentially equidistant between two 鈥榓nchor鈥?metal ions: a rhombic dipolar interaction tensor, T 鈮?[T, 鈥?i>T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 鈥榓nchor鈥?metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T 鈮?[鈭?i>T, 鈥?i>T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum鈥搃ron cofactor of nitrogenase contains two [Fe鈥揌<sup>鈥?/sup>鈥揊e] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E<sub>4sub>). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E<sub>4sub>. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.