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

Abstract

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L_3Fe(μ-NH)(μ-H)FeL_3 (L_3 = [PhB(CH_2PPh_2)_3]−) complex, 3, is a novel S = 1/2 type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. ^(1,2)H and ^(14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, –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 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [−T, –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–iron cofactor of nitrogenase contains two [Fe–H––Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E_4). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E_4. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

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