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 L3Fe(μ-NH)(μ-H)FeL3 (L3 = [PhB(CH2PPh2)3]-) complex, 3, is a novel S = ½ type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. 1,2H and 14,15N 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 (E4). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E4. This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

Insight into the Iron–Molybdenum Cofactor of Nitrogenase from Synthetic Iron Complexes with Sulfur, Carbon, and Hydride Ligands

“…6,86 Analogous ENDOR experiments on isolable Fe-H compounds with terminal hydrides showed an axial tensor, while bridging hydrides gave a rhombic tensor. 6,175,176 This combination of experiments validated the idea that that the signals from the FeMoco-hydride (“E 4 ”) are most consistent with bridging hydrides in E 4 H 4 . 6 However, the active form might have transient terminal hydrides, as is currently believed for hydrogenase enzymes based on synthetic modeling studies.…”

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

“…84 Many challenges remain in defining the factors that are important for the multielectron reduction of N 2 and CO catalyzed by nitrogenase. It will be important to elucidate the possible roles of iron-hydrides 119 as well as the influence of the protein cavity. One step in this direction is the observation that the isolated V–Fe and Mo–Fe cofactors also hydrogenate CO and cyanide.…”

Brønsted–Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

“…Hydride and dihydrogen complexes of iron likely form at the active site of FeFe and FeNi hydrogenase enzymes. [431][432][433] These enzymes have carbonyl and cyanide ligands on the iron that participate in the catalytic splitting of H 2 into protons and electrons, dihydrogen oxidation, and the reverse process, hydrogen production. Presumably these reactions take place at the pH level of biological media, near 7 in aqueous solution but possibly higher in the interior of the enzyme that might have a dielectric constant of 8, as low as THF as a solvent.…”