Kα high-energy-resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) provides a powerful tool for overcoming the limitations of conventional XAS to identify the electronic structure and coordination environment of metalloprotein active sites. Herein, Fe Kα HERFD XAS is applied to the diiron active site of soluble methane monooxygenase (sMMO) and to a series of high-valent diiron model complexes, including diamond-core [FeIV2(μ-O)2(L)2](ClO4)4] (3) and open-core [(O=FeIV-O-FeIV(OH)(L)2](ClO4)3 (4) models (where, L = tris(3,5-dimethyl-4-methoxypyridyl-2-methyl)amine) (TPA∗)). Pronounced differences in the HERFD XAS pre-edge energies and intensities are observed for the open versus closed Fe2O2 cores in the model compounds. These differences are reproduced by time-dependent density functional theory (TDDFT) calculations and allow for the pre-edge energies and intensity to be directly correlated with the local active site geometric and electronic structure. A comparison of the model complex HERFD XAS data to that of MMOHQ (the key intermediate in methane oxidation) is supportive of an open-core structure. Specifically, the large pre-edge area observed for MMOHQ may be rationalized by invoking an open-core structure with a terminal FeIV=O motif, though further modulations of the core structure due to the protein environment cannot be ruled out. The present study thus motivates the need for additional experimental and theoretical studies to unambiguously assess the active site conformation of MMOHQ.
Bibliographical noteFunding Information:
Former and current members of the Molecular Theory and Spectroscopy Department and Inorganic Spectroscopy Department at MPI CEC are thanked for helpful discussions regarding the computational studies and for assistance with data collection. Financial support was provided by the Max Planck Society (S.D.). J.D.L. and L.Q. acknowledge the National Institutes of Health for funding (grants GM118030 to J.D.L and GM38767 to L.Q. and postdoctoral fellowship GM113333 to C.J.A.). R.G.C. acknowledges the International Max Planck Research School on Reactive Structure Analysis for Chemical Reactions (IMPRS RECHARGE) for funding. The European Synchrotron Radiation Facility is acknowledged for providing beamtime and technical support.
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