Despite decades of intense research, the core structure of the methane C-H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase (sMMO) remains controversial, with conflicting reports supporting either a ‘diamond’ diiron core structure or an open-core structure. Early EXAFS data assigned a short 2.46 Å Fe-Fe distance to Q (Shu, et al., Science 1997) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo, et al., JACS, 2017). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe-Fe distance, but rather a long 3.4 Å diiron distance, as observed in open-core synthetic model complexes. The previously reported 2.46 Å feature plausibly arises from a background metallic iron contribution from the experimental setup, which is eliminated in HERFD EXAFS due to the increased selectivity. Herein, we have explored the origin of the short diiron feature in partial-fluorescent yield EXAFS measurements and discuss the diagnostic features of background metallic scattering contribution to the EXAFS of dilute biological samples. Lastly, differences in sample preparation and resultant sample inhomogeneity in rapid-freeze quenched samples for EXAFS analysis are discussed. The presented approaches have broad implications for EXAFS studies of all dilute iron-containing samples.
We report the generation and spectroscopic characterization of a heterobimetallic [(TMC)FeIII-O-CrIII(OTf)4] species (1) by O2 bubbling into a mixture of Fe(TMC)(OTf)2 and Cr(OTf)2 in NCCH3. Complex 1 also formed quantitatively by adding Cr(OTf)2 to [FeIV(O)(TMC)(NCCH3)]2+. The proposed O2 activation mechanism involves the trapping by a Cr-O2 adduct by Fe(TMC)(OTf)2.
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