High spin oxoiron(IV) complexes have been proposed to be a key intermediate in numerous non-heme metalloenzymes. The successful detection of similar complexes has been reported for only two synthetic systems. A new synthetic high spin oxoiron(IV) complex is now reported that can be prepared from a well-characterized oxoiron(III) species. This new oxoiron(IV) complexes can also be prepared from a hydroxoiron(III) species via a proton-coupled electron transfer process—a first in synthetic chemistry. The oxoiron(IV) complexes has been characterized with a variety of spectroscopic methods: FTIR studies showed a feature associated with the Fe–O bond at ν(Fe16O) = 799 cm−1 that shifted to 772 cm−1 in the 18O complex; Mössbauer experiments show a signal with an δ = 0.02 mm/s and |ΔEQ | = 0.43 mm/s, electronic parameters consistent with a Fe(IV) center; and optical spectra had visible bands at λmax = 440 (εM = 3100), 550 (εM = 1900) and 808 (εM = 280) nm. In addition, the oxoiron(IV) complex gave the first observable EPR features in the parallel-mode EPR spectrum with g-values at 8.19 and 4.06. A simulation for an S = 2 species with D = 4.0(5) cm−1, E/D = 0.03, σE/D = 0.014, and gz = 2.04 generates a fit that accurately predicted the intensity, lineshape, and position of the observed signals. These results showed the EPR spectroscopy can be a useful method for determining the properties of high spin oxoiron(IV) complexes. The oxoiron(VI) complex was crystallized at −35°C and its structure was determined by X-ray diffraction methods. The complex has a trigonal bipyramidal coordination geometry with the Fe–O unit positioned within a hydrogen bonding cavity. The FeIV–O unit bond length is 1.680(1) Å, which is the longest distance yet reported for monomeric oxoiron(IV) complex.
Using a combination of density functional calculations and Mössbauer spectroscopy, we have examined chloroperoxidase compound II (CPO-II). The Mössbauer spectrum of CPO-II suggests the presence of two distinct ferryl species in an approximately 70:30 ratio. Density functional calculations and cryogenic reduction and annealing experiments allow us to assign the major species as an Fe(IV)OH intermediate. The Mössbauer parameters of the minor component are indicative of an authentic iron(IV)oxo species, but we have found the 70:30 ratio to be pH invariant. The unchanging ratio of component concentrations is in agreement with CPO-II's visible absorption spectrum, which shows no change over the enzyme's range of pH stability.
Using a combination of Mössbauer spectroscopy and density functional calculations, we have determined that the ferryl forms of P450(BM3) and P450cam are protonated at physiological pH. Density functional calculations were performed on large active-site models of these enzymes to determine the theoretical Mössbauer parameters for the ferryl and protonated ferryl (Fe(IV)OH) species. These calculations revealed a significant enlargement of the quadrupole splitting parameter upon protonation of the ferryl unit. The calculated quadrupole splittings for the protonated and unprotonated ferryl forms of P450(BM3) are DeltaE(Q) = 2.17 mm/s and DeltaE(Q) = 1.05 mm/s, respectively. For P450cam, they are DeltaE(Q) = 1.84 mm/s and DeltaE(Q) = 0.66 mm/s, respectively. The experimentally determined quadrupole splittings (P450(BM3), DeltaE(Q) = 2.16 mm/s; P450cam, DeltaE(Q) = 2.06 mm/s) are in good agreement with the values calculated for the protonated forms of the enzymes. Our results suggest that basic ferryls are a natural consequence of thiolate-ligated hemes.
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