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Characterization of the particulate methane monooxygenase metal centers in multiple redox states by X-ray absorption spectroscopy.

ABSTRACT: The integral membrane enzyme particulate methane monooxygenase (pMMO) converts methane, the most inert hydrocarbon, to methanol under ambient conditions. The 2.8-A resolution pMMO crystal structure revealed three metal sites: a mononuclear copper center, a dinuclear copper center, and a nonphysiological mononuclear zinc center. Although not found in the crystal structure, solution samples of pMMO also contain iron. We have used X-ray absorption spectroscopy to analyze the oxidation states and coordination environments of the pMMO metal centers in as-isolated (pMMO(iso)), chemically reduced (pMMO(red)), and chemically oxidized (pMMO(ox)) samples. X-ray absorption near-edge spectra (XANES) indicate that pMMO(iso) contains both Cu(I) and Cu(II) and that the pMMO Cu centers can undergo redox chemistry. Extended X-ray absorption fine structure (EXAFS) analysis reveals a Cu-Cu interaction in all redox forms of the enzyme. The Cu-Cu distance increases from 2.51 to 2.65 A upon reduction, concomitant with an increase in the average Cu-O/N bond lengths. Appropriate Cu2 model complexes were used to refine and validate the EXAFS fitting protocols for pMMO(iso). Analysis of Fe EXAFS data combined with electron paramagnetic resonance (EPR) spectra indicates that Fe, present as Fe(III), is consistent with heme impurities. These findings are complementary to the crystallographic data and provide new insight into the oxidation states and possible electronic structures of the pMMO Cu ions.

SUBMITTER: Lieberman RL

PROVIDER: S-EPMC2864602 | biostudies-literature | 2006 Oct

REPOSITORIES: biostudies-literature

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Characterization of the particulate methane monooxygenase metal centers in multiple redox states by X-ray absorption spectroscopy.

Lieberman Raquel L RL Kondapalli Kalyan C KC Shrestha Deepak B DB Hakemian Amanda S AS Smith Stephen M SM Telser Joshua J Kuzelka Jane J Gupta Rajeev R Borovik A S AS Lippard Stephen J SJ Hoffman Brian M BM Rosenzweig Amy C AC Stemmler Timothy L TL

Inorganic chemistry 20061001 20

The integral membrane enzyme particulate methane monooxygenase (pMMO) converts methane, the most inert hydrocarbon, to methanol under ambient conditions. The 2.8-A resolution pMMO crystal structure revealed three metal sites: a mononuclear copper center, a dinuclear copper center, and a nonphysiological mononuclear zinc center. Although not found in the crystal structure, solution samples of pMMO also contain iron. We have used X-ray absorption spectroscopy to analyze the oxidation states and co ...[more]

PMID: 16999437

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Project description:Nitrogen K-edge X-ray absorption spectra (XAS) were obtained for 19 transition metal complexes bearing bipyridine, ethylenediamine, ammine, and nitride ligands. Time-dependent density functional theory (TDDFT) and DFT/restricted open configuration interaction singles (DFT/ROCIS) calculations were found to predict relative N K-edge XAS peak energies with good fidelity to experiment. The average difference (|ΔE|) between experimental and linear corrected calculated energies were found to be 0.55 ± 0.05 eV and 0.46 ± 0.04 eV, respectively, using the B3LYP hybrid density functional and scalar relativistically recontracted ZORA-def2-TZVP(-f) basis set. Deconvolution of these global correlations into individual N-donor ligand classes gave improved agreement between experiment and theory with |ΔE| less than 0.4 eV for all ligand classes in the case of DFT/ROCIS. In addition, calibration method-dependent values for the N 1s → 2p radial dipole integral of 25.4 ± 1.7 and 26.8 ± 1.9 are obtained, affording means to estimate the nitrogen 2p character in unfilled frontier molecular orbitals. For the complexes studied, nitrogen covalency values correlate well to those calculated by hybrid DFT with an R 2 = 0.92 ± 0.01. Additionally, as a test case, a well-characterized PNP ligand framework (PNP = N[2-P(CHMe2)2-4-methylphenyl]2 1-) coordinated to NiII is investigated for its ability to act as a redox non-innocent ligand. Upon oxidation of (PNP)NiCl with [FeCp2](OTf) to its radical cation, [(PNP)NiCl](OTf) (OTf = triflate), a new low-energy feature emerges in the N K-edge XAS spectra. This feature is assigned as N 1s to a PNP-localized acceptor orbital exhibiting 27 ± 2% N 2p aminyl radical character, obtained using the aforementioned nitrogen covalency calibration. Combined, these data showcase a direct spectroscopic means of identifying redox-active N-donor ligands and also estimating nitrogen 2p covalency of frontier molecular orbitals in transition metal complexes.

Dataset Information

Characterization of the particulate methane monooxygenase metal centers in multiple redox states by X-ray absorption spectroscopy.

Publications

Characterization of the particulate methane monooxygenase metal centers in multiple redox states by X-ray absorption spectroscopy.

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