Resonant Inelastic X-Ray Scattering Determination of the Electronic Structure of Oxyhemoglobin and Its Model Complex

James Yan, Thomas Kroll, Michael Baker, Samuel Wilson, Richard Decreau, Marcus Lundberg, Dimosthenis Sokaras, Pieter Glatzel, Britt Hedman, Keith Hodgson, Edward I. Solomon

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    Abstract

    Hemoglobin and myoglobin are oxygen-binding proteins with S =
    0 heme fFeO2g8 active sites. The electronic structure of these sites
    has been the subject of much debate. This study utilizes Fe K-edge
    X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic Xray
    scattering (RIXS) to study oxyhemoglobin and a related heme
    fFeO2g8 model compound, [(pfp)Fe(1-MeIm)(O2)] (pfp = mesotetra(
    α,α,α,α-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm =
    1-methylimidazole) (pfpO2), which was previously analyzed using
    L-edge XAS. The K-edge XAS and RIXS data of pfpO2 and oxyhemoglobin
    are compared with the data for low-spin FeII and
    FeIII [Fe(tpp)(Im)2]0/+ (tpp = tetra-phenyl porphyrin) compounds,
    which serve as heme references. The X-ray data show that pfpO2
    is similar to FeII, while oxyhemoglobin is qualitatively similar to
    FeIII, but with significant quantitative differences. DFT calculations
    show that the difference between pfpO2 and oxyhemoglobin is
    due to a distal histidine H-bond to O2 and the less hydrophobic
    environment in the protein, which lead to more backbonding into
    the O2. A valence bond configuration interaction multiplet model
    is used to analyze the RIXS data and show that pfpO2 is dominantly
    FeII with 6-8% FeIII character, while oxyhemoglobin has a very
    mixed wavefunction that has 50-77% FeIII character and a partially
    polarized Fe-O2 π bond.
    Original languageEnglish
    Pages (from-to)2854-2859
    Number of pages6
    JournalProceedings of the National Academy of Sciences
    Volume116
    Issue number8
    Early online date4 Feb 2019
    DOIs
    Publication statusPublished - 4 Feb 2019

    Keywords

    • x-ray spectroscopy
    • resonant inelastic x-ray scattering
    • DFT
    • oxyhemoglobin
    • electronic structure

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