Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Abstract

Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO2}8active 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 X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO2}8model compound, [(pfp)Fe(1-MeIm)(O2)] (pfp = meso-tetra(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 pfpO2and oxyhemoglobin are compared with the data for low-spin FeIIand FeIII[Fe(tpp)(Im)2]0/+(tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO2is similar to FeII, while oxyhemoglobin is qualitatively similar to FeIII, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO2and oxyhemoglobin is due to a distal histidine H bond to O2and 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 pfpO2is dominantly FeIIwith 6–8% FeIIIcharacter, while oxyhemoglobin has a very mixed wave function that has 50–77% FeIIIcharacter and a partially polarized Fe–O2π-bond.