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

Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO2}(8) 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 X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO2}(8) model compound, [(pfp) Fe(1-MeIm)(O-2)] (pfp = meso-tetra(alpha, alpha, alpha, alpha-o-pivalamido-phenyl) porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO(2)), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO(2) and oxyhemoglobin are compared with the data for low-spin Fe-II and Fe-III [ Fe(tpp)(Im)(2)](0/+) (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO(2) is similar to Fe-II, while oxyhemoglobin is qualitatively similar to Fe-III, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO(2) and oxyhemoglobin is due to a distal histidine H bond to O-2 and the less hydrophobic environment in the protein, which lead to more back-bonding into the O-2. A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO(2) is dominantly Fe-II with 6-8% Fe-III character, while oxyhemoglobin has a very mixed wave function that has 50-77% Fe-III character and a partially polarized Fe-O-2 pi-bond.