Description of the Electronic Structure of Oxyhemoglobin Using Fe L-Edge X-ray Absorption Spectroscopy

The electronic structure of oxyhemoglobin has been controversial since the discovery of the compound's diamagnetism in 1936. This study uses partial fluorescence yield Fe L-edge X-ray absorption spectroscopy (XAS) in the 3s→2p fluorescence on oxyhemoglobin solutions, measured using a transition-edge sensor detector, to obtain a quantitative experimental description of the electronic structure of the O₂-bound iron site. The spectrum is very different from typical low-spin Fe^II and Fe^III heme spectra, and multiplet simulations indicate a mixed ground configuration with ∼57% low-spin Fe^III and ∼43% low-spin Fe^II character. This is also very different from the Fe^II character found for the picket-fence porphyrin model complex. The oxyhemoglobin L-edge XAS data further show that the O₂ ligand engages in a weak σ- but strong π-bond with the iron ion, leading to the overall strong Fe-O₂ bond required for O₂ transport.