Soft-x-ray absorption spectroscopy at the $L_{2,3}$ edge of the iron center in bovine hemoglobin and hemin under physiological conditions is reported for the first time. Spectra of the same compounds in solid form are presented for comparison. Striking differences in the electronic structure of the metalloporphyrin are observed between the liquid and solid compounds. We unambiguously show that hemoglobin and hemin are in a high-spin ferric state in solution, and that the $2p$ spin-orbit coupling decreases for hemin compared to the hemoglobin, while this is not the case in solids. The spectra were simulated using ligand field multiplet theory, in good agreement with the experiment, allowing quantification of the amount of charge transfer between the porphyrin and ${\mathrm{Fe}}^{3+}$ ion in hemoglobin and in hemin.

• Received 17 October 2008

DOI:https://doi.org/10.1103/PhysRevLett.102.068103