Probing the Electronic Structure of the Hemoglobin Active Center in Physiological Solutions

Soft-x-ray absorption spectroscopy at the L2,3 edge of the iron center in bovine Hb and hemin under physiol. conditions is reported for the first time. Spectra of the same compds. in solid form are presented for comparison. Striking differences in the electronic structure of the metalloporphyrin are obsd. between the liq. and solid compds. We unambiguously show that Hb and hemin are in a high-spin ferric state in soln., and that the 2p spin-orbit coupling decreases for hemin compared to the Hb, while this is not the case in solids. The spectra were simulated using ligand field multiplet theory, in good agreement with the expt., allowing quantification of the amt. of charge transfer between the porphyrin and Fe3+ ion in Hb and in hemin.