Spin states of myoglobin heme iron in aqueous solutions at room temperature probed from porphyrins using nitrogen K-edge X-ray absorption spectroscopy.

The porphyrin CN π* peaks of myoglobin heme iron in aqueous solutions at room temperature were observed using nitrogen K-edge X-ray absorption spectroscopy (XAS), which enabled separation from the protein polypeptide peaks. The spin states of the heme iron were investigated by interpreting the CN π* peaks through inner-shell calculations. Oxymyoglobin, in which an Fe2+ ion is connected to an oxygen molecule, is in the S = 0 state. By contrast, deoxymyoglobin with an Fe2+ ion shows spin equilibrium between the S = 2 and 1 states, while metmyoglobin with an Fe3+ ion coordinated to a water molecule shows spin equilibrium between the S = 5/2 and 3/2 states. This study proposes that N K-edge XAS measurements of porphyrins are effective for determining the spin equilibrium of heme proteins, which is influenced by liquid temperature and protein structure.