Local Energy Gap Opening Induced by Hemin Dimerization in Aqueous Solution

The local electronic structure of the hemin Fe center has been investigated by X-ray absorption and emission spectroscopy (XAS/XES) for hemin in aqueous solution where hemin dimerization occurs. The XAS and XES spectra of the hemin dimer were then compared with those of the hemin monomer we previously studied in dimethyl sulfoxide solution. A local energy gap opening at the Fe sites was observed for the hemin dimer, with the occupied valence states shifted to lower binding energies, while the unoccupied valence states share the same energies as the hemin monomer. Such a gap opening is argued to originate from the Fe 3d orbital localization induced by hemin dimerization in aqueous solution.