Precise determination of zero-field splitting parameters of hemin by high-field and high-frequency electron paramagnetic resonance

Abstract

The zero-field splitting (ZFS) parameters of Fe(III) protoporphyrin IX chloride, or hemin, a model substance of hemoproteins, were determined precisely by high-field and high-frequency electron paramagnetic resonance (HFEPR). From multi-frequency measurements up to 700 GHz, multiple EPR absorptions were clearly resolved, and the rhombic component of ZFS was directly determined, for the first time, as /E/ = 0.055 ± 0.005 cm-1, in addition to the axial component D = 6.90 ± 0.01 cm-1. This finding indicates the essential role of the rhombic symmetry of excited states in the ZFS parameters.