Local Structure of Heme-Iron Studied by High-Resolution XANES: Thermal Spin Equilibrium in Myoglobin

Abstract

The local structure of heme-iron of alkaline met-myoglobin (MbOH) during the thermal spin equilibrium between high and low spin states has been studied by a high-resolution XANES. The spin-state sensitive near edge features have been found in the pre-edge ls-3d transition peak and absorption threshold region. These characteristic features are dependent on the high spin concentration and are related to the d-electron configuration and local structure of heme-iron, i. e. the distortion of octahedral ligand field caused by a displacement of heme-iron from the heme plane. The spin-state sensitive near-edge structures were also found for other myoglobin derivatives and can be used as a spin state marker. From a systematic high resolution XANES study on MbCO, MbCN and oxy-Mb), near-edge features which are sensitive to the angle between the diatomic ligand molecule and heme normal were found. These characteristic features can provide the elaborate local structure of heme-iron such as the displacement of iron atom from the hemeplane or the bond angle of ligand molecule when combined with EXAFS experiments.