The Structure-Reactivity Relationship in Heme Complexes. An Important Role of Rotational Orientations of Axial Ligands

点击翻译按钮获取中文摘要

Electron spin resonance (ESR) data for low-spin Fe(III) heme complexes are analyzed within the framework of low-symmetry (C_2) crystal field model. In the analysis, the effect of the rotational orientation angle (phi_*) of axial ligands is taken into consideration. Equations are derived which relate phi_* with the orientation angle of the rhombic field and those of the principal axes of g and hf tensors. The direction of the maximum unpaired spin density, derived from the wave functions of the Kramers doublets, is found approximately parallel to the direction of the low-symmetry field.With these angular relations, ESR data previously reported for myoglobin azide (MbN_3) and those recently observed for model heme complexes, such as Fe(TPP)(OMe)_2^- and Fe(TPP)(OMe)(CN), are reevaluated and analyzed, respectively. It is found that the direction of the maximun unpaired spin density in MbN_3 is different from that reported previously. In model complexes, the angular analysis has enabled us to determine the relative orientations of g and hf tensors, the sign of g tensor determinant, and the low-symmetry crystal field parameters. The structure-reactivity relationship in heme complexes is discussed in the light of the angular relations and emphasis is placed to an important role of the axial ligand orientation.