On The Axial Ligand Orientation of Heme Proteins and Model Compounds

Abstract

A number of recent X-ray structural studies in hemoglobin and in a large number of iron porphyrinate derivatives have demonstrated that for the six-coordinated species the imidazole ligand adopts a preferred orientation towards the porphinato nitrogen atom (eclipsed conformation) which is sterically unfavorable (average value is 16.2°). This tendency was interpreted either in terms of intramolecular interactions (crystal packing effects) and/or in terms of electronic effects (Scheidt and Chipman, 1986). In this communication we present our preliminary results of the “hanging imidazole” model (Figure 1) (Momenteau et al., 1983) by the use of 1D and 2D NMR spectroscopy.