The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory

Abstract

The binding energy of CO, O₂ and NO to isolated ferric heme, [Fe^IIIP]⁺, was studied in the presence and absence of a σ donor (N-methylimidazole and histidine) as the trans axial ligand. This study combines the experimental determination of binding enthalpies by equilibrium measurements in a low temperature ion trap using the van’t Hoff equation and high level DFT calculations. It was found that the presence of N-methylimidazole as the axial ligand on the [Fe^IIIP]⁺ porphyrin dramatically weakens the [Fe^IIIP–ligand]⁺ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the Fe^III(3d) orbitals. This trans σ donor effect is characteristic of ligation to iron in hemes in both ferrous and ferric redox forms; however, to date, this has not been observed for ferric heme.