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

The binding energy of CO, O-2 and NO to isolated ferric heme, [(FeP)-P-III](+), was studied in the presence and absence of a sigma 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 [(FeP)-P-III](+) porphyrin dramatically weakens the [(FeP)-P-III-ligand](+) bond with an up to sevenfold decrease in binding energy owing to the sigma donation by N-methylimidazole to the Fe-III(3d) orbitals. This trans sigma 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.