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

• Published in: Physical chemistry chemical physics : PCCP 2019-01, Vol.21 (4), p.1750-1760
• Main Authors: Aarabi, Mohammad, Omidyan, Reza, Soorkia, Satchin, Grégoire, Gilles, Broquier, Michel, Crestoni, Maria-Elisa, de la Lande, Aurélien, Soep, Benoît, Shafizadeh, Niloufar
• Format: Article
• Language: English
• Subjects: Binding energy; Binding Sites; Chemical Physics; Chemical Sciences; Coordination chemistry; Enthalpy; Hemin - chemistry; Hemin - metabolism; Histidine; Imidazoles - chemistry; Imidazoles - metabolism; Iron - chemistry; Iron - metabolism; Ligands; Physics; Thermodynamics
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c8cp06210b

The binding energy of CO, O2 and NO to isolated ferric heme, [FeIIIP]+, 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 [FeIIIP]+ porphyrin dramatically weakens the [FeIIIP-ligand]+ bond with an up to sevenfold decrease in binding energy owing to the σ donation by N-methylimidazole to the FeIII(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.