SCHEDULED MAINTENANCE
The dramatic effect of N-methylimidazole on trans axial ligand binding to ferric heme: experiment and theory†
*a
Satchin
Soorkia,
b
Gilles
Grégoire,
b
Maria-Elisa
Crestoni,
d
Aurélien
de la Lande,
e
Benoît
Soep
f
and
Niloufar
Shafizadeh
*b
Abstract
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.
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