Theoretical modeling of the heme group with a hybrid QM/MM method

The quality of the results obtained in calculations with the hybrid QM/MM method IMOMM on systems where the heme group is partitioned in QM and MM regions is evaluated through the performance of calculations on the 4-coordinate [Fe(P)] (P = porphyrin), the 5-coordinate [Fe(P)(1−(Me)Im)] (Im = imidazole) and the 6-coordinate [Fe(P)(1−(Me)Im)(O2)] systems. The results are compared with those obtained from much more expensive pure quantum mechanics calculations on model systems. Three different properties are analyzed—namely, the optimized geometries, the binding energies of the axial ligands to the heme group, and the energy cost of the biochemically relevant out-of-plane displacement of the iron atom. Agreement is especially good in the case of optimized geometries and energy cost of out-of-plane displacements, with larger discrepancies in the case of binding energies.