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 m ethod IMOMM on systems where the heme group is partitioned in QM and MM reg ions is evaluated through the performance of calculations on the 4-coordina te [Fe(P)] (P = porphyrin), the 5-coordinate [Fe(P)(1 - (Me)Im)] (Im = imid azole) and the 6-coordinate [Fe(P)(1 - (Me)Im)(O-2)] systems. The results a re compared with those obtained from much more expensive pure quantum mecha nics calculations on model systems. Three different properties are analyzed -namely, the optimized geometries, the binding energies of the axial Ligand s 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. (C) 2000 John Wiley & Sons, Inc.