A comparative study of O2, CO, and NO binding to iron-porphyrin

Minimum-energy structures of O-2, CO, and NO iron-porphyrin (FeP) complexes, computed with the Car-Parrinello molecular dynamics, agree well with the available experimental data for synthetic heme models. The diatomic molecule induces a 0.3-0.4 Angstrom displacement of the Fe atom out of the porphyrin nitrogen (Np) plane and a doming of the overall porphyrin ring. The energy of the iron-diatomic bond increases in the order Fe-O-2 (9 kcal/mol) < Fe-CO (26 kcal/mol) < Fe-NO (35 kcal/mol). The presence of an imidazole axial ligand increases the strength of the Fe-O-2 and Fe-CO bonds (15 and 35 kcal/mol, respectively), with few structural changes with respect to the FeP(CO) and FeP(O-2) complexes. Ln contrast, the imidazole ligand does not affect the energy of the Fe-NO bond, but induces significant structural changes with respect to the FeP(NO) complex. Similar variations in the iron-imidazole bond with respect to the addition of CO, O-2, and NO are also discussed. (C) 1998 John Wiley & Sons, Inc.