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Role of axial ligand in the electronic structure of model compound I complexes
Author(s) -
Du Ping,
Loew Gilda H.
Publication year - 1992
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560440212
Subject(s) - chemistry , radical ion , porphyrin , ligand (biochemistry) , stereochemistry , molecular orbital , crystallography , computational chemistry , photochemistry , molecule , ion , organic chemistry , receptor , biochemistry
A quantum chemical study of the two low‐lying quartet states of seven model compound I iron–porphyrin complexes with varying axial ligands has been carried out using the INDO method. The varying axial ligands included in this study are five that are models for those in the intact enzymes: imidazole and imidazolate (model peroxidase HRP and CCP), CH 3 CONH 2 (Gln175 mutant of CCP), PhO −1 (catalase), CH 3 S −1 (P450), and two that have been used in biomimetics of these enzymes: Cl −1 (hemin) and PhS −1 (model P450s). The purpose of these studies was to determine the role of the axial ligands in determining (i) the relative energies of the two nearly degenerate quartet electronic states of compound I, involved either as an a 1 u or a 2 u porphyrin π cation radical and (ii) the electron and spin distributions in the a 1 u and a 2 u radical cations of compound I. For most of the model complexes, including both HRP‐I and CAT‐I, a moderate effect of the axial ligand on the relative energy of these two states was observed and the a 1 u radical cation was found to be the ground state. The energy order of these two radical cations, however, was reversed in the P450‐I model complexes, indicating an association of the unique property of the FeO bond breaking with an a 2 u radical cation. The symmetry‐allowed overlap between the FeO and 3 a 2 u orbitals may lower the activation energy for the FeO bond cleavage in P450‐I. However, the calculated electronic and spin properties, including the unpaired spin and net charge on the oxygen and the FeO bond overlap density, important determinants of the reactivity of this complex in the ligand–FeO region, are very similar for all complexes and in both cation states. © 1992 John Wiley & Sons, Inc.