THEORETICAL-STUDY ON THE ELECTRONIC-SPECTRA OF MODEL COMPOUND-II COMPLEXES OF PEROXIDASES

A quantum chemical study of the electronic structure and spectra of four compound II iron porphyrin complexes with varying axial ligands, Cl- (Hemin), pyridine, imidazole, and imidazolate (model peroxidase), has been carried out using the INDO/S method. These calculations confirm the nature of the triplet ground state as one in which the two unpaired electrons occupy the Fe-O-pi* orbitals that are primarily Fe(IV) 3d(xz) and 3d(yz). About 20% of the unpaired spin density is calculated to be on the oxygen atom, in agreement with the experimental estimate of about 25%. The calculated spectra show that charge-transfer (CT) transitions exist for all compound II complexes. The excitation energies for these CT transitions are estimated to be between the Q and B (Soret) bands and to have moderate oscillator strengths. Although a description of the CT excited state for each of the model systems is complex, an electron transition from the porphyrin-tau orbitals to the Fe-0 tau* orbitals is a common component. For Hemin-II, an additional component of the CT transition wave function is an excitation from the Cl 3p(z) orbital to the porphyrin 4e(g) orbitals. For the pyridine and imidazole complexes, there is a mixing in the CT of an excitation from the porphyrin-tau to a ligand-tau* orbital. For the complex with an imidazolate ligand, mixing of porphyrin tau --> tau* excitations was found. The fact that the model compound II complexes with both a neutral imidazole and a deprotonated imidazolate ligand have CT transitions between Q and B does not allow us to use spectral properties to predict the actual form of this intermediate in the reaction mechanism of protein.