Ground and excited states of hemoglobin CO and horseradish peroxidase CO: SAC/SAC-CI study

The SAC (symmetry adapted cluster)/SAC-Cl method is applied to the calculations of the ground and excited states of carboxyheme complexes with two kinds of proximal ligands, imidazole (model hemoglobin and myoglobin) and imidazolate (model peroxidases). The Mulliken population analysis for the ground state of horseradish peroxidase CO (HRPCO) shows that the electron push effect from proximal to distal sites causes a charge polarization between the distal ligand and CO in comparison with the model hemoglobin CO (HbCO). The excited states of the HbCO and HRPCO are calculated up to 7.8 and 7.1 eV, respectively. The calculated excitation energies and oscillator strengths correspond well with the observed electronic spectra of these two compounds. The Q bands are well described by the excitations within Gouterman's four orbitals. The higher side shoulder of the B band is named as the n band since it is assigned to a distinct electronic structure. For the B, n, N, L, and M bands, Gouterman's four orbitals plus some lower porphyrin pi orbitals give dominant contributions. The states higher than the M band are characterized by the d-pi, pi-d*, d-CO*, and pi-CO*, etc., and therefore their intensities are predicted to be small. Some important differences in the excited states of HRPCO and HbCO are observed. In HRPCO, the lower porphyrin pi orbitals mix with imidazole sigma orbitals and are unstabilized, so that the B band energy is lower than that of HbCO, and the nature of the N band is very different between them. The different peaks in the spectrum of HbCO are correlated with those of free-base porphine (FBP) and Mg-porphine (MgP) studied previously in our laboratory.