O2-binding to heme: electronic structure and spectrum of oxyheme, studied by multiconfigurational methods.

We have studied the ground state of a realistic model of oxyheme with multiconfigurational second-order perturbation theory (CASPT2). Our results show that the ground-state electronic structure is strongly multiconfigurational in character. Thus, the wavefunction is a mixture of many different configurations, of which the three most important ones are approximately 1FeII-1O2 (70%), FeIV-2O2(2-) (12%) and 3FeII-3O2 (3%). Thus, the wavefunction is dominated by closed-shell configurations, as suggested by Pauling, whereas the Weiss 2FeIII-2O2- configuration is not encountered among the 10 most important configurations. However, many other states are also important for this multiconfigurational wavefunction. Moreover, the traditional view is based on an oversimplified picture of the atomic-orbital contributions to the molecular orbitals. Thus, the population analysis indicates that all five iron orbitals are significantly occupied (by 0.5-2.0 electrons) and that the total occupation is most similar to the 3FeII-3O2 picture. The net charge on O2 is small, -0.20 e. Thus, it is quite meaningless to discuss which is the best valence-bond description of this inherently multiconfigurational system. Finally, we have calculated the eleven lowest ligand-field excited states of oxyheme and assigned the experimental spectrum of oxyhemoglobin with an average error of 0.24 eV.