Ozone model for bonding of an O2 to heme in oxyhemoglobin

Several rather different models of the Fe-o2 bond in oxyhemoglobin have previously been proposed, none of which provide a satisfactory explanation of several properties. We propose a new model for the bonding of an O2 to the Fe of myoglobin and hemoglobin and report ab initio generalized valence bond and configuration interaction calculations on FeO2 that corroborate this model. Our model is based closely upon the bonding in ozone which recent theoretical studies have shown to be basically a biradical with a singlet state stabilized by a three-center four-electron pi bond. In this model, the facile formation and dissociation of the Fe-O2 bond is easily rationalized since the O2 always retains its triplet ground state character. The ozone model leads naturally to a large negative electric field gradient (in agreement with Mössbauer studies) and to z-polarized (perpendicular to the heme) charge transfer transitions. It also suggests that the 1.3 eV transition, present in HbO2 and absent in HbCO, is due to a porphyrin-to-Fe transition, analogous to that of ferric hemoglobins (e.g., HbCN).