Ozone model for bonding of an O₂ to heme in oxyhemoglobin

Several rather different models of the Fe O₂ bonds in oxyhemoglobin have previously been proposed, none of which provide a satisfactory explanation of several properties. The authors 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. The 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 Mossbauer 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).