Theoretical studies of oxygen binding

We discussed the bonding of O2 to hemoglobin using results of ab initio calculations of idealized portions of the Hb molecule. The bond between Fe and O2 is formed by coupling a triplet state (intermediate spin state) of Fe to the triplet ground state of O2 (analogous to the bonding of O to O2 in ozone). The coordination sphere of the Fe reduces the energy separation between the quintet, triplet, and singlet states, making an intermediate spin state accessible for bond formation. This provides the mechanism by which an O2 molecule can easily and reversibly bind to Hb. Neither the diamagnetic (t2g) excited state of Fe nor the excited singlet state of O2 play a role in the formation of the FeO2 bond. We also discussed the role of the Fe intra-atomic exchange terms and show how they serve to store electronic energy upon bond formation. An example was given, illustrating how this stored electronic energy can then be used to drive enzymatic reactions. Metal atoms such as ferrous Fe are capable of existing in several distinct electronic configurations, depending upon the ligands. Our objective here has been to illustrate the different characteristics of these Fe configurations and to indicate why various axial ligands stabilize particular Fe configurations. In addition, we have sketched the type of orbital descriptions arising from theoretical wavefunctions and illustrated how to use these descriptions to predict chemical phenomena.