Oxygen binding on hemoglobin and the electronic state of heme-iron

To study the biological function of hemoproteins, the electronic state of the central iron ion was determined by means of magnetic measurements. The temperature dependence of the magnetic susceptibility, the field dependence of the magnetization (paramagnetic saturation) under a high magnetic field of a superconducting magnet and the torque of a single crystal were studied. The analysis of the data was made on the basis of the ligand field theory, and the electronic state of the central iron ion was determined. In the case of hemoglobin, binding of O2to the heme is studied by the Hietler-London calculation with the three ionic structures Fe²⁺-O2, Fe³⁺-O2-, Fe+-O2+. Hamiltonian matrix elements are evaluated by the method of "Atoms in a Molecule". It is shown from the calculation that one of the triplet-triplet binding states of heme-O2becomes the lowest in energy at a distance of about 1.5 Å.