Heitler-London Calculation on the Bonding of Oxygen to Hemoglobin. I. Singlet States in the Case of O-O Axis Parallel to Heme Plane

Bonding of O 2 to heme is studied by Heitler-London calculation with three ionic structures (Fe 2+ –O 2 , Fe 3+ –O 2 - and Fe + –O 2 + ). The basic configurations are constructed from the low-lying terms of Fe 2+ , Fe 3+ and Fe + in the heme, and those of O 2 , O 2 - and O 2 + . Hamiltonian matrix elements are evaluated with the method of “Atoms in Molecule.” The empirically determined term energies of O 2 are used and the electronic structure of Fe atom in the heme is estimated with the ligand field theory. The molecular integrals necessary for the evaluation of interaction part are computed with the Gaussian type SCF atomic orbitals. As O 2 approaches to the heme, one of the 1 A 2 states, which is the combination of the excited 3 E term of the heme and the ground 3 Σ g - term of O 2 , becomes the ground state. In this state the negative exchange interaction between O 2 and Fe atom is effective in lowering the energy.