BINDING OF OXYGEN AND CARBON-MONOXIDE TO HEMOGLOBIN - ANALYSIS OF THE GROUND AND EXCITED-STATES

Two quantum mechanical calculations on iron-porphyrin complexes that are models for the active site in oxy-and carboxyhemoglobin are reported. The first calculation uses an extended Pariser-Parr-Pople (PPP) Hamiltonian and includes configuration interaction among singly and doubly excited configurations; the second employs the Xa multiple scattering method. A critical comparison is made between the results of these methods and others (extended Hiickel, ab initio Hartree-Fock) that have been used to examine such complexes. Ground-state properties, including the energy and charge distribution, are examined. It is found that correlation effects involving doubly excited configurations must be included to obtain a singlet ground state for the oxygen complex; there is only a small effect from these added configurations on the ground-state charge distribution. The FeC>2 unit is shown to be well represented as an equal mixture of Fe2+(S = 0), O2CS = 0) and Fe2+(S = 1), O2(S = 1) valence-state pairs; the latter resembles ozone in certain respects. The FeCO unit corresponds closely to an idealized Fe2+(S = 0), CO(S = 0) species. Calculated Mossbauer splittings and infrared stretching frequencies in approximate agreement with the experimental values for both complexes provide support for the present treatment. A detailed analysis of the excited states is presented, and the results are compared with the available data for ten transitions in oxyhemoglobin and five in carboxyhemoglobin. For oxyhemoglobin, in addition to the well-known porphyrin π → π transitions, iron d→d transitions and a variety of charge-transfer transitions are identified. Extended Hiickel, PPP, and Xa calculations agree that an unoccupied FeO2 π orbital plays an important role in the low-energy spectrum. In carboxyhemoglobin no such low-lying orbital is present and a much simpler spectrum results. © 1979, American Chemical Society. All rights reserved.