The electronic structure of ferrous heme with imidazole as the fifth ligand. II. Explanation of MCD spectra from the near IR region to the Soret region

On the basis of the ASMO–CI calculation, the MCD associated with x- and y-polarized electronic transitions in the ferrous heme of the high-spin type is studied by computational and analytical methods. The ground state 5Eη is split into five substates by the mixing of the other low-lying quintets, 5Eξ and 5B2, through the spin–orbit interaction. The temperature dependent factor in ellipticity is proportional to the z component of orbital angular momentum averaged over the five substates. For excited states, 28 configurations of one-electron excitations from the three low-lying quintets are considered; 12 π→π (3a2u, 1a1u→4eg), 14 d→π (u,v→4eg; d̄π→2̄b̄1u, 3̄b̄2u), and 2 d→d (d̄π→ū,v̄) configurations. In the configuration interaction among them, the spin–orbit interactions of the 3d electrons are also taken into account. With the use of the energy eigenvalues and eigenfunctions, MCD curves are computed in the range of wavelength 400–1000 nm. Observed MCD spectra of deoxymyoglobin are well reproduced especially at low temperature. The theoretical consideration is also developed for the essential mechanism underlying the complex shapes of the MCD spectra with the use of the perturbation analysis and the group theory. Basically, the present theory is also applicable to ferric compounds of the low-spin type.