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

摘要

On the basis of the ASMOndash;CI calculation, the MCD associated withxhyphen; andyhyphen;polarized electronic transitions in the ferrous heme of the highhyphen;spin type is studied by computational and analytical methods. The ground state5Eeegr;is split into five substates by the mixing of the other lowhyphen;lying quintets,5Exgr;and5B2, through the spinndash;orbit interaction. The temperature dependent factor in ellipticity is proportional to thezcomponent of orbital angular momentum averaged over the five substates. For excited states, 28 configurations of onehyphen;electron excitations from the three lowhyphen;lying quintets are considered; 12 pgr;rarr;pgr; (3a2u, 1a1urarr;4eg), 14drarr;pgr; (u,vrarr;4eg; dmacr;pgr;rarr;2macr;bmacr;1u, 3macr;bmacr;2u), and 2drarr;d(dmacr;pgr;rarr;umacr;,vmacr;) configurations. In the configuration interaction among them, the spinndash;orbit interactions of the 3delectrons are also taken into account. With the use of the energy eigenvalues and eigenfunctions, MCD curves are computed in the range of wavelength 400ndash;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 lowhyphen;spin type.