Comparative Quantum-Chemical Analysis of the Electronic Structures of the Valence Regions of Active Sites in Cytochromes f and c.

A comparative analysis of densities of states has been carried out for the valence regions of the hemes and clusters of cytochromes f and c using the ZINDO1 semiempirical quantum-chemical method. The molecular orbitals of these structures are formed from the p atomic orbitals of nitrogen and carbon of the porphyrin ring, making equal contributions. For systems with negative charges, more than half of all added electrons are distributed over the porphyrin parts of the molecules. The molecular orbital energies of the valence regions of the corresponding clusters and hemes are nearly the same. In iron porphyrin, as well as in heme f and cytochrome cluster f, the lowest unoccupied molecular orbital is doubly degenerate. In heme c and cytochrome cluster c, the degeneracy is lifted because of the asymmetry of the nearest aminoacid environment and substituents in the porphyrin ring.