Optical activity of hemoproteins in the Soret region. Circular dichroism of the heme undecapeptide of cytochrome c in aqueous solution.

Different possible mechanism for generation of optical activity of hemoproteins in the Soret region are reconsidered. The heme undecapeptide of cytochrome c does not contain aromatic amino acid residues, so its considerable optical activity cannot be due to coupling of heme pi pi * transitions with those of aromatic residues. CD data for the heme undecapeptide and for ferrimyoglobin and some of their complexes with small molecules are presented and critically compared. Symmetrically coordinated imidazole complexes show rotational strengths of the same magnitude as those of corresponding nonsymmetrically coordinated compounds. Inherent chirality in the bound heme is inferred to be a significant source of optical activity in the heme undecapeptide. Theoretical calculations based upon a molecular dynamics simulation support this proposal. Coupled oscillator interactions with the peptide pi pi * transitions and with the high-energy transitions in the peptide groups and thioether sulfurs, as modeled by polarizabilities, also make significant contributions. These same mechanisms must also be considered in hemoproteins in general.