Conformation of biological macromolecules. Circular dichroism and magnetic circular dichroism studies of metmyoglobin and its derivatives.

The circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of horse heart metmyoglobin and the following derivatives were measured in the Soret and near ultraviolet regions: metmyoglobin and its peroxide compound, and hydroxide, cyanide, azide, and fluoride derivatives. The heme-related CD bands in the Soret and near ultraviolet wavelength regions were altered by ligand substitution, though their relationships to the magnetic moment were quite different. In the Soret region, the CD peak had no definite relation to the magnetic moment, while in the near ultraviolet region the magnitude of the CD peak decreased with the magnetic moment. The MCD peak in the Soret and near Ultraviolet regions also varied with ligand substitution. The magnetic ellipticity decreased with the magnetic moment in both wavelength regions. There was a more quantitative correlation between the magnetic ellipticity and the magnetic moment in the near ultraviolet region than in the Soret region. Metmyoglobin peroxide compound exhibited slightly different behavior in the MCD spectrum from other derivatives. It is suggested that the heme iron of the metmyoglobin peroxide compound is in an oxidation state other than the ferric state and that the porphyrin structure of metmyoglobin may be modified by the reaction with hydrogen peroxide.