Far infrared spectroscopy on hemoproteins: A model compound study from 1800–100 cm

Infrared spectroscopy measurements on different hemoproteins and models of the active side have been completed for the spectral range from 1800 to 100 cm⁻¹ giving an overview on the contributions expected in the low frequency range. Little is known of the low frequency contribution of proteins in infrared. In order to detect the contributions of heme centers and protein moiety, a systematic study of the infrared spectroscopic properties of the porphyrin ring, the ferric porphyrines with different ligands (hemine and hematine), a heme with 11 amino acids (microperoxidase-11), cytochrome c and cytochrome c oxidase are compared at different pD values and an overview on the relative contributions of hemes, their ligands and the protein site can be provided in the low frequency region. Beside the well know amide I and II modes, the low frequency range is found to be dominated by the amide IV and VI mode around 530/580 cm⁻¹ for cytochrome c and cytochrome c oxidase, as well as further proteins like ferrodoxin. Below 300 cm⁻¹ amide VII modes, doming modes of the heme site and hydrogen-bonding signatures overlap to a broad peak with covering 100–250 cm⁻¹. As clear markers for the iron ligands, bands can be depicted at 388/378 cm⁻¹ (FeN, histidine ligand) and 345 cm⁻¹ FeCl. Furthermore the ring vibration of the protonated histidine is determined at 623 cm⁻¹.