A Model Compound Study of the CO-Adduct of Cytochrome P-450

Abstract A model compound was prepared having the same optical properties in the Soret and visible regions as the CO-adduct of ferrous cytochrome P-450. Obligate for the formation of this spectral species absorbing at 450 nm (emm = 93) and 555 nm (emm = 23) in a dimethylsulfoxide-ethanol solvent system was the presence of ferrous heme, CO, thiol, and a strong base. Decreasing the dielectric constant of the solvent, which would be equivalent to increasing the hydrophobicity of the heme environment in the protein, causes a small shift in the Soret absorption to the blue with no change in emm. The absence of thiol or strong base resulted in the formation of typical heme·CO absorptions at 413 nm (emm = 150), 535 nm (emm = 25), and 566 nm (emm = 30). It is concluded that the sulfur-containing axial ligand in the model compound and also in ferrous cytochromes P-450·CO is a mercaptide anion.