Systematic Trends in Metalloporphyrin Optical Spectra

We report the effects of axial ligands (L) on the optical spectra of cobalt-substituted myoglobin, horseradish peroxidase, and cytochrome P450_cam. In addition, we present data for a series of octapyrrole-substituted zinc porphyrin complexes. The response of a metalloporphyrin to the change of ligand is more complicated than previously noted and is not the same for pyrrole-substituted porphyrins as it is for meso-tetraaryl-substituted compounds. Perturbations by the ligand not only shift the overall spectrum as previously observed but also systematically change the frequency difference between the Soret (B) and the α-β (Q) bands. The observed trends in band frequencies can nevertheless be readily understood within the four-orbital model of Gouterman. Interpretation of band intensities is less clear.