The use of near-infrared charge-transfer transitions of low-spin ferric chlorins in axial ligand assignment.

The near-infrared magnetic circular dichroism spectra of some low-spin derivatives of ferric-octaethylchlorin substituted myoglobin have been recorded at cryogenic temperatures. The spectra, which include some of the lowest energy charge-transfer transitions ever observed for hemes, are clearly dependent upon the nature of the axial ligands present. While the results indicate that such spectra may have some practical utility in axial ligand assignment, as is now quite common practice for iron-porphyrin systems, there are some severe practical limitations to this protocol documented in the case of iron-chlorins.