Cytochromes c′:: Biological models for the S = 3/2,5/2 spin-state admixture?

The electronic ground state of the ferricytochromes c' has been and continues to be the subject of debate, and evidence for a quantum mechanically admixed S=3/2,5/2 ground state in these molecules has often been challenged. This review covers work on model heme complexes in which the properties of the S=3/2,5/2 spin-state admixture have been established, and then, the studies related to the ground-state properties of the heme iron centers in the ferricytochromes c'. The results of studies on the proteins are discussed in light of the knowledge accumulated on five- and six-coordinate ferric porphyrin model complexes.