The absorption spectra, magnetic moments and the binding of iron in some haemoproteins

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Abstract
A classification of the absorption bands of ferric porphyrin complexes is suggested. There are two groups of absorption bands, one group of two due to charge transfer to high-spin Fe3+ ions, and another group of three due to the absorption bands of the porphyrin. Only the second group of bands appears in low-spin complexes. An analysis of the absorption spectrum can be used to estimate the amounts of low-spin and high-spin ferric iron present in complexes which are equilibrium mixtures. The relationships between the properties of known groups in the fifth and sixth co-ordination positions and the absorption spectra of iron porphyrin complexes are described. These relationships lead to a set of diagnostic rules by which the type of group bound to the iron porphyrin can be recognized. The rules for ferric and ferrous porphyrin derivatives differ and are separately discussed. Magnetic, oxidation-reduction and some chemical properties provide additional evidence upon which the diagnosis of co-ordinating groups can be based. As an illustration of the diagnostic method, the nature of the groups binding the iron in the haemoproteins catalase, peroxidase, haemoglobin and myoglobin is discussed., as is the co-ordination in the peroxide compounds II and III of catalase and peroxidase.