Other Electronic Properties of Ferric Haemoproteins

In this section we shall review briefly other techniques which are proving valuable as probes into the electronic structure of the haem chromophore. 

Iron compounds are particularly well-suited for studies of the Moss-bauer Effect (80) and a number of haemoglobin derivatives have been investigated by means of this technique (81, 5). Mossbauer spectra should, in principle, yield information about spin-states, but the results as yet are not very conclusive (82, 10). Better results are being obtained with the application of a magnetic field (82). 

Optical rotatory dispersion and circular dichroism (83, 84) can often be of great value, and the spectra are particularly sensitive to the conformation of the protein. Much work remains to be done in this field before the results can be definitely interpreted in terms of the electronic structure of the metal. Magneto-optical rotation and magnetic circular dichroism (MOR and MCD), which are beginning to be applied to porphyrins and haemoproteins, offer much greater promise (30, 85). 

It is of interest to note that the relative intensities of the MOR peaks corresponding to the Soret and a—/ bands (30) are close to that expected from Simpson s theory of the electronic structure of the porphyrin chromophore (29). 

The MOR of the a—/ system is about seven times as intense as that of the Soret band, and is much stronger than that of the high-spin ferric bands at 16 kK and 20 kK. This could be of value in the interpretation of the visible spectra of ferric haemoproteins. 

---