A fair beginning has been made in understanding the Mössbauer spectra of many types of haem proteins. The diamagnetic compounds have all the usual difficulties associated with the calculation of any quadrupole spectra, with the added complication of larger molecules and much less crystallographic data. In the ferric paramagnets the large molecules are quite helpful in that they prevent interaction between neighbouring sites and make possible long electron spin-relaxation times. The resulting magnetic Mössbauer spectra contain a large amount of information and, by their complexity, tend to be a guard against ambiguity in interpretation. The theoretical problem of the magnetic hyperfine interaction appears more complex than the quadrupole interaction, but this apparent complexity is only superficial in the sense that it requires more complex mathematical manipulations. The underlying physical problem is much simpler in the magnetic case because it involves interactions only with the relatively few unpaired electrons, and is insensitive to all others and has no lattice contribution. Fortunately the unpaired electrons are those which lie high in energy, and are most likely to be involved in the chemical activity of the enzyme. Most of the results on ferric haems have been fairly satisfactory, and it seems likely that many of the uncertainties will be cleared up by further work. From the physicist's point of view it appears reasonable to expect that the search for better agreement between theory and paramagnetic spectra may be a route to a better understanding of the factors affecting the quadrupole interaction. The integral spin paramagnets are an intermediate case, usually requiring large applied field to reveal their magnetic properties. No high-spin ferrous haems have yet been subjected to detailed magnetic experimental or theoretical investigation, but this should be a fruitful field.

Lang, G. (1970). Mössbauer spectroscopy of haem proteins. Quarterly Reviews of Biophysics, 3(1), 1–60.