Electron paramagnetic resonance studies of a range of ferritins and haemosiderins

The major iron-storage proteins, ferritin and haemosiderin, comprise a protein skin which envelops closely packed Fe^III largely in the form of iron oxyhydroxide. When fully loaded there can be ca. 4500 iron atoms in ferritin and in haemosiderin, but full loading is rare. The EPR spectra comprise very broad features stretching from near-zero field to well beyond the free-spin region. It is commonly supposed that the two proteins give rise to two quite different spectra, that for ferritin being dominated by a maximum in the derivative spectrum at g ≈ 6 (feature A) and that for haemosiderin by a maximum at g ≈ 2.2 (feature B). In attempts to obtain a clearer understanding of these spectral differences we have compared X- and Q-band spectra for a range of ferritins and haemosiderins, and find that the Q-band features are much better defined, with an optimal sensitivity at ca. 150 K. We find no clear distinction between ferritin and haemosiderin samples, some of which have a dominant A component and some a dominant B component. Electron microscopy has been used to estimate the loading of the proteins studied, and the results are discussed in terms of the dominance of either the A or B features.