Magnetic circular dichroism spectroscopy of the iron cores of ferritin and bacterioferritin

Ferritin and bacterioferritin (BFR) are iron-storage proteins that contain mineralized non-haem iron cores which have a maximum diameter of 80 A. The relationships between the magnetic properties and chemical characteristics of these cores are not fully understood, and to help define these we have used magnetic circular dichroism (MCD) spectroscopy in study of these molecules. The MCD spectra reported here are the first examples where this technique has been applied to a study of both the ferritin and the BFR core. It is clear that the two core types have different spectral characteristics in accord with their different chemical composition. At low temperature (<4K) the ferritin core displays a form of magnetic hysteresis which is not observed when the temperature is raised to 77 K. In contrast, the BFR core does not give rise to such an effect even at very low temperature. The data appear to indicate that the ferritin core is highly anisotropic at low temperature whereas the BFR core is not, and that spin relaxation between the ground state energy levels of the ferritin core is exceedingly slow.