Synthesis, structure and magnetic properties of ferritin cores with varying composition and degrees of structural order: models for iron oxide deposits in iron-overload diseases

The cage-like protein ferritin was used to form nanoscale iron-containing mineral particles in vitro with different structures and compositions by reconstituting the metal-free protein (apoferritin) with iron at different temperatures and in the presence of different quantities of phosphate, The products of reconstitution were studied with inductively coupled plasma spectrometry, transmission electron microscopy, electron diffraction, extended X-ray absorption fine structure analysis, and Mossbauer spectroscopy. Reconstitution at 4 degrees C resulted in poorly ordered core structures while reconstitution at 55 degrees C resulted in more ordered structures based on that of the mineral ferrihydrite. The more ordered structure of the 55 degrees C ferritin resulted in stronger magnetic exchange interactions between the iron atoms within each core and a larger magnetic anisotropy energy per core. Incorporation of phosphate within the core structure reduced the core density. This also reduced the strength of the magnetic exchange interactions between the iron atoms. High levels of phosphate within the core resulted in cores with no measurable periodicity within their structure. This in turn caused a reduction in the magnetic anisotropy energy per core. The ability to tailor the degree of structural order and phosphate content of ferritin cores in vitro makes available a range of model materials for a more comprehensive study of the structural and magnetic correlations found in nanoscale iron biominerals in vivo such as native ferritins and haemosiderins deposited in iron-overloaded tissues.