A comparison of an undecairon(III) complex with the ferritin iron core

The iron core of ferritin is comprised of up to 4,500 Fe(III) atoms as Fe2O3.nH2O, which is maintained in solution by a surrounding, spherical coat of protein. Organisms as diverse as bacteria and man use the ferritin iron-protein complex as a reservoir of stored iron for other essential proteins. To extend studies of the steps in polynuclear iron core formation, a recently characterized undecairon(III) oxo-hydroxo aggregate [Fe11 complex] (Gorun et al., J. Am. Chem. Soc. 109, 3337 [1987]) was examined by x-ray absorption spectroscopy as a model for an intermediate. The results, which are comparable to the previous x-ray diffraction studies, show near neighbors (Fe-O) at 1.90 A that are distinct from those in ferritin and a longer distance of 2.02 A. However, contributions from neighbors (Fe-C) known to exist at ca. 2.7 A were obscured by a highly ordered Fe-Fe interaction and were not detectable in the Fe11 complex in contrast to a previously characterized Fe(III) cluster bound to the protein coat. Of the two Fe-Fe interactions detectable in the Fe11 complex, the shortest, at 3.0 A is particularly interesting, occurring at the same distance as a full shell (CN = 6) in ferritin, but having fewer Fe neighbors (CN = 2-3) characteristic of an intermediate in core formation. The incomplete Fe-Fe shell is much more ordered than in ferritin, suggesting that the disorder in ferritin cores may be associated with the later steps of the core growth. Differences between the Fe11 complex and the full core of ferritin indicate the possibility of intermediates in ferritin iron formation that might be like Fe11.