Structure of the iron-containing core in ferritin by the extended x-ray absorption fine structure technique

Abstract

Extended X-ray absorption fine structure (EXAFS) measurements are used to determine the structure of the iron-containing core of ferritin. By comparing the EXAFS from ferritin with that from an Fe-glycine model compound, it is found that at room temperature the irons are surrounded by 6.4 +- 0.6 oxygens at 1.95 +- 0.02 A which are likely in a distorted octahedral arrangement. Each iron also has 7 +- 1 iron neighbors at an average distance of 3.29 +- 0.05 A. Considerable structural disorder was found which increased when the ferritin solution was frozen, indicating a possible phase transition occurring at lower temperatures. Combining these results with the known stoichiometry and density it is shown that the structure for the iron core is a layered arrangement with the iron in the interstices between two nearly close-packed layers of oxygens with approximate sixfold rotational symmetry, and that these compact O--Fe--O layers are only weakly bound to adjacent layers. The known phosphorus component is accounted for by terminating the layer into a strip whose width naturally explains the size of the core. The ferritin core consists, in this picture, of a strip folded back and forth upon itself in the form of a pleat.more » Measurements are also presented for two forms of the polymer of Spiro and Saltman, and it is found that only one form is possibly similar to ferritin.« less