Extended X-ray absorption fine structure (EXAFS) studies of hydroxo(oxo)iron aggregates and minerals, and a critique of their use as models for ferritin

Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)(6)(O(2)CPh)(15)] (= Fe-11 and two clusters that crystallise in the same lattice (compound 1), namely, [Fe-17(mu(3)-O)(4)(mu(3)-OH)(6)(mu(2)-OH)(10)(heidi)(8)(H2O)(12)](3+) and [Fe-19(mu(3)-O)(6)(mu(3)-OH)(6)(mu(2)-OH)(8)(heidi)(10)(H2O)(12)](1+). The suitability of these systems as models for the structure of the core of ferritin has been tested by comparing their Fr K-edge EXAFS and X-ray crystallographic results with the Ee K-edge EXAFS data on horse spleen ferritin. The interpretative procedure for the EXAFS analysis was optimised by using the X-ray crystallographic data for compound 1 as a basis. This protocol was then used to interpret the Fe K-edge EXAFS spectra of alpha- and gamma-Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe It-edge EXAFS data on Fr-11 were also considered. The Fe ... Fe distances provide a clear indication of the nature of the hydroxo(oxo)iron assembly, It was found that the iron-iron interactions are the most reliable guide. Clusters are shown to be mure appropriate than infinite lattices as structural models for the core of ferritin.