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

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Abstract
Structural models for the hydroxo(oxo)iron core of ferritin include extended mineral structures and cluster systems such as [Fe11O6(OH)6(O2CPh)15] ( = Fe11) and two clusters that crystallise in the same lattice (compound 1), namely, [Fe17(μ3‐O)4(μ3‐OH) 6(μ2‐OH)10(heidi)8(H2O)12]3+and[Fe19(μ3‐O)6(μ3‐OH)6(μ2‐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 Fe K‐edge EXAFS and X‐ray crystallographic results with the Fe 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 α‐ and γ‐Fe(O)OH and to reinterpret that previously recorded for horse spleen ferritin. The published Fe K‐edge EXAFS data on Fe11 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 more appropriate than infinite lattices as structural models for the core of ferritin.