Mineralization constrained biological

The defined architecture of the metalloprotein ferritin, a natural complex of iron oxide, is found in almost all domains of life and has been used as a constrained reaction vessel for the synthesis of a number of non-natural metal oxides [28, 34]. The protein ferritin consists of 24 subunits that self-assemble into a cage, consisting of a threefold hydrophilic channel coordinated to a fourfold hydrophobic channel [20, 28]. In biology, Fe(ll) is introduced into the core of the apoprotein through its hydrophiUc charmels where the ferrous ion is catalytically oxidized to a less-soluble ferric ion, Fe(lll) [20]. The ferric ion then undergoes a series of hydrolytic polymerizations to form the insoluble ferric oxyhydroxide mineral (ferrihydrite), which is physically constrained by the size of the protein cage (12 nm outer diameter, 8nm inner diameter) [35]. The enzyme ferrous oxidase is coordinated within the protein cage, the interior and exterior of which is electrostatically dissimilar, to produce spatially defined minerals. 

---