Photochemical reactivity of ferritin for Cr(VI) reduction

The iron storage protein ferritin was used to catalyze the photoreduction of aqueous Cr(VI) species to Cr(III). Ferritin is a 24 subunit protein of roughly spherical shape with outer and inner diameters of approximately 12 and 8 nm, respectively. The native mineral core of ferritin is the ferric oxyhydroxide ferrihydrite (Fe(O)OH). Fe(O)OH particles that were used in these experiments ranged from 5 to 7.5 nm in diameter. The ferritin protein without the Fe(O)OH core (i.e., apoferritin) was inactive toward Cr(VI) reduction under our experimental conditions, suggesting that the Fe(O)OH provided the active catalytic sites in the redox chemistry. Experiments using photon band-pass filters suggested that the reaction occurred out of a photoinduced electron-hole pair and the optical band gap for the Fe(O)OH semiconductor was determined to be in the range 2.5-3.5 eV. Comparison of ferritin and protein-free Fe(O)OH mineral nanoparticles indicated that ferritin provided a photocatalyst with significantly more stability to aggregation and the loss of catalytic activity.