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Zinc selenide nanoparticles (ZnSe NPs) were synthesized in the cavity of the cage-shaped protein apoferritin by designing a slow chemical reaction system, which employs tetraaminezinc ion and selenourea. The chemical synthesis of ZnSe NPs was realized in a spatially selective manner from an aqueous solution, and ZnSe cores were formed in almost all apoferritin cavities with little bulk precipitation. Three factors are found to be important for ZnSe NP synthesis in the apoferritin cavity: (1) the threefold channel, which selectively introduces zinc ion into the apoferritin cavity, (2) the apoferritin internal potential, which favors zinc ion accumulation in the cavity, and (3) the nucleation site, which nucleates ZnSe inside the cavity. The characterization of the synthesized ZnSe NPs by X-ray powder diffraction and energy-dispersive spectrometry revealed that the synthesized NPs are a collection of cubic ZnSe polycrystals. It was shown that the 500 degrees C heat treatment for 1 h under nitrogen gas transformed the polycrystalline ZnSe core into a single crystal, and single-crystal ZnSe NPs free of protein were obtained.

The iron storage protein, apoferritin, has a cavity in which iron is oxidized and stored as a hydrated oxide core. The size of the core is about 7 nm in diameter and is regulated by the cavity size. The cavity can be utilized as a nanoreactor to grow inorganic crystals. We incubated apoferritin in nickel or chromium salt solutions to fabricate hydroxide nanoparticles in the cavity. By using a solution containing dissolved carbon dioxide and by precisely controlling the pH, we succeeded in fabricating nickel and chromium cores. During the hydroxylation process of nickel ions a large portion of the apoferritin precipitated through bulk precipitation of nickel hydroxide. Bulk precipitation was suppressed by adding ammonium ions. However, even in the presence of ammonium ions the core did not form using a degassed solution. We concluded that carbonate ions were indispensable for core formation and that the ammonium ions prevented precipitation in the bulk solution. The optimized condition for nickel core formation was 0.3 mg/mL horse spleen apoferritin and 5 mM ammonium nickel sulfate in water containing dissolved carbon dioxide. The pH was maintained at 8.65 using two buffer solutions: 150 mM HEPES (pH 7.5) and 195 mM CAPSO (pH 9.5) with 20 mM ammonium at 23 degrees C. The pH had not changed after 48 h. After 24 h of incubation, all apoferritins remained in the supernatant and all of them had cores. Recombinant L-ferritin showed less precipitation even above a pH of 8.65. A chromium core was formed under the following conditions: 0.1 mg/mL apoferritin, 1 mM ammonium chromium sulfate, 100 mM HEPES (pH 7.5) with a solution containing dissolved carbon dioxide. About 80% of the supernatant apoferritin (0.07 mg/mL) formed a core. In nickel and chromium core formation, carbonate ions would play an important role in accelerating the hydroxylation in the apoferritin cavity compared to the bulk solution outside.

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CdSe nanoparticles were synthesized in the cavity of the cage-shaped protein, apoferritin, by designing the slow chemical reaction system of Cd2+ and Se2−. The cavity size of apoferritin molecules is around 7 nm and the diameter of synthesized CdSe nanoparticles is about 6 nm with small size dispersion. CdSe nanoparticles as prepared are multi-crystalline and can be made into a single crystal by the heat-treatment under nitrogen gas. This bio-template synthesis provides a new method to synthesize uniform CdSe nanoparticles.

Adhesion force analysis using atomic force microscopy clearly revealed for the first time the mechanism underlying the specific binding between a titanium surface and ferritin possessing the sequence of Ti-binding peptide in its N-terminal domain. Our results proved that the specific binding is due to double electrostatic bonds between charged residue and surface groups of the substrate. Furthermore, it is also demonstrated that the accretion of surfactant reduces nonspecific interactions, dramatically enhancing the selectivity and specificity of Ti-binding peptide.

X-ray fibre diffraction analysis of bacterial flagellar filaments has allowed the subunit packing and secondary structure arrangement in the filament core to be determined. The central hole, presumably a channel for flagellin transport, is large enough to accommodate the folded elongated flagellin molecules during their transport to the distal end for filament growth.

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