Short Poster Lecture
The iron storage protein ferritin is well known for its ability to mineralize iron oxides in its interior cavity. In this study, the mineralization behavior of individual subunits of ferritin was investigated, which could be utilized for the controlled mineralization of surfaces without the constrained spaces of the cavities of whole ferritin proteins. Our approach is based on simulation experiments, which suggest that the subunits arrange themselves on silica surfaces with the active side facing away from the interface, although non-covalent adsorption energies might be small. In experimental studies, the mineralization of iron oxide nuclei was studied at ferritin subunits of the H- and L-chain type which were covalently immobilized at the surface of silica nanoparticles. Covalent attachment of the subunits to the substrate was realized using the EDC/NHS cross-linking protocol to immobilize the proteins on previously silanized SiO2 particles. According to the conformational analysis obtained by circular dichroism spectroscopy and BeStSel analysis, the subunits undergo structural changes upon adsorption and immobilization. Mineralization was initiated by the addition of ammonium iron (II) sulfate hexahydrate ((NH4)2Fe(SO4)2∙6H2O) as source for iron ions and trimethylamine-N-oxide as oxidant. Despite the structural changes during adsorption, the mineralization experiments show that the samples with covalently bound H or L subunits induce the formation of iron oxide nuclei, probably acting as passive template for heterogeneous nucleation. The nuclei then grow into a continuous film during the multistep mineralization procedure. In this process, nucleation and crystal growth in solution is kinetically suppressed. The obtained mineral phases are magnetically active, as shown by SQUID experiments.
Accordingly, we demonstrate that H and L chains are able to induce the formation of distinct, magnetic iron oxide crystals (magnetite and/or maghemite) on silica surfaces. This clearly shows that even disassembled, individual H and L subunits can selectively induce iron oxide nucleation and mineralization. Utilizing the mineralization behavior of immobilized Fn subunits opens further possibilities for the fabrication of functional nanomaterials and two-dimensional arrays with a potential for a broad range of magnetic, catalytic, and biomedical sensing applications.