Iron oxide crystal formation on a substrate modified with the Mms6 protein from magnetotactic bacteria
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Iron oxide crystal formation on a substrate modified with the Mms6 protein from magnetotactic bacteria Atsushi Arakaki, Fukashi Masuda and Tadashi Matsunaga Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei-shi, Tokyo 184-8588, Japan ABSTRACT Mms6 is a small acidic protein which is tightly bound to magnetite in the bacterium Magnetospirillum magneticum AMB-1. Mms6 has been previously shown to promote ironbinding capacity as well as modulate the size and morphology of magnetic iron oxide crystals in vitro. In this study, we synthesized iron oxide crystals by using a monolayer-modified substrate. A self-assembled monolayer of octadecyltrimethoxysilane was modified on a silicon substrate. Recombinant Mms6 protein was attached to the substrate through the hydrophobic interactions between the protein molecules and the monolayer. The immobilization of protein molecules on the substrate surface was confirmed by fluorescent labeling of these molecules and subsequent fluorescence microscopy. This protein-modified substrate was then used as a template for iron oxide crystal formation in a ferrous solution. Scanning electron microscopy revealed sitespecific formation of iron oxide crystals in substrate regions with immobilized proteins. This use of proteins might provide an alternative method for the bottom-up fabrication of nano-sized magnetic particles. INTRODUCTION Magnetite is a magnetic iron oxide with the formula Fe3O4, and has numerous applications in the fields of electronics and biotechnology. The methods for the controlled formation of Fe3O4 crystals on solid surfaces include molecular beam epitaxy 1, sputtering 2, pulsed laser deposition 3, and solid reaction 4. However, these methods require the use of large equipments or extremely high temperatures in reactions. Given the increasing demand for magnetic nanomaterials, simpler alternative processes are required for the site-specific formation of magnetite crystals. Magnetotactic bacteria synthesize iron oxide magnetic nanoparticles under relatively mild conditions 5. The sizes and morphologies of these nano-sized magnetic crystals are highly conserved within bacterial species or strains, implying the presence of a biologically controlled mineralization process at the genetic level. Genome sequencing and proteome analysis of magnetotactic bacteria have helped characterize several novel proteins involved in magnetite biosynthesis in these microorganisms 6. Mms6 represents a class of proteins that are tightly bound to the magnetite surface in the bacterium Magnetospirillum magneticum AMB-1 7. Mms6 is amphiphilic, and consists of a hydrophobic N-terminal LG-rich region and a hydrophilic Cterminal region containing several acidic amino acids. Competitive iron-binding analysis with other inorganic cations suggests that the C-terminal region is an iron-binding site. Furthermore, magnetite has been formed by chemical synthetic methods in the presence of Mms6, producing uniform crystals with sizes ranging from 20 nm to 30 nm 7,8. Althoug
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