Protein‐Mediated Synthesis of Uniform Superparamagnetic Magnetite Nanocrystals

Abstract

Magnetite nanocrystals are synthesized in the presence of a recombinant Mms6 protein thought to be involved in the biomineralization of bacterial magnetite magnetosomes, the mammalian iron‐storage protein, ferritin, and two proteins not known to bind iron, lipocalin (Lcn2) and bovine serum albumin (BSA). To mimic the conditions at which magnetite nanocrystals are formed in magnetotactic bacteria, magnetite synthesis is performed in a polymeric gel to slow down the diffusion rates of the reagents. Recombinant Mms6 facilitates formation of ca. 30 nm single‐domain, uniform magnetite nanocrystals in solution, as verified by using transmission electron microscopy analysis and magnetization measurements. The nanocrystals formed in the presence of ferritin, Lcn2, and BSA, do not exhibit the uniform sizes and shapes observed for those produced in the presence of Mms6. Mms6‐derived magnetite nanoparticles show the largest magnetization values above the blocking temperature, as well as the largest magnetic susceptibility compared to those of the nanomaterials synthesized with other proteins. The latter is indicative of a substantial effective magnetic moment per particle, which is consistent with the presence of magnetite with a well‐defined crystalline structure. The combination of electron microscopy analysis and magnetic measurements confirms our hypothesis that Mms6 promotes the shape‐selective formation of uniform superparamagnetic nanocrystals. This provides a unique bioinspired route for synthesis of uniform magnetite nanocrystals.

Self-Assembly and Biphasic Iron-Binding Characteristics of Mms6, A Bacterial Protein That Promotes the Formation of Superparamagnetic Magnetite Nanoparticles of Uniform Size and Shape

“…The polyhistidine tag attached to the N-terminal domain has previously been shown not to promote the formation of superparamagnetic nanoparticles. 9 Two Distinct Phases of Iron Binding by Mms6. The high affinity constant was determined at pH 7.5 using the Fe 3+ -citrate equilibrium to establish accurate low levels of free Fe 3+ .…”

Magnetic irreversibility and the Verwey transition in nanocrystalline bacterial magnetite

“…͑4͒ Although the degree of crystallinity in magnetite nanoparticles certainly plays a role ͑no magnetic signature at T V was found in ferritin-templated amorphous nanoparticles 46 ͒, the Verwey transition remains smeared even in perfect, but uncorrelated, nanoparticles.…”

Morphological Transformations in the Magnetite Biomineralizing Protein Mms6 in Iron Solutions: A Small-Angle X-ray Scattering Study

“…28 In this SAXS study, we investigate the interactions of Mms6 with iron salts in Tris/KCl solutions under very similar conditions to room temperature in vitro synthesis of magnetite. 20,21 The role of pH in the morphology of the protein is also reported as a control in that the presence of iron in solutions affects the pH. The work presented here deals with the interaction of the biomineralization protein with the iron ions, which is the first step in the formation of magnetic nanoparticles using this bioinspired approach.…”

“…31 In contrast, in the presence of Mms6, magnetite slowly grows to ∼30-nm-diameter superparamagnetic nanoparticles of uniform size and cuboctahedral shapes. 20,21 The interactions between Mms6 and iron are believed to be the initial steps of biomineralization, and also to be important in determining the shape of the magnetite nanoparticles. 16,21 The Mms6−iron interactions at various pH values have been previously investigated on aqueous surfaces, where Mms6 was deposited on the surface of an iron solute subphase to form a monolayer, taking advantage of its amphiphilic behavior.…”

“…15−19 It has been shown to promote the formation of uniform superparamagnetic magnetite nanocrystals at room temperature and mild conditions in vitro. 15,20,21 In addition, Mms6 promotes in vitro synthesis of other advanced functionalized materials, such as cobalt ferrite, cobalt doped magnetite, and magnetic nanoparticle arrays. 22−26 Such magnetic materials synthesized with Mms6 have many promising potential applications in targeted drug delivery, magnetic resonance imaging (MRI) contrast agents, and high density data storage.…”