Biomimetic magnetite formation: from biocombinatorial approaches to mineralization effects

Understanding microbial biomineralization at the molecular level: recent advances

Microbial biomineralization is a phenomenon involving deposition of inorganic minerals inside or around microbial cells as a direct consequence of biogeochemical cycling. The microbial metabolic processes often create environmental conditions conducive for the precipitation of silicate, carbonate or phosphate, ferrate forms of ubiquitous inorganic ions. Till date the fundamental mechanisms underpinning two of the major types of microbial biomineralization such as, microbially controlled and microbially induced remains poorly understood. While microbially-controlled mineralization (MCM) depends entirely on the genetic makeup of the cell, microbially-induced mineralization (MIM) is dependent on factors such as cell morphology, cell surface structures and extracellular polymeric substances (EPS). In recent years, the organic template-mediated nucleation of inorganic minerals has been considered as an underlying mechanism based on the principles of solid-state bioinorganic chemistry. The present review thus attempts to provide a comprehensive and critical overview on the recent progress in holistic understanding of both MCM and MIM, which involves, organic-inorganic biomolecular interactions that lead to template formation, biomineral nucleation and crystallization. Also, the operation of specific metabolic pathways and molecular operons in directing microbial biomineralization have been discussed. Unravelling these molecular mechanisms of biomineralization can help in the biomimetic synthesis of minerals for potential therapeutic applications, and facilitating the engineering of microorganisms for commercial production of biominerals.

Crystal-Chemical and Biological Controls of Elemental Incorporation into Magnetite Nanocrystals

Magnetite nanoparticles possess numerous fundamental, biomedical, and industrial applications, many of which depend on tuning the magnetic properties. This is often achieved by the incorporation of trace and minor elements into the magnetite lattice. Such incorporation was shown to depend strongly on the magnetite formation pathway (i.e., abiotic vs biological), but the mechanisms controlling element partitioning between magnetite and its surrounding precipitation solution remain to be elucidated. Here, we used a combination of theoretical modeling (lattice and crystal field theories) and experimental evidence (high-resolution inductively coupled plasma-mass spectrometry and X-ray absorption spectroscopy) to demonstrate that element incorporation into abiotic magnetite nanoparticles is controlled principally by cation size and valence. Elements from the first series of transition metals (Cr to Zn) constituted exceptions to this finding, as their incorporation appeared to be also controlled by the energy levels of their unfilled 3d orbitals, in line with crystal field mechanisms. We finally show that element incorporation into biological magnetite nanoparticles produced by magnetotactic bacteria (MTB) cannot be explained by crystal-chemical parameters alone, which points to the biological control exerted by the bacteria over the element transfer between the MTB growth medium and the intracellular environment. This screening effect generates biological magnetite with a purer chemical composition in comparison to the abiotic materials formed in a solution of similar composition. Our work establishes a theoretical framework for understanding the crystal-chemical and biological controls of trace and minor cation incorporation into magnetite, thereby providing predictive methods to tailor the composition of magnetite nanoparticles for improved control over magnetic properties.

Engineered Bacteria with Genetic Circuits Accumulating Nanomagnets as MRI Contrast Agents

The demand for highly efficient cancer diagnostic tools increases alongside the high cancer incidence nowadays. Moreover, there is an imperative need for novel cancer treatment therapies that lack the side effects of conventional treatment options. Developments in this aspect employ magnetic nanoparticles (MNPs) for biomedical applications due to their stability, biocompatibility, and magnetic properties. Certain organisms, including many bacteria, can synthesize magnetic nanocrystals, which help their spatial orientation and survival by sensing the earth's geomagnetic field. This work aims to convert Escherichia coli to accumulate magnetite, which can further be coupled with drug delivery modules. The authors design magnetite accumulating bacterial machines using genetic circuitries hiring Mms6 with iron-binding activity and essential in magnetite crystal formation. The work demonstrates that the combinatorial effect of Mms6 with ferroxidase, iron transporter protein, and material binding peptide enhances the paramagnetic behavior of the cells in magnetic resonance imaging (MRI) measurements. Cellular machines are also engineered to display Mms6 peptide on the cell surface via an autotransporter protein that shows augmented MRI performance. The findings are promising for endowing a probiotic bacterium, able to accumulate magnetite intracellularly or extracellularly, serving as a theranostics agent for cancer diagnostics via MRI scanning and hyperthermia treatment.

Path-Dependent Anisotropic Colloidal Assembly of Magnetic Nanocomposite-Protein Complexes

Anisotropic self-assembly of nanoparticles (NPs) stems from the fine-tuning of their surface functionality and NP interaction. Strategies involving ligand interaction, protein interaction, and external stimulus have been developed. However, robust construction of monodispersed magnetic NPs to tens of microns of anisotropically aligned colloidal assembly triggered by adsorbed protein intermolecular interaction is yet to be elucidated. Here, we present the NP-protein interaction, magnetic force, and protein corona intermolecular interaction serially but independently induced path-dependent self-assembly of 100 nm Fe₃O₄@SiO₂ nanocomposites. Dynamic formation of the micron-sized anisotropic magnetic assembly was reproducibly realized in a continuous medium in a controllable manner. Formation of the primary globular clusters upon the unique NP-protein complexes with the help of ions acts as the prerequisite for the anisotropic colloidal assembly, followed by the magnetic force-driven pre-organization and protein intermolecular electrostatic interaction-mediated elongation. The protein concentration rather than the protein original structure plays a more pivotal role in the NP-protein interaction and subsequent colloidal assembly process. Two typical serum proteins fibrinogen and bovine serum albumin enable formation of the anisotropic colloidal assembly but with a different subtle morphology. Furthermore, the obtained micron-sized magnetic colloidal assembly can be dissociated rapidly by adding a negative electrolyte in the medium due to the interference in the NP-protein interaction. However, the self-assembly process can be recycled based on the dissociated colloidal assembly.

Magnetite-binding proteins from the magnetotactic bacterium Desulfamplus magnetovallimortis BW-1

Magnetite-binding proteins are in high demand for the functionalization of magnetic nanoparticles. Binding analysis of six previously uncharacterized proteins from the magnetotactic Deltaproteobacterium Desulfamplus magnetovallimortis BW-1 identified two new magnetite-binding proteins (Mad10, Mad11). These proteins can be utilized as affinity tags for the immobilization of recombinant fusion proteins to magnetite.

Wettability of Magnetite Nanoparticles Guides Growth from Stabilized Amorphous Ferrihydrite

Crystal formation via amorphous precursors is a long-sought-after gateway to engineer nanoparticles with well-controlled size and morphology. Biomineralizing organisms, like magnetotactic bacteria, follow such a nonclassical crystallization pathway to produce magnetite nanoparticles with sophistication unmatched by synthetic efforts at ambient conditions. Here, using in situ small-angle X-ray scattering, we demonstrate how the addition of poly(arginine) in the synthetic formation of magnetite nanoparticles induces a biomineralization-reminiscent pathway. The addition of poly(arginine) stabilizes an amorphous ferrihydrite precursor, shifting the magnetite formation pathway from thermodynamic to kinetic control. Altering the energetic landscape of magnetite formation by catalyzing the pH-dependent precursor attachment, we tune magnetite nanoparticle size continuously, exceeding sizes observed in magnetotactic bacteria. This mechanistic shift we uncover here further allows for crystal morphology control by adjusting the pH-dependent interfacial interaction between liquidlike ferrihydrite and nascent magnetite nanoparticles, establishing a new strategy to control nanoparticle morphology. Synthesizing compact single crystals at wetting conditions and unique semicontinuous single-crystalline nanoparticles at dewetting conditions in combination with an improved control over magnetite crystallite size, we demonstrate the versatility of bio-inspired, kinetically controlled nanoparticle formation pathways.

Iron phosphate mediated magnetite synthesis: a bioinspired approach

The biomineralization of intracellular magnetite in magnetotactic bacteria (MTB) is an area of active investigation. Previous work has provided evidence that magnetite biomineralization begins with the formation of an amorphous phosphate-rich ferric hydroxide precursor phase followed by the eventual formation of magnetite within specialized vesicles (magnetosomes) through redox chemical reactions. Although important progress has been made in elucidating the different steps and possible precursor phases involved in the biomineralization process, many questions still remain. Here, we present a novel in vitro method to form magnetite directly from a mixed valence iron phosphate precursor, without the involvement of other known iron hydroxide precursors such as ferrihydrite. Our results corroborate the idea that phosphate containing phases likely play an iron storage role during magnetite biomineralization. Further, our results help elucidate the influence of phosphate ions on iron chemistry in groundwater and wastewater treatment.

Magnetite was synthesized from a mixed valence iron phosphate precursor through a novel mechanism inspired by biomineralization in magnetotactic bacteria.

Magnetite-Arginine Nanoparticles as a Multifunctional Biomedical Tool

Iron oxide nanoparticles are a promising platform for biomedical applications, both in terms of diagnostics and therapeutics. In addition, arginine-rich polypeptides are known to penetrate across cell membranes. Here, we thus introduce a system based on magnetite nanoparticles and the polypeptide poly-l-arginine (polyR-Fe₃O₄). We show that the hybrid nanoparticles exhibit a low cytotoxicity that is comparable to Resovist^®, a commercially available drug. PolyR-Fe₃O₄ particles perform very well in diagnostic applications, such as magnetic particle imaging (1.7 and 1.35 higher signal respectively for the 3rd and 11th harmonic when compared to Resovist^®), or as contrast agents for magnetic resonance imaging (R2/R1 ratio of 17 as compared to 11 at 0.94 T for Resovist^®). Moreover, these novel particles can also be used for therapeutic purposes such as hyperthermia, achieving a specific heating power ratio of 208 W/g as compared to 83 W/g for Feridex^®, another commercially available product. Therefore, we envision such materials to play a role in the future theranostic applications, where the arginine ability to deliver cargo into the cell can be coupled to the magnetite imaging properties and cancer fighting activity.

Development and characterization of magnetic eggshell membranes for lead removal from wastewater

An increasing concern for natural resources preservation and environmental safety is the removal of heavy metals from contaminated water. It is essential to develop simple procedures that use ecofriendly materials with high removal capacities. In this context, we have synthesized a new hybrid material in which eggshell membranes (ESMs) act as nucleation sites for magnetite nanoparticles (MNPs) precipitation in the presence of an external magnetic field. As a result, ESM was transformed into a magnetic biomaterial (MESM) in order to combine the Pb adsorption abilities of both MNPs and ESM and to facilitate collection of the bioadsorbant using an external magnetic field. This green co-precipitation method produced long strands of bead-like 50 nm superparamagnetic MNPs decorating the ESM fibers. When MESM were incubated in Pb(NO₃)₂ solutions, the hybrid material displayed a 2.5-fold increase in binding constant with respect to that of ESM alone, and a 10-fold increased capacity to remove Pb ions from aqueous solution. The manufactured MESMs present a maximum loading capacity of 0.066 ± 0.009 mg Pb/mg MNPs at 25 °C, which is increased up to 0.15 ± 0.05 mg Pb/mg MNPs at 45 °C. Moreover, the MESM system is very stable, since incubation in 1% HCl solution resulted in rapid Pb desorption, while MNP release from the MESM during the same period was negligible. Altogether, these results suggest that MESM could be utilized as an efficient nanoremediation agent for separation/removal of heavy metal ions or other charged pollutants from contaminated waters, with facile recovery for recycling.

Magnetic Particles-Based Analytical Platforms for Food Safety Monitoring

Magnetic nanoparticles (MNPs) have attracted growing interest as versatile materials for the development of analytical detection and separation platforms for food safety monitoring. This review discusses recent advances in the synthesis, functionalization and applications of MNPs in bioanalysis. A special emphasis is given to the use of MNPs as an immobilization support for biomolecules and as a target capture and pre-concentration to increase selectivity and sensitivity of analytical platforms for the monitoring of food contaminants. General principles and examples of MNP-based platforms for separation, amplification and detection of analytes of interest in food, including organic and inorganic constituents are discussed.

Shaping Magnetite with Poly-l-arginine and pH: From Small Single Crystals to Large Mesocrystals

Control over particle size, size distribution, and colloidal stability are central aims in producing functional nanomaterials. Recently, biomimetic approaches have been successfully used to enhance control over properties in the synthesis of those materials. Magnetotactic bacteria produce protein-stabilized magnetite away from its thermodynamic equilibrium structure. Mimicking the bacteria's proteins using poly-l-arginine we show that by simply increasing the pH, the dimensions of magnetite increase and a single- to mesocrystal transformation is induced. Using synchrotron X-ray diffraction and transmission electron microscopy, we show that magnetite nanoparticles with narrow size distributions and average diameters of 10 ± 2 nm for pH 9, 20 ± 2 nm for pH 10, and up to 40 ± 4 nm for pH 11 can be synthesized. We thus selectively produce superparamagnetic and stable single-domain particles merely by controlling the pH. Remarkably, while an increase in pH brings about a thermodynamically driven decrease in size for magnetite without additives, this dependency on pH is inverted when poly-l-arginine is present.

Biomimetic Mineralization of Magnetic Iron Oxide Nanoparticles Mediated by Bi-Functional Copolypeptides

Magnetite (Fe₃O₄) nanoparticles are widely used in multiple biomedical applications due to their magnetic properties depending on the size, shape and organization of the crystals. However, the crystal growth and morphology of Fe₃O₄ nanoparticles remain difficult to control without using organic solvent or a high temperature. Inspired by the natural biomineralization process, a 14-mer bi-functional copolypeptide, leveraging the affinity of binding Fe₃O₄ together with targeting ovarian cancer cell A2780, was used as a template in the biomimetic mineralization of magnetite. Alongside this, a ginger extract was applied as an antioxidant and a size-conditioning agent of Fe₃O₄ crystals. As a result of the cooperative effects of the peptide and the ginger extract, the size and dispersibility of Fe₃O₄ were controlled based on the interaction of the amino acid and the ginger extract. Our study also demonstrated that the obtained particles with superparamagnetism could selectively be taken up by A2780 cells. In summary, the Fe₃O₄-QY-G nanoparticles may have potential applications in targeting tumor therapy or angiography.

Mineralization of magnetic nano-tape in self-organized nanospace composed of nucleopeptides and peptides

Self-organization based on specific interactions has been actively studied for the formation of highly ordered and hierarchical structures on the molecular scale.

Synthesis, Characterization, and Applications of Magnetic Nanoparticles Featuring Polyzwitterionic Coatings

Throughout the last decades, magnetic nanoparticles (MNP) have gained tremendous interest in different fields of applications like biomedicine (e.g., magnetic resonance imaging (MRI), drug delivery, hyperthermia), but also more technical applications (e.g., catalysis, waste water treatment) have been pursued. Different surfactants and polymers are extensively used for surface coating of MNP to passivate the surface and avoid or decrease agglomeration, decrease or modulate biomolecule absorption, and in most cases increase dispersion stability. For this purpose, electrostatic or steric repulsion can be exploited and, in that regard, surface charge is the most important (hybrid) particle property. Therefore, polyelectrolytes are of great interest for nanoparticle coating, as they are able to stabilize the particles in dispersion by electrostatic repulsion due to their high charge densities. In this review article, we focus on polyzwitterions as a subclass of polyelectrolytes and their use as coating materials for MNP. In the context of biomedical applications, polyzwitterions are widely used as they exhibit antifouling properties and thus can lead to minimized protein adsorption and also long circulation times.

Biotemplated synthesis of magnetic filaments

With the aim of creating one-dimensional magnetic nanostructures, we genetically engineered flagellar filaments produced by Salmonella bacteria to display iron- or magnetite-binding sites, and used the mutant filaments as templates for both nucleation and attachment of the magnetic iron oxide magnetite. Although nucleation from solution and attachment of nanoparticles to a pre-existing surface are two different processes, non-classical crystal nucleation pathways have been increasingly recognized in biological systems, and in many cases nucleation and particle attachment cannot be clearly distinguished. In this study we tested the magnetite-nucleating ability of four types of mutant flagella previously shown to be efficient binders of magnetite nanoparticles, and we used two other mutant flagella that were engineered to periodically display known iron-binding oligopeptides on their surfaces. All mutant filaments were demonstrated to be efficient as templates for the synthesis of one-dimensional magnetic nanostructures under ambient conditions. Both approaches resulted in similar final products, with randomly oriented magnetite nanoparticles partially covering the filamentous biological templates. In an external magnetic field, the viscosity of a suspension of the produced magnetic filaments showed a twofold increase relative to the control sample. The results of magnetic susceptibility measurements were also consistent with the magnetic nanoparticles occurring in linear structures. Our study demonstrates that biological templating can be used to produce one-dimensional magnetic nanostructures under benign conditions, and that modified flagellar filaments can be used for creating model systems in which crystal nucleation from solution can be experimentally studied.

Single crystalline superstructured stable single domain magnetite nanoparticles

Magnetite nanoparticles exhibit magnetic properties that are size and organization dependent and, for applications that rely on their magnetic state, they usually have to be monodisperse. Forming such particles, however, has remained a challenge. Here, we synthesize 40 nm particles of magnetite in the presence of polyarginine and show that they are composed of 10 nm building blocks, yet diffract like single crystals. We use both bulk magnetic measurements and magnetic induction maps recorded from individual particles using off-axis electron holography to show that each 40 nm particle typically contains a single magnetic domain. The magnetic state is therefore determined primarily by the size of the superstructure and not by the sizes of the constituent sub-units. Our results fundamentally demonstrate the structure – property relationship in a magnetic mesoparticle.

Magnetite-Binding Flagellar Filaments Displaying the MamI Loop Motif

This work aimed at developing a novel method for fabricating 1 D magnetite nanostructures with the help of mutated flagellar filaments. We constructed four different flagellin mutants displaying magnetite-binding motifs: two contained fragments of magnetosome-associated proteins from magnetotactic bacteria (MamI and Mms6), and synthetic sequences were used for the other two. A magnetic selection method identified the MamI mutant as having the highest binding affinity to magnetite. Filaments built from MamI loop-containing flagellin subunits were used as templates to form chains of magnetite nanoparticles along the filament by capturing them from suspension. Our study represents a proof-of-concept that flagellar filaments can be engineered to facilitate formation of 1 D magnetite nanostructures under ambient conditions. In addition, it proves the interaction between MamI and magnetite, with implications for the role of this protein in magnetotactic bacteria.

Bioinspired synthesis of magnetite nanoparticles

Magnetite (Fe3O4) is a widespread magnetic iron oxide encountered in many biological and geological systems, and also in many technological applications. The magnetic properties of magnetite crystals depend strongly on the size and shape of its crystals. Hence, engineering magnetite nanoparticles with specific shapes and sizes allows tuning their properties to specific applications in a wide variety of fields, including catalysis, magnetic storage, targeted drug delivery, cancer diagnostics and magnetic resonance imaging (MRI). However, synthesis of magnetite with a specific size, shape and a narrow crystal size distribution is notoriously difficult without using high temperatures and non-aqueous media. Nevertheless, living organisms such as chitons and magnetotactic bacteria are able to form magnetite crystals with well controlled sizes and shapes under ambient conditions and in aqueous media. In these biomineralization processes the organisms use a twofold strategy to control magnetite formation: the mineral is formed from a poorly crystalline precursor phase, and nucleation and growth are controlled through the interaction of the mineral with biomolecular templates and additives. Taking inspiration from this biological strategy is a promising route to achieve control over the kinetics of magnetite crystallization under ambient conditions and in aqueous media. In this review we first summarize the main characteristics of magnetite and what is known about the mechanisms of magnetite biomineralization. We then describe the most common routes to synthesize magnetite and subsequently will introduce recent efforts in bioinspired magnetite synthesis. We describe how the use of poorly ordered, more soluble precursors such as ferrihydrite (FeH) or white rust (Fe(OH)2) can be employed to control the solution supersaturation, setting the conditions for continued growth. Further, we show how the use of various organic additives such as proteins, peptides and polymers allows for either the promotion or inhibition of magnetite nucleation and growth processes. At last we discuss how the formation of magnetite-based organic-inorganic hybrids leads to new functional nanomaterials.

Virus-Templated Near-Amorphous Iron Oxide Nanotubes

We present a simple synthesis of iron oxide nanotubes, grown under very mild conditions from a solution containing Fe(II) and Fe(III), on rod-shaped tobacco mosaic virus templates. Their well-defined shape and surface chemistry suggest that these robust bionanoparticles are a versatile platform for synthesis of small, thin mineral tubes, which was achieved efficiently. Various characterization tools were used to explore the iron oxide in detail: Electron microscopy (SEM, TEM), magnetometry (SQUID-VSM), diffraction (XRD, TEM-SAED), electron spectroscopies (EELS, EDX, XPS), and X-ray absorption (XANES with EXAFS analysis). They allowed determination of the structure, crystallinity, magnetic properties, and composition of the tubes. The protein surface of the viral templates was crucial to nucleate iron oxide, exhibiting analogies to biomineralization in natural compartments such as ferritin cages.

Bioinspired magnetite synthesis via solid precursor phases

Living organisms often exploit solid but poorly ordered mineral phases as precursors in the biomineralization of their inorganic body parts.

Living organisms often exploit solid but poorly ordered mineral phases as precursors in the biomineralization of their inorganic body parts. Generally speaking, such precursor-based approaches allow the organisms – without the need of high supersaturation levels – to accumulate significant quantities of mineral material at the desired place and time, where they can be molded and crystallized into the right morphology and structure. This strategy is also of interest in the field of bioinspired materials science, as it potentially enables the bottom-up creation of novel materials with equal or improved functionality as compared to Nature, in water and at ambient temperatures. Also for the biomineralization of magnetite (Fe₃O₄) such a strategy has been reported: ferrihydrite, a poorly crystalline iron oxide, has been identified as a precursor for the final magnetite phase in the magnetosomes of magnetotactic bacteria as well as in the outer layers of chiton teeth. In this perspective, we discuss the efforts of us and others to understand and tune the nucleation and growth of magnetite crystals to date, in aqueous, room-temperature syntheses and employing different solid precursor phases. The various examples demonstrate the importance of the precursor approach in controlling the different properties of magnetite nanoparticles.

From bacteria to mollusks: the principles underlying the biomineralization of iron oxide materials

Various organisms possess a genetic program that enables the controlled formation of a mineral, a process termed biomineralization. The variety of biological material architectures is mind-boggling and arises from the ability of organisms to exert control over crystal nucleation and growth. The structure and composition of biominerals equip biomineralizing organisms with properties and functionalities that abiotically formed materials, made of the same mineral, usually lack. Therefore, elucidating the mechanisms underlying biomineralization and morphogenesis is of interdisciplinary interest to extract design principles that will enable the biomimetic formation of functional materials with similar capabilities. Herein, we summarize what is known about iron oxides formed by bacteria and mollusks for their magnetic and mechanical properties. We describe the chemical and biological machineries that are involved in controlling mineral precipitation and organization and show how these organisms are able to form highly complex structures under physiological conditions.

Phage display selected magnetite interacting Adhirons for shape controlled nanoparticle synthesis

Adhirons are robust, well expressing, peptide display scaffold proteins, developed as an effective alternative to traditional antibody binding proteins for highly specific molecular recognition applications. This paper reports for the first time the use of these versatile proteins for material binding, and as tools for controlling material synthesis on the nanoscale. A phage library of Adhirons, each displaying two variable binding loops, was screened to identify specific proteins able to interact with [100] faces of cubic magnetite nanoparticles. The selected variable regions display a strong preference for basic residues such as lysine. Molecular dynamics simulations of amino acid adsorption onto a [100] magnetite surface provides a rationale for these interactions, with the lowest adsorption energy observed with lysine. These proteins direct the shape of the forming nanoparticles towards a cubic morphology in room temperature magnetite precipitation reactions, in stark contrast to the high temperature, harsh reaction conditions currently used to produce cubic nanoparticles. These effects demonstrate the utility of the selected Adhirons as novel magnetite mineralization control agents using ambient aqueous conditions. The approach we outline with artificial protein scaffolds has the potential to develop into a toolkit of novel additives for wider nanomaterial fabrication.

Crystal growth of bullet-shaped magnetite in magnetotactic bacteria of the Nitrospirae phylum

Magnetotactic bacteria (MTB) are known to produce single-domain magnetite or greigite crystals within intracellular membrane organelles and to navigate along the Earth's magnetic field lines. MTB have been suggested as being one of the most ancient biomineralizing metabolisms on the Earth and they represent a fundamental model of intracellular biomineralization. Moreover, the determination of their specific crystallographic signature (e.g. structure and morphology) is essential for palaeoenvironmental and ancient-life studies. Yet, the mechanisms of MTB biomineralization remain poorly understood, although this process has been extensively studied in several cultured MTB strains in the Proteobacteria phylum. Here, we show a comprehensive transmission electron microscopy (TEM) study of magnetic and structural properties down to atomic scales on bullet-shaped magnetites produced by the uncultured strain MYR-1 belonging to the Nitrospirae phylum, a deeply branching phylogenetic MTB group. We observed a multiple-step crystal growth of MYR-1 magnetite: initial isotropic growth forming cubo-octahedral particles (less than approx. 40 nm), subsequent anisotropic growth and a systematic final elongation along [001] direction. During the crystal growth, one major {111} face is well developed and preserved at the larger basal end of the crystal. The basal {111} face appears to be terminated by a tetrahedral-octahedral-mixed iron surface, suggesting dimensional advantages for binding protein(s), which may template the crystallization of magnetite. This study offers new insights for understanding magnetite biomineralization within the Nitrospirae phylum.

Crystal structure of the magnetobacterial protein MtxA C-terminal domain reveals a new sequence-structure relationship

Magnetotactic bacteria (MTB) are a diverse group of aquatic bacteria that have the magnetotaxis ability to align themselves along the geomagnetic field lines and to navigate to a microoxic zone at the bottom of chemically stratified natural water. This special navigation is the result of a unique linear assembly of a specialized organelle, the magnetosome, which contains a biomineralized magnetic nanocrystal enveloped by a cytoplasmic membrane. The Magnetospirillum gryphiswaldense MtxA protein (MGR_0208) was suggested to play a role in bacterial magnetotaxis due to its gene location in an operon together with putative signal transduction genes. Since no homology is found for MtxA, and to better understand the role and function of MtxA in MTBés magnetotaxis, we initiated structural and functional studies of MtxA via X-ray crystallography and deletion mutagenesis. Here, we present the crystal structure of the MtxA C-terminal domain and provide new insights into its sequence-structure relationship.

Genetic and biochemical investigations of the role of MamP in redox control of iron biomineralization in Magnetospirillum magneticum

Magnetotactic bacteria have evolved complex subcellular machinery to construct linear chains of magnetite nanocrystals that allow the host cell to sense direction. Each mixed-valent iron nanoparticle is mineralized from soluble iron within a membrane-encapsulated vesicle termed the magnetosome, which serves as a specialized compartment that regulates the iron, redox, and pH environment of the growing mineral. To dissect the biological components that control this process, we have carried out a genetic and biochemical study of proteins proposed to function in iron mineralization. In this study, we show that the redox sites of c-type cytochromes of the Magnetospirillum magneticum AMB-1 magnetosome island, MamP and MamT, are essential to their physiological function and that ablation of one or both heme motifs leads to loss of function, suggesting that their ability to carry out redox chemistry in vivo is important. We also develop a method to heterologously express fully heme-loaded MamP from AMB-1 for in vitro biochemical studies, which show that its Fe(III)-Fe(II) redox couple is set at an unusual potential (-89 ± 11 mV) compared with other related cytochromes involved in iron reduction or oxidation. Despite its low reduction potential, it remains competent to oxidize Fe(II) to Fe(III) and mineralize iron to produce mixed-valent iron oxides. Finally, in vitro mineralization experiments suggest that Mms mineral-templating peptides from AMB-1 can modulate the iron redox chemistry of MamP.

Magnetic particle hyperthermia--a promising tumour therapy?

We present a critical review of the state of the art of magnetic particle hyperthermia (MPH) as a minimal invasive tumour therapy. Magnetic principles of heating mechanisms are discussed with respect to the optimum choice of nanoparticle properties. In particular, the relation between superparamagnetic and ferrimagnetic single domain nanoparticles is clarified in order to choose the appropriate particle size distribution and the role of particle mobility for the relaxation path is discussed. Knowledge of the effect of particle properties for achieving high specific heating power provides necessary guidelines for development of nanoparticles tailored for tumour therapy. Nanoscale heat transfer processes are discussed with respect to the achievable temperature increase in cancer cells. The need to realize a well-controlled temperature distribution in tumour tissue represents the most serious problem of MPH, at present. Visionary concepts of particle administration, in particular by means of antibody targeting, are far from clinical practice, yet. On the basis of current knowledge of treating cancer by thermal damaging, this article elucidates possibilities, prospects, and challenges for establishment of MPH as a standard medical procedure.