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1. Micromagnetic and morphological characterization of heteropolymer human ferritin cores

   https://doi.org/10.1039/d2na00544a  

   Longo, Thomas ; Kim, Steve ; Srivastava, Ayush K. ; Hurley, Lauren ; Ji, Kaixuan ; Viescas, Arthur J. ; Flint, Nicholas ; Foucher, Alexandre C. ; Yates, Douglas ; Stach, Eric A. ; et al ( October 2022 , Nanoscale Advances)

   The physical properties of in vitro iron-reconstituted and genetically engineered human heteropolymer ferritins were investigated. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electron energy-loss spectroscopy (EELS), and 57 Fe Mössbauer spectroscopy were employed to ascertain (1) the microstructural, electronic, and micromagnetic properties of the nanosized iron cores, and (2) the effect of the H and L ferritin subunit ratios on these properties. Mössbauer spectroscopic signatures indicate that all iron within the core is in the high spin ferric state. Variable temperature Mössbauer spectroscopy for H-rich (H 21 /L 3 ) and L-rich (H 2 /L 22 ) ferritins reconstituted at 1000 57 Fe/protein indicates superparamagnetic behavior with blocking temperatures of 19 K and 28 K, while HAADF-STEM measurements give average core diameters of (3.7 ± 0.6) nm and (5.9 ± 1.0) nm, respectively. Most significantly, H-rich proteins reveal elongated, dumbbell, and crescent-shaped cores, while L-rich proteins present spherical cores, pointing to a correlation between core shape and protein shell composition. Assuming an attempt time for spin reversal of τ 0 = 10 −11 s, the Néel–Brown formula for spin-relaxation time predicts effective magnetic anisotropy energy densities of 6.83 × 10 4 J m −3 and 2.75 × 10 4 J m −3 for H-rich and L-rich proteins, respectively, due to differences in surface and shape contributions to magnetic anisotropy in the two heteropolymers. The observed differences in shape, size, and effective magnetic anisotropies of the derived biomineral cores are discussed in terms of the iron nucleation sites within the interior surface of the heteropolymer shells for H-rich and L-rich proteins. Overall, our results imply that site-directed nucleation and core growth within the protein cavity play a determinant role in the resulting core morphology. Our findings have relevance to iron biomineralization processes in nature and the growth of designer's magnetic nanoparticles within recombinant apoferritin nano-templates for nanotechnology. 

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2. Ferritin: Versatile Host, Nanoreactor, and Delivery Agent

   https://doi.org/10.1002/ijch.201600017  

   Pulsipher, Katherine W. ; Dmochowski, Ivan J. ( July 2016 , Israel Journal of Chemistry)

   The ferritin family of proteins can be thought of as natural, nano‐sized containers. Their native function is to oxidize and store iron as a hydrated ferric oxide mineral, which can be removed to give apoferritin. The hollow interior (D=5–8 nm) provides an excellent template for inorganic nanoparticle (NP) synthesis. In addition to preparing new coresin situ, apoferritins can be disassembled and reassembled around a NP of interest. Ferritin encapsulation increases NP biocompatibility and allows site‐specific functionalization. We and others have prepared many NP‐filled ferritins, as well as ferritins loaded with small molecules of interest. Uniquely, apoferritin serves as a model for a general anesthetic‐protein binding site, which has led to the discovery of the first fluorescent general anesthetic and improved understanding of anesthetic mechanisms. Finally, ferritin conjugates have exciting applications for targeted delivery, which we have explored in the vascular endothelium.

    

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3. Reaction of O 2 with a diiron protein generates a mixed-valent Fe 2+ /Fe 3+ center and peroxide

   https://doi.org/10.1073/pnas.1809913116  

   Bradley, Justin M. ; Svistunenko, Dimitri A. ; Pullin, Jacob ; Hill, Natalie ; Stuart, Rhona K. ; Palenik, Brian ; Wilson, Michael T. ; Hemmings, Andrew M. ; Moore, Geoffrey R. ; Le Brun, Nick E. ( February 2019 , Proceedings of the National Academy of Sciences)

   The gene encoding the cyanobacterial ferritinSynFtn is up-regulated in response to copper stress. Here, we show that, whileSynFtn does not interact directly with copper, it is highly unusual in several ways. First, its catalytic diiron ferroxidase center is unlike those of all other characterized prokaryotic ferritins and instead resembles an animal H-chain ferritin center. Second, as demonstrated by kinetic, spectroscopic, and high-resolution X-ray crystallographic data, reaction of O₂with the di-Fe²⁺center results in a direct, one-electron oxidation to a mixed-valent Fe²⁺/Fe³⁺form. Iron–O₂chemistry of this type is currently unknown among the growing family of proteins that bind a diiron site within a four α-helical bundle in general and ferritins in particular. The mixed-valent form, which slowly oxidized to the more usual di-Fe³⁺form, is an intermediate that is continually generated during mineralization. Peroxide, rather than superoxide, is shown to be the product of O₂reduction, implying that ferroxidase centers function in pairs via long-range electron transfer through the protein resulting in reduction of O₂bound at only one of the centers. We show that electron transfer is mediated by the transient formation of a radical on Tyr40, which lies ∼4 Å from the diiron center. As well as demonstrating an expansion of the iron–O₂chemistry known to occur in nature, these data are also highly relevant to the question of whether all ferritins mineralize iron via a common mechanism, providing unequivocal proof that they do not.

    

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4. Designed and biologically active protein lattices

   https://doi.org/10.1038/s41467-021-23966-4  

   Wang, Shih-Ting ; Minevich, Brian ; Liu, Jianfang ; Zhang, Honghu ; Nykypanchuk, Dmytro ; Byrnes, James ; Liu, Wu ; Bershadsky, Lev ; Liu, Qun ; Wang, Tong ; et al ( December 2021 , Nature Communications)

   Versatile methods to organize proteins in space are required to enable complex biomaterials, engineered biomolecular scaffolds, cell-free biology, and hybrid nanoscale systems. Here, we demonstrate how the tailored encapsulation of proteins in DNA-based voxels can be combined with programmable assembly that directs these voxels into biologically functional protein arrays with prescribed and ordered two-dimensional (2D) and three-dimensional (3D) organizations. We apply the presented concept to ferritin, an iron storage protein, and its iron-free analog, apoferritin, in order to form single-layers, double-layers, as well as several types of 3D protein lattices. Our study demonstrates that internal voxel design and inter-voxel encoding can be effectively employed to create protein lattices with designed organization, as confirmed by in situ X-ray scattering and cryo-electron microscopy 3D imaging. The assembled protein arrays maintain structural stability and biological activity in environments relevant for protein functionality. The framework design of the arrays then allows small molecules to access the ferritins and their iron cores and convert them into apoferritin arrays through the release of iron ions. The presented study introduces a platform approach for creating bio-active protein-containing ordered nanomaterials with desired 2D and 3D organizations.

    

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5. Asymmetric nanoparticle oxidation observed in-situ by the evolution of diffraction contrast

   https://doi.org/10.1088/2515-7639/ad025f  

   Poerwoprajitno, Agus R. ; Baradwaj, Nitish ; Singh, Manish Kumar ; Carter, C. Barry ; Huber, Dale L. ; Kalia, Rajiv ; Watt, John ( October 2023 , Journal of Physics: Materials)

   The use of transmission electron microscopy (TEM) to observe real-time structural and compositional changes has proven to be a valuable tool for understanding the dynamic behavior of nanomaterials. However, identifying the nanoparticles of interest typically require an obvious change in position, size, or structure, as compositional changes may not be noticeable during the experiment. Oxidation or reduction can often result in subtle volume changes only, so elucidating mechanisms in real-time requires atomic-scale resolution orin-situelectron energy loss spectroscopy, which may not be widely accessible. Here, by monitoring the evolution of diffraction contrast, we can observe both structural and compositional changes in iron oxide nanoparticles, specifically the oxidation from a wüstite-magnetite (FeO@Fe₃O₄) core–shell nanoparticle to single crystalline magnetite, Fe₃O₄nanoparticle. Thein-situTEM images reveal a distinctive light and dark contrast known as the ‘Ashby-Brown contrast’, which is a result of coherent strain across the core–shell interface. As the nanoparticles fully oxidize to Fe₃O₄, the diffraction contrast evolves and then disappears completely, which is then confirmed by modeling and simulation of TEM images. This represents a new, simplified approach to tracking the oxidation or reduction mechanisms of nanoparticles usingin-situTEM experiments.

    

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