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Assembly of nanoparticles into superlattices yields nanomaterials with novel properties. We have recently shown that engineered protein cages are excellent building blocks for the assembly of inorganic nanoparticles into highly structured hybrid materials, with unprecedented precision. In this study, we show that the protein matrix, composed of surface-charged protein cages, can be readily tuned to achieve a number of different crystalline assemblies. Simply by altering the assembly conditions, different types of crystalline structures were produced, without the need to further modify the cages. Future work can utilize these new protein scaffolds to create nanoparticle superlattices with various assembly geometries and thus tune the functionality of these hybrid materials.

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This study focuses on the design and characterization of binary nanoparticle superlattices: Two differently sized, supercharged protein nanocages are used to create a matrix for nanoparticle arrangement. We have previously established the assembly of protein nanocages of the same size. Here, we present another approach for multicomponent biohybrid material synthesis by successfully assembling two differently sized supercharged protein nanocages with different symmetries. Typically, the ordered assembly of objects with nonmatching symmetry is challenging, but our electrostatic-based approach overcomes the symmetry mismatch by exploiting electrostatic interactions between oppositely charged cages. Moreover, our study showcases the use of nanoparticles as a contrast enhancer in an elegant way to gain insights into the structural details of crystalline biohybrid materials. The assembled materials were characterized with various methods, including transmission electron microscopy (TEM) and single-crystal small-angle X-ray diffraction (SC-SAXD). We employed cryo-plasma-focused ion beam milling (cryo-PFIB) to prepare lamellae for the investigation of nanoparticle sublattices via electron cryo-tomography. Importantly, we refined superlattice structure data obtained from single-crystal SAXD experiments, providing conclusive evidence of the final assembly type. Our findings highlight the versatility of protein nanocages for creating distinctive types of binary superlattices. Because the nanoparticles do not influence the type of assembly, protein cage matrices can combine various nanoparticles in the solid state. This study not only contributes to the expanding repertoire of nanoparticle assembly methods but also demonstrates the power of advanced characterization techniques in elucidating the structural intricacies of these biohybrid materials.

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Nanomaterials with a defined composition and structure can be synthesized by exploiting natural templates or biomolecular matrices. In the present work, we use protein nanocages derived from human ferritin as a nanoscale building block for the assembly of gold nanoparticles and fluorescent molecules in the solid state. As a generalizable strategy, we show that prior to material synthesis, the cargo can be encapsulated into the protein nanocages using a dis- and reassembly approach. Toward this end, a new ligand system for gold nanoparticles enables efficient encapsulation of these particles into the nanocages. The gold nanoparticle-loaded protein nanocages are co-assembled with fluorophore-loaded protein nanocages. Binary superlattices are formed because two oppositely charged ferritin nanocages are used as templates for the assembly. The binary crystals show strong exciton-plasmon coupling between the encapsulated fluorophores and gold nanoparticles, which was spatially resolved with fluorescence lifetime imaging. The strategy outlined here offers a modular approach toward binary nanomaterials with highly ordered building blocks.

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Correction for 'Peptide-directed encapsulation of inorganic nanoparticles into protein containers' by Tobias Beck et al., Nanoscale, 2018, 10, 22917-22926.

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The assembly of molecular building blocks into highly ordered structures is crucial, both in nature and for the development of novel functional materials. In nature, noncovalent interactions, such as hydrogen bonds or hydrophobic interactions, enable the reversible assembly of biopolymers, such as DNA or proteins. Inspired by these design principles, scientists have created biohybrid materials that employ natural building blocks and their assembly properties. Thus, structures and materials are attainable that cannot be made through other synthetic procedures. Herein, we review current concepts and highlight recent advances.

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Biomolecules can be combined with inorganic compounds to unite biological features with the chemical and physical properties of abiotic materials. In particular, protein containers, with their inherent ability to encapsulate cargo molecules, are perfect platforms for the generation of multifunctional assemblies. However, encapsulation of foreign cargo is immensely challenging due to the lack of specific interactions between cargo and container. Here, we demonstrate that the highly specific cargo-loading mechanism of the bacterial nanocompartment encapsulin can be employed for encapsulation of artificial cargo like inorganic nanoparticles. For this purpose, container-filling gold nanoparticles were decorated with a small number of encapsulin cargo-loading peptides. By lock-and-key interaction between the peptides and the peptide-binding pockets on the inner container surface, the nanoparticles are encapsulated into encapsulin with extremely high efficiency. Most notably, peptide binding is independent from external factors such as ionic strength. Cargo-loading peptides may serve as generally applicable tool for efficient and specific encapsulation of cargo molecules into a proteinaceous compartment.

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Proteins are a powerful scaffold for the organisation of inorganic components in the solid state. Due to the defined atomic structure of the protein scaffold, organometallic complexes and nanoparticles can be arranged with high precision. Moreover, crystalline protein materials are porous and enable confinement of inorganic components on the nanoscale. In this Frontier article, we summarize current strategies for the synthesis of bioinorganic materials and highlight recent examples for application of these materials. Future perspectives are discussed such as tunability of material properties through protein design, and the possibility to produce protein materials in a sustainable manner.

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The construction of defined nanostructured catalysts is challenging. In previous work, we established a strategy to assemble binary nanoparticle superlattices with oppositely charged protein containers as building blocks. Here, we show that these free-standing nanoparticle superlattices are catalytically active. The metal oxide nanoparticles inside the protein scaffold are accessible for a range of substrates and show oxidase-like and peroxidase-like activity. The stable superlattices can be reused for several reaction cycles. In contrast to bulk nanoparticle-based catalysts, which are prone to aggregation and difficult to characterize, nanoparticle superlattices based on engineered protein containers provide an innovative synthetic route to structurally defined heterogeneous catalysts with control over nanoparticle size and composition.

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The gas-phase structures and parameters describing acetyl methyl torsion of N-ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier-transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C1 symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm(-1) of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The (14) N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.

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