Self-Assembly of Electrostatic Cocrystals from Supercharged Fusion Peptides and Protein Cages

Assembly Requirements for the Construction of Large-Scale Binary Protein Structures

The precise assembly of multiple biomacromolecules into well-defined structures and materials is of great importance for various biomedical and nanobiotechnological applications. In this study, we investigate the assembly requirements for two-component materials using charged protein nanocages as building blocks. To achieve this, we designed several variants of ferritin nanocages to determine the surface characteristics necessary for the formation of large-scale binary three-dimensional (3D) assemblies. These nanocage variants were employed in protein crystallization experiments and macromolecular crystallography analyses, complemented by computational methods. Through the screening of nanocage variant combinations at various ionic strengths, we identified three essential features for successful assembly: (1) the presence of a favored crystal contact region, (2) the presence of a charged patch not involved in crystal contacts, and (3) sufficient distinctiveness between the nanocages. Surprisingly, the absence of noncrystal contact mediating patches had a detrimental effect on the assemblies, highlighting their unexpected importance. Intriguingly, we observed the formation of not only binary structures but also both negatively and positively charged unitary structures under previously exclusively binary conditions. Overall, our findings will inform future design strategies by providing some design rules, showcasing the utility of supercharging symmetric building blocks in facilitating the assembly of biomacromolecules into large-scale binary 3D assemblies.

Supercharged Fluorescent Protein-Apoferritin Cocrystals for Lighting Applications

The application of fluorescent proteins (FPs) in optoelectronics is hindered by the need for effective protocols to stabilize them under device preparation and operational conditions. Factors such as high temperatures, irradiation, and organic solvent exposure contribute to the denaturation of FPs, resulting in a low device performance. Herein, we focus on addressing the photoinduced heat generation associated with FP motion and rapid heat transfer. This leads to device temperatures of approximately 65 °C, causing FP-denaturation and a subsequent loss of device functionality. We present a FP stabilization strategy involving the integration of electrostatically self-assembled FP-apoferritin cocrystals within a silicone-based color down-converting filter. Three key achievements characterize this approach: (i) an engineering strategy to design positively supercharged FPs (+22) without compromising photoluminescence and thermal stability compared to their native form, (ii) a carefully developed crystallization protocol resulting in highly emissive cocrystals that retain the essential photoluminescence features of the FPs, and (iii) a strong reduction of the device's working temperature to 40 °C, leading to a 40-fold increase in Bio-HLEDs stability compared to reference devices.

Virus-like particles nanoreactors: from catalysis towards bio-applications

Virus-like particles (VLPs) are self-assembled supramolecular structures found in nature, often used for compartmentalization. Exploiting their inherent properties, including precise nanoscale structures, monodispersity, and high stability, these architectures have been widely used as nanocarriers to protect or enrich catalysts, facilitating catalytic reactions and avoiding interference from the bulk solutions. In this review, we summarize the current progress of virus-like particles (VLPs)-based nanoreactors. First, we briefly introduce the physicochemical properties of the most commonly used virus particles to understand their roles in catalytic reactions beyond the confined space. Next, we summarize the self-assembly of nanoreactors forming higher-order hierarchical structures, highlighting the emerging field of nanoreactors as artificial organelles and their potential biomedical applications. Finally, we discuss the current findings and future perspectives of VLPs-based nanoreactors.

Advances in Peptide-Based Hydrogel for Tissue Engineering

The development of peptide-based materials has emerged as one of the most challenging aspects of biomaterials in recent years. It has been widely acknowledged that peptide-based materials can be used in a broad range of biomedical applications, particularly in tissue engineering. Among them, hydrogels have been attracting considerable interest in tissue engineering because they mimic tissue formation conditions by providing a three-dimensional environment and a high water content. It has been found that peptide-based hydrogels have received more attention due to mimicking proteins, particularly extracellular matrix proteins, as well as the wide variety of applications they are capable of serving. It is without a doubt that peptide-based hydrogels have become the leading biomaterials of today owing to their tunable mechanical stability, high water content, and high biocompatibility. Here, we discuss in detail various types of peptide-based materials, emphasizing peptide-based hydrogels, and then we examine in detail how hydrogels are formed, paying particular attention to the peptide structures that are incorporated into the final structure. Following that, we discuss the self-assembly and formation of hydrogels under various conditions, as well as the parameters to be considered as critical factors, which include pH, amino acid composi- tion within the sequence, and cross-linking techniques. Further, recent studies on the development of peptide-based hydrogels and their applications in tissue engineering are reviewed.

Polypyrrole-Stabilized Polypeptide for Eco-Friendly Supercapacitors

As an energy storage technology, supercapacitors (SCs) have become an important part of many electronic systems because of their high-power density, long cycle life, and maintenance-free characteristics. However, the widespread development and use of electronics, including SCs, have led to the generation of a large amount of e-waste. In addition, achieving compatibility between stability and biodegradability has been a prominent challenge for implantable electronics. Therefore, environmentally friendly SCs based on polypyrrole (PPy)-stabilized polypeptide (FF) are demonstrated in this study. The fully degradable SC has a layer-by-layer structure, including polylactic acid/chitosan (PLA-C) support layers, current collectors (Mg), FF/PPy composite layers, and a polyvinyl alcohol/phosphate buffer solution (PVA/PBS) hydrogel. It has the advantages of being light, thin, flexible, and biocompatible. After 5000 cycles in air, the capacitance retention remains at up to 94.7%. The device could stably operate for 7 days in a liquid environment and completely degrade in vitro within 90 days without any adverse effect on the environment. This work has important implications for eco-friendly electronics and will have a significant impact on the implantable biomedical electronics.

Supercharged Protein Nanosheets for Cell Expansion on Bioemulsions

Cell culture at liquid-liquid interfaces, for example, at the surface of oil microdroplets, is an attractive strategy to scale up adherent cell manufacturing while replacing the use of microplastics. Such a process requires the adhesion of cells at interfaces stabilized and reinforced by protein nanosheets displaying not only high elasticity but also presenting cell adhesive ligands able to bind integrin receptors. In this report, supercharged albumins are found to form strong elastic protein nanosheets when co-assembling with the co-surfactant pentafluorobenzoyl chloride (PFBC) and mediate extracellular matrix (ECM) protein adsorption and cell adhesion. The interfacial mechanical properties and elasticity of supercharged nanosheets are characterized by interfacial rheology, and behaviors are compared to those of native bovine serum albumin, human serum albumin, and α-lactalbumin. The impact of PFBC on such assembly is investigated. ECM protein adsorption to resulting supercharged nanosheets is then quantified via surface plasmon resonance and fluorescence microscopy, demonstrating that the dual role supercharged albumins are proposed to play as scaffold protein structuring liquid-liquid interfaces and substrates for the capture of ECM molecules. Finally, the adhesion and proliferation of primary human epidermal stem cells are investigated, at pinned droplets, as well as on bioemulsions stabilized by corresponding supercharged nanosheets. This study demonstrates the potential of supercharged proteins for the engineering of biointerfaces for stem cell manufacturing and draws structure-property relationships that will guide further engineering of associated systems.

Hierarchical Materials from High Information Content Macromolecular Building Blocks: Construction, Dynamic Interventions, and Prediction

Hierarchical materials that exhibit order over multiple length scales are ubiquitous in nature. Because hierarchy gives rise to unique properties and functions, many have sought inspiration from nature when designing and fabricating hierarchical matter. More and more, however, nature's own high-information content building blocks, proteins, peptides, and peptidomimetics, are being coopted to build hierarchy because the information that determines structure, function, and interfacial interactions can be readily encoded in these versatile macromolecules. Here, we take stock of recent progress in the rational design and characterization of hierarchical materials produced from high-information content blocks with a focus on stimuli-responsive and "smart" architectures. We also review advances in the use of computational simulations and data-driven predictions to shed light on how the side chain chemistry and conformational flexibility of macromolecular blocks drive the emergence of order and the acquisition of hierarchy and also on how ionic, solvent, and surface effects influence the outcomes of assembly. Continued progress in the above areas will ultimately usher in an era where an understanding of designed interactions, surface effects, and solution conditions can be harnessed to achieve predictive materials synthesis across scale and drive emergent phenomena in the self-assembly and reconfiguration of high-information content building blocks.

Understanding Supramolecular Assembly of Supercharged Proteins

Ordered supramolecular assemblies have recently been created using electrostatic interactions between oppositely charged proteins. Despite recent progress, the fundamental mechanisms governing the assembly of oppositely supercharged proteins are not fully understood. Here, we use a combination of experiments and computational modeling to systematically study the supramolecular assembly process for a series of oppositely supercharged green fluorescent protein variants. We show that net charge is a sufficient molecular descriptor to predict the interaction fate of oppositely charged proteins under a given set of solution conditions (e.g., ionic strength), but the assembled supramolecular structures critically depend on surface charge distributions. Interestingly, our results show that a large excess of charge is necessary to nucleate assembly and that charged residues not directly involved in interprotein interactions contribute to a substantial fraction (∼30%) of the interaction energy between oppositely charged proteins via long-range electrostatic interactions. Dynamic subunit exchange experiments further show that relatively small, 16-subunit assemblies of oppositely charged proteins have kinetic lifetimes on the order of ∼10-40 min, which is governed by protein composition and solution conditions. Broadly, our results inform how protein supercharging can be used to create different ordered supramolecular assemblies from a single parent protein building block.

Dynamic, Polymer-Integrated Crystals for Efficient, Reversible Protein Encapsulation

Crystalline materials are increasingly being used as platforms for encapsulating proteins to create stable, functional materials. However, the uptake efficiencies and stimuli-responsiveness of crystalline frameworks are limited by their rigidities. We have recently reported a new form of materials, polymer-integrated crystals (PIX), which combine the structural order of protein crystals with the dynamic, stimuli-responsive properties of synthetic polymers. Here we show that the crystallinity, flexibility, and chemical tunability of PIX can be exploited to encapsulate guest proteins with high loading efficiencies (up to 46% w/w). The electrostatic host-guest interactions enable reversible, pH-controlled uptake/release of guest proteins as well as the mutual stabilization of the host and the guest, thus creating a uniquely synergistic platform toward the development of functional biomaterials and the controlled delivery of biological macromolecules.

Protein Cages: From Fundamentals to Advanced Applications

Proteins that self-assemble into polyhedral shell-like structures are useful molecular containers both in nature and in the laboratory. Here we review efforts to repurpose diverse protein cages, including viral capsids, ferritins, bacterial microcompartments, and designed capsules, as vaccines, drug delivery vehicles, targeted imaging agents, nanoreactors, templates for controlled materials synthesis, building blocks for higher-order architectures, and more. A deep understanding of the principles underlying the construction, function, and evolution of natural systems has been key to tailoring selective cargo encapsulation and interactions with both biological systems and synthetic materials through protein engineering and directed evolution. The ability to adapt and design increasingly sophisticated capsid structures and functions stands to benefit the fields of catalysis, materials science, and medicine.

Protein Assembly by Design

Proteins are nature's primary building blocks for the construction of sophisticated molecular machines and dynamic materials, ranging from protein complexes such as photosystem II and nitrogenase that drive biogeochemical cycles to cytoskeletal assemblies and muscle fibers for motion. Such natural systems have inspired extensive efforts in the rational design of artificial protein assemblies in the last two decades. As molecular building blocks, proteins are highly complex, in terms of both their three-dimensional structures and chemical compositions. To enable control over the self-assembly of such complex molecules, scientists have devised many creative strategies by combining tools and principles of experimental and computational biophysics, supramolecular chemistry, inorganic chemistry, materials science, and polymer chemistry, among others. Owing to these innovative strategies, what started as a purely structure-building exercise two decades ago has, in short order, led to artificial protein assemblies with unprecedented structures and functions and protein-based materials with unusual properties. Our goal in this review is to give an overview of this exciting and highly interdisciplinary area of research, first outlining the design strategies and tools that have been devised for controlling protein self-assembly, then describing the diverse structures of artificial protein assemblies, and finally highlighting the emergent properties and functions of these assemblies.

Substrate Partitioning into Protein Macromolecular Frameworks for Enhanced Catalytic Turnover

Spatial partitioning of chemical processes is an important attribute of many biological systems, the effect of which is reflected in the high efficiency of enzymes found within otherwise chaotic cellular environments. Barriers, often provided through the formation of compartments or phase segregation, gate the access of macromolecules and small molecules within the cell and provide an added level of metabolic control. Taking inspiration from nature, we have designed virus-like particles (VLPs) as nanoreactor compartments that sequester enzyme catalysts and have used these as building blocks to construct 3D protein macromolecular framework (PMF) materials, which are structurally characterized using small-angle X-ray scattering (SAXS). The highly charged PMFs form a separate phase in suspension, and by tuning the ionic strength, we show positively charged molecules preferentially partition into the PMF, while negatively charged molecules are excluded. This molecular partitioning was exploited to tune the catalytic activity of enzymes enclosed within the individual particles in the PMF, the results of which showed that positively charged substrates had turnover rates that were 8500× faster than their negatively charged counterparts. Moreover, the catalytic PMF led to cooperative behavior resulting in charge dependent trends opposite to those observed with individual P22 nanoreactor particles.

Virus-Based Supramolecular Structure and Materials: Concept and Prospects

Rodlike and spherelike viruses are various monodisperse nanoparticles that can display small molecules or polymers with unique distribution following chemical modifications. Because of the monodisperse property, aggregates in synthetic protein-polymer nanoparticles could be eliminated, thus improving the probability for application in protein-polymer drug. In addition, the monodisperse virus could direct the growth of metal materials or inorganic materials, finding applications in hydrogel, drug delivery, and optoelectronic and catalysis materials. Benefiting from the advantages, the virus or viruslike particles have been widely explored in the field of supramolecular chemistry. In this review, we describe the modification and application of virus and viruslike particles in surpramolecular structures and biomedical research.

Designed and biologically active protein lattices

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.

Protein cages as building blocks for superstructures

Proteins naturally self-assemble to function. Protein cages result from the self-assembly of multiple protein subunits that interact to form hollow symmetrical structures with functions that range from cargo storage to catalysis. Driven by self-assembly, building elegant higher-order superstructures with protein cages as building blocks has been an increasingly attractive field in recent years. It presents an engineering challenge not only at the molecular level but also at the supramolecular level. The higher-order constructs are proposed to provide access to diverse functional materials. Focussing on design strategy as a perspective, current work on protein cage supramolecular self-assembly are reviewed from three principles that are electrostatic, metal-ligand coordination and inherent symmetry. The review also summarises possible applications of the superstructure architecture built using modified protein cages.

Light-Induced Reversible Hierarchical Self-Assembly of Amphiphilic Diblock Copolymers into Microscopic Vesicles with Tunable Optical and Nanocarrier Properties

In contrast to the conventional hierarchical self-assembly process, effective methods to enable reversible hierarchical self-assembly of block copolymers are comparatively few and limited in scope. Herein, we report, for the first time, a simple yet robust strategy for light-induced reversible hierarchical self-assembly of an amphiphilic diblock copolymer, poly(4-vinylpyridine)-block-poly[6-[4-(4-butyloxyphenylazo)phenoxy]hexyl methacrylate] (denoted P4VP-b-PAzoMA). The hierarchical structures are constructed via a two-step self-assembly process (first-level reverse micelles, second-level compound micelles, and rearrangement into micrometer-sized vesicles) driven by use of solvent. Intriguingly, because of reversible photoinduced trans-to-cis isomerization of azobenzene moieties in PAzoMA, the vesicles could disassemble into subunits upon UV light and then recover the nearly identical vesicular morphology upon visible light. Such a reversible hierarchical self-assembly process is accompanied by reversible fluorescence, encapsulation, and controlled release of dyes and can be used as a template for the synthesis of nanoparticles. Clearly, the ability to render the light-enabled reversible hierarchical self-assembly provides a unique platform for smart delivery vehicles and templates for nanomaterials.

Electrostatic Self-Assembly of Protein Cage Arrays

Protein and peptide cages are nanoscale containers, which are of particular interest in nanoscience due to their well-defined dimensions and enclosed central cavities that can be filled with material that is protected from the outside environment. Ferritin is a typical example of protein cage, formed by 24 polypeptide chains that self-assemble into a hollow, roughly spherical protein cage with external and internal diameters of approximately 12 nm and 8 nm, respectively. The interior cavity of ferritin provides a unique reaction vessel to carry out reactions separated from the exterior environment. In nature, the cavity is utilized for sequestration and biomineralization to render iron inert and safe by shielding from the external environment. Materials scientists have been inspired by this system and exploited a range of ferritin superfamily proteins as supramolecular templates to encapsulate cargoes ranging from cancer drugs to therapeutic proteins. Interesting possibilities arise if such containers can themselves be arranged into even higher-order structures such as crystalline arrays. Here, we describe how crystalline arrays of negatively charged ferritin protein cages can be built by taking advantage of electrostatic interactions with cationic gold nanoparticles.

Naked micelles: well-defined polymer nanoparticles from photo-cleavable block copolymer micelles

We report that photo-cleaving the corona of block copolymer micelles provides well-defined nanoparticles with surface charges accessible for electrostatic modification.

Supercharged Proteins and Polypeptides

Electrostatic interactions play a vital role in nature. Biomacromolecules such as proteins are orchestrated by electrostatics, among other intermolecular forces, to assemble and organize biochemistry. Natural proteins with a high net charge exist in a folded state or are unstructured and can be an inspiration for scientists to artificially supercharge other protein entities. Recent findings show that supercharging proteins allows for control of their properties such as temperature resistance and catalytic activity. One elegant method to transfer the favorable properties of supercharged proteins to other proteins is the fabrication of fusions. Genetically engineered, supercharged unstructured polypeptides (SUPs) are just one promising fusion tool. SUPs can also be complexed with artificial entities to yield thermotropic and lyotropic liquid crystals and liquids. These architectures represent novel bulk materials that are sensitive to external stimuli. Interestingly, SUPs undergo fluid-fluid phase separation to form coacervates. These coacervates can even be directly generated in living cells or can be combined with dissipative fiber assemblies that induce life-like features. Supercharged proteins and SUPs are developed into exciting classes of materials. Their synthesis, structures, and properties are summarized. Moreover, potential applications are highlighted and challenges are discussed.

Hierarchical Self-Assembly of Proteins Through Rationally Designed Supramolecular Interfaces

With the increasing advances in the basic understanding of pathogenesis mechanism and fabrication of advanced biological materials, protein nanomaterials are being developed for their potential bioengineering research and biomedical applications. Among different fabrication strategies, supramolecular self-assembly provides a versatile approach to construct hierarchical nanostructures from polyhedral cages, filaments, tubules, monolayer sheets to even cubic crystals through rationally designed supramolecular interfaces. In this mini review, we will briefly recall recent progress in reconstituting protein interfaces for hierarchical self-assembly and classify by the types of designed protein-protein interactions into receptor-ligand recognition, electrostatic interaction, metal coordination, and non-specific interaction networks. Moreover, some attempts on functionalization of protein superstructures for bioengineering and/or biomedical applications are also shortly discussed. We believe this mini review will outline the stream of hierarchical self-assembly of proteins through rationally designed supramolecular interfaces, which would open minds in visualizing protein-protein recognition and assembly in living cells and organisms, and even constructing multifarious functional bionanomaterials.

Agglomeration of Viruses by Cationic Lignin Particles for Facilitated Water Purification

Virus contamination of water is a threat to human health in many countries. Current solutions for inactivation of viruses mainly rely on environmentally burdensome chemical oxidation or energy-intensive ultraviolet irradiation, which may create toxic secondary products. Here, we show that renewable plant biomass-sourced colloidal lignin particles (CLPs) can be used as agglomeration agents to facilitate removal of viruses from water. We used dynamic light scattering (DLS), electrophoretic mobility shift assay (EMSA), atomic force microscopy and transmission electron microscopy (AFM, TEM), and UV spectrophotometry to quantify and visualize adherence of cowpea chlorotic mottle viruses (CCMVs) on CLPs. Our results show that CCMVs form agglomerated complexes with CLPs that, unlike pristine virus particles, can be easily removed from water either by filtration or centrifugation. Additionally, cationic particles formed by adsorption of quaternary amine-modified softwood kraft lignin on the CLPs were also evaluated to improve the binding interactions with these anionic viruses. We foresee that due to their moderate production cost, and high availability of lignin as a side-stream from biorefineries, CLPs could be an alternative water pretreatment material in a large variety of systems such as filters, packed columns, or flocculants.

From stars to stripes: RNA-directed shaping of plant viral protein templates-structural synthetic virology for smart biohybrid nanostructures

The self-assembly of viral building blocks bears exciting prospects for fabricating new types of bionanoparticles with multivalent protein shells. These enable a spatially controlled immobilization of functionalities at highest surface densities-an increasing demand worldwide for applications from vaccination to tissue engineering, biocatalysis, and sensing. Certain plant viruses hold particular promise because they are sustainably available, biodegradable, nonpathogenic for mammals, and amenable to in vitro self-organization of virus-like particles. This offers great opportunities for their redesign into novel "green" carrier systems by spatial and structural synthetic biology approaches, as worked out here for the robust nanotubular tobacco mosaic virus (TMV) as prime example. Natural TMV of 300 x 18 nm is built from more than 2,100 identical coat proteins (CPs) helically arranged around a 6,395 nucleotides ssRNA. In vitro, TMV-like particles (TLPs) may self-assemble also from modified CPs and RNAs if the latter contain an Origin of Assembly structure, which initiates a bidirectional encapsidation. By way of tailored RNA, the process can be reprogrammed to yield uncommon shapes such as branched nanoobjects. The nonsymmetric mechanism also proceeds on 3'-terminally immobilized RNA and can integrate distinct CP types in blends or serially. Other emerging plant virus-deduced systems include the usually isometric cowpea chlorotic mottle virus (CCMV) with further strikingly altered structures up to "cherrybombs" with protruding nucleic acids. Cartoon strips and pictorial descriptions of major RNA-based strategies induct the reader into a rare field of nanoconstruction that can give rise to utile soft-matter architectures for complex tasks. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Biology-Inspired Nanomaterials > Nucleic Acid-Based Structures.

Highly ordered protein cage assemblies: A toolkit for new materials

Protein capsids are specialized and versatile natural macromolecules with exceptional properties. Their homogenous, spherical, rod-like or toroidal geometry, and spatially directed functionalities make them intriguing building blocks for self-assembled nanostructures. High degrees of functionality and modifiability allow for their assembly via non-covalent interactions, such as electrostatic and coordination bonding, enabling controlled self-assembly into higher-order structures. These assembly processes are sensitive to the molecules used and the surrounding conditions, making it possible to tune the chemical and physical properties of the resultant material and generate multifunctional and environmentally sensitive systems. These materials have numerous potential applications, including catalysis and drug delivery. This article is categorized under: Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Three-Dimensional Protein Cage Array Capable of Active Enzyme Capture and Artificial Chaperone Activity

Development of protein cages for encapsulation of active enzyme cargoes and their subsequent arrangement into a controllable three-dimensional array is highly desirable. However, cargo capture is typically challenging because of difficulties in achieving reversible assembly/disassembly of protein cages in mild conditions. Herein we show that by using an unusual ferritin cage protein that undergoes triggerable assembly under mild conditions, we can achieve reversible filling with protein cargoes including an active enzyme. We demonstrate that these filled cages can be arrayed in three-dimensional crystal lattices and have an additional chaperone-like effect, increasing both thermostability and enzymatic activity of the encapsulated enzyme.

Virus capsid assembly across different length scales inspire the development of virus-based biomaterials

In biology, there are an abundant number of self-assembled structures organized according to hierarchical levels of complexity. In some examples, the assemblies formed at each level exhibit unique properties and behaviors not present in individual components. Viruses are an example of such where first individual subunits come together to form a capsid structure, some utilizing a scaffolding protein to template or catalyze the capsid formation. Increasing the level of complexity, the viral capsids can then be used as building blocks of higher-level assemblies. This has inspired scientists to design and construct virus capsid-based functional nano-materials. This review provides some insight into the assembly of virus capsids across several length scales, and certain properties that arise at different levels, providing examples found in naturally occurring systems and those that are synthetically designed.

Plant virus-based materials for biomedical applications: Trends and prospects

Nanomaterials composed of plant viral components are finding their way into medical technology and health care, as they offer singular properties. Precisely shaped, tailored virus nanoparticles (VNPs) with multivalent protein surfaces are efficiently loaded with functional compounds such as contrast agents and drugs, and serve as carrier templates and targeting vehicles displaying e.g. peptides and synthetic molecules. Multiple modifications enable uses including vaccination, biosensing, tissue engineering, intravital delivery and theranostics. Novel concepts exploit self-organization capacities of viral building blocks into hierarchical 2D and 3D structures, and their conversion into biocompatible, biodegradable units. High yields of VNPs and proteins can be harvested from plants after a few days so that various products have reached or are close to commercialization. The article delineates potentials and limitations of biomedical plant VNP uses, integrating perspectives of chemistry, biomaterials sciences, molecular plant virology and process engineering.

Functional protein nanostructures: a chemical toolbox

Nature has evolved an optimal synthetic factory in the form of translational and posttranslational processes by which millions of proteins with defined primary sequences and 3D structures can be built. Nature's toolkit gives rise to protein building blocks, which dictates their spatial arrangement to form functional protein nanostructures that serve a myriad of functions in cells, ranging from biocatalysis, formation of structural networks, and regulation of biochemical processes, to sensing. With the advent of chemical tools for site-selective protein modifications and recombinant engineering, there is a rapid development to develop and apply synthetic methods for creating structurally defined, functional protein nanostructures for a broad range of applications in the fields of catalysis, materials and biomedical sciences. In this review, design principles and structural features for achieving and characterizing functional protein nanostructures by synthetic approaches are summarized. The synthetic customization of protein building blocks, the design and introduction of recognition units and linkers and subsequent assembly into structurally defined protein architectures are discussed herein. Key examples of these supramolecular protein nanostructures, their unique functions and resultant impact for biomedical applications are highlighted.

pH-Dependent Polyelectrolyte Bridging of Charged Nanoparticles

Systems comprised of polyelectrolytes and charged nanoparticles are of great technological interest, being common components in formulations among other uses. The colloidal stability of formulations is an important issue, and thus a lot of effort has been made to study the interactions of individual components in these systems. Here, the complexation and adsorption of an annealed (pH-dependent) polyelectrolyte to two spherical nanoparticles has been studied using coarse-grained Monte Carlo simulations. This has been done mainly by varying the solution pH and separation distance (concentration) between the nanoparticles. The polyelectrolyte charge distribution is seen to vary with nanoparticle separation distance, and its ability to bridge both nanoparticles changes with pH. The flexible polyelectrolyte creates compact, multilink bridges at short nanoparticle separation distances and evolves to a stretched single-link bridge at longer distances, where a larger fraction of the polyelectrolyte wraps around the nanoparticles. The annealed polyelectrolyte is also compared with a quenched polyelectrolyte of similar fixed fractional charge. Here, a difference is found in the adsorption ability at low pH/ionization due to the ability of the annealed polyelectrolytes to concentrate charges in the vicinity of the nanoparticle. At intermediate polyelectrolyte charge fractions and with increasing nanoparticle separation distances, the annealed system is able to link nanoparticles at larger distances as compared to the quenched, in good agreement with experimental observations. The results in this work contribute to the understanding of the effect of annealed polyelectrolytes and pH variations in the phase behavior of polyelectrolyte-nanoparticle systems, potentially aiding in the design and optimization of pH-responsive systems.