Understanding Self-Assembly of Silica-Precipitating Peptides to Control Silica Particle Morphology

Exploiting Silica-Binding and Silica-Forming Proteins as Versatile Tools for One-Step Enzyme Immobilization on Siliceous Materials

Enzyme immobilization has emerged as an essential technique in industrial applications of enzymes. Silica (SiO2) serves as a prominent support material for enzyme immobilization. Recent advancements have led to the development of various silica-binding proteins (SBPs) and silica-forming proteins (SFPs) that are invaluable tools in immobilizing enzymes on siliceous materials in a fast and simple manner. SBPs facilitate the immobilization of enzymes with controlled orientation on silica surfaces, while SFPs enable the biomimetic synthesis and encapsulation of enzymes within silica particles. In this review, we explore recent advances in the use of SBPs and SFPs in enzyme applications. We provide a comprehensive overview of their mechanisms and sequence characteristics relevant to enzyme immobilization. Additionally, we summarize the recombinant production and immobilization procedures for enzymes with SBPs or SFPs. We then categorize the available SBPs and SFPs into naturally occurring and artificially engineered types, presenting recent examples that demonstrate their utilization in enzyme immobilization. Our review highlights the strengths and limitations of various SBPs and SFPs and sheds light on future directions for the development of tailor-made biocatalytic silica.

The structure of His15 acetamide-modified hen egg-white lysozyme: a nice surprise from an old friend

Hen egg-white lysozyme (HEWL) is a small polycationic protein which is highly soluble and stable. This has led to it becoming a `molecular laboratory' where chemical biological operations and structural techniques are tested. To date, HEWL accounts for 1233 PDB entries, roughly 0.5% of the total, making it the best-represented protein in the PDB. With the aim of unambiguously identifying the N atom of the His15 side chain that is most reactive towards iodoacetamide, the structure of chemically modified HEWL was determined by crystallizing it using the `15 minutes lysozyme' protocol. This protocol invariably yields tetragonal crystals of the unmodified protein. To our surprise, we found that the crystals of the modified protein had similar unit-cell parameters but that refinement was only possible when considering an orthorhombic system.

A pH-Dependent Phase Separation Drives Polyamine-Mediated Silicification from Undersaturated Solutions

Silica polymerization from its soluble monomers is fundamental to many chemical processes. Although industrial methods require harsh conditions and concentrated precursors, biological silica precipitation occurs under ambient conditions from dilute solutions. The hallmark of biosilica is the presence of amine-rich organic macromolecules, but their functional role remains elusive. Here, we show a pH-dependent stimulatory effect of such polyamines on silica polymerization. Notably, this process is decoupled from the saturation degree, allowing the synthesis of polymer-silica hybrid products with controlled network morphologies from undersaturated solutions. The data suggest a two-step phase separation process. First, an associative liquid-liquid phase separation forms a micrometer-size dense phase. Second, silica undergoes a liquid-to-solid transition in the supersaturated condensates to form a bicontinuous silica structure. This study can inspire "soft chemistry" routes to design organic-inorganic nanomaterials with regulatory principles optimized by evolution.

Understanding Biomineralization Mechanisms to Produce Size-Controlled, Tailored Nanocrystals for Optoelectronic and Catalytic Applications: A Review

Biomineralization, the use of biological systems to produce inorganic materials, has recently become an attractive approach for the sustainable manufacturing of functional nanomaterials. Relying on proteins or other biomolecules, biomineralization occurs under ambient temperatures and pressures, which presents an easily scalable, economical, and environmentally friendly method for nanoparticle synthesis. Biomineralized nanocrystals are quickly approaching a quality applicable for catalytic and optoelectronic applications, replacing materials synthesized using expensive traditional routes. Here, we review the current state of development for producing functional nanocrystals using biomineralization and distill the wide variety of biosynthetic pathways into two main approaches: templating and catalysis. Throughout, we compare and contrast biomineralization and traditional syntheses, highlighting optimizations from traditional syntheses that can be implemented to improve biomineralized nanocrystal properties such as size and morphology, making them competitive with chemically synthesized state-of-the-art functional nanomaterials.

An Atomistic View on the Mechanism of Diatom Peptide-Guided Biomimetic Silica Formation

Deciphering nature's remarkable way of encoding functions in its biominerals holds the potential to enable the rational development of nature-inspired materials with tailored properties. However, the complex processes that convert solution-state precursors into solid biomaterials remain largely unknown. In this study, an unconventional approach is presented to characterize these precursors for the diatom-derived peptides R5 and synthetic Silaffin-1A1 (synSil-1A1). These molecules can form defined supramolecular assemblies in solution, which act as templates for solid silica structures. Using a tailored structural biology toolbox, the structure-function relationships of these self-assemblies are unveiled. NMR-derived constraints are employed to enable a recently developed fractal-cluster formalism and then reveal the architecture of the peptide assemblies in atomistic detail. Finally, by monitoring the self-assembly activities during silica formation at simultaneous high temporal and residue resolution using real-time spectroscopy, the mechanism is elucidated underlying template-driven silica formation. Thus, it is demonstrated how to exercise morphology control over bioinorganic solids by manipulating the template architectures. It is found that the morphology of the templates is translated into the shape of bioinorganic particles via a mechanism that includes silica nucleation on the solution-state complexes' surfaces followed by complete surface coating and particle precipitation.

Biphasic NMR of Hyperpolarized Suspensions-Real-Time Monitoring of Solute-to-Solid Conversion to Watch Materials Grow

Nuclear magnetic resonance (NMR) spectroscopy is a key method for the determination of molecular structures. Due to its intrinsically high (i.e., atomistic) resolution and versatility, it has found numerous applications for investigating gases, liquids, and solids. However, liquid-state NMR has found little application for suspensions of solid particles as the resonances of such systems are excessively broadened, typically beyond the detection threshold. Herein, we propose a route to overcoming this critical limitation by enhancing the signals of particle suspensions by >3.000-fold using dissolution dynamic nuclear polarization (d-DNP) coupled with rapid solid precipitation. For the proof-of-concept series of experiments, we employed calcium phosphate (CaP) as a model system. By d-DNP, we boosted the signals of phosphate 31P spins before rapid CaP precipitation inside the NMR spectrometer, leading to the inclusion of the hyperpolarized phosphate into CaP-nucleated solid particles within milliseconds. With our approach, within only 1 s of acquisition time, we obtained spectra of biphasic systems, i.e., micrometer-sized dilute solid CaP particles coexisting with their solution-state precursors. Thus, this work is a step toward real-time characterization of the solid-solution equilibrium. Finally, integrating the hyperpolarized data with molecular dynamics simulations and electron microscopy enabled us to shed light on the CaP formation mechanism in atomistic detail.

A Biomimetic, Silaffin R5-Based Antigen Delivery Platform

Nature offers a wide range of evolutionary optimized materials that combine unique properties with intrinsic biocompatibility and that can be exploited as biomimetic materials. The R5 and RRIL peptides employed here are derived from silaffin proteins that play a crucial role in the biomineralization of marine diatom silica shells and are also able to form silica materials in vitro. Here, we demonstrate the application of biomimetic silica particles as a vaccine delivery and adjuvant platform by linking the precipitating peptides R5 and the RRIL motif to a variety of peptide antigens. The resulting antigen-loaded silica particles combine the advantages of biomaterial-based vaccines with the proven intracellular uptake of silica particles. These particles induce NETosis in human neutrophils as well as IL-6 and TNF-α secretion in murine bone marrow-derived dendritic cells.