Rationally Tailored Mesoporous Hosts for Optimal Protein Encapsulation

Enhancing Biocatalysis: Metal-Organic Frameworks as Multifunctional Enzyme Hosts

ConspectusEnzymes are highly efficient and selective catalysts that operate under mild conditions, making them invaluable for various chemical transformations. However, their limitations, such as instability and high cost, call for advancements in enzyme immobilization and the development of suitable host materials. Metal-organic frameworks (MOFs), characterized by high porosity, crystallinity, and tunability, are promising candidates for enzyme encapsulation. Among these, zirconium-based MOFs (Zr-MOFs) stand out due to their exceptional structural diversity and chemical stability. The physical and chemical properties of Zr-MOFs can be tuned and characterized with atomic precision, and their interactions with enzymes can be analyzed through a range of techniques spanning from chemistry and materials science to biochemistry. This tunable platform provides opportunities to systematically investigate the impact of encapsulation on the stability and activity of enzymes in order to develop design rules for enzyme hosts.In this Account, we discuss experimentally validated concepts for designing MOF hosts based on their structural properties and enzyme encapsulation mechanisms. We present methods to enhance enzyme catalytic performance through encapsulation and strategies for creating multifunctional enzyme@MOF systems via host modifications. We start by highlighting the importance of host structural design that maximizes substrate diffusion and enzyme availability, with particular focus on MOFs containing hierarchical mesoporous structures such as those in the csq topology. We then delve into the encapsulation process and host-guest interactions examined through techniques such as microscopy, calorimetry, and computational methods, which provide guidelines to fine-tune the local pore chemical environment to enhance enzyme stability and catalytic activity. Techniques found in biochemistry, such as isothermal titration calorimetry (ITC) and confocal laser scanning microscopy (CLSM), were developed to investigate enzyme encapsulation mechanisms, revealing high-entropy-driven host-guest affinity. Additionally, we discuss cases in which enzyme@MOF systems demonstrated enhanced catalytic activities and multifunctional capabilities. Encapsulated enzymes have demonstrated improved thermal and chemical stabilities compared to their free counterparts, maintaining activity under conditions that typically lead to denaturation. Additionally, the highly tunable nature of the MOF platforms allows them to support more complex systems such as tandem reactions, enabling applications in biophotocatalysis, bioelectrocatalysis, and targeted therapeutic protein delivery.The versatility of enzyme@MOFs promises extensive applications in both research and industrial processes across fields including biotechnology, pharmaceutical development, and environmental science. We provide an outlook for promising directions for enzyme@MOF research, with the aim of continuing innovation and exploration. We hope that this Account can benefit chemists, biologists, and material scientists toward designing efficient and adaptable next-generation biocatalytic composite materials.

Dynamic Polymer/Metal-Organic Framework Hybrid Microcapsules for Self-Healing Anticorrosion Coatings

An ideal microcapsule effectively preserves an active substance and can rapidly release it to elicit a self-healing anticorrosion effect. However, the development of highly efficient microcapsules remains a challenge. In this study, polymer/metal-organic framework hybrid microcapsules with dynamic properties were constructed as self-healing anticorrosion coatings. The shell of the microcapsule consisted of flexible polydopamine and a hard crystalline zeolitic imidazolate framework-8 (ZIF-8) layer. The corrosion inhibitor 8-hydroxyquinoline (8-HQ) was trapped in the microcapsules and remained unreleased because the ZIF-8 layer acted as a molecular sieve. When the coating was surrounded by an acidic environment, the ZIF-8 nanocrystals in the shell dissociated, followed by the release of 8-HQ. A dense protective layer was formed on the steel surface to suppress extensive corrosion propagation. The |Z|0.01Hz value of the self-healing coating increased from 1.9 × 104 Ω cm2 to 2.2 × 106 Ω cm2 within 48 h and remained at this level until 120 h post application. This value is 3 orders of magnitude higher than that of a pure epoxy coating under the same conditions. Compared with conventional coatings, the novel dynamic microcapsules enable the application of self-healing coatings that can withstand harsh acidic environments without human intervention.

Recyclable Enzymatic Hydrolysis with Metal-Organic Framework Stabilized Humicola insolens Cutinase (HiC) for Potential PET Upcycling

The degradation and recycling of plastics, such as poly(ethylene terephthalate) (PET), often require energy-intensive processes with significant waste generation. Moreover, prevalent methods primarily entail physical recycling, yielding subpar materials. In contrast, upcycling involves breaking down polymers into monomers, generating valuable chemicals and materials for alternative products. Enzyme-catalyzed depolymerization presents a promising approach to break down PET without the need for extreme conditions and unstable or toxic metal catalysts, which are typical of traditional recycling methods. However, the practical application of enzymes has been hindered by their high cost and low stability. In this study, we stabilized the enzyme Humicola insolens cutinase (HiC) by encapsulating it within a mesoporous zirconium-based metal-organic framework, NU-1000. HiC@NU-1000 exhibited a quantitative degradation of the PET surrogate, ethylene glycol dibenzoate (EGDB), with greater selectivity than native HiC in producing the fully hydrolyzed product benzoic acid in partial organic solvent. Additionally, the heterogeneous catalyst is also active toward the hydrolysis of PET and has demonstrated recyclability for at least four catalytic cycles. The HiC@NU-1000 model system represents a promising approach to stabilize industrially relevant enzymes under conditions involving elevated temperatures and organic solvents, offering a potential solution for relevant protein-related applications.

Enhancing Kinase Activity Detection with a Programmable Lanthanide Metal-Organic Framework via ATP-to-ADP Conversion

Precise modulation of host-guest interactions between programmable Ln-MOFs (lanthanide metal-organic frameworks) and phosphate analytes holds immense promise for enabling novel functionalities in biosensing. However, the intricate relationship between these functionalities and structures remains largely elusive. Understanding this correlation is crucial for advancing the rational design of fluorescent biosensor technology. Presently, there exists a large research gap concerning the utilization of Ln-MOFsto monitor the conversion of ATP to ADP, which poses a limitation for kinase detection. In this work, we delve into the potential of Ln-MOFs to amplify the fluorescence response during the kinase-mediated ATP-to-ADP conversion. Six Eu-MOFs were synthesized and Eu-TPTC ([1,1':4',1″]-terphenyl-3,3'',5,5''-tetracarboxylic acid) was selected as a ratiometric fluorescent probe, which is most suitable for high-precision detection of creatine kinase activity through the differential response from ATP to ADP. The molecular -level mechanism was confirmed by density functional theory. Furthermore, a simple paper chip-based platform was constructed to realize the fast (20 min) and sensitive (limit of detection is 0.34 U/L) creatine kinase activity detection in biological samples. Ln-MOF-phosphate interactions offer promising avenues for kinase activity assays and hold the potential for precise customization of analytical chemistry.

Mining for Metal-Organic Systems: Chemistry Frontiers of Th-, U-, and Zr-Materials

The conceptual framework presented in this Perspective overviews the design principles of innovative thorium-based materials that could address urgent needs of the medicinal, nuclear energy, and waste remediation sectors from the lens of zirconium and uranium analogs. We survey the intersections of Zr, Th, and U chemistry with a focus on how the intrinsic behavior of each metal translates to broader material properties, including, but not limited to, structural and topological diversity, preferential metal-ligand binding, and reactivity. On the example of several classes of materials, including organometallic complexes, polyoxometalates, and the primary focus of this Perspective, metal-organic frameworks (MOFs), the design principles that govern the preparation of Zr-, Th-, and U-compounds, including oxophilicity, variation in oxidation states, and stable coordination environments have been considered. Further, we highlight how the impact of the mentioned variables may shift throughout the progression from discrete molecular systems to extended structures. We discuss the common assumption that zirconium-organic materials are typically considered a close analog of thorium-based congeners in areas such as material design and preparation. Through consideration of fundamental chemistry principles, we shed light on the relationships between Zr-, Th-, and U-based materials and highlight how a critical analysis of their distinct properties can be used to target a desired material performance. As a result, we provide a detailed understanding of Th-based materials chemistry by anchoring their fundamental properties between two well-studied reference points, zirconium- and uranium-containing analogs.

Co-immobilization of whole cells and enzymes by covalent organic framework for biocatalysis process intensification

Co-immobilization of cells and enzymes is often essential for the cascade biocatalytic processes of industrial-scale feasibility but remains a vast challenge. Herein, we create a facile co-immobilization platform integrating enzymes and cells in covalent organic frameworks (COFs) to realize the highly efficient cascade of inulinase and E. coli for bioconversion of natural products. Enzymes can be uniformly immobilized in the COF armor, which coats on the cell surface to produce cascade biocatalysts with high efficiency, stability and recyclability. Furthermore, this one-pot in situ synthesis process facilitates a gram-scale fabrication of enzyme-cell biocatalysts, which can generate a continuous-flow device conversing inulin to D-allulose, achieving space-time yield of 161.28 g L-1 d-1 and high stability (remaining >90% initial catalytic efficiency after 7 days of continuous reaction). The created platform is applied for various cells (e.g., E. coli, Yeast) and enzymes, demonstrating excellent universality. This study paves a pathway to break the bottleneck of extra- and intracellular catalysis, creates a high-performance and customizable platform for enzyme-cell cascade biomanufacturing, and expands the scope of biocatalysis process intensification.

A Synergetic Pore Compartmentalization and Hydrophobization Strategy for Synchronously Boosting the Stability and Activity of Enzyme

Spatial immobilization of fragile enzymes using a nanocarrier is an efficient means to design heterogeneous biocatalysts, presenting superior stability and recyclability to pristine enzymes. An immobilized enzyme, however, usually compromises its catalytic activity because of inevasible mass transfer issues and the unfavorable conformation changes in a confined environment. Here, we describe a synergetic metal-organic framework pore-engineering strategy to trap lipase (an important hydrolase), which confers lipase-boosted stability and activity simultaneously. The hierarchically porous NU-1003, featuring interconnected mesopore and micropore channels, is precisely modified by chain-adjustable fatty acids on its mesopore channel, into which lipase is trapped. The interconnected pore structure ensures efficient communication between trapped lipase and exterior media, while the fatty acid-mediated hydrophobic pore can activate the opening conformation of lipase by interfacial interaction. Such dual pore compartmentalization and hydrophobization activation effects render the catalytic center of trapped lipase highly accessible, resulting in 1.57-fold and 2.46-fold activities as native lipase on ester hydrolysis and enantioselective catalysis. In addition, the feasibility of these heterogeneous biocatalysts for kinetic resolution of enantiomer is also validated, showing much higher efficiency than native lipase.

Selective Removal of Denatured Proteins Using MOF Nanopores

Here we present, for the first time, the selective adsorption of denatured proteins using a metal-organic framework (MOF), demonstrating promising potential for protein purification. Typical proteins, such as lysozyme and carbonic anhydrase B, enter the pores of MIL-101 through their narrow apertures when they are denatured to an unfolded state. Selective adsorption is achieved by finely tuning two key features: the sizes of the aperture and cage of the MOF nanopores, which are responsible for sorting unfolded polypeptide chains and inhibiting the translocation of the native form into the pores, respectively. By leveraging this selective adsorption, we successfully purified a mixture of native and denatured proteins by adding MOF to the mixture, achieving a native purity of over 99%.

Friends or Foes: Fundamental Principles of Th-Organic Scaffold Chemistry Using Zr-Analogs as a Guide

The fundamental interest in actinide chemistry, particularly for the development of thorium-based materials, is experiencing a renaissance owing to the recent and rapidly growing attention to fuel cycle reactors, radiological daughters for nuclear medicine, and efficient nuclear stockpile development. Herein, we uncover fundamental principles of thorium chemistry on the example of Th-based extended structures such as metal-organic frameworks in comparison with the discrete systems and zirconium extended analogs, demonstrating remarkable over two-and-half-year chemical stability of Th-based frameworks as a function of metal node connectivity, amount of defects, and conformational linker rigidity through comprehensive spectroscopic and crystallographic analysis as well as theoretical modeling. Despite exceptional chemical stability, we report the first example of studies focusing on the reactivity of the most chemically stable Th-based frameworks in comparison with the discrete Th-based systems such as metal-organic complexes and a cage, contrasting multicycle recyclability and selectivity (>97%) of the extended structures in comparison with the molecular compounds. Overall, the presented work not only establishes the conceptual foundation for evaluating the capabilities of Th-based materials but also represents a milestone for their multifaceted future and foreshadows their potential to shape the next era of actinide chemistry.

Biomolecules meet organic frameworks: from synthesis strategies to diverse applications

Biomolecules are essential in pharmaceuticals, biocatalysts, biomaterials, etc., but unfortunately they are extremely susceptible to extraneous conditions. When biomolecules meet porous organic frameworks, significantly improved thermal, chemical, and mechanical stabilities are not only acquired for raw biomolecules, but also molecule sieving, substrate enrichment, chirality property, and other functionalities are additionally introduced for application expansions. In addition, the intriguing synergistic effect stemming from elaborate and concerted interactions between biomolecules and frameworks can further enhance application performances. In this paper, the synthesis strategies of the so-called bio-organic frameworks (BOFs) in recent years are systematically reviewed and classified. Additionally, their broad applications in biomedicine, catalysis, separation, sensing, and imaging are introduced and discussed. Before ending, the current challenges and prospects in the future for this infancy-stage but significant research field are also provided. We hope that this review will offer a concise but comprehensive vision of designing and constructing multifunctional BOF materials as well as their full explorations in various fields.