Filling Polymersomes with Polymers by Peroxidase-Catalyzed Atom Transfer Radical Polymerization

Biocatalytic Synthesis Using Self-Assembled Polymeric Nano- and Microreactors

Biocatalysis is increasingly being explored for the sustainable development of green industry. Though enzymes show great industrial potential with their high efficiency, specificity, and selectivity, they suffer from poor usability and stability under abiological conditions. To solve these problems, researchers have fabricated nano- and micro-sized biocatalytic reactors based on the self-assembly of various polymers, leading to highly stable, functional, and reusable biocatalytic systems. This Review highlights recent progress in self-assembled polymeric nano- and microreactors for biocatalytic synthesis, including polymersomes, reverse micelles, polymer emulsions, Pickering emulsions, and static emulsions. We categorize these reactors into monophasic and biphasic systems and discuss their structural characteristics and latest successes with representative examples. We also consider the challenges and potential solutions associated with the future development of this field.

Peroxidase Activity of Myoglobin Variants Reconstituted with Artificial Cofactors

Myoglobin (Mb) can react with hydrogen peroxide (H₂ O₂ ) to form a highly active intermediate compound and catalyse oxidation reactions. To enhance this activity, known as pseudo-peroxidase activity, previous studies have focused on the modification of key amino acid residues of Mb or the heme cofactor. In this work, the Mb scaffold (apo-Mb) was systematically reconstituted with a set of cofactors based on six metal ions and two ligands. These Mb variants were fully characterised by UV-Vis spectroscopy, circular dichroism (CD) spectroscopy, inductively coupled plasma mass spectrometry (ICP-MS) and native mass spectrometry (nMS). The steady-state kinetics of guaiacol oxidation and 2,4,6-trichlorophenol (TCP) dehalogenation catalysed by Mb variants were determined. Mb variants with iron chlorin e6 (Fe-Ce6) and manganese chlorin e6 (Mn-Ce6) cofactors were found to have improved catalytic efficiency for both guaiacol and TCP substrates in comparison with wild-type Mb, i. e. Fe-protoporphyrin IX-Mb. Furthermore, the selected cofactors were incorporated into the scaffold of a Mb mutant, swMb H64D. Enhanced peroxidase activity for both substrates were found via the reconstitution of Fe-Ce6 into the mutant scaffold.

Enzyme Catalysis for Reversible Deactivation Radical Polymerization

Enzyme catalysis has been increasingly utilized in reversible deactivation radical polymerization (Enz-RDRP) on account of its mildness, efficiency, and sustainability. In this Minireview we discuss the key roles enzymes play in RDRP, including their ATRPase, initiase, deoxygenation, and photoenzyme activities. We use selected examples to highlight applications of Enz-RDRP in surface brush fabrication, sensing, polymerization-induced self-assembly, and high-throughput synthesis. We also give our reflections on the challenges and future directions of this emerging area.

Kinetic Processes in Enzymatic Nanoreactors for In Vivo Detoxification

Enzymatic nanoreactors are enzyme-encapsulated nanobodies that are capable of performing biosynthetic or catabolic reactions. For this paper, we focused on therapeutic enzyme nanoreactors for the neutralization of toxicants, paying special attention to the inactivation of organophosphorus compounds (OP). Therapeutic enzymes that are capable of detoxifying OPs are known as bioscavengers. The encapsulation of injectable bioscavengers by nanoparticles was first used to prevent fast clearance and the immune response to heterologous enzymes. The aim of enzyme nanoreactors is also to provide a high concentration of the reactive enzyme in stable nanocontainers. Under these conditions, the detoxification reaction takes place inside the compartment, where the enzyme concentration is much higher than in the toxicant diffusing across the nanoreactor membrane. Thus, the determination of the concentration of the encapsulated enzyme is an important issue in nanoreactor biotechnology. The implications of second-order reaction conditions, the nanoreactor’s permeability in terms of substrates, and the reaction products and their possible osmotic, viscosity, and crowding effects are also examined.

Artificial Melanogenesis by Confining Melanin/Polydopamine Production inside Polymersomes

Melanin and polydopamine are potent biopolymers for the development of biomedical nanosystems. However, applications of melanin or polydopamine-based nanoparticles are limited by drawbacks related to a compromised colloidal stability over long time periods and associated cytotoxicity. To overcome these hurdles, a novel strategy is proposed that mimics the confinement of natural melanin in melanosomes. Melanosome mimics are developed by co-encapsulating the melanin/polydopamine precursors L-DOPA/dopamine with melanogenic enzyme Tyrosinase within polymersomes. The conditions of polymersome formation are optimized to obtain melanin/polydopamine polymerization within the cavity of the polymersomes. Similar to native melanosomes, polymersomes containing melanin/polydopamine show long-term colloidal stability, cell-compatibility, and potential for cell photoprotection. This novel kind of artificial melanogenesis is expected to inspire new applications of the confined melanin/polydopamine biopolymers.

Polymeric Nanoreactors as Emerging Nanoplatforms for Cancer Precise Nanomedicine

How to precisely detect and effectively cure cancer which is defined as precise nanomedicine has drawn great attention worldwide. Polymeric nanoreactors which can in situ catalyze inert species into activated ones, can greatly increase imaging quality and enhance therapeutic effects along with decreased background interference and reduced serious side effects. After a brief introduction, the design and preparation of polymeric nanoreactors are discussed from the following aspects, that is, solvent-switch, pH-tuning, film rehydration, hard template, electrostatic interaction, and polymerization-induced self-assembly (PISA). Subsequently, the biomedical applications of these nanoreactors in the fields of cancer imaging, cancer therapy, and cancer theranostics are highlighted. The last but not least, conclusions and future perspectives about polymeric nanoreactors are given. It is believed that polymeric nanoreactors can bring a great opportunity for future fabrication and clinical translation of precise nanomedicine.

Development of Environmentally Friendly Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) is one of the most successful techniques for the preparation of well-defined polymers with controllable molecular weights, narrow molecular weight distributions, specific macromolecular architectures, and precisely designed functionalities. ATRP usually involves transition-metal complex as catalyst. As the most commonly used copper complex catalyst is usually biologically toxic and environmentally unsafe, considerable interest has been focused on iron complex, enzyme, and metal-free catalysts owing to their low toxicity, inexpensive cost, commercial availability and environmental friendliness. This review aims to provide a comprehensive understanding of iron catalyst used in normal, reverse, AGET, ICAR, GAMA, and SARA ATRP, enzyme as well as metal-free catalyst mediated ATRP in the point of view of catalytic activity, initiation efficiency, and polymerization controllability. The principle of ATRP and the development of iron ligand are briefly discussed. The recent development of enzyme-mediated ATRP, the latest research progress on metal-free ATRP, and the application of metal-free ATRP in interdisciplinary areas are highlighted in sections. The prospects and challenges of these three ATRP techniques are also described in the review.

Stabilizing Enzymes within Polymersomes by Coencapsulation of Trehalose

Enzymes are essential biocatalysts and very attractive as therapeutics. However, their functionality is strictly related to their stability, which is significantly affected by the environmental changes occurring during their usage or long-term storage. Therefore, maintaining the activity of enzymes is essential when they are exposed to high temperature during usage or when they are stored for extended periods of time. Here, we stabilize and protect enzymes by coencapsulating them with trehalose into polymersomes. The anhydrobiotic disaccharide preserved up to about 81% of the enzyme's original activity when laccase/trehalose-loaded nanoreactors were kept desiccated for 2 months at room temperature and 75% of its activity when heated at 50 °C for 3 weeks. Moreover, the applicability of laccase/trehalose-loaded nanoreactors as catalysts for bleaching of the textile dyes orange G, toluidine blue O, and indigo was proven. Our results demonstrate the advantages of coencapsulating trehalose within polymersomes to stabilize enzymes in dehydrated state for extended periods of time, preserving their activity even when heated to elevated temperature.

Enzyme-mediated reversible deactivation radical polymerization for functional materials: principles, synthesis, and applications

Enzymes provide a potential and highly efficient way to mediate the formation of various functional polymer materials with wide applications.

Biocatalytic ATRP in solution and on surfaces

The promiscuity of enzymes allows for their implementation as catalysts for non-native chemical transformations. Utilizing the redox activity of metalloenzymes under activator regenerated by electron transfer (ARGET) ATRP conditions, well-controlled and defined polymers can be generated. In this chapter, we review bioATRP in solution and on surfaces and provide experimental protocols for hemoglobin-catalyzed ATRP and for surface-initiated biocatalytic ATRP. This chapter highlights the polymerization of acrylate and acrylamide monomers and provides detailed experimental protocols for the characterization of the polymers and of the polymer brushes.

Chlorophyll derivatives as catalysts and comonomers for atom transfer radical polymerizations

Derivatives of chlorophyll were investigated as both catalysts and comonomers to generate well-defined polymers with narrow dispersities under AGET ATRP conditions.

Live Follow-Up of Enzymatic Reactions Inside the Cavities of Synthetic Giant Unilamellar Vesicles Equipped with Membrane Proteins Mimicking Cell Architecture

Compartmentalization of functional biological units, cells, and organelles serves as an inspiration for the development of biomimetic materials with unprecedented properties and applications in biosensing and medicine. Because of the complexity of cells, the design of ideal functional materials remains a challenge. An elegant strategy to obtain cell-like compartments as novel materials with biofunctionality is the combination of synthetic micrometer-sized giant unilamellar vesicles (GUVs) with biomolecules because it enables studying the behavior of biomolecules and processes within confined cavities. Here we introduce a functional cell-mimetic compartment formed by insertion of the model biopore bacterial membrane protein OmpF in thick synthetic membranes of an artificial GUV compartment that encloses-as a model-the oxidative enzyme horseradish peroxidase. In this manner, a simple and robust cell mimic is designed: the biopore serves as a gate that allows substrates to enter cavities of the GUVs, where they are converted into products by the encapsulated enzyme and then released in the environments of GUVs. Our bioequipped GUVs facilitate the control of specific catalytic reactions in confined microscale spaces mimicking cell size and architecture and thus provide a straightforward approach serving to obtain deeper insights into biological processes inside cells in real time.

Repurposing Biocatalysts to Control Radical Polymerizations

Reversible-deactivation radical polymerizations (controlled radical polymerizations) have revolutionized and revitalized the field of polymer synthesis. While enzymes and other biologically derived catalysts have long been known to initiate free radical polymerizations, the ability of peroxidases, hemoglobin, laccases, enzyme-mimetics, chlorophylls, heme, red blood cells, bacteria, and other biocatalysts to control or initiate reversible-deactivation radical polymerizations has only been described recently. Here, the scope of biocatalytic atom transfer radical polymerizations (bioATRP), enzyme-initiated reversible addition-fragmentation chain transfer radical polymerizations (bioRAFT), biocatalytic organometallic-mediated radical polymerizations (bioOMRP), and biocatalytic reversible complexation mediated polymerizations (bioRCMP) is critically reviewed, and the potential of these reactions for the environmentally friendly synthesis of precision polymers, for the preparation of functional nanostructures, for the modification of surfaces, and for biosensing is discussed.

Oxidoreductase-Initiated Radical Polymerizations to Design Hydrogels and Micro/Nanogels: Mechanism, Molding, and Applications

Due to their 3D cross-linked networks and tunable physicochemical properties, polymer hydrogels with different sizes are applied widely in tissue engineering, drug-delivery systems, pollution regulation, ionic conducting electrolytes, agricultural drought-resistance, cosmetics, and the food industry. Novel, environmentally friendly, and efficient oxidoreductase-initiated radical polymerizations to design hydrogels and micro/nanogels have gained increasing attention. Herein, the recent advances on the use of novel enzyme-initiated systems for hydrogel polymerization, including the mechanisms, and molding of polymeric and hybrid-polymeric networks are reviewed. Preliminary progress related to interfacial enzymatic polymerization for the generation of hybrid micro/nanogels is introduced as an emerging initiating approach. In addition, certain biological applications in tissue engineering, bioimaging, and therapy are demonstrated step by step. Finally, some perspectives on the safety profile of enzymatic formed hydrogels, new enzymatic systems, and potential theranostic applications are discussed.

Inhibition of lysozyme's polymerization activity using a polymer structural mimic

Hen egg white lysozyme (HEWL) is a green catalyst capable of polymerizing the formation of 2-ethynylpyridine. 1,3-di(2-pyridyl)propane (DPP) is a mimic of the polymer repeating unit and a polymerization inhibitor. DPP's interaction with HEWL reveals structural insight into the mechanism of polymerization.

Controlling Enzymatic Polymerization from Surfaces with Switchable Bioaffinity

The affinity of surfaces toward proteins is found to be a key parameter to govern the synthesis of polymer brushes by surface-initiated biocatalytic atom transfer radical polymerization (SI-bioATRP). While the "ATRPase" hemoglobin (Hb) stimulates only a relatively slow growth of protein repellent brushes, the synthesis of thermoresponsive grafts can be regulated by switching the polymer's attraction toward proteins across its lower critical solution temperature (LCST). Poly(N-isopropylacrylamide) (PNIPAM) brushes are synthesized in discrete steps of thickness at temperatures above LCST, while the biocatalyst layer is refreshed at T < LCST. Multistep surface-initiated biocatalytic ATRP demonstrates a high degree of control, results in high chain end group fidelity and enables the synthesis of multiblock copolymer brushes under fully aqueous conditions. The activity of Hb can be further modulated by tuning the accessibility of the heme pocket within the protein. Hence, the multistep polymerization is accelerated at acid pH, where the enzyme undergoes a transition from its native to a molten globule conformation. The controlled synthesis of polymer brushes by multistep SI-bioATRP highlights how a biocatalytic synthesis of grafted polymer films can be precisely controlled through the modulation of the polymer's interfacial physicochemical properties, in particular of the affinity of the surface toward proteins. This is not only of importance to gain a predictive understanding of surface-confined enzymatic polymerizations, but also represents a new way to translate bioadhesion into a controlled functionalization of materials.

Enzymatic Catalysis at Nanoscale: Enzyme-Coated Nanoparticles as Colloidal Biocatalysts for Polymerization Reactions

Enzyme-catalyzed controlled radical polymerization represents a powerful approach for the polymerization of a wide variety of water-soluble monomers. However, in such an enzyme-based polymerization system, the macromolecular catalyst (i.e., enzyme) has to be separated from the polymer product. Here, we present a compelling approach for the separation of the two macromolecular species, by taking the catalyst out of the molecular domain and locating it in the colloidal domain, ensuring quasi-homogeneous catalysis as well as easy separation of precious biocatalysts. We report on gold nanoparticles coated with horseradish peroxidase that can catalyze the polymerization of various monomers (e.g., N-isopropylacrylamide), yielding thermoresponsive polymers. Strikingly, these biocatalyst-coated nanoparticles can be recovered completely and reused in more than three independent polymerization cycles, without significant loss of their catalytic activity.

Biomimetic Strategy To Reversibly Trigger Functionality of Catalytic Nanocompartments by the Insertion of pH-Responsive Biovalves

We describe an innovative strategy to generate catalytic compartments with triggered functionality at the nanoscale level by combining pH-reversible biovalves and enzyme-loaded synthetic compartments. The biovalve has been engineered by the attachment of stimuli-responsive peptides to a genetically modified channel porin, enabling a reversible change of the molecular flow through the pores of the porin in response to a pH change in the local environment. The biovalve functionality triggers the reaction inside the cavity of the enzyme-loaded compartments by switching the in situ activity of the enzymes on/off based on a reversible change of the permeability of the membrane, which blocks or allows the passage of substrates and products. The complex functionality of our catalytic compartments is based on the preservation of the integrity of the compartments to protect encapsulated enzymes. An increase of the in situ activity compared to that of the free enzyme and a reversible on/off switch of the activity upon the presence of a specific stimulus is achieved. This strategy provides straightforward solutions for the development of catalytic nanocompartments efficiently producing desired molecules in a controlled, stimuli-responsive manner with high potential in areas, such as medicine, analytical chemistry, and catalysis.

Deuterohemin-Peptide Enzyme Mimic-Embedded Metal-Organic Frameworks through Biomimetic Mineralization with Efficient ATRP Catalytic Activity

An enzyme mimic harboring iron porphyrin (DhHP-6) embedded in zeolite imidazolate framework-8 (ZIF-8) was constructed through a biomimetic mineralization approach to obtain composite DhHP-6@ZIF-8. The composite was then used as a catalyst in the atom transfer radical polymerization (ATRP) of poly(ethylene glycol) methyl ether methacrylate (PEGMA₅₀₀) in which poly(PEGMA₅₀₀) could be synthesized with monomer conversion of 76.1% and M_n of 45 900 g/mol, stronger than that obtained when using free DhHP-6 as a catalyst. More importantly, it could efficiently overcome the drawbacks of free DhHP-6 and achieve the easy separation of DhHP-6 from the catalytic system and the elimination of iron residues in the synthesized polymer. In addition, it exhibited an enhanced recyclability with monomer conversion of 75.7% after five cycles and favorable stability during the ATRP reaction with <3.0% of DhHP-6 release within 100 h. Thus, the enzyme mimic-ZIF-8 composite developed through biomimetic mineralization can be potentially used as an effective catalyst for preparing well-defined polymers with biomedical applications.

Spectrophotometric Quantification of Peroxidase with p-Phenylene-diamine for Analyzing Peroxidase-Encapsulating Lipid Vesicles

A spectrophotometric assay for the determination of horseradish peroxidase (HRP) in aqueous solution with p-phenylenediamine (PPD, benzene-1,4-diamine) as electron donor substrate and hydrogen peroxide (H2O2) as oxidant was developed. The oxidation of PPD by HRP/H2O2 leads to the formation of Bandrowski's base ((3E,6E)-3,6-bis[(4-aminophenyl)imino]cyclohexa-1,4-diene-1,4-diamine), which can be quantified by following the increase in absorbance at 500 nm. The assay was applied for monitoring the activity of HRP inside ≈180 nm-sized lipid vesicles (liposomes), prepared from POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and purified by size exclusion chromatography. Because of the high POPC bilayer permeability of PPD and H2O2, the HRP-catalyzed oxidation of PPD occurs inside the vesicles once PPD and H2O2 are added to the vesicle suspension. In contrast, if instead of PPD the bilayer-impermeable substrate ABTS2- (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate)) is used, the oxidation of ABTS2- inside the vesicles does not occur. Therefore, using PPD and ABTS2- in separate assays allows distinguishing between vesicle-trapped HRP and HRP in the external bulk solution. In this way, the storage stability of HRP-containing POPC vesicles was investigated in terms of HRP leakage and activity of entrapped HRP. It was found that pH 7.0 suspensions of POPC vesicles (2.2 mM POPC) containing on average about 12 HRP molecules per vesicle are stable for at least 1 month without any significant HRP leakage, if stored at 4 °C. Such high stability is beneficial not only for bioanalytical applications but also for exploring the kinetic properties of vesicle-entrapped HRP through simple spectrophotometric absorption measurements with PPD as a sensitive and cheap substrate.

Lysozyme-catalyzed formation of a conjugated polyacetylene

Hen egg white lysozyme catalyzes the polymerization of 2-ethynylpyridine in water as the singular protein catalyst. This marks the first time a protein has been observed generating conjugated polymers from alkynes.

Biocatalytic atom transfer radical polymerization in a protein cage nanoreactor

The ATRP-catalyzing enzyme horseradish peroxidase was encapsulated into the protein cage thermosome resulting in an all-protein nanoreactor system for controlled radical polymerizations.

Surface Immobilization of pH-Responsive Polymer Brushes on Mesoporous Silica Nanoparticles by Enzyme Mimetic Catalytic ATRP for Controlled Cargo Release

Peroxidase mimetic catalytic atom transfer radical polymerization (ATRP) was first used to install tertiary amine-functionalized polymer brushes on the surface of mesoporous silica nanoparticles (MSNs) in a facile and highly efficient manner. Poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) brushes-grafted MSNs were fabricated by biocompatible deuterohemin-β-Ala-His-Thr-Val-Glu-Lys (DhHP-6)-catalyzed surface-initiated ATRP (SI-ATRP). The resulting organic⁻inorganic hybrid nanocarriers were fully characterized by Fourier transform-infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), SEM, TEM, Elemental analysis, Zeta-potential, and N₂ adsorption⁻desorption isotherms, which demonstrated the successful coating of pH-responsive polymers on the MSN surface. Rhodamine 6G (Rh6G) dyes were further loaded within the mesopores of this nanocarrier, and the release of Rh6G out of MSNs in a controlled fashion was achieved upon lowing the solution pH. The electrostatic repulsion of positively-charged tertiary ammonium of PDMAEMAs in acidic environments induced the stretching out of polymer brushes on MSN surfaces, thus opening the gates to allow cargo diffusion out of the mesopores of MSNs.

Radical-Mediated Enzymatic Polymerizations

Polymerization reactions are commonly effected by exposing monomer formulations to some initiation stimulus such as elevated temperature, light, or a chemical reactant. Increasingly, these polymerization reactions are mediated by enzymes--catalytic proteins--owing to their reaction efficiency under mild conditions as well as their environmental friendliness. The utilization of enzymes, particularly oxidases and peroxidases, for generating radicals via reduction-oxidation mechanisms is especially common for initiating radical-mediated polymerization reactions, including vinyl chain-growth polymerization, atom transfer radical polymerization, thiol-ene step-growth polymerization, and polymerization via oxidative coupling. While enzyme-mediated polymerization is useful for the production of materials intended for subsequent use, it is especially well-suited for in situ polymerizations, where the polymer is formed in the place where it will be utilized. Such polymerizations are especially useful for biomedical adhesives and for sensing applications.

Enzyme-mediated polymerization inside engineered protein cages

Engineered variants of the capsid-forming enzyme lumazine synthase, AaLS, were used as nanoreactors for an enzyme-mediated polymerization.

Laccase-catalyzed controlled radical polymerization of N-vinylimidazole

Laccase from Trametes versicolor catalyzes the controlled radical polymerization of N-vinylimidazole, yielding narrowly dispersed, metal-free polymers.

Enzyme mimetic-catalyzed ATRP and its application in block copolymer synthesis combined with enzymatic ring-opening polymerization

Use of an enzyme mimetic (DhHP-6) as an ATRP catalyst for the synthesis of a series of functional polymers.