Enzyme Initiated Radical Polymerizations

Co-immobilization of laccase and mediator through a self-initiated one-pot process for enhanced conversion of malachite green

Laccase is a green biocatalyst. It works with molecular oxygen and produces water as the only by-product. However, its practical application is far less than satisfactory due to the low stability/poor reusability of free laccase and the potential secondary pollution caused by dissolved mediators. To address those bottlenecks in laccase-based catalysis, a novel biocatalyst (Immo-LMS) was fabricated by simultaneously immobilizing both laccase and a mediator (acetylacetone, abbreviated as AA) into a hydrogel through the laccase-AA initiated polymerization. This self-initiated immobilization process avoided the forced conformational change of laccase in the passive embedding to pre-existing carriers. Resulting from the effective cooperation of laccase and AA, the Immo-LMS had the highest substrate conversion quantity to malachite green, followed by the sole immobilized laccase and the immobilized laccase with an external mediator. Besides the improved activity, the Immo-LMS showed enhanced stability. The good performance of the Immo-LMS suggests that the co-immobilization of laccase and mediator through the self-initiated one-pot process was a promising strategy for the immobilization of laccase, which is expected to be helpful to cut down the running cost as well as the potential toxicity that come from mediators in the practical application of laccase.

Enzymes as Green Catalysts for Precision Macromolecular Synthesis

The present article comprehensively reviews the macromolecular synthesis using enzymes as catalysts. Among the six main classes of enzymes, the three classes, oxidoreductases, transferases, and hydrolases, have been employed as catalysts for the in vitro macromolecular synthesis and modification reactions. Appropriate design of reaction including monomer and enzyme catalyst produces macromolecules with precisely controlled structure, similarly as in vivo enzymatic reactions. The reaction controls the product structure with respect to substrate selectivity, chemo-selectivity, regio-selectivity, stereoselectivity, and choro-selectivity. Oxidoreductases catalyze various oxidation polymerizations of aromatic compounds as well as vinyl polymerizations. Transferases are effective catalysts for producing polysaccharide having a variety of structure and polyesters. Hydrolases catalyzing the bond-cleaving of macromolecules in vivo, catalyze the reverse reaction for bond forming in vitro to give various polysaccharides and functionalized polyesters. The enzymatic polymerizations allowed the first in vitro synthesis of natural polysaccharides having complicated structures like cellulose, amylose, xylan, chitin, hyaluronan, and chondroitin. These polymerizations are "green" with several respects; nontoxicity of enzyme, high catalyst efficiency, selective reactions under mild conditions using green solvents and renewable starting materials, and producing minimal byproducts. Thus, the enzymatic polymerization is desirable for the environment and contributes to "green polymer chemistry" for maintaining sustainable society.

Well-Defined Macromolecules Using Horseradish Peroxidase as a RAFT Initiase

Enzymatic catalysis and control over macromolecular architectures from reversible addition-fragmentation chain transfer polymerization (RAFT) are combined to give a new method of making polymers. Horseradish peroxidase (HRP) is used to catalytically generate radicals using hydrogen peroxide and acetylacetone as a mediator. RAFT is used to control the polymer structure. HRP catalyzed RAFT polymerization gives acrylate and acrylamide polymers with relatively narrow molecular weight distributions. The polymerization is rapid, typically exceeding 90% monomer conversion in 30 min. Complex macromolecular architectures including a block copolymer and a protein-polymer conjugate are synthesized using HRP to catalytically initiate RAFT polymerization.

Characterization of the laccase-mediated oligomerization of 4-hydroxybenzoic acid

The curious (lac)case of four dimers – how minor 4-hydroxybenzoic acid dimers can be of major importance.

Label-free polymerization amplified potentiometric sensing platform for radical reactions using polyion sensitive membrane electrodes as transducers

Based on the cascade amplification abilities of radical polymerization reactions, an amplified potentiometric sensing platform for radical reactions was developed.

Enzymatic polymerization of dihydroquercetin using bilirubin oxidase

Dihydroquercetin (or taxifolin) is one of the most famous flavonoids and is abundant in Siberian larch (Larix sibirica). The oxidative polymerization of dihydroquercetin (DHQ) using bilirubin oxidase as a biocatalyst was investigated and some physicochemical properties of the products were studied. DHQ oligomers (oligoDHQ) with molecular mass of 2800 and polydispersity of 8.6 were obtained by enzymatic reaction under optimal conditions. The oligomers appeared to be soluble in dimethylsulfoxide, dimethylformamide, and methanol. UV-visible spectra of oligoDHQ in dimethylsulfoxide indicated the presence of highly conjugated bonds. The synthesized oligoDHQ was also characterized by FTIR and (1)H and (13)C NMR spectroscopy. Comparison of NMR spectra of oligoDHQ with DHQ monomer and the parent flavonoids revealed irregular structure of a polymer formed via the enzymatic oxidation of DHQ followed by nonselective radical polymerization. As compared with the monomer, oligoDHQ demonstrated higher thermal stability and high antioxidant activity.

Rapid and direct spectrophotometric method for kinetics studies and routine assay of peroxidase based on aniline diazo substrates

Peroxidases are ubiquitous enzymes that play an important role in living organisms. Current spectrophotometrically based peroxidase assay methods are based on the production of chromophoric substances at the end of the enzymatic reaction. The ambiguity regarding the formation and identity of the final chromophoric product and its possible reactions with other molecules have raised concerns about the accuracy of these methods. This can be of serious concern in inhibition studies. A novel spectrophotometric assay for peroxidase, based on direct measurement of a soluble aniline diazo substrate, is introduced. In addition to the routine assays, this method can be used in comprehensive kinetics studies. 4-[(4-Sulfophenyl)azo]aniline (λmax = 390 nm, ɛ = 32 880 M(-1) cm(-1) at pH 4.5 to 9) was introduced for routine assay of peroxidase. This compound is commercially available and is indexed as a food dye. Using this method, a detection limit of 0.05 nmol mL(-1) was achieved for peroxidase.

A 2,5-Dihydroxybenzoic Acid-Gelatin Conjugate: The Synthesis, Antiviral Activity and Mechanism of Antiviral Action Against Two Alphaherpesviruses

Various natural and synthetic polyanionic polymers with different chemical structures are known to exhibit potent antiviral activity in vitro toward a variety of enveloped viruses and may be considered as promising therapeutic agents. A water-soluble conjugate of 2,5-dihydroxybezoic acid (2,5-DHBA) with gelatin was synthesized by laccase-catalyzed oxidation of 2,5-DHBA in the presence of gelatin, and its antiviral activity against pseudorabies virus (PRV) and bovine herpesvirus type 1 (BoHV-1), two members of the Alphaherpesvirinae subfamily, was studied. The conjugate produced no direct cytotoxic effect on cells, and did not inhibit cell growth at concentrations up to 1000 µg/mL. It exhibited potent antiviral activity against PRV (IC50, 1.5-15 µg/mL for different virus strains) and BoHV-1 (IC50, 0.5-0.7 µg/mL). When present during virus adsorption, the conjugate strongly inhibited the attachment of PRV and BoHV-1 to cells. The 2,5-DHBA-gelatin conjugate had no direct virucidal effect on the viruses and did not influence their penetration into cells, cell-to-cell spread, production of infectious virus particles in cells, and expression of PRV glycoproteins E and B. The results indicated that the 2,5-DHBA-gelatin conjugate strongly inhibits the adsorption of alphaherpesviruses to cells and can be a promising synthetic polymer for the development of antiviral formulations against alphaherpesvirus infections.

Dual-Enzyme-Loaded Multifunctional Hybrid Nanogel System for Pathological Responsive Ultrasound Imaging and T2-Weighted Magnetic Resonance Imaging

A dual-enzyme-loaded multifunctional hybrid nanogel probe (SPIO@GCS/acryl/biotin-CAT/SOD-gel, or SGC) has been developed for dual-modality pathological responsive ultrasound (US) imaging and enhanced T2-weighted magnetic resonance (MR) imaging. This probe is composed of functionalized superparamagnetic iron oxide particles, a dual enzyme species (catalase and superoxide dismutase), and a polysaccharide cationic polymer glycol chitosan gel. The dual-modality US/MR imaging capabilities of the hybrid nanogel for responsive US imaging and enhanced T2-weighted MR imaging have been evaluated both in vitro and in vivo. These results show that the hybrid nanogel SGC can exhibit efficient dual-enzyme biocatalysis with pathological species for responsive US imaging. SGC also demonstrates increased accumulation in acidic environments for enhanced T2-weighted MR imaging. Further research on these nanogel systems may lead to the development of more efficient US/MR contrast agents.

Laccase-Catalyzed Surface Modification of Thermo-Mechanical Pulp (TMP) for the Production of Wood Fiber Insulation Boards Using Industrial Process Water

Low-density wood fiber insulation boards are traditionally manufactured in a wet process using a closed water circuit (process water). The water of these industrial processes contains natural phenolic extractives, aside from small amounts of admixtures (e.g., binders and paraffin). The suitability of two fungal laccases and one bacterial laccase was determined by biochemical characterization considering stability and substrate spectra. In a series of laboratory scale experiments, the selected commercial laccase from Myceliophtora thermophila was used to catalyze the surface modification of thermo-mechanical pulp (TMP) using process water. The laccase catalyzed the covalent binding of the phenolic compounds of the process water onto the wood fiber surface and led to change of the surface chemistry directly via crosslinking of lignin moieties. Although a complete substitution of the binder was not accomplished by laccase, the combined use of laccase and latex significantly improved the mechanical strength properties of wood fiber boards. The enzymatically-treated TMP showed better interactions with the synthetic binder, as shown by FTIR-analysis. Moreover, the enzyme is extensively stable in the process water and the approach requires no fresh water as well as no cost-intensive mediator. By applying a second-order polynomial model in combination with the genetic algorithm (GA), the required amount of laccase and synthetic latex could be optimized enabling the reduction of the binder by 40%.

Oxidation and nitration of mononitrophenols by a DyP-type peroxidase

Substantial conversion of nitrophenols, typical high-redox potential phenolic substrates, by heme peroxidases has only been reported for lignin peroxidase (LiP) so far. But also a dye-decolorizing peroxidase of Auricularia auricula-judae (AauDyP) was found to be capable of acting on (i) ortho-nitrophenol (oNP), (ii) meta-nitrophenol (mNP) and (iii) para-nitrophenol (pNP). The pH dependency for pNP oxidation showed an optimum at pH 4.5, which is typical for phenol conversion by DyPs and other heme peroxidases. In the case of oNP and pNP conversion, dinitrophenols (2,4-DNP and 2,6-DNP) were identified as products and for pNP additionally p-benzoquinone. Moreover, indications were found for the formation of random polymerization products originating from initially formed phenoxy radical intermediates. Nitration was examined using (15)N-labeled pNP and Na(14)NO2 as an additional source of nitro-groups. Products were identified by HPLC-MS, and mass-to-charge ratios were evaluated to clarify the origin of nitro-groups. The additional nitrogen in DNPs formed during enzymatic conversion was found to originate both from (15)N-pNP and (14)NO2Na. Based on these results, a hypothetical reaction scheme and a catalytically responsible confine of the enzyme's active site are postulated.

How to enjoy laccases

An analysis of the scientific literature published in the last 10 years reveals a constant growth of laccase applicative research in several industrial fields followed by the publication of a great number of patents. The Green Chemistry journal devoted the cover of its September 2014 issue to a laccase as greener alternative for chemical oxidation. This indicates that laccase "never-ending story" has found a new promising trend within the constant search for efficient (bio)catalysts able to meet the 12 green chemistry principles. A survey of ancient and cutting-edge uses of laccase in different industrial sectors is offered in this review with the aim both to underline their potential and to provide inspiration for new ones. Applications in textile and food fields have been deeply described, as well as examples concerning polymer synthesis and laccase-catalysed grafting. Recent applications in pharmaceutical and cosmetic industry have also been reviewed.

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.

Engineering a horseradish peroxidase C stable to radical attacks by mutating multiple radical coupling sites

Peroxidases have great potential as industrial biocatalysts. In particular, the oxidative polymerization of phenolic compounds catalyzed by peroxidases has been extensively examined because of the advantage of this method over other conventional chemical methods. However, the industrial application of peroxidases is often limited because of their rapid inactivation by phenoxyl radicals during oxidative polymerization. In this work, we report a novel protein engineering approach to improve the radical stability of horseradish peroxidase isozyme C (HRPC). Phenylalanine residues that are vulnerable to modification by the phenoxyl radicals were identified using mass spectrometry analysis. UV-Vis and CD spectra showed that radical coupling did not change the secondary structure or the active site of HRPC. Four phenylalanine (Phe) residues (F68, F142, F143, and F179) were each mutated to alanine residues to generate single mutants to examine the role of these sites in radical coupling. Despite marginal improvement of radical stability, each single mutant still exhibited rapid radical inactivation. To further reduce inactivation by radical coupling, the four substitution mutations were combined in F68A/F142A/F143A/F179A. This mutant demonstrated dramatic enhancement of radical stability by retaining 41% of its initial activity compared to the wild-type, which was completely inactivated. Structure and sequence alignment revealed that radical-vulnerable Phe residues of HPRC are conserved in homologous peroxidases, which showed the same rapid inactivation tendency as HRPC. Based on our site-directed mutagenesis and biochemical characterization, we have shown that engineering radical-vulnerable residues to eliminate multiple radical coupling can be a good strategy to improve the stability of peroxidases against radical attack.

Polymerization amplified detection for nanoparticle-based biosensing

Efficient signal amplification processes are key to the design of sensitive assays for biomolecule detection. Here, we describe a new assay platform that takes advantage of both polymerization reactions and the aggregation of nanoparticles to amplify signal. In our design, a cascade is set up in which radicals generated by either enzymes or metal ions are polymerized to form polymers that can entangle multiple gold nanoparticles (AuNPs) into aggregates, resulting in a visible color change. Less than 0.05% monomer-to-polymer conversion is required to initiate aggregation, providing high sensitivity toward the radical generating species. Good sensitivity of this assay toward horseradish peroxidase, catalase, and parts per billion concentrations of iron and copper is shown. Incorporation of the oxygen-consuming enzyme glucose oxidase (GOx), enables this assay to be performed in open air conditions at ambient temperature. We anticipate that such a design will provide a useful platform for sensitive detection of a broad range of biomolecules through polymerization-based amplification.

Peroxidase activity in scutella of maize in association with anatomical changes during germination and grain storage

The embryo of the maize grain (Zea mays L.) is separated from the starchy endosperm by a fibrous structure, which is called the fibrous layer (FL). Using histochemical staining, it was determined that the FL is composed of collapsed cellular layers that contain phenols, neutral lipids, and 1,3-β-glucan. Due to its composition, the FL prevents free diffusion and separates the embryo from the endosperm during germination. Twenty-four hours after imbibition, the scutellum epidermis initiated a series of asynchronous spatial modifications, including cell growth, the perforation of cell walls, increased peroxidase activity in the apoplastic space, and elevated levels of superoxide, phenols, and other components that interact with the fibrous layer, enabling its transformation in addition to the free flow between compartments. During storage at high relative humidity levels, which leads to fast or slow deterioration depending on the temperature, the activity of phenol peroxidase in the scutellum was associated with a loss of vigor and reduced germination capacity when compared with low temperature and low relative humidity conditions. Such deterioration is associated with alterations in autofluorescent emissions from endogenous compounds in the scutellum, indicating changes in the microenvironment or in the differential proportions of epidermal and FL components.

Oxygen-Mediated Enzymatic Polymerization of Thiol-Ene Hydrogels

Materials that solidify in response to an initiation stimulus are currently utilized in several biomedical and surgical applications; however, their clinical adoption would be more widespread with improved physical properties and biocompatibility. One chemistry that is particularly promising is based on the thiol-ene addition reaction, a radical-mediated step-growth polymerization that is resistant to oxygen inhibition and thus is an excellent candidate for materials that polymerize upon exposure to aerobic conditions. Here, thiol-ene-based hydrogels are polymerized by exposing aqueous solutions of multi-functional thiol and allyl ether PEG monomers, in combination with enzymatic radical initiating systems, to air. An initiating system based on glucose oxidase, glucose, and Fe²⁺ is initially investigated where, in the presence of glucose, the glucose oxidase reduces oxygen to hydrogen peroxide which is then further reduced by Fe²⁺ to yield hydroxyl radicals capable of initiating thiol-ene polymerization. While this system is shown to effectively initiate polymerization after exposure to oxygen, the polymerization rate does not monotonically increase with raised Fe²⁺ concentration owing to inhibitory reactions that retard polymerization at higher Fe²⁺ concentrations. Conversely, replacing the Fe²⁺ with horseradish peroxidase affords an initiating system is that is not subject to the iron-mediated inhibitory reactions and enables increased polymerization rates to be attained.

Horseradish peroxidase (HRP) as a tool in green chemistry

The horseradish peroxidase (HRP) potential in organic synthesis.

Laccase-catalyzed synthesis of aniline oligomers and their application for the protection of copper against corrosion

A new method for the enzymatic synthesis of oligoaniline soluble in organic solutions is developed. Aniline oligomers showed a high inhibition of copper corrosion in aqueous HCl and NaCl solutions.

An antioxidant bioinspired phenolic polymer for efficient stabilization of polyethylene

The synthesis, structural characterization and properties of a new bioinspired phenolic polymer (polyCAME) produced by oxidative polymerization of caffeic acid methyl ester (CAME) with horseradish peroxidase (HRP)-H2O2 is reported as a new sustainable stabilizer toward polyethylene (PE) thermal and photo-oxidative degradation. PolyCAME exhibits high stability toward decarboxylation and oxidative degradation during the thermal processes associated with PE film preparation. Characterization of PE films by thermal methods, photo-oxidative treatments combined with chemiluminescence, and FTIR spectroscopy and mechanical tests indicate a significant effect of polyCAME on PE durability. Data from antioxidant capacity tests suggest that the protective effects of polyCAME are due to the potent scavenging activity on aggressive OH radicals, the efficient H-atom donor properties inducing free radical quenching, and the ferric ion reducing ability. PolyCAME is thus proposed as a novel easily accessible, eco-friendly, and biocompatible biomaterial for a sustainable approach to the stabilization of PE films in packaging and other applications.

Hemoglobin and red blood cells catalyze atom transfer radical polymerization

Hemoglobin (Hb) is a promiscuous protein that not only transports oxygen, but also catalyzes several biotransformations. A novel in vitro catalytic activity of Hb is described. Bovine Hb and human erythrocytes were found to display ATRPase activity, i.e., they catalyzed the polymerization of vinyl monomers under conditions typical for atom transfer radical polymerization (ATRP). N-isopropylacrylamide (NIPAAm), poly(ethylene glycol) methyl ether acrylate (PEGA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) were polymerized using organobromine initiators and the reducing agent ascorbic acid in acidic aqueous solution. In order to avoid chain transfer from polymer radicals to Hb's cysteine residues, the accessible cysteines were blocked by a reaction with a maleimide. The formation of polymers with bromine chain ends, relatively low polydispersity indices (PDI), first order kinetics and an increase in the molecular weight of poly(PEGA) and poly(PEGMA) upon conversion indicate that control of the polymerization by Hb occurred via reversible atom transfer between the protein and the growing polymer chain. For poly(PEGA) and poly(PEGMA), the reactions proceeded with a good to moderate degree of control. Sodium dodecyl sulfate (SDS) gel electrophoresis, circular dichroism spectroscopy, and time-resolved ultraviolet-visible (UV-vis) spectroscopy revealed that the protein was stable during polymerization, and only underwent minor conformational changes. As Hb and erythrocytes are readily available, environmentally friendly, and nontoxic, their ATRPase activity is a useful tool for synthetic polymer chemistry. Moreover, this novel activity enhances the understanding of Hb's redox chemistry in the presence of organobromine compounds.

Photoreaction of a hydroxyalkyphenone with the membrane of polymersomes: a versatile method to generate semipermeable nanoreactors

Block copolymer vesicles can be turned into nanoreactors when a catalyst is encapsulated in these hollow nanostructures. However the membranes of these polymersomes are most often impermeable to small organic molecules, while applications as nanoreactor, as artificial organelles, or as drug-delivery devices require an exchange of substances between the outside and the inside of polymersomes. Here, a simple and versatile method is presented to render polymersomes semipermeable. It does not require complex membrane proteins or pose requirements on the chemical nature of the polymers. Vesicles made from three different amphiphilic block copolymers (α,ω-hydroxy-end-capped poly(2-methyl-2-oxazoline)-block-poly(dimethylsiloxane)-block-poly(2-methyl-2-oxazoline) (PMOXA-b-PDMS-b-PMOXA), α,ω-acrylate-end-capped PMOXA-b-PDMS-b-PMOXA, and poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PB)) were reacted with externally added 2-hydroxy-4'-2-(hydroxyethoxy)-2-methylpropiophenone under UV-irradiation. The photoreactive compound incorporated into the block copolymer membranes independently of their chemical nature or the presence of double bonds. This treatment of polymersomes resulted in substantial increase in permeability for organic compounds while not disturbing the size and the shape of the vesicles. Permeability was assessed by encapsulating horseradish peroxidase into vesicles and measuring the accessibility of substrates to the enzyme. The permeability of photoreacted polymersomes for ABTS, AEC, pyrogallol, and TMB was determined to be between 1.9 and 38.2 nm s(-1). It correlated with the hydrophobicity of the compounds. Moreover, fluorescent dyes were released at higher rates from permeabilized polymersomes compared to unmodified ones. The permeabilized nanoreactors retained their ability to protect encapsulated biocatalysts from degradation by proteases.

α-Glucosidase inhibiton and antiglycation activity of laccase-catalyzed catechin polymers

Catechin polymers were produced by laccase (12 U/mL) in a mixture of sodium acetate buffer (1% (+)-catechin, 100 mM, pH 5) and methanol (buffer:methanol = 95:5, v/v). The freeze-dried catechin polymers were recovered from the precipitate after dialysis followed by centrifugation. Catechin polymers extracted with 20% ethanol had potent inhibitory activity against α-glucosidase with an IC50 value of 4 μg/mL, and they were present as a mixture of dimers, trimers, and tetramers. The antihyperglycemic effect of the catechin polymers was confirmed by an oral maltose tolerance test. The catechin polymers also had significantly improved antiglycation and superoxide dismutase-like activities compared to those of (+)-catechin. Since formation of advanced glycation end products and oxidative stress are accelerated in hyperglycemic conditions, we suggest that enzymatic production of catechin polymers could have a potential protective effect in type 2 diabetes, diabetic complications, and other free radical related diseases.