Enzymatic grafting of functional molecules to the lignin model dibenzodioxocin and lignocellulose material

Enzymatic Modification of Cellulose To Unlock Its Exploitation in Advanced Materials

Nowadays natural biopolymers have a wide variety of uses in various industrial applications, such as food, adhesives and composite materials. Among them, cellulose has attracted the interest of researchers due to its properties: high strength and flexibility, biocompatibility and nontoxicity. Despite that, in many cases its practical use is limited because of poor solubility and/or an unsuitable hydrophilic/hydrophobic balance. In this context, enzymatic modification appears as a powerful strategy to overcome these problems through selective, green and environmentally friendly processes. This minireview discusses the different methods developed for the enzymatic modification of cellulose, emphasizing the type of reaction, the enzymes used (laccases, esterases, lipases, hexokinases, etc.), and the properties and applications of the cellulose derivatives obtained. Considering that cellulose is the most abundant natural polymer on Earth and can be derived from residual lignocellulosic biomass, the impact of its use in bio-based process following the logic of the circular economy is relevant.

Laccase-Enzyme Treated Flax Fibre for Use in Natural Fibre Epoxy Composites

Natural fibres have a high potential as reinforcement of polymer matrices, as they combine a high specific strength and modulus with sustainable production and reasonable prices. Modifying the fibre surface is a common method to increase the adhesion and thereby enhance the mechanical properties of composites. In this study, a novel sustainable surface treatment is presented: the fungal enzyme laccase was utilised with the aim of covalently binding the coupling agent dopamine to flax fibre surfaces. The goal is to improve the interfacial strength towards an epoxy matrix. SEM and AFM micrographs showed that the modification changes the surface morphology, indicating a deposition of dopamine on the surface. Fibre tensile tests, which were performed to check whether the fibre structure was damaged during the treatment, showed that no decrease in tensile strength or modulus occurred. Single fibre pullout tests showed a 30% increase in interfacial shear strength (IFSS) due to the laccase-mediated bonding of the coupling agent dopamine. These results demonstrate that a laccase + dopamine treatment modifies flax fibres sustainably and increases the interfacial strength towards epoxy.

Mold resistance of bamboo after laccase-catalyzed attachment of thymol and proposed mechanism of attachment

Laccase-catalyzed attachment of functional molecules onto the surface of bamboo represents an alternative, green approach to improve performance. Although treatment of bamboo with thymol improved resistance to mold, using laccase to fix the same concentration of thymol to the surface of the bamboo could increase both the antifungal activity and resistance to leaching. Leaching of thymol was reduced by as much as 48.4% when laccase was used in thymol fixation. To make clear the mechanisms of fixation, reaction of thymol catalyzed with laccase, was investigated using Fourier-transform infrared spectroscopy, ¹H-NMR, high-resolution mass spectrometry and thermogravimetric analysis, respectively. Results show that thymol oligomer (l-thymol) was formed with ether linkages, which resist water leaching. Although further confirmatory studies are needed, it seems that ether linkages were the main connection of thymol to lignin. This study demonstrates that laccase catalysis is a promising strategy to functionalize the surface of bamboo in order to bestow new properties suitable for a wide range of applications.

Schematic diagram of laccase-catalysed fixation of natural antimicrobial phenol to bamboo.

Functionalization of Bacterial Cellulose Nonwoven by Poly(fluorophenol) to Improve Its Hydrophobicity and Durability

The present study aims to improve the hydrophobicity and durability of bacterial cellulose (BC) nonwoven by functionalization with poly(fluorophenol). To this end, laccase was first entrapped onto BC and then used to polymerize the fluorophenol {4-[4-(trifluoromethyl) phenoxy] phenol} in-situ. The polymerization of fluorophenol by laccase was confirmed through 1H NMR and MALDI-TOF analyses. The effect of poly(fluorophenol) on BC nonwoven was determined by evaluation of the surface hydrophobicity and olephobicity properties such as water contact angle (WCA), oil contact angle (OCA), surface energy and water/oil absorption time. After BC functionalization with poly(fluorophenol) (20 mM), the WCA increased from 54.5 ± 1.2° to 120 ± 1.5° while the surface energy decreased (11.58 ± 1.4 mN/m). The OCA was also increased from 46.5 ± 2.5° to 87 ± 2° along to the decrease of surface energy (8.7 ± 1.5°). X-ray photoelectron spectroscopy (XPS) analysis confirmed an increase in the fluorine content in BC from 5.27 to 17.57%. The findings confirmed the polymerization of fluorophenol by laccase and its entrapment onto a BC nanofiber structure. The durability of the functionalization with poly(fluorophenol) was confirmed by evaluating the washing fastness, tensile strength after washing and dimensional stability. The results indicate that the functionalized BC nonwoven had higher tensile strength (×10 times) and better dimensional stability (30%) than the non-functionalized BC nonwoven material.

Thermophilic enzyme systems for efficient conversion of lignocellulose to valuable products: Structural insights and future perspectives for esterases and oxidative catalysts

Thermophilic enzyme systems are of major importance nowadays in all industrial processes due to their great performance at elevated temperatures. In the present review, an overview of the current knowledge on the properties of thermophilic and thermotolerant carbohydrate esterases and oxidative enzymes with great thermostability is provided, with respect to their potential use in biotechnological applications. A special focus is given to the lytic polysaccharide monooxygenases that are able to oxidatively cleave lignocellulose through the use of oxygen or hydrogen peroxide as co-substrate and a reducing agent as electron donor. Structural characteristics of the enzymes, including active site conformation and surface properties are discussed and correlated with their substrate specificity and thermostability properties.

Laccase-Mediated Grafting on Biopolymers and Synthetic Polymers: A Critical Review

Laccase-mediated grafting on lignocelluloses has gained considerable attention as an environmentally benign method to covalently modify wood, paper and cork. In recent decades this technique has also been employed to modify fibres with a polysaccharide backbone, such as cellulose or chitosan, to infer colouration, antimicrobial activity or antioxidant activity to the material. The scope of this approach has been further widened by researchers, who apply mediators or high redox potential laccases and those that modify synthetic polymers and proteins. In all cases, the methodology relies on one- or two-electron oxidation of the surface functional groups or of the graftable molecule in solution. However, similar results can very often be achieved through simple deposition, even after extensive washing. This unintended adsorption of the active substance could have an adverse effect on the durability of the applied coating. Differentiating between actual covalent binding and adsorption is therefore essential, but proves to be challenging. This review not only covers excellent research on the topic of laccase-mediated grafting over the last five to ten years, but also provides a critical comparison to highlight either the lack or presence of compelling evidence for covalent grafting.

Elucidating the mechanism behind the laccase-mediated modification of poly(ethersulfone)

Laccase-mediated oligomerisation of 4-hydroxybenzoic acid (4-HBA) derivatives and simultaneous in situ surface modification has proven to be a cost-effective, easily applicable and eco-friendly strategy for preventing biofouling of poly(ethersulfone) (PES) water filtration membranes. Modification of the membrane surface has previously been hypothesised to occur through covalent bonding of enzymatically generated phenolic radicals to the polymeric membrane. The current study shows, however, that in situ formation of soluble phenolic oligomers does not result in covalent membrane modification. We studied in situ laccase-mediated oligomerisation of custom-synthesised positively charged and commercially available negatively charged monomeric phenols, and demonstrated that their mode of binding to PES is not covalent. In addition, soluble, non-soluble and on-resin PES model compounds were synthesised and used in the laccase-mediated oligomerisation of 4-HBA. Covalent bond formation between these model compounds and (oligomeric) 4-HBA could not be observed either. Furthermore, extensive washing of PES membranes modified through laccase-mediated oligomerisation of 4-HBA resulted in substantial discolouration of the membrane surface, showing that the layer of oligomerised phenolics could easily be removed. Altogether, it was concluded that laccase-assisted modification of PES membranes resulted from strong physical adsorption of phenolic oligomers and polymers rather than from covalent bonding of those.

The mechanism behind the laccase-mediated functionalisation of poly(ethersulfone) was studied using a multifaceted approach, which revealed that surface modification had occurred through strong physical adsorption, rather than through grafting of phenolic oligomers.

Laccase catalysis for the synthesis of bioactive compounds

The demand for compounds of therapeutic value is increasing mainly because of new applications of bioactive compounds in medicine, pharmaceutical, agricultural, and food industries. This has necessitated the search for cost-effective methods for producing bioactive compounds and therefore the intensification of the search for enzymatic approaches in organic synthesis. Laccase is one of the enzymes that have shown encouraging potential as biocatalysts in the synthesis of bioactive compounds. Laccases are multicopper oxidases with a diverse range of catalytic activities revolving around synthesis and degradative reactions. They have attracted much attention as potential industrial catalysts in organic synthesis mainly because they are essentially green catalysts with a diverse substrate range. Their reaction only requires molecular oxygen and releases water as the only by-product. Laccase catalysis involves the abstraction of a single electron from their substrates to produce reactive radicals. The free radicals subsequently undergo homo- and hetero-coupling to form dimeric, oligomeric, polymeric, or cross-coupling products which have practical implications in organic synthesis. Consequently, there is a growing body of research focused on the synthetic applications of laccases such as organic synthesis, hair and textile dyeing, polymer synthesis, and grafting processes. This paper reviews the major advances in laccase-mediated synthesis of bioactive compounds, the mechanisms of enzymatic coupling, structure-activity relationships of synthesized compounds, and the challenges that might guide future research directions.

Conversion of lignin into value-added materials and chemicals via laccase-assisted copolymerization

With today's environmental concerns and the diminishing supply of the world's petroleum-based chemicals and materials, much focus has been directed toward alternative sources. Woody biomass presents a promising option due to its sheer abundance, renewability, and biodegradability. Lignin, a highly irregular polyphenolic compound, is one of the major chemical constituents of woody biomass and is the second most abundant biopolymer on Earth, surpassed only by cellulose. The pulp and paper and cellulosic ethanol industries produce lignin on the scale of millions of tons each year as a by-product. Traditionally, lignin has been viewed as a waste material and burned as an inefficient fuel. However, in recent decades, research has focused on more economical ways to convert lignin into value-added commodities, such as biofuels, biomaterials, and biochemicals, thus developing and strengthening the concept of fully integrated biorefineries. Owing to the phenolic structure of lignin, it is possible to enzymatically graft molecules onto its surface using laccases (benzenediol:oxygen oxidoreductases, EC 1.10.3.2) to create exciting novel biomaterials. These environmentally friendly enzymes use oxygen as their only co-substrate and produce water as their sole by-product, and have thus found great industrial application. This mini-review highlights recent advances in the field of laccase-facilitated functionalization of lignin as well as promising future directions for lignin-based polymers.

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.

Surface functionalization of coconut fibers by enzymatic biografting of syringaldehyde for the development of biocomposites

Surface modification of coconut fibers was carried out by laccase-assisted biografting of syringaldehyde for their use as reinforcing material in the preparation of biocomposites.

Novel, biocatalytically produced hydroxytyrosol dimer protects against ultraviolet-induced cell death in human immortalized keratinocytes

Compounds derived from botanicals, such as olive trees, have been shown to possess various qualities that make them function as ideal antioxidants and, in doing so, protect them against the damaging effect of ultraviolet (UV)-derived oxidative stress. The aim of this study was to biocatalytically synthesize a dimeric product (compound II) from a known botanical, 3-hydroxytyrosol, and test it for its antioxidant ability using a human immortalized keratinocyte cell line (HaCaT). 2,2-Diphenyl-picryhydrazyl (DPPH) antioxidant assays showed 33 and 86.7% radical scavenging activity for 3-hydroxytyrosol and its dimer, respectively. The ferric-reducing antioxidant power (FRAP) assay corroborated this by showing a 3-fold higher antioxidant activity for the dimer than 3-hydroxytyrosol. Western blot analyses, showing cells exposed to 500 μM of the dimeric product when ultraviolet A (UVA)-irradiated, increased the anti-apoptotic protein Bcl-2 expression by 16% and reduced the pro-apoptotic protein Bax by 87.5%. Collectively, the data show that the dimeric product of 3-hydroxytyrosol is a more effective antioxidant and could be considered for use in skin-care products, health, and nutraceuticals.

Protection of wood from microorganisms by laccase-catalyzed iodination

In the present work, Norway spruce wood (Picea abies L.) was reacted with a commercial Trametes versicolor laccase in the presence of potassium iodide salt or the phenolic compounds thymol and isoeugenol to impart an antimicrobial property to the wood surface. In order to assess the efficacy of the wood treatment, a leaching of the iodinated and polymerized wood and two biotests including bacteria, a yeast, blue stain fungi, and wood decay fungi were performed. After laccase-catalyzed oxidation of the phenols, the antimicrobial effect was significantly reduced. In contrast, the enzymatic oxidation of iodide (I(-)) to iodine (I(2)) in the presence of wood led to an enhanced resistance of the wood surface against all microorganisms, even after exposure to leaching. The efficiency of the enzymatic wood iodination was comparable to that of a chemical wood preservative, VP 7/260a. The modification of the lignocellulose by the laccase-catalyzed iodination was assessed by the Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) technique. The intensities of the selected lignin-associated bands and carbohydrate reference bands were analyzed, and the results indicated a structural change in the lignin matrix. The results suggest that the laccase-catalyzed iodination of the wood surface presents an efficient and ecofriendly method for wood protection.

A new procedure for the hydrophobization of cellulose fibre using laccase and a hydrophobic phenolic compound

A new biotechnological procedure using laccase in combination with a hydrophobic phenolic compound (lauryl gallate) for the hydrophobization of cellulose fibres and internal sizing of paper was developed. Cellulose fibres from hardwood kraft pulp were incubated with laccase (Lac), in combination with lauryl gallate (LG). The Lac-LG treatment resulted in the internal sizing of paper, and also in significantly reduced water penetration in the handsheets and wettability of the paper surface. Paper was found not to be effectively rendered hydrophobic by LG alone. SEM images of the fibre network revealed the presence of the sizing agent: a product of the reaction between laccase and lauryl gallate. Binding of lauryl gallate to cellulose fibres was suggested by the increase in kappa number of the pulp and further confirmed by IR spectroscopy.