Fungal laccase grafts acrylamide onto lignin in presence of peroxides

Functional screening pipeline to uncover laccase-like multicopper oxidase enzymes that transform industrial lignins

Laccase-like multicopper oxidases are recognized for their potential to alter the reactivity of lignins for application in value-added products. Typically, model compounds are employed to discover such enzymes; however, they do not represent the complexity of industrial lignin substrates. In this work, a screening pipeline was developed to test enzymes simultaneously on model compounds and industrial lignins. A total of 12 lignin-active fungal multicopper oxidases were discovered, including 9 enzymes active under alkaline conditions (pH 11.0). Principal component analysis revealed the poor ability of model compounds to predict enzyme performance on industrial lignins. Additionally, sequence similarity analyses grouped these enzymes with Auxiliary Activity-1 sub-families with few previously characterized members, underscoring their taxonomic novelty. Correlation between the lignin-activity of these enzymes and their taxonomic origin, however, was not observed. These are critical insights to bridge the gap between enzyme discovery and application for industrial lignin valorization.

Enzymatic synthesis of kraft lignin-acrylate copolymers using an alkaline tolerant laccase

Abstract

Softwood kraft lignin is a major bioresource relevant to the production of sustainable bio-based products. Continued challenges to lignin valorization, however, include poor solubility in organic solvents and in aqueous solutions at neutral pH. Herein, an alkaline tolerant laccase was used to graft acrylate functionalities onto softwood kraft lignin, which is expected to enhance the reactivity of lignin with isocyanate when producing bio-based polyurethanes. Proton nuclear magnetic resonance, Fourier-transform infrared spectroscopy, and high-performance liquid chromatography were used to confirm successful grafting of the acrylate monomer onto lignin and verify the importance of including tert-butyl hydroperoxide as an initiator in the grafting reaction. Laccase-mediated grafting of softwood kraft lignin under alkaline conditions produced lignin products with approximately 30% higher hydroxyl value and higher reactivity toward isocyanate. The reported enzymatic and aqueous process presents an opportunity for the sustainable valorization of softwood kraft lignin.

Key points

• Softwood kraft lignin displayed high phenolic hydroxyl content, polydispersity index and average molecular weight

• Grafting hydroxyethyl acrylate (HEA) monomer onto kraft lignin by laccase was successful at 60 °C and alkaline conditions

• Lignin-HEA grafted copolymer showed an increase in total OH value and an increase in average molecular weight

Supplementary information

The online version contains supplementary material available at 10.1007/s00253-022-11916-z.

Laccase as a Tool in Building Advanced Lignin-Based Materials

Lignin is an abundant natural feedstock that offers great potential as a renewable substitute for fossil-based resources. Its polyaromatic structure and unique properties have attracted significant research efforts. The advantages of an enzymatic over chemical or thermal approach to construct or deconstruct lignins are that it operates in mild conditions, requires less energy, and usually uses non-toxic chemicals. Laccase is a widely investigated oxidative enzyme that can catalyze the polymerization and depolymerization of lignin. Its dual nature causes a challenge in controlling the overall direction of lignin-laccase catalysis. In this Review, the factors that affect laccase-catalyzed lignin polymerization were summarized, evaluated, and compared to identify key features that favor lignin polymerization. In addition, a critical assessment of the conditions that enable production of novel lignin hybrids via laccase-catalyzed grafting was presented. To assess the industrial relevance of laccase-assisted lignin valorization, patented applications were surveyed and industrial challenges and opportunities were analyzed. Finally, our perspective in realizing the full potential of laccase in building lignin-based materials for advanced applications was deduced from analysis of the limitations governing laccase-assisted lignin polymerization and grafting.

Lignin Biopolymers in the Age of Controlled Polymerization

Polymers made from natural biomass are gaining interest due to the rising environmental concerns and depletion of petrochemical resources. Lignin isolated from lignocellulosic biomass is the second most abundant natural polymer next to cellulose. The paper pulp process produces industrial lignin as a byproduct that is mostly used for energy and has less significant utility in materials applications. High abundance, rich chemical functionalities, CO2 neutrality, reinforcing properties, antioxidant and UV blocking abilities, as well as environmental friendliness, make lignin an interesting substrate for materials and chemical development. However, poor processability, low reactivity, and intrinsic structural heterogeneity limit lignins' polymeric applications in high-performance advanced materials. With the advent of controlled polymerization methods such as ATRP, RAFT, and ADMET, there has been a great interest in academia and industry to make value-added polymeric materials from lignin. This review focuses on recent investigations that utilize controlled polymerization methods to generate novel lignin-based polymeric materials. Polymers developed from lignin-based monomers, various polymer grafting technologies, copolymer properties, and their applications are discussed.

Hydrophobic functionalization of jute fabrics by enzymatic-assisted grafting of vinyl copolymers

Mechanism of grafting of vinyl monomers onto the lignin molecules of jute fabrics.

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.

Surface functionalization of natural lignin isolated from Aloe barbadensis Miller biomass by atom transfer radical polymerization for enhanced anticancer efficacy

Lignin (LIG), one of the major natural polymers in the biomass is widely used for various industrial and biomedical applications, mainly in its modified form of grafted lignin.

Jute hydrophobization via laccase-catalyzed grafting of fluorophenol and fluoroamine

The figure mechanism of the 4-[4-(trifluoromethyl)phenoxy]phenol (TFMPP) and 1H,1H-perfluorononylamine (PFNL) grafting onto the lignins of jute fabrics.

Engineering highly stretchable lignin-based electrospun nanofibers for potential biomedical applications

The incorporation of lignin–PMMA copolymers into PCL nanofibers significantly improved the mechanical properties and biocompatibility of the nanofibrous composites.

Surface modification of cellulose fibers: towards wood composites by biomimetics

A biomimetic approach was taken for studying the adsorption of a model copolymer (pullulan abietate, DS 0.027), representing the lignin-carbohydrate complex, to a model surface for cellulose fibers (Langmuir-Blodgett thin films of regenerated cellulose). Adsorption results were assayed using surface plasmon resonance spectroscopy (SPR) and atomic force microscopy (AFM). Rapid, spontaneous, and desorption-resistant surface modification resulted. This effort is viewed as a critical first step towards the permanent surface modification of cellulose fibers with a layer of molecules amenable to either enzymatic crosslinking for improved wood composites or thermoplastic consolidation.

Degradation of acrylic copolymers by white-rot fungi

Various water-soluble homopolymers and copolymers of acrylamide (AAm) and acrylic acid (AA) which contained phenolic sites, such as guaiacol, lignin sulfonate (LS) and 3,4-dihydroxybenzoic acid (3,4-DHBA), were tested with regard to their degradability by white-rot fungi. Compared with Phanerochaete chrysosporium, Pleurotus ostreatus caused a significantly higher decrease in the average molecular weight ( Mw) of most of the copolymers and the homopolymer under the applied culture conditions. However, the Mw of poly(guaiacol/AAm) increased significantly during incubation with Pl ostreatus. P. chrysosporium was able to reduce only the Mw of the poly(LS/AA) to a significant degree and not that of the other polymers. The mineralization rate of AAm and AA copolymers and terpolymers of AAm, AA and phenolics (LS, 3,4-DHBA, guiacol), which were tested with P. ostreatus and Trametes versicolor, turned out to be low (0.8-3.2%). While the rates of mineralization were similar among all polymers, the decrease in radioactivity from the culture media was higher with the terpolymers bearing phenolic sites. UV spectra of the culture media revealed that the phenolic sites in the terpolymers were significantly degraded by both fungi. Obviously, the degradation of phenolics within the polymer chain caused a higher decrease in Mw but did not significantly increase the mineralization rate.

The effect of ions on the enzymatically induced synthesis of lignin graft copolymers

The effect of different ions which are constituents of technical lignin sulfonates (LS) on chemo-enzymatic graft co-polymerization was determined. The application of the iron chelator desferrioxamine in the initial reaction mixture revealed that iron impurities of LS which catalyzed a Fenton-like reaction were crucial for the initiation of grafting, whereas calcium or chloride ions showed no such effect. The addition of laccase (ATCC 11235) to the reaction mixture which contained desferrioxamine caused a significantly higher yield compared to the control; this indicates a crucial effect of laccase with regard to the initiation of copolymerization. The involvement of laccase in the initiation of the graft copolymerization was additionally confirmed by the application of low molecular weight phenolics instead of LS. In the presence of the lignin-like substrates, 3,4-dihydroxybenzoic acid and guaiacol, the rate of the decomposition of t-butylhydroperoxide was significantly enhanced by laccase. It can be assumed that the enzymatically generated phenoxy radicals mediate the production of oxygen centered radicals (alkoxy or peroxy) which initiate grafting.

Chemo-enzymatic synthesis and characterization of graft copolymers from lignin and acrylic compounds

Chemo-enzymatic initiation of graft copolymerization of acrylic compounds onto different technical lignosulfonates (LS) was compared to a Fenton-like system (ferrous ion, t-BHP). The enzyme tested was a phenoloxidase laccase (EC 1.10.3.2) from the white rot basidomycete Trametes versicolor. Most applied lignins were successfully grafted, resulting in a polymer yield of more than 90%. The effect of initiator concentration and the lignin/monomer ratio on the yield and M(w) of enzymatically grafted polymers were studied. The homopolymer proportion in the enzymatically produced grafts of Ca-LS and acrylic acid was 5 to 6x lower than those initiated by the Fenton-like reagent; no such differences were observed for Na-LS.