Enzymatic surface functionalisation of lignocellulosic materials with tannins for enhancing antibacterial properties

Enhancement of the biological activity of hydroxytyrosol through its oxidation by laccase from Trametes versicolor

The laccase-catalyzed oxidation of hydroxytyrosol (HT) towards the formation of its bioactive oligomer derivatives was investigated. The biocatalytic oligomerization was catalyzed by laccase from Trametes versicolor in aqueous or various water-miscible organic solvents and deep eutectic solvent (DES)-based media. Mass Spectroscopy and Nuclear Magnetic Resonance were used for the characterization of the products. The solvent system used significantly affects the degree of HT oligomerization. The use of 50 % v/v methanol favored the production of the HT dimer, while other organic solvents as well as DESs led to the formation of hydroxytyrosol trimer and other oligomers. In vitro studies showed that the HT dimer exhibits 3- to 4-fold enhanced antibacterial activity against Gram-positive and Gram-negative bacteria compared to the parent compound. Moreover, the ability of HT dimer to inhibit the activity of soybean lipoxygenase and Candida rugosa lipase was 1.5-fold higher than HT, while molecular docking supported these results. Furthermore, HT dimer showed reduced cytotoxicity against HEK293 cells and exhibited a strong ability to inhibit ROS formation. The enhanced bioactivity of HT dimer indicates that this compound could be considered for use in cosmetics, skin-care products, and nutraceuticals.

Future Antimicrobials: Natural and Functionalized Phenolics

With incidence of antimicrobial resistance rising globally, there is a continuous need for development of new antimicrobial molecules. Phenolic compounds having a versatile scaffold that allows for a broad range of chemical additions; they also exhibit potent antimicrobial activities which can be enhanced significantly through functionalization. Synthetic routes such as esterification, phosphorylation, hydroxylation or enzymatic conjugation may increase the antimicrobial activity of compounds and reduce minimal concentrations needed. With potent action mechanisms interfering with bacterial cell wall synthesis, DNA replication or enzyme production, phenolics can target multiple sites in bacteria, leading to a much higher sensitivity of cells towards these natural compounds. The current review summarizes some of the most important knowledge on functionalization of natural phenolic compounds and the effects on their antimicrobial activity.

Amphiphilic sodium alginate-polylysine hydrogel with high antibacterial efficiency in a wide pH range

Bacterial infection is a major pathological factor leading to persistent wounds. With the aging of population, wound infection has gradually become a global health-issue. The wound site environment is complicated, and the pH changes dynamically during healing. Therefore, there is an urgent need for new antibacterial materials that can adapt to a wide pH range. To achieve this goal, we developed a thymol-oligomeric tannic acid/amphiphilic sodium alginate-polylysine hydrogel film, which exhibited excellent antibacterial efficacy in the pH range from 4 to 9, achieving the highest achievable 99.993 % (4.2 log units) and 99.62 % (2.4 log units) against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli, respectively. The hydrogel films exhibited excellent cytocompatibility, suggesting that the materials are promising as a novel wound healing material without the concern of biosafety.

Bioinspired adhesive coatings from polyethylenimine and tannic acid complexes exhibiting antifogging, self-cleaning, and antibacterial capabilities

In this work, we develop a simple yet robust method to fabricate a bioinspired adhesive coating based on polyethyleneimine (PEI) and tannic acid (TA) complexes, exhibiting excellent antifogging, self-cleaning, and antibacterial properties. The polyethyleneimine-tannic acid (PEI-TA) complexes coating combined with the bioinspired adhesive property from TA can be effectively and stably coated onto various substrates through a one-step deposition process, and the hydrophilicity of the coated substrates can be significantly enhanced with their water contact angle less than 10°. The bioinspired adhesive coating endows the coated substrates with outstanding antifogging and self-cleaning performance. Moreover, it is found that the PEI-TA coated safety goggles display excellent durability and antifogging capability compared to the commercial antifogging safety goggles and commercial antifogging agents coated safety goggles under 65 ℃ vapor condition for 2 h. Furthermore, the PEI-TA coatings show superior antibacterial activities for Gram-negative Escherichiak coli and Gram-positive Staphylococcus aureus. The antifogging, self-cleaning, and antibacterial coating provides widely potential application prospects in optical and medical devices.

HPLC/MS-MS Identification of Oak Quercus aegilops Root Tannins

Tannins are natural polyphenolic compounds widely distributed in the plant kingdom in the leaves, bark, fruits, and other parts. They have various biological functions in humans and animals and are used mainly in the pharmaceutical and cosmetic industries. The aim of this work was to isolate, extract, purify, and identify the tannins from the root bark of a common oak tree (Quercus aegilops L.) in Jordan and around the Mediterranean. The results showed that at least one form of ellagitannin (ellagic acid ester), quercitrin, afzelechin, valoneic acid, trigalloyl glucose, and catechin was identified in addition to two unidentified compounds. Results of this work can help in developing an ESI MS/MS search library for the constituents of the tannins of oak (Quercus aegilops L.) root bark.

Catechol-Based Antimicrobial Polymers

Catechol is a key constituent in mussel adhesive proteins and is responsible for strong adhesive property and crosslinking formation. Plant-based polyphenols are also capable of chemical interactions similar to those of catechol and are inherently antimicrobial. This review reports a series of catechol-based antimicrobial polymers classified according to their antimicrobial mechanisms. Catechol is utilized as a surface anchoring group for adhering monomers and polymers of known antimicrobial properties onto various types of surfaces. Additionally, catechol's ability to form strong complexes with metal ions and nanoparticles was utilized to sequester these antimicrobial agents into coatings and polymer matrices. During catechol oxidation, reactive oxygen species (ROS) is generated as a byproduct, and the use of the generated ROS for antimicrobial applications was also introduced. Finally, polymers that utilized the innate antimicrobial property of halogenated catechols and polyphenols were reviewed.

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.

Production and characterisation of a novel actinobacterial DyP-type peroxidase and its application in coupling of phenolic monomers

The extracellular peroxidase from Streptomyces albidoflavus BSII#1 was purified to near homogeneity using sequential steps of acid and acetone precipitation, followed by ultrafiltration. The purified peroxidase was characterised and tested for the ability to catalyse coupling reactions between selected phenolic monomer pairs. A 46-fold purification of the peroxidase was achieved, and it was shown to be a 46 kDa haem peroxidase. Unlike other actinobacteria-derived peroxidases, it was only inhibited (27 % inhibition) by relatively high concentrations of sodium azide (5 mM) and was capable of oxidising eleven (2,4-dichlorophenol, 2,6-dimethoxyphenol, 4-tert-butylcatechol, ABTS, caffeic acid, catechol, guaiacol, l-DOPA, o-aminophenol, phenol, pyrogallol) of the seventeen substrates tested. The peroxidase remained stable at temperatures of up to 80 °C for 60 min and retained >50 % activity after 24 h between pH 5.0-9.0, but was most sensitive to incubation with hydrogen peroxide (H₂O₂; 0.01 mM), l-cysteine (0.02 mM) and ascorbate (0.05 mM) for one hour. It was significantly inhibited by all organic solvents tested (p ≤ 0.05). The K_m and V_max values of the partially purified peroxidase with the substrate 2,4-DCP were 0.95 mM and 0.12 mmol min^-1, respectively. The dyes reactive blue 4, reactive black 5, and Azure B, were all decolourised to a certain extent: approximately 30 % decolourisation was observed after 24 h (1 μM dye). The peroxidase successfully catalysed coupling reactions between several phenolic monomer pairs including catechin-caffeic acid, catechin-catechol, catechin-guaiacol and guaiacol-syringaldazine under the non-optimised conditions used in this study. Genome sequencing confirmed the identity of strain BSII#1 as a S. albidoflavus strain. In addition, the genome sequence revealed the presence of one peroxidase gene that includes the twin arginine translocation signal sequence of extracellular proteins. Functional studies confirmed that the peroxidase produced by S. albidoflavus BSII#1 is part of the dye-decolourising peroxidase (DyP-type) family.

Structure and Bioactive Properties of Novel Textile Dyes Synthesised by Fungal Laccase

Novel sustainable processes involving oxidative enzymatic catalysts are considered as an alternative for classical organic chemistry. The unique physicochemical and bioactive properties of novel bio-products can be obtained using fungal laccase as catalyst. Among them are textile biodyes synthesised during oxidation of substrates belonging to the amine and methoxy organic derivatives. The process of synthesis occurs in mild conditions of pH, temperature, and pressure, and without using harmful oxidants. The effect of fungal laccase activity on the substrates mixture transformation efficiency was analysed in terms of antimicrobial dye synthesis on a large scale. Three new phenazine dyes, obtained in the presence of laccase from Cerrena unicolor, were studied for their structure and properties. The phenazine core structure of the products was a result of tri-molecular transformation of aminomethoxybenzoic acid and aminonaphthalene sulfonic acid isomers. One of the compounds from the synthesised dye, namely 10-((2-carboxy-6-methoxyphenyl)amino)-11-methoxybenzo[a]phenazine-8-carboxylic acid, was able to inhibit the growth of Staphylococcus aureus. The high concentration of substrates (5 g/L) was efficiently transformed during 72 h in the mild conditions of pH 4 with the use of laccase with an activity of 200 U per g of the substrates mixture. The new bioactive dye exhibited excellent dyeing properties with concomitant antibacterial and antioxidative activity. The proposed enzyme-mediated synthesis represents an alternative eco-friendly route for the synthesis of novel antimicrobial compounds with high importance for the medical textile industry.

Laccase-mediated hydrophilization of lignin decreases unproductive enzyme binding but limits subsequent enzymatic hydrolysis at high substrate concentrations

One of the predominant mechanisms by which lignin restricts effective enzymatic deconstruction of lignocellulosic materials is the unproductive adsorption of enzymes. Although this inhibition can be partially mitigated through hydrophilization of lignin during thermochemical pretreatment, these types of treatments could potentially worsen slurry rheology, consequently making it more difficult to process the material at high substrate concentrations. In the work reported here, laccases were used to specifically modify lignin hydrophilicity within steam-pretreated substrate via in situ phenolic compound grafting. While lignin hydrophilization reduced unproductive enzyme adsorption, high-solids hydrolysis efficiency decreased significantly due to mass transfer limitations. It was apparent that low-solids hydrolysis experiments were a poor predictor of substrate digestibility at high-solids conditions and that substrate-water interactions impacted both substrate digestibility and slurry rheology.

Production of highly thermo-tolerant laccase from novel thermophilic bacterium Bacillus sp. PC-3 and its application in functionalization of chitosan film

In this study, a novel thermophilic bacterial strain was isolated from Tattapani hot spring located in the Chhattisgarh state of India. The laccase was produced via submerged fermentation and purified by ammonium sulfate precipitation and anion exchange chromatography up to 13.7 fold. The 16S rRNA gene sequence and biochemical analysis revealed that the bacterial isolate is Bacillus sp. strain PC-3. The activity of extracellular crude laccase was determined to be 11.2 U/mL using 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) as a substrate. The SDS-PAGE revealed that the enzyme consists of single subunit with molecular size of 36 kDa. The laccase exhibited the maximum enzyme activity at temperature of 60°C and pH 7. Moreover, the laccase retained 99.1% of its original activity for 180 min and exhibited half-life of 3.75 h at 60°C. Similarly, the laccase retained 95% activity at pH 7 for 240 min and displayed significant activity at wider pH range. In addition, the laccase was used for functionalization of chitosan film and characterized for antioxidant and antimicrobial activity. Interestingly, the functionalized chitosan film showed the improved antioxidant and antimicrobial activity.

Enzyme biotechnology in degradation and modification of plant cell wall polymers

Lignocelluloses are abundant raw materials for production of fuels, chemicals and materials. The purpose of this paper is to review the enzyme-types and enzyme-technologies studied and applied in the processing of the lignocelluloses into different products. The enzymes here are mostly glycoside hydrolases, esterases and different redox enzymes. Enzymatic hydrolysis of lignocellulosic polysaccharides to platform sugars has been widely studied leading to development of advanced commercial products for this purpose. Restricted hydrolysis or oxidation of cellulosic fibers have been applied in processing of pulps to paper products, nanocelluloses and textile fibers. Oxidation, transglycosylation and derivatization have been utilized in functionalization of fibers, cellulosic surfaces and polysaccharides. Enzymatic polymerization, depolymerization and grafting methods are being developed for lignin valorization.

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.

Heavy Metal-Free Tannin from Bark for Sustainable Energy Storage

A novel renewable cathode made from earth abundant, low-cost materials can contribute to the intermittent storage needs of renewable energy-based society. In this work, we report for the first-time tannin from Nature as a cathode material. Our approach exploits the charge storage mechanism of the redox active quinone moiety. Tannins extracted from tree bark using environmental friendly aqueous solvents have the highest phenol content (5.56 mol g^-1) among all the natural phenolic biopolymers, 5000 times higher than lignin. Tannins coupled with a conductive polymer polypyrrole acquire high specific capacitance values of 370 F g^-1 at 0.5 A g^-1 as well as excellent rate performance of 196 F g^-1 at 25 A g^-1. Additionally, we employed carbonized wood as an electrode substrate to produce a sustainable electrochemical device with dramatically improved performance compared to conventional devices. The high surface area provided by the well-aligned, cellular porosity of wood-derived substrate combined with the high mobility of ions and electrons in the carbonized cell walls and deposited tannin can achieve an areal capacitance of 4.6 F cm^-2 at 1 mA cm^-2, which is 1.5 times higher than activated wood carbon.

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.

Electrophoretic deposition of colloidal particles on Mg with cytocompatibility, antibacterial performance, and corrosion resistance

Magnesium (Mg) has recently received increasing attention due to its unique biological performance, including cytocompatibility, antibacterial and biodegradable properties. However, rapid corrosion in physiological environment and potential toxicity limits its clinical applications. To improve the corrosion resistance meanwhile not compromise other excellent performance, self-assembled colloidal particles were deposited onto magnesium surfaces in ethanol by a simple and effective electrophoretic deposition (EPD) method. The fabricated functional nanostructured coatings were investigated using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analyses, and scanning electron microscopy (SEM). The electrochemical test, pH value, and Mg ion concentration data show that the corrosion resistance of Mg samples is enhanced appreciably after surface treatment. In vitro cellular response and antibacterial capability of the modified Mg substrates are performed. Significantly increased cell adhesion and viability are observed from the coated Mg samples, and the amounts of adherent bacteria on the treated Mg surfaces diminish remarkably compared to the bare Mg. Furthermore, the bare and coated Mg samples were implanted in New Zealand white rabbits for 12 weeks to examine the in vivo long-term corrosion performance and in situ inflammation behavior. The experiment results confirmed that compared with bare Mg substrate the corrosion and foreign-body reactions of the coated Mg samples were suppressed. The above results suggested that our coatings, which effectively enhance the biocompatibility, antimicrobial properties, and corrosion resistance of Mg substrate, provide a simple and practical strategy to expedite clinical acceptance of biodegradableMg and its alloys.

STATEMENT OF SIGNIFICANCE

Biomedical Mg metals have been considered as promising biodegradable implants because of their intended functions, such as cytocompatibility, antibacterial, and biodegradable properties. However, rapid corrosion in physiological environment limits their clinical applications. Alloying and surface coatings have been used to reduce the degradation rate. But this would compromise other excellent performance of Mg samples, including antibacterial and anti-inflammatory activity. Thus, while the rapid degradation of Mg samples must be solved, good antibacterial property and acceptable cytocompatibility are also necessary. In this study, polymer-based coatings were fabricated on Mg surfaces by electrophoretic deposition of poly(isobornyl acrylate-co-dimethylaminoethyl methacrylate)/tannic acid (P(ISA-co-DMA)/TA) colloidal particles. It suggested that the coating materials effectively improved the biocompatibility, antimicrobial behavior, and corrosion resistance of biomedical Mg.

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.

Studying the effects of laccase-catalysed grafting of ferulic acid on sisal pulp fibers

Functionalization of sisal specialty pulp fibers by laccase-catalysed grafting of ferulic acid (FRC) was investigated. To this end, the extent of phenol coupling to fibers under different reaction conditions (laccase and FRC rates, and time) was evaluated in terms of pulp properties including kappa number (expressed as the combined contributions of lignin and hexenuronic acids), brightness, Klason lignin and surface anionic charge after Soxhlet extraction of acetone-treated pulp. The specific treatment resulting in the highest degree of grafting was then used in a comparative study of the effects of applying the laccase-FRC system to refined and unrefined pulp with a view to confirming whether the increased surface area obtained by effect of fibrillation would lead to enhanced grafting. Based on the results, refining the pulp prior to the enzyme treatment resulted in increased grafting which in turn led to handsheets with improved strength-related properties (particularly wet tensile strength) relative to control samples.

Laccase-catalysed oxidations of naturally occurring phenols: from in vivo biosynthetic pathways to green synthetic applications

Laccases are oxidases that contain several copper atoms, and catalyse single-electron oxidations of phenolic compounds with concomitant reduction of oxygen to water. The enzymes are particularly widespread in ligninolytic basidiomycetes, but also occur in certain prokaryotes, insects and plants. Depending on the species, laccases are involved in various biosynthetic processes contributing to carbon recycling in land ecosystems and the morphogenesis of biomatrices, wherein low-molecular-weight naturally occurring phenols serve as key enzyme substrates. Studies of these in vivo synthetic pathways have afforded new insights into fungal laccase applicability in green synthetic chemistry. Thus, we here review fungal laccase-catalysed oxidations of naturally occurring phenols that are particularly relevant to the synthesis of fine organic chemicals, and we discuss how the discovered synthetic strategies mimic laccase-involved in vivo pathways, thus enhancing the green nature of such reactions. Laccase-catalysed in vivo processes yield several types of biopolymers, including those of cuticles, lignin, polyflavonoids, humus and the melanin pigments, using natural mono- or poly-phenols as building blocks. The in vivo synthetic pathways involve either phenoxyl radical-mediated coupling or cross-linking reactions, and can be adapted to the design of in vitro oxidative processes involving fungal laccases in organic synthesis; the laccase substrates and the synthetic mechanisms reflect in vivo processes. Notably, such in vitro synthetic pathways can also reproduce physicochemical properties (e.g. those of chromophores, and radical-scavenging, hydration and antimicrobial activities) found in natural biomaterials. Careful study of laccase-associated in vivo metabolic pathways has been rewarded by the discovery of novel green applications for fungal laccases. This review comprehensively summarizes the available data on laccase-catalysed biosynthetic pathways and associated applications in fine chemical syntheses.