Mechanism of non-phenolic substrate oxidation by the fungal laccase Type 1 copper site from Trametes versicolor: the case of benzo[a]pyrene and anthracene

Laccases (EC 1.10.3.2) are multicopper oxidases with the capability to oxidize diverse phenolic and non-phenolic substrates. While the molecular mechanism of their activity towards phenolic substrates is well-established, their reactivity towards non-phenolic substrates, such as polycyclic aromatic hydrocarbons (PAHs), remains unclear. To elucidate the oxidation mechanism of PAHs, particularly the activation mechanism of the sp2 aromatic C-H bond, we conducted a density functional theory investigation on the oxidation of two PAHs (anthracene and benzo[a]pyrene) using an extensive model of the T1 copper catalytic site of the fungal laccase from Trametes versicolor.

Efficient molecular doping of polymeric semiconductors improved by coupled reaction

Exploring chemical doping method to improve the electrical conductivity of polymers is still very attractive for researchers. In this work, we report a developed method of doping a polymer semiconductor aided by the coupled reaction that commonly exists in biological systems where a non-spontaneous reaction is driven by a spontaneous reaction. During the doping process, the chemical reaction between the dopant and the polymer is promoted by introducing a thermodynamically favorable reaction via adding additives that are highly reactive to the reduction product of the dopant to form a coupled reaction, thus significantly improving the electrical conductivity of polymers by 3-7 orders. This coupled reaction doping process shows the potential of wide applications in exploring efficient doping systems to prepare functional conducting polymers, which could be a powerful tool for modern organic electronics.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

An Artifact of Perfluoroalkyl Acid (PFAA) Removal Attributed to Sorption Processes in a Laccase Mediator System

Fungi and laccase mediator systems (LMSs) have a proven track record of oxidizing recalcitrant organic compounds. There has been considerable interest in applying LMSs to the treatment of perfluoroalkyl acids (PFAAs), a class of ubiquitous and persistent environmental contaminants. Some laboratory experiments have indicated modest losses of PFAAs over extended periods, but there have been no clear demonstrations of a transformation mechanism or the kinetics that would be needed for remediation applications. We set out to determine if this was a question of identifying and optimizing a rate-limiting step but discovered that observed losses of PFAAs were experimental artifacts. While unable to replicate the oxidation of PFAAs, we show that interactions of the PFAA compounds with laccase and laccase mediator mixtures could cause an artifact that mimics transformation (≲60%) of PFAAs. Furthermore, we employed a surrogate compound, carbamazepine (CBZ), and electron paramagnetic resonance spectroscopy to probe the formation of the radical species that had been proposed to be responsible for contaminant oxidation. We confirmed that under conditions where sufficient radical concentrations were produced to oxidize CBZ, no PFAA removal took place.

Recent Theoretical Insights into the Oxidative Degradation of Biopolymers and Plastics by Metalloenzymes

Molecular modeling techniques have become indispensable in many fields of molecular sciences in which the details related to mechanisms and reactivity need to be studied at an atomistic level. This review article provides a collection of computational modeling works on a topic of enormous interest and urgent relevance: the properties of metalloenzymes involved in the degradation and valorization of natural biopolymers and synthetic plastics on the basis of both circular biofuel production and bioremediation strategies. In particular, we will focus on lytic polysaccharide monooxygenase, laccases, and various heme peroxidases involved in the processing of polysaccharides, lignins, rubbers, and some synthetic polymers. Special attention will be dedicated to the interaction between these enzymes and their substrate studied at different levels of theory, starting from classical molecular docking and molecular dynamics techniques up to techniques based on quantum chemistry.

Laccase-assisted degradation of emerging recalcitrant compounds - A review

The main objective of this review is to provide up to date, brief, irrefutable, organized data on the conducted experiments on a range of emerging recalcitrant compounds such as Diclofenac (DCF), Chlorophenols (CPs), tetracycline (TCs), Triclosan (TCS), Bisphenol A (BPA) and Carbamazepine (CBZ). These compounds were selected from the categories of pharmaceutical contaminants (PCs), endocrine disruptors (EDs) and personal care products (PCPs) on the basis of their toxicity and concentration retained in the environment. In this context, detailed mechanism of laccase mediated degradation has been conversed that laccase assisted degradation occurs by one electron oxidation involving redox potential as underlying element of the process. Further, converging towards biotechnology, laccase immobilization increased removal efficiency, storage and reusability through various experimentally conducted studies. Laccase is being considered noteworthy as mediators facilitate laccase in oxidation of non-phenolic compounds and thereby increasing its substrate range which is being discussed in further in the review. The laccase assisted degradation mechanism of each compound has been elucidated but further studies to undercover proper degradation mechanisms needs to be performed.

A Horseradish Peroxidase-Mediator System for Benzylic C-H Activation

Enzyme-mediator systems generate radical intermediates that abstract hydrogen atoms under mild conditions. These systems have been employed extensively for alcohol oxidation, primarily in biomass degradation, but they are underexplored for direct activation of C(sp3)-H bonds in alkyl groups. Here, we combine horseradish peroxidase (HRP), H2O2, and redox mediator N-hydroxyphthalimide (NHPI) for C(sp3)-H functionalization of alkylbenzene-type substrates. The HRP-NHPI system is >10-fold more active than existing enzyme-mediator systems in converting alkylbenzenes to ketones and aldehydes under air, and it operates from 0-50 °C and in numerous aqueous-organic solvent mixtures. The benzylic substrate radical can be trapped through a reaction with NHPI, demonstrating the formation of benzylic products beyond ketones. Furthermore, we demonstrate a one-pot, two-step enzymatic cascade for converting alkylbenzenes to benzylic amines. Overall, the HRP-NHPI system enables the selective benzylic C-H functionalization of diverse substrates under mild conditions using a straightforward procedure.

Nickel-Catalyzed C(sp3)-C(sp3) Cross-Electrophile Coupling of In Situ Generated NHP Esters with Unactivated Alkyl Bromides

The formation of C(sp3)-C(sp3) bonds by cross-coupling remains a challenge in synthesis. Here, we demonstrate a two-step, one-pot protocol for the in situ generation of N-hydroxyphthalimide esters and their nickel-catalyzed cross-electrophile coupling with unactivated alkyl bromides for the construction of 1°/1 ° C(sp3)-C(sp3) bonds. The conditions tolerate an array of functional groups, and mechanistic studies indicate that both substrates are converted to alkyl radicals during the reaction.

Occurrence, environmental fate, ecological issues, and redefining of endocrine disruptive estrogens in water resources

The growing persistence of estrogenic pollutants in water resources is a worrying concern because of their endocrine disrupting activities and potentially hazardous consequences on the environmental matrices, ecology, and human health, even at low concentration. The long-term persistence of steroidal estrogens leads to their bioaccumulation in aquatic organisms that can further reach to humans via food chain route. Considering the toxicity of steroidal estrogens, it is important to mitigate these environmentally related hazardous contaminants. So far, several treatment methods, like adsorption, oxidation, irradiation, and electrochemical techniques have been proposed to eliminate estrogens from aqueous ecosystems. Nevertheless, high operational costs, insufficient removal, generation of toxic sludge, and the necessity of skilled maintenance and operating workers are the major hindrances associated with large scale applications. Bioremediation of steroidal estrogens using enzyme-based biocatalytic system has recently emerged as a promising alternative to remove and bio-transform estrogens from aqueous systems. However, the current literature lacks a critique focusing specifically and comprehensively on steroidal estrogens. The presented review is a critical assessment of the existing literature on steroid-based endocrine disruptive estrogens. A detailed description about the occurrence and eco-fate of steroidal estrogens is given with representative examples. The later half of the review stresses on the redefining (removal) of endocrine disruptive estrogens in water resources with particular reference to enzyme-based approaches.

Fungal Laccases: The Forefront of Enzymes for Sustainability

Enzymatic catalysis is one of the main pillars of sustainability for industrial production. Enzyme application allows minimization of the use of toxic solvents and to valorize the agro-industrial residues through reuse. In addition, they are safe and energy efficient. Nonetheless, their use in biotechnological processes is still hindered by the cost, stability, and low rate of recycling and reuse. Among the many industrial enzymes, fungal laccases (LCs) are perfect candidates to serve as a biotechnological tool as they are outstanding, versatile catalytic oxidants, only requiring molecular oxygen to function. LCs are able to degrade phenolic components of lignin, allowing them to efficiently reuse the lignocellulosic biomass for the production of enzymes, bioactive compounds, or clean energy, while minimizing the use of chemicals. Therefore, this review aims to give an overview of fungal LC, a promising green and sustainable enzyme, its mechanism of action, advantages, disadvantages, and solutions for its use as a tool to reduce the environmental and economic impact of industrial processes with a particular insight on the reuse of agro-wastes.

Laccases: Versatile Biocatalysts for the Synthesis of Heterocyclic Cores

Laccases are multicopper oxidases that have shown a great potential in various biotechnological and green chemistry processes mainly due to their high relative non-specific oxidation of phenols, arylamines and some inorganic metals, and their high redox potentials that can span from 500 to 800 mV vs. SHE. Other advantages of laccases include the use of readily available oxygen as a second substrate, the formation of water as a side-product and no requirement for cofactors. Importantly, addition of low-molecular-weight redox mediators that act as electron shuttles, promoting the oxidation of complex bulky substrates and/or of higher redox potential than the enzymes themselves, can further expand their substrate scope, in the so-called laccase-mediated systems (LMS). Laccase bioprocesses can be designed for efficiency at both acidic and basic conditions since it is known that fungal and bacterial laccases exhibit distinct optimal pH values for the similar phenolic and aromatic amines. This review covers studies on the synthesis of five- and six-membered ring heterocyclic cores, such as benzimidazoles, benzofurans, benzothiazoles, quinazoline and quinazolinone, phenazine, phenoxazine, phenoxazinone and phenothiazine derivatives. The enzymes used and the reaction protocols are briefly outlined, and the mechanistic pathways described.

Hydrogen Atom Transfer from Benzyl Alcohols to N-Oxyl Radicals. Reactivity Parameters

A model for predicting the rate constants of hydrogen atom transfer (HAT) from the α-C-H bond of p-substituted benzyl alcohols to N-oxyl radicals was proposed. To quantify the factors governing the reactivity of both N-oxyl radicals and benzyl alcohols, multivariate regression analysis was performed using various combinations of reactivity parameters. The analysis was based on a 2D array of 35 HAT reactions, the rate constants of which span 4 orders of magnitude. The proposed polyparameter equation approximates the experimental rate constants of reactions with high accuracy using three independent parameters: Brown and Okamoto's substituent constants σ⁺ in alcohol molecules and the spin population on O and N atoms in the N-O^• fragment of N-oxyl radicals [calculated by DFT/B3LYP/6-31G(d,p)]. The rate constants of HAT reactions from p-substituted benzyl alcohols to a series of aryl-substituted phthalimide-N-oxyl radicals containing either electron-withdrawing or electron-donating substituents (4-Cl, 4-HOOC, 4-CH₃O), quinolinimide-N-oxyl, benzotriazole-N-oxyl, and violuric acid radicals were experimentally determined at 30 °C in acetonitrile.

Characterization of 2,3,7,8-tetrachlorodibenzo-p-dioxin biodegradation by extracellular lignin-modifying enzymes from ligninolytic fungus

Ligninolytic fungi secrete extracellular lignin-modifying enzymes (LME) that degrade plant polymers for fungal nutrition but that are, because of their broad substrate specificity, also applicable for the degradation of many hazardous pollutants. Laccase is one of the most well characterized LME and is involved in the removal and degradation of recalcitrant aromatic compounds with or without the assistance of laccase-mediators. The Ligninolytic fungus Rigidoporus sp. FMD21 can degrade 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) with a half-life of 6.2 days. Using Rigidoporus sp. FMD21 crude extracellular enzyme extract (ExE) that mainly consisted of laccase, 77.4% of 2,3,7,8-TCDD was degraded within 36 days. The degradation rate did not depend on the 2,3,7,8-TCDD concentration in the tested range between 0.005 and 0.5 pgTEQ/μL. 2,3,7,8-TCDD was analysed by DR-CALUX® bioassay and the degradation was confirmed by GC-HRMS. In this study, we found evidence for cleavage of the diaryl ether bond in the 2,3,7,8-TCDD molecule and here we propose a new degradation mechanism in which 3,4-dichlorophenol is the main metabolite of 2,3,7,8-TCDD degradation by FMD21's ExE. Six laccase-mediators were tested. Three of them 1-hydroxybenzotriazole (HBT), syringaldehyde (Syr) and violuric acid (Vio) showed an equipotent added effect on 2,3,7,8-TCDD degradation by ExE, however only in case of Vio a level of significance was reached. The others showed no effect or negatively impacted degradation. In conclusion, we have shown that Rigidoporus sp. FMD21 produces extracellular enzymes, mainly laccases that apparently are able to degrade the highly recalcitrant and most toxic 2,3,7,8-congener of TCDD via diaryl bond cleavage into 3,4-dichlorophenol.

An efficient and practical aerobic oxidation of benzylic methylenes by recyclable N-hydroxyimide

An efficient and practical benzylic aerobic oxidation catalyzed by cheap and simple N-hydroxyimide organocatalyst has been achieved with high yields and broad substrate scope. The organocatalyst used can be recycled and reused by simple workup and only minute amount (1 mol% in most cases) of simple iron salt is used as promoter. Phenyl substrates with mild and strong electron-withdrawing group could also be oxygenated in high yields as well as other benzylic methylenes. Influence of substituents, gram-scale application, catalysts decay and general mechanism of this methodology has also been discussed.

An efficient and practical benzylic aerobic oxidation catalyzed by cheap and simple N-hydroxyimide organocatalyst has been achieved with high yields and broad substrate scope.

Electrochemical characteristics of the decolorization of three dyes by laccase mediator system (LMS) with synthetic and natural mediators

Laccase mediator system (LMS), a very attractive candidate for refractory organics biodegradation, harbors tremendous potential on industry application. However, the performance of LMS usually varies with the discrepancy of mediators and substrates in their chemical structures. Here, we adopt electrochemical analysis that is able to assess the degradation performance of various LMS on three different dyes by quantitative analysis of reaction outcome. Two mechanisms were suggested to explain the grafting of three mediators (1-Hydroxybenzotriazole, Violuric Acid and Acetosyringone), involving the transformation of proton or electron to produce active moieties, which subsequently react with target substrates. A thorough electrochemical insight into the redox features of mediators and its change in the presence of laccase and substrates were carried out using electrochemical analysis. The effectiveness of each kind of LMS on substrates was preliminarily evaluated by analyzing the change of the peak current and potential of mediators. The actual conversion rate of dyes was used to verify the analysis results, which confirms the important role of the stability of the oxidized form as well as their redox potential of the mediators in determining the mechanism of substrate oxidation. The application of electrochemical analysis in efficiency evaluation of LMS shed new light on effective selection of suitable mediators for degradation of refractory organics. It was therefore possible to prejudge the efficacy of LMS by analyzing the electrochemical parameters of target substances and mediators, which undoubtedly has broad further application prospects of LMS.

Persistence and impact of steroidal estrogens on the environment and their laccase-assisted removal

Steroidal estrogens are widespread water contaminants with potential carcinogenic and endocrine-disrupting activities. The World Health Organization has listed estrogens as group 1 carcinogens. These contaminants are of substantial concern because of potential threats to human health, and aquatic organisms on long-term exposure. A range of methods, including oxidation, adsorption, electrochemical, and irradiation techniques have been employed for their remediation from aqueous systems. However, inadequate removal, toxic sludge generation, high operating costs, and the requisite for skilled operating and maintenance personnel commercially hampered the application of many methods. An interesting alternative treatment approach based on the use of oxidoreductases, particularly laccases, has recently gained amicability for the biotransformation of emerging pollutants. The use of immobilized enzymes is more cost-effective from an industrial perspective due to improved catalytic stability, reusability, reduction of product inhibition, and easier product separation. This review provides comprehensive knowledge on the use of laccases in the biodegradation of steroidal estrogens, including estrone, 17β-estradiol, and 17α-ethinylestradiol with endocrine-disrupting potency from the environment. After an overview of estrogens and catalytic properties of laccase, the use of free, as well as immobilized laccases with a particular emphasis on estrogens removal by laccase-based fed-batch, packed bed bioreactors, and membrane reactors, is discussed. A comparison of existing treatment technologies with enzyme technology for the removal of estrogens from different environmental matrices is made. Lastly, along with concluding remarks, future research direction aimed at bridging knowledge gaps for estrogenic compounds removal are also proposed in this very important research area.

Degradation of Aflatoxin B1 and Zearalenone by Bacterial and Fungal Laccases in Presence of Structurally Defined Chemicals and Complex Natural Mediators

Aflatoxin B₁ (AFB₁) and zearalenone (ZEN) exert deleterious effects to human and animal health. In this study, the ability of a CotA laccase from Bacillus subtilis (BsCotA) to degrade these two mycotoxins was first investigated. Among the nine structurally defined chemical compounds, methyl syringate was the most efficient mediator assisting BsCotA to degrade AFB₁ (98.0%) and ZEN (100.0%). BsCotA could also use plant extracts, including the Epimedium brevicornu, Cucumis sativus L., Lavandula angustifolia, and Schizonepeta tenuifolia extracts to degrade AFB₁ and ZEN. Using hydra and BLYES as indicators, it was demonstrated that the degraded products of AFB₁ and ZEN using the laccase/mediator systems were detoxified. Finally, a laccase of fungal origin was also able to degrade AFB₁ and ZEN in the presence of the discovered mediators. The findings shed light on the possibility of using laccases and a mediator, particularly a natural plant-derived complex mediator, to simultaneously degrade AFB₁ and ZEN contaminants in food and feed.

The impact of lignin sulfonation on its reactivity with laccase and laccase/HBT

This study shows and explains how sulfonation of lignin influences its reactivity with laccase and LMS (with mediator HBT), and what consequences this has for the overall outcome of laccase and LMS treatments.

Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions

N-Oxyl compounds represent a diverse group of reagents that find widespread use as catalysts for the selective oxidation of organic molecules in both laboratory and industrial applications. While turnover of N-oxyl catalysts in oxidation reactions may be accomplished with a variety of stoichiometric oxidants, N-oxyl reagents have also been extensively used as catalysts under electrochemical conditions in the absence of chemical oxidants. Several classes of N-oxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate a wide range of electrosynthetic reactions. Electrochemical studies also provide insights into the structural properties and mechanisms of chemical and electrochemical catalysis by N-oxyl compounds. This review provides a comprehensive survey of the electrochemical properties and electrocatalytic applications of aminoxyls, imidoxyls, and related reagents, of which the two prototypical and widely used examples are 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) and phthalimide N-oxyl (PINO).

Removal of pharmaceutical compounds in water and wastewater using fungal oxidoreductase enzymes

Due to recalcitrance of some pharmaceutically active compounds (PhACs), conventional wastewater treatment is not able to remove them effectively. Therefore, their occurrence in surface water and potential environmental impact has raised serious global concern. Biological transformation of these contaminants using white-rot fungi (WRF) and their oxidoreductase enzymes has been proposed as a low cost and environmentally friendly solution for water treatment. The removal performance of PhACs by a fungal culture is dependent on several factors, such as fungal species, the secreted enzymes, molecular structure of target compounds, culture medium composition, etc. In recent 20 years, numerous researchers tried to elucidate the removal mechanisms and the effects of important operational parameters such as temperature and pH on the enzymatic treatment of PhACs. This review summarizes and analyzes the studies performed on PhACs removal from spiked pure water and real wastewaters using oxidoreductase enzymes and the data related to degradation efficiencies of the most studied compounds. The review also offers an insight into enzymes immobilization, fungal reactors, mediators, degradation mechanisms and transformation products (TPs) of PhACs. In brief, higher hydrophobicity and having electron-donating groups, such as amine and hydroxyl in molecular structure leads to more effective degradation of PhACs by fungal cultures. For recalcitrant compounds, using redox mediators, such as syringaldehyde increases the degradation efficiency, however they may cause toxicity in the effluent and deactivate the enzyme. Immobilization of enzymes on supports can enhance the performance of enzyme in terms of reusability and stability. However, the immobilization strategy should be carefully selected to reduce the cost and enable regeneration. Still, further studies are needed to elucidate the mechanisms involved in enzymatic degradation and the toxicity levels of TPs and also to optimize the whole treatment strategy to have economical and technical competitiveness.

Laccase/Mediator Systems: Their Reactivity toward Phenolic Lignin Structures

Laccase-mediator systems (LMS) have been widely studied for their capacity to oxidize the nonphenolic subunits of lignin (70-90% of the polymer). The phenolic subunits (10-30% of the polymer), which can also be oxidized without mediators, have received considerably less attention. Consequently, it remains unclear to what extent the presence of a mediator influences the reactions of the phenolic subunits of lignin. To get more insight in this, UHPLC-MS was used to study the reactions of a phenolic lignin dimer (GBG), initiated by a laccase from Trametes versicolor, alone or in combination with the mediators HBT and ABTS. The role of HBT was negligible, as its oxidation by laccase occurred slowly in comparison to that of GBG. Laccase and laccase/HBT oxidized GBG at a comparable rate, resulting in extensive polymerization of GBG. In contrast, laccase/ABTS converted GBG at a higher rate, as GBG was oxidized both directly by laccase but also by ABTS radical cations, which were rapidly formed by laccase. The laccase/ABTS system resulted in Cα oxidation of GBG and coupling of ABTS to GBG, rather than polymerization of GBG. Based on these results, we propose reaction pathways of phenolic lignin model compounds with laccase/HBT and laccase/ABTS.

Chitin nanocrystals prepared by oxidation of α-chitin using the O2/laccase/TEMPO system

Laccase mediator oxidation was applied to chitin at pH 6.8 and 30 °C to prepare chitin nanocrystals with a catalytic amount of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO). When 40 mM TEMPO and a total of 500 U laccase were added to 1 g chitin, the yield of water-insoluble oxidized chitin was more than 95%, and the carboxylate content was 0.43 mmol/g. Adsorption of laccase molecules on chitin particles occurred in a buffer at pH 6.8, which may have been caused by electrostatic interactions between positively charged C2-ammonium groups of chitin and anionically charged groups of laccase. Rod-like chitin nanocrystals (ChNCs) were obtained with average lengths and widths of 480 ± 200 nm and 24 ± 17 nm, respectively, by sonication of the oxidized chitin/water suspensions. The O₂/laccase/TEMPO oxidation caused no decrease in the degree of N-acetylation or the crystallinity of the original chitin based on FTIR and X-ray diffraction data.

Distinguishing ionic and radical mechanisms of hydroxylamine mediated electrocatalytic alcohol oxidation using NO–H bond dissociation energies

An idea is proposed to sort N-oxyl radicals with respect to their mechanisms of electrocatalytic alcohol oxidation by knowing the NO–H bond dissociation energies of their precursors.

Laccase catalyzed grafting of –N–OH type mediators to lignin via radical–radical coupling

Lignin can be functionalized with –N–OH type mediators via laccase catalysis. Three radical coupling mechanisms are suggested for this enzymatic “hetero-functionalization” which may be a new route for biomass lignin upgrading.

Two Decades of Laccases: Advancing Sustainability in the Chemical Industry

Given the current state of environmental affairs and that our future on this planet as we know it is in jeopardy, research and development into greener and more sustainable technologies within the chemical and forest products industries is at its peak. Given the global scale of these industries, the need for environmentally benign practices is propelling new green processes. These challenges are also impacting academic research and our reagents of interest are laccases. These enzymes are employed in a variety of biotechnological applications due to their native function as catalytic oxidants. They are about as green as it gets when it comes to chemical processes, requiring O₂ as their only co-substrate and producing H₂ O as the sole by-product. The following account will review our twenty year journey on the use of these enzymes within our research group, from their initial use in biobleaching of kraft pulps and for fiber modification within the pulp and paper industry, to their current application as green catalytic oxidants in the field of synthetic organic chemistry.

Fungal laccases as tools for biodegradation of industrial dyes

Laccases are blue copper oxidases, found in some plants and secreted by a wide range of ligninolytic fungi. These enzymes are well known for their ability in oxidizing several organic compounds, mainly phenolics and aromatic amines, at the expenses of molecular oxygen. Therefore, they could find application in the field of enzymatic bioremediation of many industrial wastewaters, and in particular to bleach and/or detoxify dye-containing effluents. Not all industrial dyes behave as laccase substrates, but this limitation is often overcome by the judicious use of redox mediators. These could substantially widen the application range of laccases as bioremediation tools. The present study encompasses the main properties of the most used industrial dyes as related to their chemical classification, fungal laccases and their molecular and catalytic features, the use of redox mediators, limitations and perspectives of the use of fungal laccases for industrial dye bleaching.

Oxidative cleavage of 1-aryl-isochroman derivatives using the Trametes villosa laccase/1-hydroxybenzotriazole system

Dimethyl carbonate was firstly used as a co-solvent in a green oxidative cleavage of 1-aryl-isochroman derivatives yielding useful synthetic intermediates of drugs.

Aerobic oxidation catalysis with stable radicals

Selective oxidation reactions are challenging when carried out on an industrial scale. Many traditional methods are undesirable from an environmental or safety point of view. There is a need to develop sustainable catalytic approaches that use molecular oxygen as the terminal oxidant. This review will discuss the use of stable radicals (primarily nitroxyl radicals) in aerobic oxidation catalysis. We will discuss the important advances that have occurred in recent years, highlighting the catalytic performance, mechanistic insights and the expanding synthetic utility of these catalytic systems.

Properties of bacterial laccases and their application in bioremediation of industrial wastes

The bioremediation process of industrial waste can be made more efficient using ligninolytic laccase enzymes, which are obtained from fungi, bacteria, higher plants, insects, and also in lichen. Laccase are catalyzed in the mono-electronic oxidation of a substrate from the expenditure of molecular oxygen. This enzyme belongs to the multicopper oxidases and participates in the cross linking of monomers, involved in the degradation of wide range industrial pollutants. In recent years, these enzymes have gained application in pulp and paper, textile and food industries. There are numerous reviews on laccases; however, a lot of information is still unknown due to their broad range of functions and applications. In this review, the bacterial laccases are focused for the bioremediation of various industrial pollutants. A brief description on structural molecular and physicochemical properties has been made. Moreover, the mechanism by which the reaction is catalyzed, the physical basis of thermostability and enantioselectivity, which requires more attention from researchers, and applications of laccase in various fields of biotechnology are pointed out.

Improving the performance of a biofuel cell cathode with laccase-containing culture supernatant from Pycnoporus sanguineus

Laccases are multicopper oxidoreductases that can be used in biofuel cells to improve cathode performance by cathodic oxygen reduction. Here we present a laccase from the ligninolytic white-rot fungus Pycnoporus sanguineus that, in contrast to the Trametes versicolor laccase, can be produced in the absence of inducers in a standard culture medium. After 7days of cultivation the activity of this laccase in culture supernatant reached 2.5U/ml, which is high enough for direct application of the supernatant in biofuel cells. The highest current density of 115.0±3.5μA/cm(2) at 400mV vs. SCE was obtained at pH 5 with a buckypaper cathode with a laccase-containing culture supernatant. The enzyme also showed electrocatalytic activity at pH 6 and 7. These results not only present a new cost-efficient laccase for improving cathode performance, but also show that new laccases with different catalytic properties can be suitable for biofuel cells.

Enzymatic delignification: an attempt for lignin degradation from lignocellulosic feedstock

Burgeoning population growth and an increased demand for transportation and industrialization has led to the excessive use of fossil fuels, which in turn leads to higher levels of greenhouse gas emissions and contributes to global warming.

Deracemisation of profenol core by combining laccase/TEMPO-mediated oxidation and alcohol dehydrogenase-catalysed dynamic kinetic resolution

A one-pot two-step chemoenzymatic protocol to deracemise a profen-like derivative has been designed.

Candida parapsilosis ATCC 7330 mediated oxidation of aromatic (activated) primary alcohols to aldehydes

A green, simple and high yielding [up to 86% yield] procedure is developed for the oxidation of aromatic (activated) primary alcohols to aldehydes using whole cells of Candida parapsilosis ATCC 7330.

Mechanism of alcohol oxidation mediated by copper(II) and nitroxyl radicals

2,2'-Bipyridine-ligated copper complexes, in combination with TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), are highly effective catalysts for aerobic alcohol oxidation. Considerable uncertainty and debate exist over the mechanism of alcohol oxidation mediated by Cu(II) and TEMPO. Here, we report experimental and density functional theory (DFT) computational studies that distinguish among numerous previously proposed mechanistic pathways. Oxidation of various classes of radical-probe substrates shows that long-lived radicals are not formed in the reaction. DFT computational studies support this conclusion. A bimolecular pathway involving hydrogen-atom-transfer from a Cu(II)-alkoxide to a nitroxyl radical is higher in energy than hydrogen transfer from a Cu(II)-alkoxide to a coordinated nitroxyl species. The data presented here reconcile a collection of diverse and seemingly contradictory experimental and computational data reported previously in the literature. The resulting Oppenauer-like reaction pathway further explains experimental trends in the relative reactivity of different classes of alcohols (benzylic versus aliphatic and primary versus secondary), as well as the different reactivity observed between TEMPO and bicyclic nitroxyls, such as ABNO (ABNO = 9-azabicyclo[3.3.1]nonane N-oxyl).