Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase–ABTS oxidations: an apparent paradox

Dye-decolorizing peroxidase of Thermobifida halotolerance displays complex kinetics with both substrate inhibition and apparent positive cooperativity

Dye-decolorizing peroxidases (DyPs) have been intensively investigated for the purpose of industrial dye decolourization and lignin degradation. Unfortunately, the characterization of these peroxidases is hampered by their non-Michaelis-Menten kinetics, exemplified by substrate inhibition and/or positive cooperativity. Although often observed, the underlying mechanisms behind the unusual kinetics of DyPs are poorly understood. Here we studied the kinetics of the oxidation of 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), hydroquinones, and anthraquinone dyes by DyP from the bacterium Thermobifida halotolerans (ThDyP) and solved its crystal structure. We also provide rate equations for different kinetic mechanisms explaining the complex kinetics of heme peroxidases. Kinetic studies along with the analysis of the structure of ThDyP suggest that the substrate inhibition is caused by the non-productive binding of ABTS to the enzyme resting state. Strong irreversible inactivation of ThDyP by H2O2 in the absence of ABTS suggests that the substrate inhibition by H2O2 may be caused by the non-productive binding of H2O2 to compound I. Positive cooperativity was observed only with the oxidation of ABTS but not with the two electron-donating substrates. Although the conventional mechanism of cooperativity cannot be excluded, we propose that the oxidation of ABTS assumes the simultaneous binding of two ABTS molecules to reduce compound I to the enzyme resting state, and this causes the apparent positive cooperativity.

Enzymatic Dimerization-Induced Self-Assembly of Alanine-Tyramine Conjugates into Versatile, Uniform, Enzyme-Loaded Organic Nanoparticles

Herein, we report a novel enzymatic dimerization-induced self-assembly (e-DISA) procedure that converts alanine-tyramine conjugates into highly uniform enzyme-loaded nanoparticles (NPs) or nanocontainers by the action of horseradish peroxidase (HRP) in an aqueous medium under ambient conditions. The NP formation was possible with both enantiomers of alanine, and the average diameter could be varied from 150 nm to 250 nm (with a 5-12 % standard deviation of as-prepared samples) depending on the precursor concentration. About 60 % of the added HRP enzyme was entrapped within the NPs and was subsequently utilized for post-synthetic modification of the NPs with phenolic compounds such as tyramine or tannic acid. One-pot multi-enzyme entrapment of glucose oxidase (GOx) and peroxidase (HRP) within the NPs was also achieved. These GOx-HRP loaded NPs allowed multimodal detection of glucose, including that present in human saliva, with a limit of detection (LoD) of 740 nM through fluorimetry. The NPs exhibited good cytocompatibility and were stable to changes in pH (acidic to basic), temperature, ultrasonication, and even the presence of organic solvent (EtOH) to a certain extent, since they are stabilized by intermolecular hydrogen bonding, π-π, and CH-π interactions. The proposed e-DISA procedure can be widely expanded through the design of diverse enzyme-responsive precursors.

Unusual Oligomeric Laccase-like Oxidases from Ascomycete Curvularia geniculata VKM F-3561 Polymerizing Phenylpropanoids and Phenolic Compounds under Neutral Environmental Conditions

The unique oligomeric alkaliphilic laccase-like oxidases of the ascomycete C. geniculata VKM F-3561 (with molecular masses about 1035 and 870 kDa) were purified and characterized for the first time. The ability of the enzymes to oxidize phenylpropanoids and phenolic compounds under neutral environmental conditions with the formation of previously unknown di-, tri-, and tetrameric products of transformation was shown. The possibility to obtain industrially valuable compounds (dihydroxybenzyl alcohol and hydroxytyrosol) from caffeic acid using laccase-like oxidases of C. geniculata VKM F-3561 has been shown. Complete nucleotide sequence of the laccase gene, which is expressed at the peak of alkaliphilic laccase activity of the fungus, and its promoter region were determined. Based on the phylogenetic analysis of the nucleotide sequence, the nearest relationship of the isolated laccase gene with similar genes of fungi of the genera Alternaria, Bipolaris, and Cochliobolus was shown. Homologous model of the laccase structure was predicted and a proton channel was found, which was presumably responsible for the accumulation and transport of protons to T2/T3-copper center in the alkaliphilic laccase molecule and providing the functional activity of the enzyme in the neutral alkaline environment conditions.

Recombinant Lignin Peroxidase with Superior Thermal Stability and Melanin Decolorization Efficiency in a Typical Human Skin-Mimicking Environment

Recently, the desire for a safe and effective method for skin whitening has been growing in the cosmetics industry. Commonly used tyrosinase-inhibiting chemical reagents exhibit side effects. Thus, recent studies have focused on performing melanin decolorization with enzymes as an alternative due to the low toxicity of enzymes and their ability to decolorize melanin selectively. Herein, 10 different isozymes were expressed as recombinant lignin peroxidases (LiPs) from Phanerochaete chrysosporium (PcLiPs), and PcLiP isozyme 4 (PcLiP04) was selected due to its high stability and activity at pH 5.5 and 37 °C, which is close to human skin conditions. In vitro melanin decolorization results indicated that PcLiP04 exhibited at least 2.9-fold higher efficiency than that of well-known lignin peroxidase (PcLiP01) in a typical human skin-mimicking environment. The interaction force between melanin films measured by a surface forces apparatus (SFA) revealed that the decolorization of melanin by PcLiP04 harbors a disrupted structure, possibly interrupting π-π stacking and/or hydrogen bonds. In addition, a 3D reconstructed human pigmented epidermis skin model showed a decrease in melanin area to 59.8% using PcLiP04, which suggests that PcLiP04 exhibits a strong potential for skin whitening.

The Laccase of Myrothecium roridum VKM F-3565: A New Look at Fungal Laccase Tolerance to Neutral and Alkaline Conditions

Most of the currently known fungal laccases show their maximum activity under acidic environmental conditions. It is known that a decrease in the activity of a typical laccase at neutral or alkaline pH values is the result of an increase in the binding of the hydroxide anion to the T2/T3 copper center, which prevents the transfer of an electron from the T1 Cu to the trinuclear copper center. However, evolutionary pressure has resolved the existing limitations in the catalytic mechanism of laccase, allowing such enzymes to be functionally active under neutral/alkaline pH conditions, thereby giving fungi an advantage for their survival. Combined molecular and biochemical studies, homological modeling, calculation of the electrostatic potential on the Connolly surface at pH 5.0 and 7.0, and structural analysis of the novel alkaliphilic laccase of Myrothecium roridum VKM F-3565 and alkaliphilic and acidophilic fungal laccases with a known structure allowed a new intramolecular channel near the one of the catalytic aspartate residues at T2-copper atom to be found. The amino acid residues of alkaliphilic laccases forming this channel can presumably serve as proton donors for catalytic aspartates under neutral conditions, thus ensuring proper functioning. For the first time for ascomycetous laccases, the production of new trimeric products of phenylpropanoid condensation under neutral conditions has been shown, which could have a potential for use in pharmacology.

Time-dependent electrochemical characteristics of a phenolic and non-phenolic compound in the presence of laccase/ABTS system

The laccase/ABTS system has found several industrial applications ranging from biodeterioration to biodegradation and bioremediation. However, the capability of the laccase/ABTS system varies depending upon the type of substrate used. Voltammetric studies involving two widely used substrates, i.e., veratryl alcohol (VA) and alkali lignin (AL), were performed to gain new insight into the electrochemical behavior of the reactions. The individual electrochemical reactions established the differential nature of the two compounds over a concentration range, along with the mediator ABTS producing a distinguishing effect on their oxidative reactions, which was further studied over a 12hour period. It was followed by the reaction of both the compounds against the laccase/ABTS system that helped verify the role of the enzyme and the mediator in the electron transfer process and elucidate the mediated oxidations carried out by laccase against the phenolic and non-phenolic substrate through the process of cyclic voltammetry.

Intrinsic Multienzyme-like Activities of the Nanoparticles of Mn and Fe Cyano-Bridged Assemblies

This study investigates the intrinsic multienzyme-like properties of the non-stabilized nanocrystalline nanoparticles of manganese-doped Prussian blue (Mn-PB) nanozymes and Prussian blue (PB) nanozymes in chemical and electrocatalytic transformations of reactive oxygen species. The effect of manganese doping on the structural, biomimetic, and electrocatalytic properties of cyano-bridged assemblies is also discussed.

Electrochemical analysis of Catechol polymerization in presence of Trametes versicolor laccase and the mediator ABTS

The phenolic compound catechol has found various applications in the industry but is often discharged untreated in industrial effluents. Catechol is highly toxic and adversely affects the environment. This has increased extensive investigation into elucidating the effects of various synthetic elements or different biocatalysts on catechol, thereby leading the way to its bioremediation. Hence, an electrochemical-based study on catechol in the presence of the enzyme laccase could provide a basic understanding of the unique characteristics exhibited by catechol, thus facilitating a distinct perspective to its subsequent treatment and removal. The present study focuses on the electrochemical characterization of catechol based on the oxidation of laccase and the redox mediator 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS). Catechol exhibited distinct electrochemical behavior across various concentrations. The unique electroactive nature of ABTS assisted in the polymerization of catechol which was found to be concentration-dependent. Laccase produced a higher oxidation-reduction rate, thereby producing a much more stable condition for the polymerization of catechol. However, with the laccase-mediator system (LMS), the catechol polymerization rate was distinctly higher and more gradual with the enzyme utilizing the electroactive species produced by ABTS to increase the electron transfer and producing a combinatorial impact on the phenolic compound. This study could rightly serve as the building block in developing future technologies like wastewater treatment and biosensors for catechol bioremediation.

Biodegradation of sulfonamide antibiotics through the heterologous expression of laccases from bacteria and investigation of their potential degradation pathways

In this study, seven laccase genes from different bacteria were linked with the signal peptides PelB, Lpp or Ompa for heterologous expression in E. coli. The recombinant strains were applied for the removal of sulfadiazine (SDZ), sulfamethazine (SMZ), and sulfamethoxazole (SMX). The results obtained for different signal peptides did not provide insights into the removal mechanism. The removal ratios of SDZ, SMZ, and SMX obtained with the recombinant strain 6#P at 60 h were around 92.0%, 89.0%, and 88.0%, respectively. The degradation pathways of sulfonamides have been proposed, including SO₂ elimination, hydroxylation, oxidation, pyrimidine ring cleavage, and N-S bond cleavage. Different mediators participate in the degradation of antibiotics through different mechanisms, and different antibiotics have different responses to the same mediator. The addition of 2,2'-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) slightly promoted the removal of sulfonamides by most recombinant strains with different signal peptides, especially for the recombinant strain 2#O. The removal of sulfonamides by 1-hydroxybenzotriazole (HBT) varied with the recombinant strains. Syringaldehyde (SA) had a slight inhibitory effect on the removal of sulfonamides, with the most significant effect on strains 7#L and 7#O.

Elucidating fungal Rigidoporus species FMD21 lignin-modifying enzyme genes and 2,3,7,8-tetrachlorodibenzo-p-dioxin degradation by laccase isozymes

White-rot fungus Rigidoporus sp. FMD21 is a lignin-modifying enzyme producing fungus that can degrade dioxin. Extracellular enzymes from FMD21 include laccase and manganese peroxidase which are promising enzymes for myco-remediation because of their wide substrate specificity and mild catalysis conditions. The FMD21 genome was sequenced using Ion Torrent technology and consists of 38.98 Mbps with a GC content of 47.4 %. Gene prediction using Augustus with Basidiomycota reference setting resulted in 8245 genes. Functional gene annotations were carried out by using several programs and databases. We focused on laccase and ligninolytic peroxidase genes, which are most likely involved in the degradation of aromatic pollutants. The genome of FMD21 contains 12 predicted laccase genes (10 out of 12 predicted as full length) and 13 putative ligninolytic peroxidases which were annotated as MnP or versatile peroxidases. Four predicted laccases showed a higher than 65 % binding chance to 2,3,7,8-TCDD with the highest at 72 % in in silico docking analysis. Heterologous expressed laccases showed activity towards three tested substrates included ABTS, guaiacol and 2,6-DMP. ABTS displayed two-stage oxidation which differed from natural FMD21 laccases. 2,3,7,8-TCDD was degraded by 50 % after two weeks of enzymatic treatment by three out of five laccase isozymes which were natural laccases secreted by FMD21. In this study, we provide direct evidence for the 2,3,7,8-TCDD biodegradation capability of fungal laccases.

Protective and inhibitory effects of acteoside from Abeliophyllum distichum Nakai against oxidative DNA damage

Abeliophyllum distichum Nakai is a Korean endemic plant of the Oleaceae family that contains acteoside, a glycosylated caffeic acid, with neuroprotective, anti‑inflammatory and antibacterial properties. Previous studies, involving Accelerated Chromatographic Isolation, a high‑performance liquid chromatography‑photodiode array detector and a liquid chromatograph‑mass selective detector, isolated and identified acteoside in A. distichum (AAD) and documented its antioxidant and anti‑inflammatory activities. The aim of the present study was to determine whether AAD could protect from DNA damage by reducing oxidative stress. AAD treatment protected plasmid DNA against damage to DNA double‑strands induced by reactive oxygen species (ROS) and decreased the levels of phosphorylated p53 and γ‑H2AX in ROS‑treated NIH 3T3 cells. These findings suggested that AAD could reduce ROS‑mediated cellular damage and may represent an effective, natural antioxidant with the ability to protect genetic material.

ABTS/PP Decolorization Assay of Antioxidant Capacity Reaction Pathways

The 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•+) radical cation-based assays are among the most abundant antioxidant capacity assays, together with the 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-based assays according to the Scopus citation rates. The main objective of this review was to elucidate the reaction pathways that underlie the ABTS/potassium persulfate decolorization assay of antioxidant capacity. Comparative analysis of the literature data showed that there are two principal reaction pathways. Some antioxidants, at least of phenolic nature, can form coupling adducts with ABTS^•+, whereas others can undergo oxidation without coupling, thus the coupling is a specific reaction for certain antioxidants. These coupling adducts can undergo further oxidative degradation, leading to hydrazindyilidene-like and/or imine-like adducts with 3-ethyl-2-oxo-1,3-benzothiazoline-6-sulfonate and 3-ethyl-2-imino-1,3-benzothiazoline-6-sulfonate as marker compounds, respectively. The extent to which the coupling reaction contributes to the total antioxidant capacity, as well as the specificity and relevance of oxidation products, requires further in-depth elucidation. Undoubtedly, there are questions as to the overall application of this assay and this review adds to them, as specific reactions such as coupling might bias a comparison between antioxidants. Nevertheless, ABTS-based assays can still be recommended with certain reservations, particularly for tracking changes in the same antioxidant system during storage and processing.

Biomimetic synthesis of galantamine via laccase/TEMPO mediated oxidative coupling

Laccase-mediated intramolecular oxidative radical coupling of N-formyl-2-bromo-O-methylnorbelladine afforded a novel and isolable spirocyclohexadienonic intermediate of galantamine.

Immobilization of laccase on hollow mesoporous carbon nanospheres: Noteworthy immobilization, excellent stability and efficacious for antibiotic contaminants removal

In this study, the hollow mesoporous carbon spheres (HMCs) were synthesized and modified for laccase (Lac) immobilization, and the structural characteristics of HMCs materials were determined by FESEM, TEM and FTIR etc. The maximum loading of Lac on the HMCs materials could reach 835 mg/g, meanwhile, the immobilized Lac exhibited excellent thermo-stability, pH stability, storage stability and reusability. The antibiotics removal experiments indicated that the immobilized Lac possess efficient removal efficiency for both tetracycline hydrochloride (TCH) and ciprofloxacin hydrochloride (CPH) in the presence of redox mediator. The synergy of the adsorption by HMCs and the degradation by Lac could be the reasons for the high removal of antibiotics. Meanwhile, for investigating degradation mechanism, the degradation product analysis and molecular docking method had been introduced to this study. According to the degradation products, dehydroxylation and demethylation are major degradation reactions for TCH degradation, and the oxidation of the piperazinyl substituent and hydroxylation are the major degradation for CPH degradation. The docking results showed that some important residues played the key role in the degradation process. This study indicated that the immobilization of Lac on HMCs could be potentially applied in environmental remediation of antibiotics.

Synthesis of Azobenzene Dyes Mediated by CotA Laccase

An eco-friendly protocol for the synthesis of azobenzene dyes by oxidative coupling of primary aromatic amines is reported. As efficient biocatalytic systems, CotA laccase and CotA laccase/ABTS (2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)) enable the oxidation of various substituted anilines, in aqueous medium, ambient atmosphere and under mild reaction conditions of pH and temperature. A series of azobenzene dyes were prepared in good to excellent yields in an one-pot reaction. A mechanistic proposal for the formation of the azo derivatives is presented. Our strategy offers an alternative approach for the direct synthesis of azobenzene dyes, avoiding the harsh conditions generally required for most of the traditional synthetic methods.

Immobilization of a Laccase/2,2'-azino-bis-(3-ethylbenzothiazoline)-6-sulfonic Acid System to Layered Double Hydroxide/Alginate Biohybrid Beads for Biodegradation of Malachite Green Dye

The application of laccase-mediator-based catalysis is limited owing to the high cost of laccases and mediators and the potential toxicity of free mediators. Here, a novel biocatalyst (Im-LMS) was fabricated by immobilizing both laccase and a mediator (2,2'-azino-bis-[3-ethylbenzothiazoline]-6-sulfonic acid) on layered double hydroxide/alginate biohybrid beads. The catalytic activity of Im-LMS was evaluated for dye decolorization using malachite green. The decolorization yields of malachite green by Im-LMS and the free laccase-mediator system were 92% within 120 min and 90% within 90 min. Malachite green solution was detoxified completely after biodegradation by Im-LMS. Following eight reuse cycles of Im-LMS for dye treatment, a decolorization yield of 79% was obtained. The activity of Im-LMS was almost completely stable after being stored for 10 days. The recyclability and stability of Im-LMS will be helpful for reducing the running cost and potential toxicity associated with mediators to facilitate practical applications.

Role of fungal laccase in iodide oxidation in soils

Previously, we hypothesized that microbial laccase oxidizes iodide (I^-) in soils to molecular iodine (I₂) or hypoiodous acid (HIO), both of which are easily incorporated into natural soil organic matter, and thus plays a role in iodine sorption on soils. In this study, soil iodide oxidase activity was determined by a colorimetric assay to evaluate if laccase is responsible for iodide oxidation in soils. Three types of Japanese soil showed significant iodide oxidase activities (0.751-2.87 mU g soil^-1) at pH 4.0, which decreased with increasing pH, until it was no longer detected at pH 5.5. The activity was inhibited strongly by autoclaving or by the addition of common laccase inhibitors. Similar tendency of inhibition was observed in soil laccase activity, which was determined with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a substrate. Significant positive correlations (R² values of 0.855-0.896) between iodide oxidase activity and laccase activity were observed in two of three soils. Commercially available fungal laccases showed only very low iodide oxidase activities (4.68-18.0 mU mg^-1), but enhanced activities of 102-739 mU mg^-1 were observed in the presence of redox mediators. Finally, we successfully isolated fungal strains with iodide-oxidizing phenotype in the presence of redox mediators. Polyacrylamide gel electrophoresis of the culture supernatant of Scytalidium sp. strain UMS and subsequent active stain revealed that the fungal laccase actually oxidized iodide in the presence of redox mediators. These results suggest that at least part of iodide in soils is oxidized by fungal laccase through the laccase-mediator system.

Fungal lignin peroxidase does not produce the veratryl alcohol cation radical as a diffusible ligninolytic oxidant

Peroxidases are considered essential agents of lignin degradation by white-rot basidiomycetes. However, low-molecular-weight oxidants likely have a primary role in lignin breakdown because many of these fungi delignify wood before its porosity has sufficiently increased for enzymes to infiltrate. It has been proposed that lignin peroxidases (LPs, EC 1.11.1.14) fulfill this role by oxidizing the secreted fungal metabolite veratryl alcohol (VA) to its aryl cation radical (VA^+•), releasing it to act as a one-electron lignin oxidant within woody plant cell walls. Here, we attached the fluorescent oxidant sensor BODIPY 581/591 throughout beads with a nominal porosity of 6 kDa and assessed whether peroxidase-generated aryl cation radical systems could oxidize the beads. As positive control, we used the 1,2,4,5-tetramethoxybenzene (TMB) cation radical, generated from TMB by horseradish peroxidase. This control oxidized the beads to depths that increased with the amount of oxidant supplied, ultimately resulting in completely oxidized beads. A reaction-diffusion computer model yielded oxidation profiles that were within the 95% confidence intervals for the data. By contrast, bead oxidation caused by VA and the LPA isozyme of Phanerochaete chrysosporium was confined to a shallow shell of LP-accessible volume at the bead surface, regardless of how much oxidant was supplied. This finding contrasted with the modeling results, which showed that if the LP/VA system were to release VA^+•, it would oxidize the bead interiors. We conclude that LPA releases insignificant quantities of VA^+• and that a different mechanism produces small ligninolytic oxidants during white rot.

Selective natural induction of laccases in Pleurotus sajor-caju, suitable for application at a biofuel cell cathode at neutral pH

Laccases are multicopper oxidoreductases with broad substrate specificity and are applied in biofuel cells at the cathode to improve its oxygen reduction performance. However, the production of laccases by e.g. fungi is often accompanied by the need of synthetic growth supplements for increased enzyme production. In this study we present a strategy for the white-rot fungus Pleurotus sajor-caju for natural laccase activity induction using lignocellulose substrates and culture supernatant of Aspergillus nidulans. P. sajor-caju laccases were secreted into the supernatant, which was directly used at a carbon-nanotube buckypaper cathode in a biofuel cell. Maximal current densities of -148±3μAcm(-2) and -102±9μAcm(-2) at 400mV were achieved at pH 5 and 7, respectively. Variations in cathode performance were observed with culture supernatants produced under different conditions due to the induction of specific laccases.

ABTS-Modified Silica Nanoparticles as Laccase Mediators for Decolorization of Indigo Carmine Dye

Efficient reuse and regeneration of spent mediators are highly desired for many of the laccases’ biotechnology applications. This investigation demonstrates that a redox mediator 2,2′-azino-bis-(3-ethylbenzothiazoline)-6-sulfonic acid (ABTS) covalently attached to silica nanoparticles (SNPs) effectively mediated dye decolorization catalyzed by laccase. Characteristics of ABTS-modified silica nanoparticles (ABTS-SNPs) were researched by scanning electron microscopy and Fourier-transformed infrared spectroscopy. When ABTS and ABTS-SNPs were used as laccase mediators, the decolorization yields of 96 and 95% were, respectively, obtained for indigo carmine dye. The results suggest that ABTS immobilized on SNPs can be used as laccase mediators as they retain almost the same efficiency as the free ABTS. The oxidized ABTS-SNPs were regenerated by their reduction reaction with ascorbic acid. Decolorization efficiency of regenerated ABTS-SNPs and their initial forms were found to be almost equivalent. Six reuse cycles for spent ABTS-SNPs were run for the treatment of indigo carmine, providing decolorization yields of 96–77%. Compared with free mediator, the immobilized mediators have the advantage of being easily recovered, regenerated, and reused making the whole process environmentally friendly.

Role of laccase as an enzymatic pretreatment method to improve lignocellulosic saccharification

The saccharification of wheat straw was improved when an incubation step was performed withTrametes versicolorlaccase (TvL) and the mediator 1-hydroxybenzotriazole (1-HBT) prior to an alkaline peroxide extraction (APE).

Effects of ionic liquids on the reaction kinetics of a laccase–mediator system

Conductivity and viscosity explain part of the effect of ionic liquids on the laccase–mediator system kinetics, but not all.

Metal-free aerobic oxidations mediated by N-hydroxyphthalimide. A concise review

Since the beginning of the century, N-hydroxyphthalimide and related compounds have been revealed to be efficient organocatalysts for free-radical processes and have found ample application in promoting the aerobic oxidation of a wide range of organic substrates. When combined with different co-catalysts, they are activated to the corresponding N-oxyl radical species and become able to promote radical chains, involving molecular oxygen, directly or indirectly. Most of the examples reported in the literature describe the use of these N-hydroxy derivatives in the presence of transition-metal complexes. However, eco-friendly standards, including the demand for highly selective transformations, impose the development of metal-free processes, especially for large-scale productions, as in the case of the oxygenation of hydrocarbons. For this reason, many efforts have been devoted in the past decade to the design of new protocols for the activation of N-hydroxy imides in the presence of nonmetal initiators. Herein we provide a concise overview of the most significant and successful examples in this field, with the final aim to furnish a useful instrument for all scientists actively involved in the O₂-mediated selective oxidation of organic compounds and looking for environmentally safe alternatives to metal catalysis.

One-electron oxidation of ferrocenes by short-lived N-oxyl radicals. The role of structural effects on the intrinsic electron transfer reactivities

A kinetic study of the one electron oxidation of substituted ferrocenes (FcX: X = H, COPh, COMe, CO(2)Et, CONH(2), CH(2)OH, Et, and Me(2)) by a series of N-oxyl radicals (succinimide-N-oxyl radical (SINO), maleimide-N-oxyl radical (MINO), 3-quinazolin-4-one-N-oxyl radical (QONO) and 3-benzotriazin-4-one-N-oxyl radical (BONO)), has been carried out in CH(3)CN. N-oxyl radicals were produced by hydrogen abstraction from the corresponding N-hydroxy derivatives by the cumyloxyl radical. With all systems, the rate constants exhibited a satisfactory fit to the Marcus equation allowing us to determine self-exchange reorganization energy values (λ(NO˙/NO(-))) which have been compared with those previously determined for the PINO/PINO(-) and BTNO/BTNO(-) couples. Even small modification of the structure of the N-oxyl radicals lead to significant variation of the λ(NO˙/NO(-)) values. The λ(NO˙/NO(-)) values increase in the order BONO < BTNO < QONO < PINO < SINO < MINO which do not parallel the order of the oxidation potentials. The higher λ(NO˙/NO(-)) values found for the MINO and SINO radicals might be in accordance with a lower degree of spin delocalization in the radicals MINO and SINO and charge delocalization in the anions MINO(-) and SINO(-) due to the absence of an aromatic ring in their structure.

Chemoselective C-4 aerobic oxidation of catechin derivatives catalyzed by the Trametes villosa laccase/1-hydroxybenzotriazole system: synthetic and mechanistic aspects

Catechin derivatives were oxidized in air in the presence of the Trametes villosa laccase/1-hydroxybenzotriazole (HBT) system in buffered water/1,4-dioxane as reaction medium. The oxidation products, flavan-3,4-diols and the corresponding C-4 ketones, are bioactive compounds and useful intermediates for the hemisynthesis of proanthocyanidins, plant polyphenols which provide beneficial health properties for humans. Determinations of oxidation potentials excluded that catechin derivatives could be directly oxidized by laccase Cu(II), while it resulted in the H-abstraction from benzylic positions being promptly promoted by the enzyme in the presence of the mediator HBT, the parent species producing in situ the reactive intermediate benzotriazole-N-oxyl (BTNO) radical. A remarkable and unexpected result for the laccase/HBT oxidative system has been the chemoselective insertion of the oxygen atom into the C-4-H bond of catechin derivatives. Mechanistic aspects of the oxidation reaction have been investigated in detail for the first time in order to corroborate these results. Since the collected experimental findings could not alone provide information useful to clarify the origin of the observed chemoselectivity, these data were expressly supplemented with information derived by suitable molecular modeling investigations. The integrated evaluation of the dissociation energies of the C-H bonds calculated both by semiempirical and DFT methods and the differential activation energies of the process estimated by a molecular modeling approach suggested that the observed selective oxidation at the C-4 carbon has a kinetic origin.

Laccases and their natural mediators: biotechnological tools for sustainable eco-friendly processes

Laccases are oxidoreductases which oxidize a variety of aromatic compounds using oxygen as the electron acceptor and producing water as by-product. The interest for these old enzymes (first described in 19th century) has progressively increased due to their outstanding biotechnological applicability. The presence of redox mediators is required for a number of biotechnological applications, providing the oxidation of complex substrates not oxidized by the enzyme alone. The efficiency of laccase-mediator systems to degrade recalcitrant compounds has been demonstrated, but still the high cost and possible toxicity of artificial mediators hamper their application at the industrial scale. Here, we present a general outlook of how alternative mediators can change this tendency. We focus on phenolic compounds related to lignin polymer that promotes the in vitro transformation of recalcitrant non-phenolic structures by laccase and are seemingly the natural mediators of laccases. The use of eco-friendly mediators easily available from lignocellulose, could contribute to the industrial implementation of laccases and the development of the 21th century biorefineries.

N-demethylation of N,N-dimethylanilines by the benzotriazole N-oxyl radical: evidence for a two-step electron transfer-proton transfer mechanism

The reaction of the benzotriazole N-oxyl radical (BTNO) with a series of 4-X-N,N-dimethylanilines (X = CN, CF(3), CO(2)CH(2)CH(3), CH(3), OC(6)H(5), OCH(3)) has been investigated in CH(3)CN. Product analysis shows that the radical, 4-X-C(6)H(4)N(CH(3))CH(2)(*), is first formed, which can lead to the N-demethylated product or the product of coupling with BTNO. Reaction rates were found to increase significantly by increasing the electron-donating power of the aryl substituents (rho(+) = -3.8). With electron-donating substituents (X = CH(3), OC(6)H(5), OCH(3)), no intermolecular deuterium kinetic isotope effect (DKIE) and a substantial intramolecular DKIE are observed. With electron-withdrawing substituents (X = CN, CF(3), CO(2)CH(2)CH(3)), substantial values of both intermolecular and intramolecular DKIEs are observed. These results can be interpreted on the basis of an electron-transfer mechanism from the N,N-dimethylanilines to the BTNO radical followed by deprotonation of the anilinium radical cation (ET-PT mechanism). By applying the Marcus equation to the kinetic data for X = CH(3), OC(6)H(5), OCH(3) (rate-determining ET), a reorganization energy for the ET reaction was determined (lambda(BTNO/DMA) = 32.1 kcal mol(-1)). From the self-exchange reorganization energy for the BTNO/BTNO(-) couple, a self-exchange reorganization energy value of 31.9 kcal mol(-1) was calculated for the DMA(*+)/DMA couple.

Structure and sensor properties of thin ordered solid films

Miniaturized gas sensors and biosensors based on nanostructured sensing elements have attracted considerable interest because these nanostructured materials can be used to significantly improve sensor sensitivity and the response time. We report here on a generic, reversible sensing platform based on hybrid nanofilms. Thin ordered Langmuir-Blodgett (LB) films built of fluorene derivatives were used as effective gas sensors for both oxidative and reductive analytes. A novel immobilization method based on thin LB films as a matrix has been developed for construction of sensing protein layers. Biomolecules can often be incorporated into and immobilized on Langmuir-Blodgett films using adsorption methods or by covalent immobilization of proteins. The sensor sensitisation was achieved by an amphiphilic N-alkyl-bis(thiophene)arylenes admixed into the film. The interlaced derivative was expected to facilitate the electron transfer, thereby enhancing the sensor sensitivity. The results suggest that this may be very promising approach for exploring the interactions between proteins and high throughput detection of phenol derivatives in wastewater.