Radical-mediated Mechanism Research Articles

Visible‐Light Mediated Tandem Oxidation–Organocatalyzed Aromatic C−S Bond Functionalization: Synthesis of 2‐(Benzo[b]thiophen‐2‐yl)benzenesulfonamide Derivatives

AbstractHerein, we demonstrated a visible light mediated tandem oxidation–organo catalyzed C−S bond functionalization protocol for the cleavage of aromatic C−S bond. The investigation encompassed the functionalization of C−S bonds within sulfones, sulfoxides, and aliphatic sulfide derivatives. However, the process of functionalizing the C−S bond in aromatic sulfides remains uncertain. The presented protocol demonstrates the C−S bond functionalization on benzo[b]benzo[4,5]thieno[2,3‐d]thiophene derivative and amines including primary and secondary amines as well. A radical mediated mechanism was proposed for the tandem oxidation–organocatalyzed C−S bond functionalization protocol.

Radical-Mediated Degradation of Thiol-Maleimide Hydrogels.

Michael addition between thiol- and maleimide-functionalized molecules is a long-standing approach used for bioconjugation, hydrogel crosslinking, and the functionalization of other advanced materials. While the simplicity of this chemistry enables facile synthesis of hydrogels, network degradation is also desirable in many instances. Here, the susceptibility of thiol-maleimide bonds to radical-mediated degradation is reported. Irreversible degradation in crosslinked materials is demonstrated using photoinitiated and chemically initiated radicals in hydrogels and linear polymers. The extent of degradation is shown to be dependent on initiator concentration. Using a model linear polymer system, the radical-mediated mechanism of degradation is elucidated, in which the thiosuccinimide crosslink is converted to a succinimide and a new thioether formed with an initiator fragment. Using laser stereolithography, high-fidelity spatiotemporal control over degradation in crosslinked gels is demonstrated. Ultimately, this work establishes a platform for controllable, radical-mediated degradation in thiol-maleimide hydrogels, further expanding their versatility as functional materials.

Phenylalanine-Based Co2+ and Cd2+ 1D Coordination Polymers: Structural Properties and Catalytic Application for Solvent-Free Aerobic Oxidation of Cycloalkene.

Two 1D coordination polymers (CPs) with general formula [M(L)(H2O)(AcO)]n, (M = Co (1) or Cd (2), AcO = acetate anion and L denotes l-phenylalanine based ligand), were synthesized and fully characterized by various spectroscopies (UV-vis, FTIR, and NMR), thermal techniques, magnetic measurements (for 1), and single-crystal and powder X-ray diffraction studies. They can be described as "ribbon-like" 1D polymers constructed through a zigzag arrangement. The polymeric structure is developed due to the coordination mode adopted by the amino acid ligand, classified as μ3-N1O1:O1:O2, which simultaneously links three metal centers. This moiety also plays an important role as a magnetic coupler between metal centers in the cobalt system, which shows a weak antiferromagnetic interaction. Both CPs have also been used in the catalytic oxidation of cyclohexene with molecular oxygen (O2) as an oxidant. Under mild conditions, both compounds demonstrated remarkable catalytic activity, with the cobalt system being more efficient than the cadmium analogue (conversion: 73 and 58% and selectivity for the major product, 2-cyclohexanone: 63 and 55%, for 1 and 2, respectively). Leaching experiments and the results obtained using a radical quencher are consistent with a radical-mediated mechanism for the Co compound. The presence of the superoxide radical was also confirmed using EPR spectroscopy and DMPO as a spin trap, which was further validated by DFT calculations. The activity observed for the Cd analogue is attributed to the organic scaffold assisted by the templating effect of the metal ion.

Pentafluorocyclopropanation of (Hetero)arenes Using Sulfonium Salts: Applications in Late‐Stage Functionalization

AbstractThe evaluation of the pentafluorocyclopropyl group as a chemotype in crop protection and medicinal chemistry has been hampered in the past by the lack of suitable methodologies that enable the practical incorporation of this moiety into advanced synthetic intermediates. Herein, we report the gram‐scale synthesis of an unprecedented sulfonium salt, 5‐(pentafluorocyclopropyl)dibenzothiophenium triflate, and its use as a versatile reagent for the photoinduced C−H pentafluorocyclopropylation of a broad series of non‐previously functionalized (hetero)arenes through a radical mediated mechanism. The scope and potential benefits of the protocol developed are further demonstrated by the late‐stage introduction of the pentafluorocyclopropyl unit into biologically relevant molecules and widely used pharmaceuticals.

Pentafluorocyclopropanation of (Hetero)arenes Using Sulfonium Salts: Applications in Late-Stage Functionalization.

The evaluation of the pentafluorocyclopropyl group as a chemotype in crop protection and medicinal chemistry has been hampered in the past by the lack of suitable methodologies that enable the practical incorporation of this moiety into advanced synthetic intermediates. Herein, we report the gram-scale synthesis of an unprecedented sulfonium salt, 5-(pentafluorocyclopropyl)dibenzothiophenium triflate, and its use as a versatile reagent for the photoinduced C-H pentafluorocyclopropylation of a broad series of non-previously functionalized (hetero)arenes through a radical mediated mechanism. The scope and potential benefits of the protocol developed are further demonstrated by the late-stage introduction of the pentafluorocyclopropyl unit into biologically relevant molecules and widely used pharmaceuticals.

Can bacteria F. nucleatum be actively involved in colon cancer progression via a radical mediated mechanism?

Outer membrane proteins of Fusobacterium nucleatum, a cancer‑leading bacteria, are considered as the factors responsible for its pathogenicity. Among them, homotrimeric autotransporter protein YadA (Yersinia adhesin A) is an important virulence factor also found in the outer membrane of pathogenic Yersinia species. In this paper, the structure and stability of certain Cu(II) complexes with YadA fragments were investigated using both, experimental and theoretical methods. Potentiometry, UV–Vis, CD, EPR, and calculations at the density functional theory (DFT) level were applied to determine the metal ion coordination sphere. Moreover, the complexes ability to DNA cleavage and reactive oxygen species (ROS) production was studied. We have shown that copper(II) complexes can cleave DNA by 1O2, O2•− and •OH, which are formed in the studied systems. However, the results of electrophoretic experiments revealed that complexes cleave DNA less effectively than free copper(II) ions. Therefore, the presence of studied peptides may prevent DNA from a Cu(II)-induced damage to some extent.

Carbon Deposit Analysis in Catalyst Deactivation, Regeneration, and Rejuvenation.

Hydrocarbon conversion catalysts suffer from deactivation by deposition or formation of carbon deposits. Carbon deposit formation is thermodynamically favored above 350 °C, even in some hydrogen-rich environments. We discuss four basic mechanisms: a carbenium-ion based mechanism taking place on acid sites of zeolites or bifunctional catalysts, a metal-induced formation of soft coke (i.e., oligomers of small olefins) on bifunctional catalysts, a radical-mediated mechanism in higher-temperature processes, and fast-growing carbon filament formation. Catalysts deactivate because carbon deposits block pores at different length scales, or directly block active sites. Some deactivated catalysts can be re-used, others can be regenerated or have to be discarded. Catalyst and process design can mitigate the effects of deactivation. New analytical tools allow for the direct observation (in some cases even under in situ or operando conditions) of the 3D-distribution of coke-type species as a function of catalyst structure and lifetime.

Electrochemical Synthesis of Isoxazolines: Method and Mechanism.

An electrochemical method for the green and practical synthesis of a broad range of substituted isoxazoline cores is presented. Both aryl and more challenging alkyl aldoximes are converted to the desired isoxazoline in an electrochemically enabled regio‐ and diastereoselective reaction with electron‐deficient alkenes. Additionally, in‐situ reaction monitoring methods compatible with electrochemistry equipment have been developed in order to probe the reaction pathway. Supporting analyses from kinetic (time‐course) modelling and density functional theory support a stepwise, radical‐mediated mechanism, and discounts hypothesised involvement of closed shell [3+2] cycloaddition pathways.

Self-healing and shape-memory epoxy thermosets based on dynamic diselenide bonds

The fabrication and development of excellent thermoset polymers with self-healing and shape memory properties is highly desirable. Herein, a novel high-strength epoxy resin (EP-SeSe) with diselenide bonds was rationally designed for rapid repair materials in building constructions and electrical packaging. The resultant resins exhibit excellent shape memory performance and self-healing performance due to the dynamic diselenide bonds (SeSe), while maintaining a high mechanical strength (tensile strength = 105 MPa and E = 2.0GPa), and exceptional thermal stability. It is especially noteworthy that the diselenide bonds of the bis(4-aminophenyl) diselenide (BAPDSe) allow EP-SeSe to exchange molecular chains in a dynamic manner, facilitating transport of chain segments while the ortho-phenyl groups prevent the side effects of weak bonds on mechanical properties. Furthermore, when the temperature exceeds Tg, EP-SeSe displays fast stress relaxation, self-healing, remodeling, and weldability due to the [2 + 1] radical-mediated mechanism of the diselenide bonds. We envision that this study provides a facile method for constructing robust multifunctional epoxy resins, which have great potential applications for self-healing and shape-memory epoxy resins.

Mechanistic Study on Oxidative DNA Damage and Modifications by Haloquinoid Carcinogenic Intermediates and Disinfection Byproducts.

Haloquinones (XQs) are a group of carcinogenic intermediates of the haloaromatic environmental pollutants and newly identified chlorination disinfection byproducts (DBPs) in drinking water. The highly reactive hydroxyl radicals/alkoxyl radicals and quinone enoxy/ketoxy radicals were found to arise in XQs and H2O2 or organic hydroperoxides system, independent of transition-metal ions. However, it was not clear whether these haloquinoid carcinogens and hydroperoxides can cause oxidative DNA damage and modifications, and if so, what are the underlying molecular mechanisms. We found that 8-oxodeoxyguanosine (8-oxodG), DNA strand breaks, and three methyl oxidation products could arise when DNA was treated with tetrachloro-1,4-benzoquinone and H2O2 via a metal-independent and intercalation-enhanced oxidation mechanism. Similar effects were observed with other XQs, which are generally more efficient than the typical Fenton system. We further extended our studies from isolated DNA to genomic DNA in living cells. We also found that potent oxidation of DNA to the more mutagenic imidazolone dIz could be induced by XQs and organic hydroperoxides such as t-butylhydroperoxide or the physiologically relevant hydroperoxide 13S-hydroperoxy-9Z,11E-octadecadienoic acid via an unprecedented quinone-enoxy radical-mediated mechanism. These findings should provide new perspectives to explain the potential genotoxicity, mutagenesis, and carcinogenicity for the ubiquitous haloquinoid carcinogenic intermediates and DBPs.

Facile synthesis of N doped ZnO Coral bundles for enhanced photocatalytic and antimicrobial activity

AbstractAn efficient single‐step facile synthesis of coral bundle‐like nanocrystalline N‐doped ZnO was developed using (NH4)2CO3 as a source of nitrogen. The method of zinc nitrate addition and concentration of ammonium carbonate (3.5 mol/L) played a pivotal role in the formation of coral bundle‐like morphology along with nitrogen doping and change in color of the prepared nanomaterial. XPS, Raman, and the peak broadening in XRD confirmed the presence of nitrogen in the crystal lattice. An enhanced absorbance in the UV‐visible DRS spectrum predicts the visible light‐ and UV light‐induced photocatalytic activities for the degradation of organics via a radical mediated mechanism as evidenced by EPR analysis. The results evidence the particle size (9‐12 nm), morphology (coral bundle like), surface roughness (33.87 nm), and surface area (44.028 m2/g) of the prepared coral bundle‐shaped nitrogen‐doped zinc oxide (CBSNZ) photocatalyst. CBSNZ showed a twofold increase in visible light photocatalytic activity when compared to ZnO and excellent antimicrobial properties when compared to the standard streptomycin and ketoconazole. The MIC for CBSNZ was very less when compared to the standards for “Salmonella typhimurium, Pseudomonas aeruginosa and Enterobacter aerogenes.” This provides a wider application of CBSNZ as an environmentally benign nanomaterial.

Potent Oxidation of DNA by Haloquinoid Disinfection Byproducts to the More Mutagenic Imidazolone dIz via an Unprecedented Haloquinone-Enoxy Radical-Mediated Mechanism.

Halogenated quinones are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. We found recently that halogenated quinones could enhance the decomposition of hydroperoxides independent of transition-metal ions and formation of the novel quinone enoxy/ketoxy radicals. Here, we show that the major oxidation product was 2-amino-5-[(2-deoxy-β-d-erythro-pentofuranosyl)amino]-4H-imidazol-4-one (dIz) when the nucleoside 2'-deoxyguanosine (dG) was treated with tetrachloro-1,4-benzoquinone (TCBQ) and t-butyl hydroperoxide (t-BuOOH). The formation of dIz was markedly inhibited by typical radical spin-trapping agents. Interestingly and unexpectedly, we found that the generated quinone enoxy radical played a critical role in dIz formation. Using [15N5]-8-oxodG, dIz was found to be produced either directly from dG or through the transient formation of 8-oxodG. Based on these data, we proposed that the production of dIz might be through an unusual haloquinone-enoxy radical-mediated mechanism. Analogous results were observed in the oxidation of ctDNA by TCBQ/t-BuOOH and when t-BuOOH was substituted by the endogenously generated physiologically relevant hydroperoxide 13S-hydroperoxy-9Z,11E-octadecadienoic acid. This is the first report that halogenated quinoid carcinogens and hydroperoxides can induce potent oxidation of dG to the more mutagenic product dIz via an unprecedented quinone-enoxy radical-mediated mechanism, which may partly explain their potential carcinogenicity.

Production of Atmospheric Organosulfates via Mineral-Mediated Photochemistry

Although organosulfates (ROSO3–) comprise a significant component of secondary organic aerosol (SOA) mass, their atmospheric formation mechanisms are not fully understood. Here, using methacrolein as a model organosulfate precursor, we present a new, mineral-mediated photochemical pathway for organosulfate formation. First, we describe studies of TiO2-catalyzed formation of the atmospherically important organosulfate hydroxyacetone sulfate from methacrolein as a function of illumination time, catalyst loading, sulfate concentration, counterion identity, and methacrolein concentration. Then, we propose a sulfate radical-mediated mechanism for organosulfate formation consistent with these observations. Finally, we show that natural Ti-containing minerals and road dust not only catalyze the formation of comparable amounts of hydroxyacetone sulfate to those formed in the presence of commercial TiO2 but also facilitate the production of additional organosulfate species. These results highlight the complex natu...

84 - The Critical Role of Quinone-Enoxy Radicals in the Production of the More Mutagenic dIz from DNA by Carcinogenic Halogenated Quinones and Hydroperoxides

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. We found recently that XQ could enhance hydroperoxide decomposition and formation of alkoxyl and quinone enoxy/ketoxy radicals metal-independently. However, it is not clear whether XQ/hydroperoxides can oxidize dG to its oxidation products, and if so, what is the underlying molecular mechanism. Here we show that the major oxidation product was dIz when dG was treated with tetrachloro-1,4-benzoquinone (TCBQ) and t-BuOOH, which was markedly inhibited by typical spin-trapping agents. Further ESR and HPLC/MS results showed that the quinone enoxy radicals played a critical role in dIz formation, and O2 was also involved in dIz formation by using oxygen-18-labelled O2. We proposed that the oxidation of dG by TCBQ/t-BuOOH might be through the following mechanism: the quinone enoxy radical may first abstract H from dG, forming dG(-H)• radical, which may combine with O2 to form its corresponding hydroperoxide, resulting in a cascade of decomposition reactions leading to dIz formation. This is the first report that XQ/hydroperoxides can induce potent oxidation of dG to dIz via a quinone-enoxy radical-mediated mechanism, which may partly explain their potential carcinogenicity.

Manganese superoxide dismutase deficiency triggers mitochondrial uncoupling and the Warburg effect.

This corrects the article DOI: 10.1038/onc.2014.355.

Anaerobic Heme Degradation: ChuY Is an Anaerobilin Reductase That Exhibits Kinetic Cooperativity.

Heme catabolism is an important biochemical process that many bacterial pathogens utilize to acquire iron. However, tetrapyrrole catabolites can be reactive and often require further processing for transport out of the cell or conversion to another useful cofactor. In previous work, we presented in vitro evidence of an anaerobic heme degradation pathway in Escherichia coli O157:H7. Consistent with reactions that have been reported for other radical S-adenosyl-l-methionine methyltransferases, ChuW transfers a methyl group to heme by a radical-mediated mechanism and catalyzes the β-scission of the porphyrin macrocycle. This facilitates iron release and the production of a new linear tetrapyrrole termed "anaerobilin". In this work, we describe the structure and function of ChuY, an enzyme expressed downstream from chuW within the same heme utilization operon. ChuY is structurally similar to biliverdin reductase and forms a dimeric complex in solution that reduces anaerobilin to the product we have termed anaerorubin. Steady state analysis of ChuY exhibits kinetic cooperativity that is best explained by a random addition mechanism with a kinetically preferred path for initial reduced nicotinamide adenine dinucleotide phosphate binding.

45 - Molecular Mechanism for the Production of the More Mutagenic DIz from DNA by Halogenated Quinoid Carcinogens and Hydroperoxides: The Critical Role of Quinone-Enoxy Radicals

Halogenated quinones (XQ) are a class of carcinogenic intermediates and newly identified chlorination disinfection byproducts in drinking water. We found recently that XQ could enhance hydroperoxide decomposition and formation of alkoxyl and quinone enoxy/ketoxy radicals metal-independently. However, it is not clear whether XQ/hydroperoxides can oxidize dG to its oxidation products, and if so, what is the underlying molecular mechanism. Here we show that the major oxidation product was dIz when dG was treated with tetrachloro-1,4-benzoquinone (TCBQ) and t-BuOOH, which was markedly inhibited by typical spin-trapping agents. Further ESR and HPLC/MS results showed that the quinone enoxy radicals played a critical role in dIz formation, and O2 was also involved in dIz formation by using oxygen-18-labelled O2. We proposed that the oxidation of dG by TCBQ/t-BuOOH might be through the following mechanism: the quinone enoxy radical may first abstract H from dG, forming dG(-H)● radical, which may combine with O2 to form its corresponding hydroperoxide, resulting in a cascade of decomposition reactions leading to dIz formation. This is the first report that XQ/hydroperoxides can induce potent oxidation of dG to dIz via a quinone-enoxy radical-mediated mechanism, which may partly explain their potential carcinogenicity.

Radical new paradigm for heme degradation in Escherichia coli O157:H7

All of the heme-degrading enzymes that have been characterized to date require molecular oxygen as a cosubstrate. Escherichia coli O157:H7 has been shown to express heme uptake and transport proteins, as well as use heme as an iron source. This enteric pathogen colonizes the anaerobic space of the lower intestine in mammals, yet no mechanism for anaerobic heme degradation has been reported. Herein we provide evidence for an oxygen-independent heme-degradation pathway. Specifically, we demonstrate that ChuW is a radical S-adenosylmethionine methyltransferase that catalyzes a radical-mediated mechanism facilitating iron liberation and the production of the tetrapyrrole product we termed "anaerobilin." We further demonstrate that anaerobilin can be used as a substrate by ChuY, an enzyme that is coexpressed with ChuW in vivo along with the heme uptake machinery. Our findings are discussed in terms of the competitive advantage this system provides for enteric bacteria, particularly those that inhabit an anaerobic niche in the intestines.

Vancomycin-sensitized photooxidation in the presence of the natural pigment vitamin B2: Interaction with excited states and photogenerated ROS

Sensitized photooxidation processes in the presence of natural pigments may provide an alternative to antibiotics degradation since these compounds are transparent to natural light irradiation, therefore, they can be degraded by the action of photosensitizers which absorb light and produce highly reactive species, especially those derived from molecular oxygen (ROS). Most antibiotics used currently belong to a group of pharmaceutical substances that have been considered a new type of contaminants due to their persistence and bioaccumulation in the environment.Objective: In this context, we decided to investigate the kinetic and mechanistic aspects of Vancomycin (Vanco) photosensitized degradation in the presence of the natural pigment Riboflavin (Vitamin B2, Rf) and the artificial dye Rose Bengal (RB) for comparative purposes.Methods: The study have been done by using Stationary photolysis, Laser flash photolysis, Time-resolved phosphorence detection of O2(1Δg) experiments and Bactericidal activity evaluation. The experiments were carried out in aqueous solution at different pH values in order to establish relationships between the structure of the compound and its susceptibility to ROS-mediated photooxidation.Results: Experimental evidence indicates that in the presence of Rf there is considerable contribution of the radical-mediated mechanism, while in the presence of RB the photooxidation process occurs exclusively through O2(1Δg) and the reactivity to this excited species increases with increasing pH of the environment.Discussion: The results obtained, have been shown that Rf can raise the photodegradation of Vanco by both the radical pathway and the O2(1Δg) mediated. Furthermore, the antibiotic is able to interact with the excited electronic states of Rf as well as O2(1Δg) generated by energy transfer between the excited triplet state of the photosensitizer and the oxygen ground state. The predominant mechanism for photodegradation of Vanco in the presence of the Rf is the radical via because of the considerable interaction with the excited triplet state of the photosensitizer demonstrated by laser flash photolysis experiments.Microbiological test on Staphylococcus aureus ATCC25923 showed that the bactericidal activity of the antibiotic on the strain studied was affected by the sensitized photodegradation process, suggesting that photoproducts generated eventually do not retain the bactericidal properties of the original antibiotic.

Comparative analysis of the secretomes of Schizophyllum commune and other wood-decay basidiomycetes during solid-state fermentation reveals its unique lignocellulose-degrading enzyme system.

BackgroundThe genome of Schizophyllum commune encodes a diverse repertoire of degradative enzymes for plant cell wall breakdown. Recent comparative genomics study suggests that this wood decayer likely has a mode of biodegradation distinct from the well-established white-rot/brown-rot models. However, much about the extracellular enzyme system secreted by S. commune during lignocellulose deconstruction remains unknown and the underlying mechanism is poorly understood. In this study, extracellular proteins of S. commune colonizing Jerusalem artichoke stalk were analyzed and compared with those of two white-rot fungi Phanerochaete chrysosporium and Ceriporiopsis subvermispora and a brown-rot fungus Gloeophyllum trabeum.ResultsUnder solid-state fermentation (SSF) conditions, S. commune displayed considerably higher levels of hydrolytic enzyme activities in comparison with those of P. chrysosporium, C. subvermispora and G. trabeum. During biodegradation process, this fungus modified the lignin polymer in a way which was consistent with a hydroxyl radical attack, similar to that of G. trabeum. The crude enzyme cocktail derived from S. commune demonstrated superior performance over a commercial enzyme preparation from Trichoderma longibrachiatum in the hydrolysis of pretreated lignocellulosic biomass at low enzyme loadings. Secretomic analysis revealed that compared with three other fungi, this species produced a higher diversity of carbohydrate-degrading enzymes, especially hemicellulases and pectinases acting on polysaccharide backbones and side chains, and a larger set of enzymes potentially supporting the generation of hydroxyl radicals. In addition, multiple non-hydrolytic proteins implicated in enhancing polysaccharide accessibility were identified in the S. commune secretome, including lytic polysaccharide monooxygenases (LPMOs) and expansin-like proteins.ConclusionsPlant lignocellulose degradation by S. commune involves a hydroxyl radical-mediated mechanism for lignocellulose modification in parallel with the synergistic system of various polysaccharide-degrading enzymes. Furthermore, the complex enzyme system of S. commune holds significant potential for application in biomass saccharification. These discoveries will help unveil the diversity of natural lignocellulose-degrading mechanisms, and advance the design of more efficient enzyme mixtures for the deconstruction of lignocellulosic feedstocks.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0461-x) contains supplementary material, which is available to authorized users.