Metal-independent reduction of hydrogen peroxide by semiquinones

Sources of acellular oxidative potential of water-soluble fine ambient particulate matter in the midwestern United States

Ambient fine particulate matter (PM2.5) is associated with numerous health complications, yet the specific PM2.5 chemical components and their emission sources contributing to these health outcomes are understudied. Our study analyzes the chemical composition of PM2.5 collected from five distinct locations at urban, roadside and rural environments in midwestern region of the United States, and associates them with five acellular oxidative potential (OP) endpoints of water-soluble PM2.5. Redox-active metals (i.e., Cu, Fe, and Mn) and carbonaceous species were correlated with most OP endpoints, suggesting their significant role in OP. We conducted a source apportionment analysis using positive matrix factorization (PMF) and found a strong disparity in the contribution of various emission sources to PM2.5 mass vs. OP. Regional secondary sources and combustion-related aerosols contributed significantly (> 75 % in total) to PM2.5 mass, but showed weaker contribution (43-69 %) to OP. Local sources such as parking emissions, industrial emissions, and agricultural activities, though accounting marginally to PM2.5 mass (< 10 % for each), significantly contributed to various OP endpoints (10-50 %). Our results demonstrate that the sources contributing to PM2.5 mass and health effects are not necessarily same, emphasizing the need for an improved air quality management strategy utilizing more health-relevant PM2.5 indicators.

Reactive Oxygen Species and Chromophoric Dissolved Organic Matter Drive the Aquatic Photochemical Pathways and Photoproducts of 6PPD-quinone under Simulated High-Latitude Conditions

The photochemical degradation pathways of 6PPD-quinone (6PPDQ, 6PPD-Q), a toxic transformation product of the tire antiozonant 6PPD, were determined under simulated sunlight conditions typical of high-latitude surface waters. Direct photochemical degradation resulted in 6PPDQ half-lives ranging from 17.5 h at 20 °C to no observable degradation over 48 h at 4 °C. Sensitization of excited triplet-state pathways using Cs+ and Ar purging demonstrated that 6PPDQ does not decompose significantly from a triplet state relative to a singlet state. However, assessment of processes involving reactive oxygen species (ROS) quenchers and sensitizers indicated that singlet oxygen and hydroxyl radical do significantly contribute to the degradation of 6PPDQ. Investigation of these processes in natural lake waters indicated no difference in attenuation rates for direct photochemical processes at 20 °C. This suggests that direct photochemical degradation will dominate in warm waters, while indirect photochemical pathways will dominate in cold waters, involving ROS mediated by chromophoric dissolved organic matter (CDOM). Overall, the aquatic photodegradation rate of 6PPDQ will be strongly influenced by the compounding effects of environmental factors such as light screening and temperature on both direct and indirect photochemical processes. Transformation products were identified via UHPLC-Orbitrap mass spectrometry, revealing four major processes: (1) oxidation and cleavage of the quinone ring in the presence of ROS, (2) dealkylation, (3) rearrangement, and (4) deamination. These data indicate that 6PPDQ can photodegrade in cool, sunlit waters under the appropriate conditions: t1/2 = 17.4 h tono observable decrease (direct); t1/2 = 5.2-11.2 h (indirect, CDOM).

Antioxidants of Non-Enzymatic Nature: Their Function in Higher Plant Cells and the Ways of Boosting Their Biosynthesis

Plants are exposed to a variety of abiotic and biotic stresses leading to increased formation of reactive oxygen species (ROS) in plant cells. ROS are capable of oxidizing proteins, pigments, lipids, nucleic acids, and other cell molecules, disrupting their functional activity. During the process of evolution, numerous antioxidant systems were formed in plants, including antioxidant enzymes and low molecular weight non-enzymatic antioxidants. Antioxidant systems perform neutralization of ROS and therefore prevent oxidative damage of cell components. In the present review, we focus on the biosynthesis of non-enzymatic antioxidants in higher plants cells such as ascorbic acid (vitamin C), glutathione, flavonoids, isoprenoids, carotenoids, tocopherol (vitamin E), ubiquinone, and plastoquinone. Their functioning and their reactivity with respect to individual ROS will be described. This review is also devoted to the modern genetic engineering methods, which are widely used to change the quantitative and qualitative content of the non-enzymatic antioxidants in cultivated plants. These methods allow various plant lines with given properties to be obtained in a rather short time. The most successful approaches for plant transgenesis and plant genome editing for the enhancement of biosynthesis and the content of these antioxidants are discussed.

H2O2-Mediated Synthesis of a Quinazolin-4(3H)-one Scaffold: A Sustainable Approach

A quinazolin-4(3H)-one ring system is a privileged heterocyclic moiety with distinctive biological properties. From this perspective, the development of an efficient strategy for the synthesis of quinazolin-4(3H)-one has always been in demand for the synthetic chemistry community. In this report, we envisaged an efficient protocol for the synthesis of quinazolin-4(3H)-one using substituted 2-amino benzamide with dimethyl sulfoxide (DMSO) as a carbon source and H2O2 as an effective oxidant. Mechanistically, the reaction proceeds through the radical approach with DMSO as one carbon source. To further substantiate the synthetic claim, the synthetic protocol has been extended to the synthesis of the anti-endotoxic active compound 3-(2-carboxyphenyl)-4-(3H)-quinazolinone.

Naphthoquinones Oxidize H2S to Polysulfides and Thiosulfate, Implications for Therapeutic Applications

1,4-Napththoquinones (NQs) are clinically relevant therapeutics that affect cell function through production of reactive oxygen species (ROS) and formation of adducts with regulatory protein thiols. Reactive sulfur species (RSS) are chemically and biologically similar to ROS and here we examine RSS production by NQ oxidation of hydrogen sulfide (H2S) using RSS-specific fluorophores, liquid chromatography-mass spectrometry, UV-Vis absorption spectrometry, oxygen-sensitive optodes, thiosulfate-specific nanoparticles, HPLC-monobromobimane derivatization, and ion chromatographic assays. We show that NQs, catalytically oxidize H2S to per- and polysulfides (H2Sn, n = 2−6), thiosulfate, sulfite and sulfate in reactions that consume oxygen and are accelerated by superoxide dismutase (SOD) and inhibited by catalase. The approximate efficacy of NQs (in decreasing order) is, 1,4-NQ ≈ juglone ≈ plumbagin > 2-methoxy-1,4-NQ ≈ menadione >> phylloquinone ≈ anthraquinone ≈ menaquinone ≈ lawsone. We propose that the most probable reactions are an initial two-electron oxidation of H2S to S0 and reduction of NQ to NQH2. S0 may react with H2S or elongate H2Sn in variety of reactions. Reoxidation of NQH2 likely involves a semiquinone radical (NQ·−) intermediate via several mechanisms involving oxygen and comproportionation to produce NQ and superoxide. Dismutation of the latter forms hydrogen peroxide which then further oxidizes RSS to sulfoxides. These findings provide the chemical background for novel sulfur-based approaches to naphthoquinone-directed therapies.

Kinetics and mechanisms for sulfamethoxazole transformation in the phenolic acid-laccase (Trametes versicolor) system

Oxidation of phenolic acids (PCs) by laccase could produce various kinds of reactive oxygen species (ROS), which is expected to have substantial impact on the transformation of antibiotics like sulfamethoxazole (SMX) in soil and aquatic environments. In this study, the formation of semiquinones radical (SQ^●-), superoxide anion radical (O₂^●-), hydrogen peroxide (H₂O₂), hydroxyl radical (^●OH), and singlet oxygen (¹O₂) in a laccase-gallic acid (GA) reaction system was confirmed. Meanwhile, GA would be transformed to its monomeric quinone and quinones of di- and tri-polymers. Transformation of SMX by laccase alone is negligible, while which was greatly enhanced in the presence of GA at the optimal pH of 5.5. The dissolved O₂ was the requisite for transformation of SMX due to its fundamental role in the formation of SQ^●-, the key species initializing the chain reactions for the generation of other ROS. The quenching experiments indicated O₂^●- and ¹O₂ were the main ROS responsible for SMX transformation. A total of thirteen products were proposed for the SMX transformation, with the pathways including the breaking of S-N bond, the cleavage of oxazole ring, electrophilic substitution, Michael addition, and condensation reactions. Moreover, the existence of electron-withdrawing substitution group on the benzene ring of PCs and less stability of SQ^●- was believed to be favorable for the transformation of SMX. The results above expand our understanding on the role of oxidation of PCs by laccase in the SMX transformation in environments and are of significance in relation to use of laccase in dealing with SMX pollution.

Reactive oxygen species in the world ocean and their impacts on marine ecosystems

Reactive oxygen species (ROS) are omnipresent in the ocean, originating from both biological (e.g., unbalanced metabolism or stress) and non-biological processes (e.g. photooxidation of colored dissolved organic matter). ROS can directly affect the growth of marine organisms, and can also influence marine biogeochemistry, thus indirectly impacting the availability of nutrients and food sources. Microbial communities and evolution are shaped by marine ROS, and in turn microorganisms influence steady-state ROS concentrations by acting as the predominant sink for marine ROS. Through their interactions with trace metals and organic matter, ROS can enhance microbial growth, but ROS can also attack biological macromolecules, causing extensive modifications with deleterious results. Several biogeochemically important taxa are vulnerable to very low ROS concentrations within the ranges measured in situ, including the globally distributed marine cyanobacterium Prochlorococcus and ammonia-oxidizing archaea of the phylum Thaumarchaeota. Finally, climate change may increase the amount of ROS in the ocean, especially in the most productive surface layers. In this review, we explore the sources of ROS and their roles in the oceans, how the dynamics of ROS might change in the future, and how this change might impact the ecology and chemistry of the future ocean.

Synthesis of Murrayaquinone-A Derivatives and Investigation of Potential Anticancer Properties

A series of novel murrayaquinone a derivatives were synthesized and their anti-cancer activity were evaluated on healthy colon cell lines (CCD-18Co), primary (Caco-2) and metastatic (DLD-1) colon cancer cell lines. The results showed that the cytotoxicity of murrayaquinone molecules is significantly high even in micromolar levels. The DNA binding, cell cycle arrest and metabolic activity studies of these molecules were also carried out and the results showed that these molecules induce apoptosis. In conclusion, the data support further studies on murrayaquinone derivatives toward selection of a candidate for cancer treatment.

Effect of Chloride and Suwannee River Fulvic Acid on Cu Speciation: Implications to Cu Redox Transformations in Simulated Natural Waters

Copper is a critical trace nutrient and, at higher concentrations, a toxicant in natural waters, with the relative rates of transformation between the Cu(I) and Cu(II) oxidation states being key to its speciation, bioavailability, and toxicity. While the influence of chloride (Cl^-) and natural organic matter on Cu speciation and associated redox transformations has been studied separately, their combined influence on Cu speciation and Cu redox transformations has not been examined. As such, in this study, we investigate the impact of Cl^- and Suwannee River fulvic acid (SRFA) on Cu(II) reduction and Cu(I) oxidation kinetics at pH 8.2. SRFA plays a dual role in providing Cu(II) reducing moieties as well as Cu ligating sites. Our results indicate that the SRFA-bound Cu(II) is less reactive than the inorganic Cu(II), and the SRFA-bound Cu(I) being much more rapidly oxidized than the inorganic Cu(I). The presence of Cl^- weakens Cu(II) binding by SRFA, thereby increasing the reactivity of Cu(II). Similarly, weakening of Cu(I) binding by SRFA and concomitant binding of Cu(I) by Cl^- stabilizes Cu(I). Our results further show that continuous formation of hydrogen peroxide occurs in the presence of Cu(II), SRFA, and Cl^- in air-saturated solution with the presence of H₂O₂ enhancing the dynamic nature of the system.

Formation and Evolution of Solvent-Extracted and Nonextractable Environmentally Persistent Free Radicals in Fly Ash of Municipal Solid Waste Incinerators

Environmentally persistent free radicals (EPFRs) are emerging contaminants occurring in combustion-borne particulates and atmospheric particulate matter, but information on their formation and behavior on fly ash from municipal solid waste (MSW) incinerators is scarce. Here, we have found that MSW-associated fly ash samples contain an EPFR concentration of 3-10 × 10¹⁵ spins g^-1, a line width (ΔH_p-p) of ∼8.6 G, and a g-factor of 2.0032-2.0038. These EPFRs are proposed to be mixtures of carbon-centered and oxygen-centered free radicals. Fractionation of the fly ash-associated EPFRs into solvent-extracted and nonextractable radicals suggests that the solvent-extracted part accounts for ∼45-73% of the total amount of EPFRs. Spin densities of solvent-extracted EPFRs correlate positively with the concentrations of Fe, Cu, Mn, Ti, and Zn, whereas similar correlations are comparatively insignificant for nonextractable EPFRs. Under natural conditions, these two types of EPFRs exhibit different stabilization that solvent-extracted EPFRs are relatively unstable, whereas the nonextractable fraction possesses a long life span. Significant correlations between concentrations of solvent-extracted EPFRs and generation of hydroxyl and superoxide radicals are found. Overall, our results suggest that the fractionated solvent-extracted and nonextractable EPFRs may experience different formation and stabilization processes and health effects.

Antioxidant and signaling functions of the plastoquinone pool in higher plants

The review covers data representing the plastoquinone pool as the component integrated in plant antioxidant defense and plant signaling. The main goal of the review is to discuss the evidence describing the plastoquinone-involved biochemical reactions, which are incorporated in maintaining the sustainability of higher plants to stress conditions. In this context, the analysis of the reactions of various redox forms of plastoquinone with oxygen species is presented. The review describes how these reactions can constitute both the antioxidant and signaling functions of the pool. Special attention is paid to the reaction of superoxide anion radicals with plastohydroquinone molecules, producing hydrogen peroxide as signal molecules. Attention is also given to the processes affecting the redox state of the plastoquinone pool because the redox state of the pool is of special importance for antioxidant defense and signaling.

Oxidation of the plastoquinone pool in chloroplast thylakoid membranes by superoxide anion radicals

The plastoquinone (PQ)-pool in chloroplast thylakoid membranes is a key electron carrier in the photosynthetic electron transport chain (PETC), and its redox state plays an essential role in the control of plant metabolism. Oxygen reduction in thylakoid membranes produces superoxide anion radicals ( O 2 · - ), which may react with the PQ-pool. Here, using isolated thylakoids, we show for the first time the oxidation of the PQ-pool by O 2 · - . The xanthine-xanthine oxidase system was used to supply O 2 · - externally to the thylakoid membrane and the redox state of the PQ-pool was monitored by tracking chlorophyll a fluorescence. We propose that, in vivo, the reaction of  O 2 · - produced in Photosystem I with reduced PQ (plastohydroquinone) creates hydrogen peroxide, which serves as a messenger that signals the redox state of the PETC.

Plastoquinol generates and scavenges reactive oxygen species in organic solvent: Potential relevance for thylakoids

The present work reports reactions of plastoquinol (PQH₂-9) and plastoquinone (PQ-9) in organic solvents and summarizes the literature to understand similar reactions in thylakoids. In thylakoids, PQH₂-9 is oxidized by the cytochrome b₆/f complex (Cyt b₆/f) but some PQH₂-9 is also oxidized by reactions in which oxygen acts as an electron acceptor and is converted to reactive oxygen species (ROS). Furthermore, PQH₂-9 reacts with ROS. Light enhances oxygen-dependent oxidation of PQH₂-9. We examined the oxidation of PQH₂-9 via dismutation of PQH₂-9 and PQ-9 and scavenging of the superoxide anion radical (O₂^-) and hydrogen peroxide (H₂O₂) by PQH₂-9. Oxidation of PQH₂-9 via dismutation to semiquinone was slow and independent of pH in organic solvents and in solvent/buffer systems, suggesting that intramembraneous oxidation of PQH₂-9 in darkness mainly proceeds via reactions catalyzed by the plastid terminal oxidase and cytochrome b₅₅₉. In the light, oxidation of PQH₂-9 by singlet oxygen and by O₂^- formed in PSI contribute significantly. In addition, Cyt b_6/f forms H₂O₂ with a PQH₂-9 dependent mechanism. Measurements of the reaction of O₂^- with PQH₂-9 and PQ-9 in acetonitrile showed that O₂^- oxidizes PQH₂-9, forming PQ-9 and several PQ-9-derived products. The rate constant of the reaction between PQH₂-9 and O₂^- was found to be 10⁴ M^-1 s^-1. H₂O₂ was found to oxidize PQH₂-9 to PQ-9, but failed to oxidize all PQH₂-9, suggesting that the oxidation of PQH₂-9 by H₂O₂ proceeds via deprotonation mechanisms producing PQH^--9, PQ^2--9 and the protonated hydrogen peroxide cation, H₃O₂⁺.

Experimental and Theoretical Reduction Potentials of Some Biologically Active ortho-Carbonyl para-Quinones

The rational design of quinones with specific redox properties is an issue of great interest because of their applications in pharmaceutical and material sciences. In this work, the electrochemical behavior of a series of four p-quinones was studied experimentally and theoretically. The first and second one-electron reduction potentials of the quinones were determined using cyclic voltammetry and correlated with those calculated by density functional theory (DFT) using three different functionals, BHandHLYP, M06-2x and PBE0. The differences among the experimental reduction potentials were explained in terms of structural effects on the stabilities of the formed species. DFT calculations accurately reproduced the first one-electron experimental reduction potentials with R² higher than 0.94. The BHandHLYP functional presented the best fit to the experimental values (R² = 0.957), followed by M06-2x (R² = 0.947) and PBE0 (R² = 0.942).

Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids

Innovative valorization of naturally abundant and renewable lignocellulosic biomass is of great importance in the pursuit of a sustainable future and biobased economy. Ionic liquids (ILs) as an important kind of green solvents and functional fluids have attracted significant attention for the catalytic transformation of lignocellulosic feedstocks into a diverse range of products. Taking advantage of some unique properties of ILs with different functions, the catalytic transformation processes can be carried out more efficiently and potentially with lower environmental impacts. Also, a new product portfolio may be derived from catalytic systems with ILs as media. This review focuses on the catalytic chemical conversion of lignocellulose and its primary ingredients (i.e., cellulose, hemicellulose, and lignin) into value-added chemicals and fuel products using ILs as the reaction media. An outlook is provided at the end of this review to highlight the challenges and opportunities associated with this interesting and important area.

H2O2/DMSO-Promoted Regioselective Synthesis of 3,3'-Bisimidazopyridinylmethanes via Intermolecular Oxidative C(sp(2))-H Bond Activation of Imidazoheterocycles

In the past decade, metal-free approaches for C-C bond formation have attracted a great deal of attention due to their ease of use and low cost. This report represents a novel and metal-free synthesis of 3,3'-bisimidazopyridinylmethanes via intermolecular oxidative C(sp(2))-H bond functionalization of imidazo[1,2-a]pyridines with dimethyl sulfoxide as the carbon synthon (CH2) using H2O2 as a mild oxidant under air. A library of 3,3'-bis(2-arylimidazo[1,2-a]pyridin-3-yl)methanes has been achieved in good to excellent yields. The present methodology has been successfully applied to imidazo[2,1-b]thiazoles and imidazo[2,1-b]benzothiazoles. Furthermore, the current approach was also extended for the synthesis of unsymmetrical 3,3'-bisimidazopyridinylmethanes under optimized reaction conditions. A mechanistic pathway is proposed on the basis of experiments with radical scavengers and DMSO-d6 and ESI-MS observations.