Effects of electron transfer mediators on the bioreduction of lepidocrocite (gamma-FeOOH) by Shewanella putrefaciens CN32

Electron transfer mediators (ETMs) such as low-molecular-mass quinones (e.g., juglone and lawsone) and humic substances are believed to play a role in many redox reactions involved in contaminant transformations and the biogeochemical cycling of many redox-active elements (e.g., Fe and Mn) in aquatic and terrestrial environments. This study examines the effects of a series of compounds representing major classes of natural and synthetic organic ETMs, including low-molecular-mass quinones, humic substances, phenazines, phenoxazines, phenothiazines, and indigo derivatives, on the bioreduction of lepidocrocite (gamma-FeOOH) by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32. Although S. putrefaciens CN32 was able to reduce lepidocrocite in the absence of exogenous ETMs, the addition of exogenous ETMs enhanced the bioreduction of lepidocrocite. In general, the rate of Fe(II) production correlated well with the reduction potentials of the ETMs. The addition of humic acids or unfractionated natural organic matter at concentrations of 10 mg organic CL(-1) resulted in, at best, a minimal enhancement of lepidocrocite bioreduction. This observation suggests that electron shuttling by humic substances is not likely to play a major role in Fe(lll) bioreduction in oligotrophic environments such as subsurface sediments with low organic C contents.     

Nuclear magnetic resonance and diffuse-reflectance infrared Fourier transform spectroscopy of biosolids-derived biocolloidal organic matter

We extracted the acid-soluble portion of municipal biosolids, fractionated it by both molecular weight (MW) and hydrophobicity, and used various solid-state nuclear magnetic resonance (NMR) methods and diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy to characterize the fractions. Spectroscopic characterization of the MW components of the biosolids-derived organic matter fractions revealed the presence of functionally distinct groups of compounds. Quantitative 13C NMR, CH spectral editing, and several two-dimensional NMR experiments show that the high-MW hydrophilic fraction in particular is structurally simple, consisting predominantly of N-acetylated polysaccharides, perhaps derived from bacterial peptidoglycans. In the high-MW hydrophobic fraction, aromatic compounds were present in addition to the N-acetylated polysaccharides. Infrared spectroscopy confirmed that hydrophilic fractions were dominated by carbohydrates and indicated that the lower-MW fractions lacked amide moieties. Complementary interpretations of the DRIFT and NMR spectra improved our knowledge of the components separated by this fractionation scheme, allowing better characterization of biosolids organic matter. Moreover, fractionation based on both MW and hydrophobicity may prove useful in detailed characterization of the structure of biosolids-derived organic matter and other similarly heterogeneous natural organic matter in soils and sediments.     

Nature and abundance of organic radicals in natural organic matter: effect of pH and irradiation

Dissolved natural organic matter (NOM) plays an essential role in freshwater geochemical and biochemical processes. A major property, its redox behavior, can be attributed to the chinone building blocks, which can form stable radicals. However, electron paramagnetic resonance (EPR) data indicating free radicals on solid NOM are sparse. Here we present EPR spectra of 23 NOM from European surface waters isolated by reverse osmosis. The organic radical concentrations of NOM ranged from 5 x 10(15) to 1.84 x 10(17) spins g(-1), and g values ranged from 2.0031 to 2.0045. Number and type of organic radicals in solid NOM are significantly influenced by the pH of raw water. EPR experiments indicate the presence of semiquinone-type radicals in coexistence with carbon-centered "aromatic" radicals, with the semiquinone-type radicals dominating at alkaline pH. Basically these processes are reversible. Organic radical concentrations in NOM adjusted to pH 6.5 before freeze-drying correlate with iron and aluminum contents. UV- and VIS-irradiation of solid NOM can lead to more than a 10-fold increase of the concentration of organic radicals. These radicals were long-lived and had the same g value as the original radical. Similar effects were not observed with isolated humic and fulvic acids, demonstrating the limited reflection of environmental properties of organic carbon by the classical isolation procedure.     

3种新型查尔酮衍生物的合成及抗氧化活性

以查尔酮为起始原料,经缩合反应得到双查尔酮类、查尔酮结构的黄酮醇类、查尔酮结构的黄酮类3 种共8 个查尔酮类衍生物化合物。分别用红外光谱、核磁共振波谱法、质谱法和元素分析法对化合物的结构进行确证。比较了查尔酮类衍生物化合物清除超氧阴离子自由基、羟自由基和1,1-二苯基-2-三硝基苯肼自由基的活性及相对还原能力,并将效果较好的化合物应用于油脂中,对其抗氧化性能进行研究。结果表明：当样品质量浓度为0.5 mg/mL时,多数查尔酮类衍生物化合物具有显著的自由基清除活性,其中双查尔酮类化合物2对油脂抗氧化作用较强,优于VC对照组。     

Free radical destruction of N-nitrosodimethylamine in water

Absolute rate constants for the reactions of the hydroxyl radical, hydrated electron, and hydrogen atom with N-nitrosodimethylamine (NDMA) in water at room temperature have been determined using electron pulse radiolysis and transient absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. Specific values of (4.30 +/- 0.12) x 10(8), (1.41 +/- 0.02) x 10(10), and (2.01 +/- 0.03) x 10(8) M(-1) s(-1) were measured, respectively. DMPO spin-trapping experiments demonstrated that the hydroxyl radical reaction with NDMA occurs by hydrogen atom abstraction from a methyl group, and the rate constant for the subsequent reaction of this radical transient with dissolved oxygen was measured as (5.3 +/- 0.6) x 10(6) M(-1) s(-1). This relatively slow rate constant implies that regeneration of the parent nitrosoamine from the oxidized transient could occur in natural waters containing dissolved organic compounds. The reaction of the hydrated electron with NDMA was to form a transient adduct anion, which could subsequently transfer this excess electron to regenerate the parent chemical. Such regeneration reactions would significantly reduce the effectiveness of any applied advanced oxidation technology remediation effort on NDMA-contaminated natural waters.     

Transformation of carbon tetrachloride by thiol reductants in the presence of quinone compounds

Quinones are present in trace amounts in natural organic matter. The addition of thiol compounds to quinones produces reactive electron-transfer species that may be important for the transformation of chlorinated hydrocarbons under sulfate-reducing conditions. This study systematically investigated the transformation of carbon tetrachloride (CCl4) in homogeneous aqueous solutions containing quinones as electron-transfer mediators and thiol compounds as bulk reductants. The thiol compounds, including sodium hydrosulfide (NaHS) and cysteine, were found to effectively transform CCl4. The transformation of CCl4 followed pseudo-first-order kinetics, and the pseudo-first-order rate constants (kobs) were (3.24 +/- 0.46) x 10(-7) and 1.04 x 10(-7) s(-1), respectively, when solutions contained NaHS and cysteine alone. Addition of quinone compounds, including anthraquinone-2,6-disulfonate (AQDS), benzoquinone (BQ), juglone (JQ), naphthoquinone (NQ), lawsone (LQ), and menadione (MQ), increased the transformation rate and efficiency of CCl4. The kobs values for CCl4 transformation in the presence of quinones were 2.6-71 times higher than those for the thiol compounds alone. The enhancement efficiency followed the order JQ > NQ > BQ > AQDS > LQ > MQ. Spectroscopic studies indicated that the quinone compounds generated various active electron-transfer mediators to transfer electrons from the bulk reductants to CCl4. BQ and NQ produced mercaptoquinones as active redox mediators that significantly enhanced the transformation rate of CCl4 in the presence of NaHS. The addition of thiol reductants produced large amounts of AQDS semiquinone radical as the electron shuttle. In addition, MQ and LQ were reduced by NaHS to give hydroquinone, which slightly enhanced the transformation efficiency of CCl4. These results clearly indicate that the enhanced efficiency of quinones for the transformation of chlorinated hydrocarbons is specifically related to the produced reactive species. Mercaptoquinone is a more active mediator than either semiquinone or hydroquinone for transferring electrons in a reducing environment containing thiol reductants.     

PhetoFate: a new approach in accounting for the contribution of indirect photolysis of pesticides and pharmaceuticals in surface waters

A new approach was developed to account for the contribution of indirect photolysis of pesticides and pharmaceuticals in which laboratory test conditions are similar to those prevalent in the aqueous environment. Rates of photolysis as a function of water composition were investigated for several aquatic contaminants. Using the laboratory-based test system, PhotoFate, the dependence of phototransformation rates on concentrations of natural water constituents that are radical producers and scavengers (nitrate, colored dissolved organic matter, bicarbonate) was studied. Mean half-lives of the model compounds in the presence of water constituents were compared to their direct photolysis half-lives to assess the contribution of photosensitized reactions to their fate in surface waters. Reactions mediated by .OH were predominant in waters with high nitrate concentrations. Colored dissolved organic matter (cDOM) acted mainly as a radiation filter and had a more important role in scavenging radicals than in their production. However, in low nitrate waters, the contribution of cDOM-derived reactive intermediates to the degradation of parent compounds became more apparent     

Mechanism of C60 photoreactivity in water: fate of triplet state and radical anion and production of reactive oxygen species

The mechanism involved with (1) energy and electron transfer by C60 in the aqueous phase during UV irradiation and (2) subsequent production of reactive oxygen species (ROS) such as singlet oxygen and superoxide radical anion was investigated. Electron paramagnetic resonance (EPR) study showed that C60 embedded in micelles of nonionic surfactant (Triton X 100) or anionic surfactant (sodium dodecylbenzenesulfonate) produced ROS, but aggregated C60 did not, consistent with our earlier findings made using indicator chemicals. Nanosecond and femtosecond laser flash photolysis showed that the aggregation of C60 significantly accelerates the decay of excited triplet state C60, which is a key intermediate for energy and electron transfer, thus blocking the pathway for ROS production. This finding suggests that C60 clusters will not contribute to oxidative damage or redox reactions in natural environment and biological systems in the same way molecular C60 in organic phase reportedly does. In contrast, C60 embedded in surfactant micelles produces ROS and the evidence is presented for the formation of C60 radical anion as an intermediate.     

Metal complexes and free radical toxins produced by Pfiesteria piscicida

Metal-containing organic toxins produced by Pfiesteria piscicida were characterized, for the first time, by corroborating data obtained from five distinct instrumental methods: nuclear magnetic resonance spectroscopy (NMR), inductively coupled plasma mass spectrometry (ICP-MS), liquid chromatography particle beam glow discharge mass spectrometry (LC/PB-G DMS), electron paramagnetic resonance spectroscopy (EPR), and X-ray absorption spectroscopy (XAS). The high toxicity of the metal-containing toxins is due to metal-mediated free radical production. This mode of activity explains the toxicity of Pfiesteria, as well as previously reported difficulty in observing the molecular target, due to the ephemeral nature of radical species. The toxins are highly labile in purified form, maintaining activity for only 2-5 days before all activity is lost. The multiple toxin congeners in active extracts are also susceptible to decomposition in the presence of white light, pH variations, and prolonged heat. These findings represent the first formal isolation and characterization of a radical forming toxic organic-ligated metal complex isolated from estuarine/marine dinoflagellates. These findings add to an increased understanding regarding the active role of metals interacting with biological systems in the estuarine environment, as well as their links and implications to human health.     

Laccase-mediated michael addition of 15N-sulfapyridine to a model humic constituent

Chemical incorporation of sulfonamide antimicrobials into natural organic matter may represent an important process influencing the fate of these synthetic, primarily agents in soil and sediment environments. We previously demonstrated that a fungal peroxidase mediates covalent coupling of sulfonamide antimicrobials to model humic constituents; reactions with the 2,6-dimethoxyphenol syringic acid produced Schiff bases (Bialk et al. Environ. Sci. TechnoL 2005, 39, 4436-4473). Here, we show that fungal laccase-mediated reaction of sulfapyridine with the orthodihydroxyphenol protocatechuic acid yields a Michael adduct. We synthesized 15N-enriched sulfapyridine to facilitate determination of the covalent linkage(s) formed between sulfapyridine and protocatechuic acid by NMR spectroscopy. 1H-(15)N heteronuclear multiple bond correlation experiments and tandem mass spectrometry demonstrated that the sulfapyridine anilinic nitrogen engaged in a Michael addition reaction to oxidized protocatechuic acid to form an anilinoquinone. Michael adducts are more stable than the previously reported imine linkages between sulfonamides and 2,6-dimethoxyphenols. Michael addition to quinone-like structures in soil organic matter is expected to diminish the mobility and biological activity of sulfonamide antimicrobials.     

Impact of vegetation on sedimentary organic matter composition and polycyclic aromatic hydrocarbon attenuation

Results from natural and engineered phytoremediation systems provide strong evidencethatvegetated soils mitigate polycyclic aromatic hydrocarbon (PAH) contamination. However, the mechanisms by which PAH mitigation occurs and the impact of plant organic matter on PAH attenuation remain unclear. This study assessed the impact of plant organic matter on PAH attenuation in labile and refractory sediments fractions from a petroleum distillate waste pit that has naturally revegetated. Samples were collected in distinct zones of barren and vegetated areas to assess changes to organic matter composition and PAH content as vegetation colonized and became established in the waste pit. Sediments were fractionated into bulk sediment and humin fractions and analyzed for organic matter composition by isotope ratio mass spectrometry (delta (13)C), 13C nuclear magnetic resonance (13C NMR), delta 14C AMS (accelerator mass spectrometry), and percent organic carbon (%TOC). Gas chromatography mass spectrometry (GC/ MS) of lipid extracts of SOM fractions provided data for PAH distribution histograms, compound weathering ratios, and alkylated and nonalkylated PAH concentrations. Inputs of biogenic plant carbon, PAH weathering, and declines in PAH concentrations are most evidentfor vegetated SOM fractions, particularly humin fractions. Sequestered PAH metabolites were also observed in vegetated humin. These results show that plant organic matter does impact PAH attenuation in both labile and refractory fractions of petroleum distillate waste.     

Comparative study of polyphenolic content and antiradical activity of cloudy and clear apple juices

Clear and cloudy apple juices from Idared and Champion varieties were studied for their radical‐scavenging effects. The polyphenolic content and composition of the juices before and after thiolysis were determined by high‐performance liquid chromatography with diode array detection. Cloudy juices, especially that prepared from Champion variety, had a higher content of procyanidins than clear juices. Radical‐scavenging activity was measured by electron paramagnetic resonance (EPR) spectroscopy using the stable 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH) radical. All juices showed long‐lasting radical‐scavenging activity, and EPR spectra were recorded over time to follow the reaction kinetics. Scavenging of DPPH showed pseudo‐first‐order kinetics, which might be expected in the presence of polymerised antioxidants that prevent closer contact between the DPPH radical and hydroxyl groups. The content of polymeric procyanidins showed a linear dependence on the rate constant, suggesting that these compounds are mainly responsible for time‐extended radical‐scavenging activity. The antioxidant properties of apple juices were much better reproduced by EPR spectroscopy than by UV–visible measurements. The former method is especially sensitive to the concentration of polymerised or bound procyanidins, whilst the latter method requires transparent (clear) samples. Apple juices, especially cloudy ones, are a rich source of natural antioxidants that may be used in the pharmaceutical or food industry. Copyright © 2006 Society of Chemical Industry     

Caffeoylquinic acid derived free radicals identified during antioxidant reactions of bitter tea (Ilex latifolia and Ilex kudincha)

In order to provide some insight into the chemical basis for the antioxidant behaviour of bitter tea, the Chinese medicinal beverage derived from leaves of Ilex kudincha or Ilex latifolia, free radicals generated during the oxidation of aqueous extracts of dried leaves have been investigated by electron paramagnetic resonance (EPR) spectroscopy. With both beverages, the major components in the EPR spectra after accelerated autoxidation under alkaline conditions or oxidation with the superoxide anion radical were comparable to those derived from reactions of caffeoylquinic acids. Thus these reaction products have sufficient stability for biological activity, and the present results suggest that such molecules contribute appreciably to the antioxidant chemistry of these beverages.     

Trimethylamine as precursor to secondary organic aerosol formation via nitrate radical reaction in the atmosphere

Amines in fine particulate matter have been detected and quantified during ambient studies of winter inversions in Logan, UT, using aerosol mass spectrometry. Amine-related compounds account for 0.5-6 microg m(-3) of fine particulate mass during some wintertime periods. The amine contributions sometimes show a clear diurnal pattern, reaching peak concentrations during the middle of the nightwhile decreasing during the morning and afternoon. Smog chamber reactions show that the reaction of tertiary amines with nitrate radical can account for this behavior in the atmosphere. The lower bound reaction rate of trimethylamine and nitrate radical is estimated at 4.4 x 10(-16) cm3/molecules/s with a conversion rate to the aerosol phase of approximately 65%. This suggests that amines could be a contributor to secondary organic aerosol formation in areas where nitrate radical is a significant player in oxidation chemistry.     

Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups

The chemical speciation of inorganic mercury (Hg) is to a great extent controlling biologically mediated processes, such as mercury methylation, in soils, sediments, and surface waters. Of utmost importance are complexation reactions with functional groups of natural organic matter (NOM), indirectly determining concentrations of bioavailable, inorganic Hg species. Two previous extended X-ray absorption fine structure (EXAFS) spectroscopic studies have revealed that reduced organic sulfur (S) and oxygen/ nitrogen (O/N) groups are involved in the complexation of Hg(II) to humic substances extracted from organic soils. In this work, covering intact organic soils and extending to much lower concentrations of Hg than before, we show that Hg is complexed by two reduced organic S groups (likely thiols) at a distance of 2.33 A in a linear configuration. Furthermore, a third reduced S (likely an organic sulfide) was indicated to contribute with a weaker second shell attraction at a distance of 2.92-3.08 A. When all high-affinity S sites, corresponding to 20-30% of total reduced organic S, were saturated, a structure involving one carbonyl-O or amino-N at 2.07 A and one carboxyl-O at 2.84 A in the first shell, and two second shell C atoms at an average distance of 3.14 A, gave the best fit to data. Similar results were obtained for humic acid extracted from an organic wetland soil. We conclude that models that are in current use to describe the biogeochemistry of mercury and to calculate thermodynamic processes need to include a two-coordinated complexation of Hg(II) to reduced organic sulfur groups in NOM in soils and waters.     

Sorption and manganese-induced oxidative coupling of hydroxylated aromatic compounds by natural geosorbents

The sorption/desorption equilibria and solvent extractabilities of phenol, o-cresol, and p-chlorophenol with respect to natural sorbents having different types of soil organic matter were investigated. Parallel tests in systems amended with birnessite (delta-MnO2), a solid-phase oxidative coupling catalyst, were also conducted. Sorption/desorption isotherms and solvent extraction data reveal that the relative isotherm linearities, desorption hysteresis, and extractabilities of these compounds are related to the geochemical nature of the sorbent organic matter and to the existence of system conditions that promote oxidative coupling reactions. When suitable coupling catalysts are present, soils containing primarily diagenetically "young" and highly amorphous organic matter (e.g., humic materials) are more likely to retain those solutes than are those containing primarily diagenetically "old" and more condensed organic matter (e.g., kerogens). The sorption/desorption properties of the solutes were significantly altered in the presence of birnessite as a result of both cross-coupling reactions with reactive soil organic matter components and self-coupling reactions with each other to form polymeric species. Under appropriate conditions, mineral-catalyzed oxidative coupling may exert a dominant influence on the sorption and transport of hydroxylated aromatic compounds in soil and sediment systems.     

Photochemical oxidation of As(III) in ferrioxalate solutions

Photochemical reactions involving aqueous Fe(III) complexes are known to generate free radical species such as OH* that are capable of oxidizing numerous inorganic and organic compounds. Recent work has shown that As(III) can be oxidized to As(V) via photochemical reactions in ferric-citrate solutions; however, the mechanisms of As(III) oxidation and the potential importance of photochemical oxidation in natural waters are poorly understood. Consequently, the objectives of this study were to evaluate oxidation rates of As(III) in irradiated ferrioxalate solutions as a function of pH, identify mechanisms of photochemical As(III) oxidation, and evaluate the oxidation of As(III) in a representative natural water containing dissolved organic C (DOC). The oxidation of As(III) was studied in irradiated ferrioxalate solutions as a function of pH (3-7), As(III), Fe(III), and 2-propanol concentration. Rates of As(III) oxidation (0.5-254 microM h(-1)) were first-order in As(III) and Fe(III) concentration and increased with decreasing pH. Experiments conducted at pH 5.0 using 2-propanol as an OH* scavenger in light and dark reactions suggested that OH* is the important free radical responsible for As(III) oxidation. Significant rates of As(III) oxidation (4-6 microM h(-1)) were also observed in a natural water sample containing DOC, indicating that photochemical oxidation of As(III) may contribute to arsenic (As) cycling in natural waters.