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Compound-specific isotope analysis (CSIA) is an established tool to track the in situ transformation of organic chemicals at contaminated sites. In this work, we evaluated the potential of multi-element CSIA to assess biodegradation of 2,3-dichloroaniline (2,3-DCA), which is a major industrial feedstock. Using controlled laboratory experiments, we determined, for the first time, negligible carbon (<0.5‰) and hydrogen (<10‰) isotope fractionation and a significant inverse nitrogen isotope fractionation (>10‰) during aerobic 2,3-DCA biodegradation by a mixed enrichment culture. The tentative identification of a glutamate conjugate of 2,3-DCA as a reaction intermediate indicates that the initial multistep enzymatic reaction may be rate-limiting. The formation of the glutamate adduct would increase the bond energy at the N atom, thus likely explaining the observed inverse N isotope fractionation. The corresponding nitrogen enrichment factor was +6.8 ± 0.6‰. This value was applied to investigate the in situ 2,3-DCA biodegradation at a contaminated site where the carbon and nitrogen isotope signatures from field samples suggested similar aerobic processes by native microorganisms. Under the assumption of the applicability of the Rayleigh model in a pilot wetland treating contaminated groundwater, the extent of biodegradation was estimated to be up to 80-90%. This study proposes multi-element CSIA as a novel application to study 2,3-DCA fate in groundwater and surface water and provides insights into biodegradation pathways.

2,4-Dinitroanisole (DNAN) is a main constituent in various new insensitive munition formulations. Although DNAN is susceptible to biotic and abiotic transformations, in many environmental instances, transformation mechanisms are difficult to resolve, distinguish, or apportion on the basis solely of analysis of concentrations. We used compound-specific isotope analysis (CSIA) to investigate the characteristic isotope fractionations of the biotic (by three microbial consortia and three pure cultures) and abiotic (by 9,10-anthrahydroquinone-2-sulfonic acid [AHQS]) transformations of DNAN. The correlations of isotope enrichment factors (Λ N/C ) for biotic transformations had a range of values from 4.93 ± 0.53 to 12.19 ± 1.23, which is entirely distinct from Λ N/C values reported previously for alkaline hydrolysis, enzymatic hydrolysis, reduction by Fe 2+ -bearing minerals and iron-oxide-bound Fe 2+ , and UV-driven phototransformations. The Λ N/C value associated with the abiotic reduction by AHQS was 38.76 ± 2.23, within the range of previously reported values for DNAN reduction by Fe 2+ -bearing minerals and iron-oxide-bound Fe 2+ , albeit the mean Λ N/C was lower. These results enhance the database of isotope effects accompanying DNAN transformations under environmentally relevant conditions, allowing better evaluation of the extents of biotic and abiotic transformations of DNAN that occur in soils, groundwaters, surface waters, and the marine environment.

Assessing the precursors and reactions leading to the carcinogenic N-nitrosodimethylamine (NDMA) during drinking water disinfection is a major challenge. Here, we investigate whether changes of 13C/12C, ²H/¹H, and 15N/14N ratios of NDMA give rise to isotope fractionation trends that can be used to infer NDMA formation pathways. We carried out compound-specific isotope analysis (CSIA) of NDMA during chloramination of four tertiary amines that produce NDMA at high yields, namely ranitidine, 5-(dimethylaminomethyl)furfuryl alcohol, N,N-dimethylthiophene-2 -methylamine, and N,N-dimethylbenzylamine. Carbon and hydrogen isotope ratios of NDMA function as fingerprints of the N(CH3)2 moiety and exhibit only minor isotope fractionation during the disinfection process. Nitrogen isotope ratios showed that NH[sub 2]Cl is the source of the N atom of the nitroso group. The large enrichment of 15N in NDMA was indicative of the isotope effects pertinent to bond-cleavage and bond-formation reactions during chloramination of the tertiary [ABSTRACT FROM AUTHOR]

15N isotope tracing is an effective and direct approach to investigate sources of nitric oxide (NO) formed in soils. However, NO is highly reactive and rapidly converted to nitrogen dioxide (NO2) in the presence of ozone, making it impossible to directly measure 15N in NO. Various wet-chemical methods for conversion of NO to nitrite (NO2 -) and nitrate (NO3 -) have been proposed for 15N analysis in high-concentration NO sources, such as combustion processes. In contrast, NO concentrations in the soil surface-near air are usually small (ppbv-range), posing major challenges to conversion efficiency and blank correction. Here, we present a modified method in which NO is oxidized quantitatively to NO2 by chromium trioxide (CrO3), before conversion to NO2 - and NO3 - in an alkaline hydrogen peroxide (H2O2) solution. A denitrifier method was used to reduce NO2 - and NO3 - in the trapping solution quantitatively to nitrous oxide (N2O) for subsequent 15N analysis. NO trapping efficiencies of >85% were obtained with 50 ppb NO in a 0.5 L min-1 air stream bubbling through a solution of 1.2 M H2O2 and 0.5 M NaOH. In a laboratory test with distinct 15NO abundances, the overall precision was 0.29‰ (δ-values) for natural abundance NO and 0.13 atom % for labeled NO, suggesting that our method can be used for both natural abundance studies and 15N labeling experiments. In a soil incubation experiment with 15NH4NO3, NH4 15NO3, or Na15NO2 amendments, we found distinct 15N abundances in NO, indicating that our method is well suited to investigate NO sources in soils. [ABSTRACT FROM AUTHOR]

A mechanistic study of the isothiourea-catalyzed enantioselective [2,3]-rearrangement of allylic ammonium ylides is described. Reaction kinetic analyses using 19F NMR and density functional theory computations have elucidated a reaction profile and allowed identification of the catalyst resting state and turnover-rate limiting step. A catalytically relevant catalyst-substrate adduct has been observed, and its constitution elucidated unambiguously by 13C and 15N isotopic labeling. Isotopic entrainment has shown the observed catalyst-substrate adduct to be a genuine intermediate on the productive cycle toward catalysis. The influence of HOBt as an additive upon the reaction, catalyst resting state, and turnover-rate limiting step has been examined. Crossover experiments have probed the reversibility of each of the proposed steps of the catalytic cycle. Computations were also used to elucidate the origins of stereocontrol, with a 1,5-S...O interaction and the catalyst stereodirecting group providing transition structure rigidification and enantioselectivity, while preference for cation-π interactions over C-H...π is responsible for diastereoselectivity. [ABSTRACT FROM AUTHOR]

Insect metamorphosis often results in substantial chemical changes that can alter contaminant concentrations and fractionate isotopes. We exposed larval mayflies (Baetis tricaudatus) and their food (periphyton) to an aqueous zinc gradient (3-340 µg Zn/l) and measured zinc concentrations at different stages of metamorphosis: larval, subimago, and imago. We also measured changes in stable isotopes (δ15N and δ13C) in unexposed mayflies. Larval zinc concentrations were positively related to aqueous zinc, increasing 9-fold across the exposure gradient. Adult zinc concentrations were also positively related to aqueous zinc, but were 7-fold lower than larvae. This relationship varied according to adult substage and sex. Tissue concentrations in female imagoes were not related to exposure concentrations, but the converse was true for all other stage-by-sex combinations. Metamorphosis also increased δ15N by ~0.8‰, but not δ13C. Thus, the main effects of metamorphosis on insect chemistry were large declines in zinc concentrations coupled with increased δ15N signatures. For zinc, this change was largely consistent across the aqueous exposure gradient. However, differences among sexes and stages suggest that caution is warranted when using nitrogen isotopes or metal concentrations measured in one insect stage (e.g., larvae) to assess risk to wildlife that feed on subsequent life stages (e.g., adults). [ABSTRACT FROM AUTHOR]

Efficient 15 N-hyperpolarization of [ 15 N 3 ]metronidazole was reported previously using the Signal Amplification By Reversible Exchange in SHield Enabled Alignment Transfer (SABRE-SHEATH) technique. This hyperpolarized FDA-approved antibiotic is a potential contrast agent because it can be administered in a large dose and because previous studies revealed long-lasting HP states with exponential decay constant T 1 values of up to 10 min. Possible hypoxia-sensing applications have been proposed using hyperpolarized [ 15 N 3 ]metronidazole. In this work, we report on the functionalization of [ 15 N 3 ]metronidazole with a fluorine-19 moiety via a one-step reaction to substitute the -OH group. SABRE-SHEATH hyperpolarization studies of fluoro-[ 15 N 3 ]metronidazole revealed efficient hyperpolarization of all three 15 N sites with maximum % P 15N values ranging from 4.2 to 6.2%, indicating efficient spin-relayed polarization transfer in microtesla fields via the network formed by 2 J 15N-15N . The corresponding 15 N to 19 F spin-relayed polarization transfer was found to be far less efficient with % P 19F of 0.16%, i.e., more than an order of magnitude lower than that of 15 N. Relaxation dynamics studies in microtesla fields support a spin-relayed polarization transfer mechanism because all 15 N and 19 F spins share the same T 1 value of ca. 16-20 s and the same magnetic field profile for the SABRE-SHEATH polarization process. We envision the use of fluoro-[ 15 N 3 ]metronidazole as a potential hypoxia sensor. It is anticipated that under hypoxic conditions, the nitro group of fluoro-[ 15 N 3 ]metronidazole undergoes electronic stepwise reduction to an amino derivative. Ab initio calculations of 15 N and 19 F chemical shifts of fluoro-[ 15 N 3 ]metronidazole and its putative hypoxia-induced metabolites clearly indicate that the chemical shift dispersions of all three 15 N sites and the 19 F site are large enough to enable the envisioned hypoxia-sensing approaches.

Through laboratory and field studies, the utility of amino acid compound-specific nitrogen isotope analysis (AA-CSIA) in avian studies is investigated. Captive American kestrels (Falco sparverius) were fed an isotopically characterized diet and patterns in δ15N values of amino acids (AAs) were compared to those in their tissues (muscle and red blood cells) and food. Based upon nitrogen isotope discrimination between diet and kestrel tissues, AAs could mostly be categorized as source AAs (retaining baseline δ15N values) and trophic AAs (showing 15N enrichment). Trophic discrimination factors based upon the source (phenylalanine, Phe) and trophic (glutamic acid, Glu) AAs were 4.1 (muscle) and 5.4 (red blood cells), lower than those reported for metazoan invertebrates. In a field study involving omnivorous herring gulls (Larus argentatus smithsonianus), egg AA isotopic patterns largely retained those observed in the laying female's tissues (muscle, red blood cells, and liver). Realistic estimates of gull trophic position were obtained using bird Glu and Phe δ15N values combined with β values (difference in Glu and Phe δ 15N in primary producers) for aquatic and terrestrial food webs. Egg fatty acids were used to weight β values for proportions of aquatic and terrestrial food in gull diets. This novel approach can be applied to generalist species that feed across ecosystem boundaries. [ABSTRACT FROM AUTHOR]

Reducing atmospheric ammonia (NH 3 ) emissions is critical to mitigating poor air quality. However, the contributions of major agricultural and non-agricultural source emissions to NH 3 at receptor sites remain uncertain in many regions, hindering the assessment and implementation of effective NH 3 reduction strategies. This study conducted simultaneous measurements of the monthly concentrations and stable nitrogen isotopes of NH x (gaseous NH 3 plus particulate NH 4 + ) at 16 sites across China. Ambient NH x concentrations averaged 21.7 ± 19.6 μg m -3 at rural sites, slightly higher than those at urban (19.2 ± 6.0 μg m -3 ) and three times of those at background (7.0 ± 6.9 μg m -3 ) sites. Based on revised δ 15 N values of the initial NH 3 , source apportionment results indicated that non-agricultural sources (traffic and waste) and agricultural sources (fertilizer and livestock) contributed 54 and 46% to NH 3 at urban sites, 51 and 49% at rural sites, and 61 and 39% at background sites, respectively. Non-agricultural sources contributed more to NH 3 at rural and background sites in cold than warm seasons, arising from traffic and waste, but were similar across seasons at urban sites. We concluded that non-agricultural sources need to be addressed when reducing ambient NH 3 across China, even in rural regions.

The nitrogen isotopic composition (δ15N) of NOx (NO + NO2) was measured during the fourth Fire Lab at Missoula Experiment (FLAME-4). The δ15N-NOx produced by burning a variety of biomass types ranged from â7 to +12‰ (vs air N2). In the laboratory experiments, two types of emissions were sampled: "stack" fires where the emissions were measured within a few seconds of production from the fire and "chamber" fires where the emissions were held in a room for 1â€"2 h and sampled continuously. For both types of emissions sampled, the primary control on δ15N-NOx is the δ15N of the biomass burned (δ15N-biomass), although differences were found for δ15N-NOx between the two types of fires. For the stack emissions, δ15N-NOx = 0.41 Ã-- δ15N-biomass +1.0 (R² = 0.83, p-value <0.001) and for the chamber fires, δ15N-NOx = 0.98 Ã-- δ15N-biomass +1.7 (R² = 0.94, p-value <0.001). While a large range of δ15N-NOx values were observed, the strong relationship between δ15N-NOx and δ15N-biomass suggests that in any given environment, the δ15N-NOx can be predicted. [ABSTRACT FROM AUTHOR]

Although diclofenac ranks among the most frequently detected pharmaceuticals in the urban water cycle, its environmental transformation reactions remain imperfectly understood. Biodegradation-induced changes in 15N/14N ratios (εN = -7.1‰ ± 0.4‰) have indicated that compound-specific isotope analysis (CSIA) may detect diclofenac degradation. This singular observation warrants exploration for further transformation reactions. The present study surveys carbon and nitrogen isotope fractionation in other environmental and engineered transformation reactions of diclofenac. While carbon isotope fractionation was generally small, observed nitrogen isotope fractionation in degradation by MnO2 (εN = -7.3‰ ± 0.3‰), photolysis (εN = +1.9‰ ± 0.1‰), and ozonation (εN = +1.5‰ ± 0.2‰) revealed distinct trends for different oxidative transformation reactions. The small, secondary isotope effect associated with ozonation suggests an attack of O3 in a molecular position distant from the N atom. Model reactants for outer-sphere single electron transfer generated large inverse nitrogen isotope fractionation (εN = +5.7‰ ± 0.3‰), ruling out this mechanism for biodegradation and transformation by MnO2. In a river model, isotope fractionation-derived degradation estimates agreed well with concentration mass balances, providing a proof-of-principle validation for assessing micropollutant degradation in river sediment. Our study highlights the prospect of combining CSIA with transformation product analysis for a better assessment of transformation reactions within the environmental life of diclofenac. [ABSTRACT FROM AUTHOR]

The nitrogen stable isotope composition of NOx (d15N-NOx) may be a useful indicator for NOx source partitioning, which would help constrain NOx source contributions in nitrogen deposition studies. However, there is large uncertainty in the d15N-NOx values for anthropogenic sources other than on-road vehicles and coal-fired energy generating units. To this end, this study presents a broad analysis of d15N-NOx from several fossil-fuel combustion sources that includes: airplanes, gasoline-powered vehicles not equipped with a three-way catalytic converter, lawn equipment, utility vehicles, urban buses, semitrucks, residential gas furnaces, and natural-gas-fired power plants. A relatively large range of d15N-NOx values was measured from -28.1‰ to 8.5‰ for individual exhaust/flue samples that generally tended to be negative due to the kinetic isotope effect associated with thermal NOx production. A negative correlation between NOx concentrations and d15N-NOx for fossil-fuel combustion sources equipped with selective catalytic reducers was observed, suggesting that the catalytic reduction of NOx increases d15N-NOx values relative to the NOx produced through fossil-fuel combustion processes. Combining the d15N-NOx measured in this study with previous published values, a d15N-NOx regional and seasonal isoscape was constructed for the contiguous U.S., which demonstrates seasonal and regional importance of various NOx sources. [ABSTRACT FROM AUTHOR]

The effects of the molecular structureand solution pH on compounds prone to lactam–lactim tautomerismhave been evaluated by 15N NMR spectroscopy. The lactam–lactimtautomerism activities of cP3O6(NH)33–and cP4O8(NH)44–showed a significant pH dependence, with theprocess being inactivated under alkaline conditions because of thedecrease in the number of hydrogen atoms by the deprotonation of theanions. The tautomerism was activated under the acidic conditionsby the increase in the number of dissociative hydrogen atoms resultingfrom the protonation of the anions. cP3O6(NH)33–has much more of a planar molecular structurethan cP4O8(NH)44–, meaning that the hydrogen atoms in cP3O6(NH)33–would be delocalized over the entirestructure to a greater extent than those in cP4O8(NH)44–. This difference in the distributionof hydrogen atoms would result in the lactam–lactim tautomerismactivity of cP3O6(NH)33–being higher than that of cP4O8(NH)44–. The results have shown that the following factorsare critical to the achievement of an efficient anhydrous proton conductor:(1) the regular molecular arrangement of highly planar molecules;(2) the existence of a large number of dissociative protons in a molecule;and (3) a molecular structure with a small energy barrier for thestructural rearrangement required of the tautomerism process. [ABSTRACT FROM AUTHOR]

A statisticaltreatment is presented of reactive collisions involvingN+ions in the 3Pjastate with H2, D2, and HD moleculesin rotation level jof the ground vibration state,leading to either the reaction products H + NH+, D + ND+, and D + NH+or H + ND+. Unlike previoustheoretical work, the fine-structure states of the N+ionsare treated on an equal footing with other internal motions. The calculatedcross sections for a given ion energy are averaged over a thermaldistribution of initial fine-structure states of N+fora temperature of 300 K and over a thermal distribution of both theinternal rotation states and translation energy of H2,D2, and HD for temperatures of 305 and 105 K in order tofacilitate a comparison with experiment. The results are presentedfor a range of collision energies from 10 meV to 1 eV. [ABSTRACT FROM AUTHOR]

In Hawaii, some of the most important commercial and recreational fishes comprise an assemblage of lutjanids and carangids called bottomfish. Despite their importance, we know little about their trophic ecology or where the mercury (Hg) that ultimately resides in their tissue originates. Here we investigated these topics, by analyzing muscle samples for mercury content, nitrogen, carbon, and amino acid specific nitrogen isotope ratios in six species distributed across different depths from the Northwestern Hawaiian Islands (NWHI) and the Main Hawaiian Islands (MHI). Fishes had different sources of nitrogen and carbon, with isotopic values suggesting benthic food sources for shallow nearshore species. High trophic position lutjanids that foraged in deeper water, benthic environments generally had higher Hg levels. Model results also suggested that benthic Hg methylation was an important source of Hg for shallow benthic feeders, while deepwater sources of mercury may be important for those with a diet that derives, at least in part, from the pelagic environment. Further, despite the lack of freshwater sources of Hg in the NWHI, statistical models explaining the variation in tissue Hg in the MHI and NWHI were nearly identical, suggesting freshwater Hg inputs were not a major source of Hg in fish tissue. [ABSTRACT FROM AUTHOR]

s-Triazine herbicides (atrazine, ametryn) are groundwater contaminants which may undergo microbial hydrolysis. Previously, inverse nitrogen isotope effects in atrazine degradation by Arthrobacter aurescens TCI (i) delivered highly characteristic (13C/12C, 15N /14N) fractionation trends for pathway identification and (ii) suggested that the s-triazine ring nitrogen was protonated in the enzyme striazine hydrolase (TrzN) where (iii) TrzN crystal structure and mutagenesis indicated H>+-transfer from the residue E241. This study tested the general validity of these conclusions for atrazine and ametryn with purified TrzN and a TrzN-E241Q site-directed mutant. TrzN-E241Q lacked activity with ametryn; otherwise, degradation consistently showed normal carbon isotope effects (εcarbon = 5.0%c ± 0.2%c (atrazine/ TrzN), εCarbon = -4.2%c ± O.S%o (atrazine/TrzN-E241Q), eaiban = -2A%o ± 0.3%e (ametryn/TrzN)) and inverse nitrogen isotope effects = 2.S%c ± 0A%c (atrazine/TrzN), εnitrogen = 2.1 %c ± 0.3%c (atrazine/TrzN-E241Q), nitrogen = 3.6%c ± 0.4%c (ametryn/TrzN)). Surprisingly, TrzN-E24lQ therefore still activated substrates through protonation implicating another proton donor besides E241. Sulfur isotope effects were larger in enzymatic (εsulfur = - 14.7%c ± 1.0% c , ametryn/TrzN) than in acidic ametryn hydrolysis (εsulfur = - 0.2%c- ± 0.0%o, pH 1.75), indicating rate-determinmg C-S bond cleavage in TrzN. Our results highlight a robust inverse 15N /14N fractionation pattern for identifying microbial s-triazine hydrolysis in the environment caused by multiple protonation options in TrzN. [ABSTRACT FROM AUTHOR]

Absolute quantification of protein targets using liquid chromatography-mass spectrometry (LC-MS) is a key component of candidate biomarker validation. One popular method combines multiple reaction monitoring (MRM) using a triple quadrupole instrument with stable isotope-labeled standards (SIS) for absolute quantification (AQUA). LC-MRM AQUA assays are sensitive and specific, but they are also expensive because of the cost of synthesizing stable isotope peptide standards. While the chemical modification approach using mass differential tags for relative and absolute quantification (mTRAQ) represents a more economical approach when quantifying large numbers of peptides, these reagents are costly and still suffer from lower throughput because only two concentration values per peptide can be obtained in a single LC-MS run. Here, we have developed and applied a set of five novel mass difference reagents, isotopic N, N-dimethyl leucine (iDiLeu). These labels contain an amine reactive group, triazine ester, are cost effective because of their synthetic simplicity, and have increased throughput compared with previous LC-MS quantification methods by allowing construction of a four-point standard curve in one run. iDiLeu-labeled peptides show remarkably similar retention time shifts, slightly lower energy thresholds for higher-energy collisional dissociation (HCD) fragmentation, and high quantification accuracy for trypsin-digested protein samples (median errors <15%). By spiking in an iDiLeu-labeled neuropeptide, allatostatin, into mouse urine matrix, two quantification methods are validated. The first uses one labeled peptide as an internal standard to normalize labeled peptide peak areas across runs (<19% error), whereas the second enables standard curve creation and analyte quantification in one run (<8% error). [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Many aquatic fish-eating birds migrate long distances and are exposed to different mercury concentrations ([Hg]) during their annual cycle. Here we examined the importance of migration on [Hg] in two colonial migratory fish-eating bird species. We determined temporal trends of [Hg] and stable isotopes of carbon (<δ13C) and nitrogen (<δ15N) during the annual cycle in Double-crested Cormorants (Phalacrocorax auritus) and Caspian Terns (Hydroprogne caspia) breeding in Lake Ontario by a repeated sampling of breast feathers and blood from recaptured individuals. We found an effect of previous winter [Hg]species, sex, and location to explain variations of Hg at breeding sites. This suggests Hg carryover from winter to summer periods and that variations of [Hg] in the summer are partially explained by [Hg] in the winter. Carryover of Hg among seasons and slow changes in [Hg] over time were found especially for individuals in high winter exposure groups, suggesting a slow depuration rate and a fast uptake rate for both species. In contrast, stable isotope values rapidly switched to reach equilibrium at a similar midpoint regardless of winter habitat or diet suggesting minimal carryover of isotopic signatures. The potential of Hg carryover from wintering sites indicates that Hg concentrations in birds at a given time may be influenced by previous exposure from distant locations. [ABSTRACT FROM AUTHOR]

Taking advantage of the high level of performance of N-alkoxycarbonyl-imines, we achieved the first example of addition of the aminomethyl radical to imine. The reaction efficiency depended on the structure of the radical precursor, whether it is an iodide or a xanthate, and an electron-withdrawing group on the nitrogen atom of the radical. This reaction allows direct introduction of an N-substituted aminomethyl group onto imine to provide 1,2-diamine as well as the short-step synthesis of ICI-199,441. [ABSTRACT FROM AUTHOR]

Understanding the variability of the natural abundance in nitrogen stable isotopes (expressed as ,δ15N) of primary uptake compartments (PUCs; e.g., epilithon or macro-phytes) is important due to the multiple applications ot stable isotopes in freshwater research and can give insights into environmental and anthropogenic factors controlling N dynamics in streams. While previous research has shown how δ15N of PUCs varies with δ15N of dissolved inorganic N (DIN) among streams, less is known about how δ15N of PUCs varies over time. Here, we examined monthly variation of δ15N of PUCs and of DIN species (nitrate and ammonium) over a year, and compared it among streams with contrasting human impacts and PUC types. Our results showed no evidence of isotopic seasonal patterns. Temporal variability in δ15N-PUCs increased with human impact, being the highest in the urban stream, probably influenced by the high variability of δ15N-DIN. Among compartments, in-stream PUCs characterized by fast turnover rates, such as filamentous algae showed the highest temporal variability in δ15N values (from -3.6 to 23.2 %o). Our study elucidates some of the environmental and biological controls of temporal variability of δ15N in streams, which should be taken into account when using stable isotopes as an ecological tool. [ABSTRACT FROM AUTHOR]

Nutrient over-enrichment is one of the classic triggering mechanisms for the occurrence of cyanobacteria blooms in aquatic ecosystems. In the Baltic Sea, cyanobacteria regularly occur in the late summer months and form nuisance accumulations in surface waters and their abundance has intensified significantly in the past 50 years attributed to human-induced eutrophication. However, the natural occurrence of cyanobacteria during the Holocene is debated. In this study, we present records of cyanobacteria pigments, water column redox proxies, and nitrogen isotopic signatures for the past ca. 8000 years from Baltic Sea sediment cores. Our results demonstrate that cyanobacteria abundance and nitrogen fixation are correlated with hypoxia occurring during three main intervals: (1) ca. 7000-4000 B.P. during the Littorina transgression, (2) ca. 1400-700 B.P. during the Medieval Climate Anomaly, and (3) from ca. 1950 A.D. to the present. Issues of preservation were investigated, and we show that organic matter and pigment profiles are not simply an artifact of preservation. These results suggest that cyanobacteria abundance is sustained during periods of hypoxia, most likely because of enhanced recycling of phosphorus in low oxygen conditions. [ABSTRACT FROM AUTHOR]

Although diclofenac is frequently found in aquatic systems, its degradability in the environment remains imperfectly understood. On the one hand, evidence from concentration analysis alone is inconclusive if an unknown hydrology impedes a distinction between degradation and dilution. On the other hand, not all transformation products may be detectable. As a new approach, we therefore developed GC-IRMS (gas chromatography-isotope-ratio mass-spectrometry) analysis for carbon and nitrogen isotope measurements of diclofenac. The method uses a derivatization step that can be conducted either online or offline, for optimized throughput or sensitivity, respectively. In combination with on-column injection, the latter method enables determination of diclofenac isotope ratios down to the sub-μgL-1 range in environmental samples. Degradation in an aerobic sediment-water system showed strong nitrogen isotope fractionation (ϵN = -7.1‰), whereas reductive diclofenac dechlorination was associated with significant carbon isotope fractionation (ϵC = -2.0‰). Hence dual element isotope analysis bears potential not only to detect diclofenac degradation, but even to distinguish both transformation pathways in the environment. In an explorative survey, analysis of commercial diclofenac products showed significant differences in carbon and nitrogen isotope ratios, demonstrating a further potential to track, and potentially even to authenticate, commercial production batches. [ABSTRACT FROM AUTHOR]

Monooxygenation is an important route of nitroaromatic compound (NAC) biodegradation and it is widely found for cometabolic transformations of NACs and other aromatic pollutants. We investigated the C and N isotope fractionation of nitrophenol monooxygenation to complement the characterization of NAC (bio)degradation pathways by compound-specific isotope analysis (CSIA). Because of the large diversity of enzymes catalyzing monooxygenations, we studied the combined C and N isotope fractionation and the corresponding 13C- and 15N-apparent kinetic isotope effects (AKIEs) of four nitrophenol-biodegrad-ing microorganisms (Bacillus spharericus JS905, Pseudomonas sp. 1A, Arthrobacter sp. JS443, Pseudomonas putida B2) in the pH range 6.1-8.6 with resting cells and crude cell extracts. While the extent of C and N isotope fractionation and the AKIE-values varied considerably for the different organisms, the correlated C and N isotope signatures (δ15N vs δ13C) revealed trends, indicative of two distinct monooxygenation pathways involving hydroxy-1,4-benzoquinone or 1,2- and 1,4-benzoquinone intermediates, respectively. The distinction was possible based on larger secondary 15N-AKIEs associated with the benzoquinone pathway. Isotope fractionation was neither masked substantially by nitrophenol speciation nor transport across cell membranes. Only when 4-nitrophenol was biodegraded by Pseudomonas sp. 1A did isotope fractionation become negligible, presumably due to rate-limiting substrate binding steps pertinent to the catalytic cycle of flavin-dependent monooxygenases. [ABSTRACT FROM AUTHOR]

Concentrations of several persistent organic pollutants (POPs: PCBs, PBDEs, OCPs) in aquatic species from the Scheldt estuary were related with factors (body size, lipids, trophic position) possibly influencing their bioaccumulation. Stable nitrogen isotope ratios (δ15N) were used as a measure for trophic position. A decreasing trend in POP levels toward the sea was observed. For POP concentrations in sediments, this trend could be attributed to a dilution effect from mixing with seawater. However, concentrations in biota more downstream were higher than expected after taking into account the dilution effect, possibly due to differences in bioavailability. Tissue concentrations were correlated with the lipid content in biota, but not with body size. Biomagnification was only significant for some PCB congeners and p,p′-DDE at the most marine sampling location (Terneuzen, L1) and for p,p′-DDD and BDE 100 at the second sampling location (Bath, L2). A significant decreasing relationship was found for γ-HCH concentrations with increasing δ15N at Terneuzen. For Antwerpen (L3), no significant relationships were detected. TMFs ranged from 0.64 for γ-HCH up to 1.60 for PCB 194. These results suggest that biomagnification was more important in the marine part of the estuary, although the presence of multiple carbon sources at the freshwater side might have led to an underestimation of the influence of trophic position. [ABSTRACT FROM AUTHOR]

High variability in the natural abundance of nitrogen stable isotopes (δ15N) has been reported for primary uptake compartments (PUCs; e.g., epilithon, filamentous algae, bryophytes, macrophytes) in human-impacted aquatic ecosystems, but the origin of this variability is not yet well understood. We examined how δ15N of different PUC types relate to δ15N of dissolved inorganic nitrogen (DIN) species (nitrate and ammonium) and to the stream nutrient concentrations in which they grow. We selected 25 reaches located across the fluvial network of La Tordera catchment (NE Spain, 868.5 km2), encompassing a gradient of human pressures from headwaters to the river valley. δ15N-PUC variability was mostly explained by location within the fluvial network and was strongly related to the δ15N of DIN species, especially of ammonium. Models were stronger for PUCs growing within the stream channel and thus using streamwater as their main source of nutrients. Regression models including nutrient concentrations improved the prediction power for δ15N-PUCs, suggesting that nutrient concentrations and stoichiometry cannot be ignored in explaining the natural abundance of nitrogen isotopes in PUCs. These results provide insights into what controls variability in δ15N of PUCs within a stream network, with implications for the application of stables isotopes as an ecological tool. [ABSTRACT FROM AUTHOR]

The catalytic activity of Lewis acid activated oxazaborolidines in the Diels-Alder reaction between cyclopentadiene and methacrolein is investigated by using the DFT method. Oxazaborolidine is not able to coordinate to methacrolein in the absence of AlBr3 because the bonding stabilization is too small to cover the destabilization arising from the deformation of the two species. Accordingly, oxazaborolidine hardly catalyzes the cycloaddition by itself. The calculations show that the attachment of AlBr3 to the nitrogen atom of oxazaborolidine enhances the Lewis acidity of its boron center and enables it to coordinate to methacrolein. When the AlBr3-assisted oxazaborolidine is once coordinated, the catalytic activity originates mainly from the oxazaborolidine framework, and to a smaller extent from the attached AlBr3 part. The Lewis acid AlBr3 plays an additional role to facilitate the reaction by reducing the overlap repulsion between the diene and the dienophile. The attachment of AlBr3 to the oxygen atom, another Lewis basic site in oxazaborolidine, also gives a stable AlBr3-oxazaborolidine complex, but the reaction catalyzed by this complex is not preferred to that catalyzed by the complex in which AlBr3 is attached to the nitrogen atom. The electrophilicity of boron center in oxazaborolidine and those in the AlBr3-oxazaborolidine complexes are compared in terms of localized reactive orbitals. [ABSTRACT FROM AUTHOR]

This Article describes the simultaneous imaging of chiral nitroxyl radicals using electron paramagnetic resonance (EPR). Chiral nitroxyl radicals could be simultaneously visualized with the labeling of isotopic nitrogen. Chiral nitroxyl radicals, hydroxyhnethyl-2,2,5,5-tetramethylpyrrolidine-i-oxyl, were visualized using the method of simultaneous EPR imaging which refers to the visualization of two kinds of molecules with unpaired electrons in a single image scan. EPR spectra of a racemic mixture of chiral nitroxyl radicals and those of the respective R and S configurations confirmed labeling by isotopic nitrogen. 1H nuclear magnetic resonance (NMR) imaging and simultaneous imaging of solutions of chiral nitroxyl radicals were performed. The advantages and limitations of simultaneous imaging using EPR are also discussed. Simultaneous imaging with chiral-labeled nitroxyl radicals is a new application of EPR imaging and may be useful for biological studies involving biologically active chiral molecules. [ABSTRACT FROM AUTHOR]

During and after the Deepwater Horizon Oil Spill (DWHOS), oysters (Crassostrea virginica) were exposed to oil and susceptible to incidental consumption of surface and subsurface oil materials. We determined the contribution of oil materials from the DWHOS to diet of oysters by comparing carbon (C) and nitrogen (N) stable isotope ratios in oyster shell to ratios in suspended particulate matter (SPM) and in fresh and weathered oil. Average δ13C and δ15N values in oyster shell (−21 ± 1‰ and 9–11‰, respectively) were consistent with consumption of naturally available SPM as opposed to values in oil (−27 ± 0.2‰, 1.6 ± 0.4‰). Stable isotope ratios in oyster adductor muscle were similar to shell for δ15N but not δ13C, suggesting either a recent shift in diet composition or differential assimilation of C between tissue types. We found no evidence of assimilation of oil-derived C and N and, therefore, no evidence of an oyster-based conduit to higher trophic levels. Trace elements in shell were inconclusive to corroborate oil exposure. These findings are not an indication that oysters were not exposed to oil; rather they imply oysters either did not consume oil-derived materials or consumed too little to be detectable compared to natural diet. [ABSTRACT FROM AUTHOR]

Dioxygenation of aromatic rings is frequently the initial step of biodegradation of organic subsurface pollutants. This process can be tracked by compound-specific isotope analysis to assess the extent of contaminant transformation, but the corresponding isotope effects, especially for dioxygenation of N-substituted, aromatic contaminants, are not well understood. We investigated the C and N isotope fractionation associated with the biodegradation of aniline and diphenylamine using pure cultures of Burkholderia sp. strain JS667, which can biodegrade both compounds, each by a distinct dioxygenase enzyme. For diphenylamine, the C and N isotope enrichment was normal with εC- and εN-values of −0.6 ± 0.1‰ and −1.0 ± 0.1‰, respectively. In contrast, N isotopes of aniline were subject to substantial inverse fractionation (εN of +13 ± 0.5‰), whereas the εC-value was identical to that of diphenylamine. A comparison of the apparent kinetic isotope effects for aniline and diphenylamine dioxygenation with those from abiotic oxidation by manganese oxide (MnO2) suggest that the oxidation of a diarylamine system leads to distinct C–N bonding changes compared to aniline regardless of reaction mechanism and oxidant involved. Combined evaluation of the C and N isotope signatures of the contaminants reveals characteristic Δδ15N/Δδ13C-trends for the identification of diphenylamine and aniline oxidation in contaminated subsurfaces and for the distinction of aniline oxidation from its formation by microbial and/or abiotic reduction of nitrobenzene. [ABSTRACT FROM AUTHOR]

We investigated the mechanisms and isotope effects associated with the N-dealkylation and N-atom oxidation of substituted N-methyl- and N,N-dimethylanilines to identify isotope fractionation trends for the assessment of oxidations of aromatic N-alkyl moieties by compound-specific isotope analysis (CSIA). In laboratory batch model systems, we determined the C, H, and N isotope enrichment factors for the oxidation by MnO2 and horseradish peroxidase (HRP), derived apparent 13C-, 2H-, and 15N-kinetic isotope effects (AKIEs), and characterized reaction products. The N-atom oxidation pathway leading to radical coupling products typically exhibited inverse 15N-AKIEs (up to 0.991) and only minor 13C- and 2H-AKIEs. Oxidative N-dealkylation, in contrast, was subject to large normal 13C- and 2H-AKIEs (up to 1.019 and 3.1, respectively) and small 15N-AKIEs. Subtle changes of the compound's electronic properties due to different types of aromatic and/or N-alkyl substituents resulted in changes of reaction mechanisms, rate-limiting step(s), and thus isotope fractionation trends. The complex sequence of electron and proton transfers during the oxidative transformation of substituted aromatic N-alkyl amines suggests highly compound- and mechanism-dependent isotope effects precluding extrapolations to other organic micropollutants reacting along the same degradation pathways. [ABSTRACT FROM AUTHOR]

Mechanistic aspects involved in the formation of N2andof N2O during the reduction of gas-phase NO and of NOxstored over a model PtBa/Al2O3NSR catalyst are investigated using unlabeled ammonia andlabeled NO. The reduction of the stored NOxspecies (labeled nitrites and nitrates) with NH3leadsto the selective formation of N2as a major product andof small amounts of nitrous oxide. Based on the nitrogen isotopicdistribution, it appears that N2formation occurs primarilythrough the statistical coupling of N-atoms formed by dissociationof NOx- and NH3-related surfaceintermediates, although an SCR-pathway (involving the coupling ofNH3- and NO-derived ad-species) is also likely to occur.It appears as well that the formation of nitrous oxide involves eitherthe coupling of two adsorbed NO molecules or the recombination ofan adsorbed NO molecule with an adsorbed NHxfragment. [ABSTRACT FROM AUTHOR]

The results of isotope ratio mass spectrometry (IRMS) on hexamethylene triperoxide diamine (HMTD) and its precursor hexamethylenetetramine (hexamine) is presented. HMTD was prepared from hexamine using several different sources of hexamine under both controlled laboratoiy conditions and in field experiments that represent the less controlled conditions that are likely to be observed in forensic casework scenarios. Precursor and product carbon isotope δ values consistently fit a linear relationship regardless of precursor or conditions. The magnitude of the isotope fractionation observed is affected by the ethdency of the reaction: with greater yielding reactions giving rise to HMTD with δ values more similar to the precursor material than lower yielding reactions. Nitrogen isotope δ values comparing precursor with product show some linearity when the reaction conditions are carefully controlled; however, results indicate a poor fit with linearity when synthesis conditions are more variable. Despite the greater variation, the HMTD product consistently has a more positive δ value compared with the hexamine precursor. The results observed from these experiments suggest hexamine reacts to form HIvITD in a 1:1 ratio. Having prepared multiple HMTD sampies from various precursors using a range of experimental conditions, we have observed results that may be useful in forensic investigations of improvised explosive materials. [ABSTRACT FROM AUTHOR]

In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate (15N/14N and 18O/16O) in the environment and (ii) investigate the potential for isotopes to elucidate the mechanism of a key N cycle enzyme, we measured the nitrate N and O isotope effects (15ϵ and 18ϵ) for nitrate reduction by two assimilatory eukaryotic nitrate reductase (eukNR) enzymes. The 15ϵ for purified extracts of NADPH eukNR from the fungus Aspergillus niger and the 15ϵ for NADH eukNR from cell homogenates of the marine diatom Thalassiosira weissflogii were indistinguisha±ble, yielding a mean 15ϵ for the enzyme of 26.6 ± 0.2%. Both forms of eukNR imparted near equivalent fractionation on N and O isotopes. The increase in 18O/16O versus the increase in 15N/14N (relative to their natural abundances) was 0.96 ± 0.01 for NADPH eukNR and 1.09 ± 0.03 for NADH eukNR. These results are the first reliable measurements of the coupled N and O isotope effects for any form of eukNR. They support the prevailing view that intracellular reduction by eukNR is the dominant step in isotope fractionation during nitrate assimilation and that it drives the 18ϵ:15ϵ ≈ 1 observed in phytoplankton cultures, suggesting that this O-to-N isotope signature will apply broadly in the environment. Our measured 15ϵ and 18ϵ may represent the intrinsic isotope effects for eukNR-mediated N-O bond rupture, a potential constraint on the nature of the enzyme's transition state. [ABSTRACT FROM AUTHOR]

Despite the potential use of δ15N as a tracer of NOx source contributions, prior documentation of δ15N of various NOx emission sources is exceedingly limited. This manuscript presents the first measurements of the nitrogen isotopic composition of NOx (δ15N-NOx) emitted from coal-fired power plants in the U.S. at typical operating conditions with and without the presence of selective catalytic reduction (SCR) and selective noncatalytic reduction (SNCR) technology. To accomplish this, a novel method for collection and isotopic analysis of coal-fired stack NOx emission samples was developed based on modifications of a historic U.S. EPA stack sampling method. At the power plants included in this study, large differences exist in the isotopic composition of NOx emitted with and without SCRs and SNCRs; further the isotopic composition of power plant NOx is higher than that of other measured NOx emission sources confirming its use as an environmental tracer. These findings indicate that gradual implementation of SCRs at power plants will result in an industry-wide increase in δ15N values of NOx and NOy oxidation products from this emission source. [ABSTRACT FROM AUTHOR]

Polyhalogenated 1′-methyl-1,2′-bipyrroles are natural products that biomagnify into upper trophic levels of marine food webs. Here we demonstrate that they are unusually enriched in 15N (δ15N from +19.3‰ to +28.1‰) relative to other biosynthetic organic compounds measured to date and the mammals from which the compounds were isolated. We argue the 15N enrichment likely stems from enriched precursors and/or fractionation during biosynthesis and is not from MBP degradation. We also consider possible sources of MBPs in light of these results. [ABSTRACT FROM AUTHOR]

With the advent of ultra-long MD simulations it becomes possible to model microsecond time-scale protein dynamics and, in particular, the exchange broadening effects (Rex) as probed by NMR relaxation dispersion measurements. This new approach allows one to identify the exchanging species, including the elusive "excited states". It further helps to map out the exchange network, which is potentially far more complex than the commonly assumed 2- or 3-site schemes. Under fast exchange conditions, this method can be useful for separating the populations of exchanging species from their respective chemical shift differences, thus paving the way for structural analyses. In this study, recent millisecond-long MD trajectory of protein BPTI (Shaw et al. Science 2010, 330, 341) is employed to simulate the time variation of amide 15N chemical shifts. The results are used to predict the exchange broadening of 15N lines and, more generally, the outcome of the relaxation dispersion measurements using Carr-Purcell-Meiboom-Gill sequence. The simulated Rex effect stems from the fast (∼10-100 μs) isomerization of the C14-C38 disulfide bond, in agreement with the prior experimental findings (Grey et al. J. Am. Chem. Soc. 2003, 125, 14324). [ABSTRACT FROM AUTHOR]

Six novel homoleptic palladium(II) and platinum(II) complexes of donor-substituted alkenol ligands [PyCHC(R)OH; Py = pyridine, R = CH3, CF3, C2F5, C3F7] of the general formula M[PyCHC(R)O]2 (M = Pd, Pt) were synthesized by reacting the deprotonated ligands with PdCl2 and K2PtCl4, respectively. Molecular structures, revealed by single-crystal X-ray diffraction analyses, showed a square-planar arrangement of ligands around palladium and platinum centers, with the pyridine-ring nitrogen atoms situated in a mutually trans position. The monomeric nature of the compounds in the solution state was confirmed by multinuclear (1H, 13C, and 19F) NMR spectroscopy. Thermal decomposition profiles recorded under a nitrogen atmosphere suggested their potential as volatile precursors to palladium and platinum materials. The volatility was increased upon elongation of the perfluoroalkyl chain, which suppressed the intermolecular interactions, as is evident in crystal packings. The volatility of these compounds was attributed to bidentate chelation of the alkenol units and cooperativity among the electron-back-donating nitrogen atom and interplay of electron-withdrawing CxFy groups, resulting in an effective steric shielding of the metal atoms. [ABSTRACT FROM AUTHOR]

Various diamonds were analyzed with photoluminescence (PL) spectra excited with synchrotron radiation in the wavelength range 160-250 nm. The emission of type IaAB diamond begins near 300 nm and extends to 700 nm; two broad lines with maximums about 419 and 469 nm correspond to energies 2.96 and 2.64 eV, respectively. The spectral features observed in the PL excitation spectra show two vibrational progressions, A and B, related to nitrogen defects in diamond. Progression A has a spacing 1266 ± 20 cm-1 and is associated with the N2 (or A) center of a nitrogen defect, whereas progression B has a spacing 1177 ± 20 cm-1 related to the N4 (or B) center of a nitrogen defect. These vibrational progressions in PL excitation spectra of N2 and N4 centers in type IaAB diamond are here identified for the first time. We suggest the use of PL spectra excited in the region 170-240 nm to analyze and to identify the type of diamond. [ABSTRACT FROM AUTHOR]

We explored the N isotope fractionation associated with the oxidation of substituted primary aromatic amines, which are often the position of initial attack in transformation processes of environmental contaminants. Apparent 15N-kinetic isotope effects, AKIEN, were determined for the oxidation of various substituted anilines in suspensions of manganese oxide (MnO2) and compared to reference experiments in homogeneous solutions and at electrode surfaces, as well as to density functional theory calculations of intrinsic KIEN for electron and hydrogen atom transfer reactions. Owing to the partial aromatic imine formation after one-electron oxidation and corresponding increase in C-N bond strength, AKIEN-values were inverse, substituent-dependent, and confined to the range between 0.992 and 0.999 in agreement with theory. However, AKIEN-values became normal once the fraction of cationic species prevailed owing to 15N-equilibrium isotope effects, EIEN, of 1.02 associated with N atom deprotonation. The observable AKIEN-values are substantially modulated by the acid/base pre-equilibria of the substituted anilines and isotope fractionation may even vanish under conditions where normal EIEN and inverse AKIEN cancel each other out. The pH-dependent trends of the AKIEN-values provide a new line of evidence for the identification of contaminant degradation processes via oxidation of primary aromatic amino groups. [ABSTRACT FROM AUTHOR]

Global models have indicated agriculturally impacted rivers and streams may be important sources of the greenhouse gas nitrous oxide (N2O). However, there is significant uncertainty in N2O budgets. Isotopic characterization can be used to help constrain N2O budgets. We present the first published measurements of the isotopic character of N2O emitted from low (2-4) order streams. Isotopic character of N2O varied seasonally, among streams, and over diel periods. On an annual basis, δ 18O of emitted N2O (+47.4 to +51.4‰; relative to VSMOW) was higher than previously reported for larger rivers, but δ 15N of emitted N2O (-16.2 to +2.4‰ among streams; relative to atmospheric N2) was similar to that of past studies. On an annual basis, all streams emitted N2O with lower δ 15N than tropospheric N2O. Given these streams have elevated nitrate concentrations which are associated with enhanced N2O fluxes, this supports the hypothesis that streams are contributing to the accumulation of 15N-depleted N2O in the troposphere. [ABSTRACT FROM AUTHOR]

Herein, we explore a new strategy in the chemo-sensor field for fluorescence amplification upon binding with metal ions based on controlled participation of the nitrogen lone pair orbitat. The basic architecture of the sensor entails a fluorophore, the sp2 hybridized nitrogen lone pair (-C=N-), and a chelator site referred to as the control part. Though nonplanar and nonfluorescent, compound IC1 achieved pseudo planatity from binding with Zn2+ as indicated by the increased fluorescence signal. Its other analogue (IC2) is also planar, and unlike IC1-Zn2+ was fluorescent with a lack of binding affinity to metal ions. The time-dependent density functional theory (TDDFT) calculations revealed that the fluorescence amplification was due to the blocking of the nitrogen lone pair orbital; unlikely geometrical rearrangements were insignificant. This could indicate a breakthrough concept in the future design of fluorescent turn-on sensors. [ABSTRACT FROM AUTHOR]

Activation of N2 by (silox)3Ta (1, silox = tBU3SiO) to afford (silox)3Ta=N-N=Ta(silox)3 (12-N2) does not occur despite ΔG°cald = -55.6 kcal/mol because of constraints ot orbital symmetry, prompting efforts at an independent synthesis that included a study of REH2 activation (E = N, P, As). Oxidative addition of REH2 to 1 afforded (silox)3HTaEHR (2-NHR, A = H, Me, nBu, C6H4-p-X (X = H, Me, NMe2); 2-PHR, A = H, Ph; 2-AsHR, R = H, Ph), which underwent 1,2-H2-elimination to form (silox)3Ta=NR (1=NF; R = H, Me, nBu, C6H4-p-X (X = H (X-ray), Me, NMe2, CF3)), (silox)3Ta=PR (1=PR; R = H, Ph), and (silox)3Ta=AsR (1=AsR; R = H, Ph). Kinetics revealed NH bond-breaking as critical, and As > N > P rates tor (silox)3HTaEHPh (2-EHPh) were attributed to (1) ΔG°calc(N) < ΔG°calc(As); (2) similar tractional reaction coordinates (ACs), but with AC shorter for N < P∼As; and (3) stronger TaE bonds for N> P∼As. Calculations of the pnictidenes aided interpretation of UV-vis spectra. Addition of H2NNH2 or H2N-N(CNC2H3Me) to 1 afforded 1=NH, obviating these routes to 12-N2, and formation of (silox)3MeTaNHNH2 (4-NHNH2) and (silox)3MeTaNH(-cNCHMeCH2) (4-NH(azir)) occurred upon exposure to (silox)3Ta=CH2 (1 =CH2). Thermolyses of 4-NHNH2 and 4-NH(azir) yielded [(silox)2TaMe](μ-NαHNβ)(μ-NγHNσH)-[Ta(silox)2] (5) and [(silox)3MeTa](μ-η²-N,N:η¹-C-NHNHCH2CH2CH2)[Ta(κ-O,C-OSitBu2CMe2CH2)(silox)2] (7, X-ray), respectively. (silox)3Ta=CPPh3 (1=CPPh3, X-ray) was a byproduct from Ph3PCH2 treatment of ito give 1=CH2. Addition of Na(silox) to [(THF)2Cl3Ta]2(μ-N2) fed to [(sitox)2ClTa](μ-N2) (B-Cl), and via subsequent methyfation, [(silox)2MeTa]2(μ-N2) (8-Me); both dimers were thermally stable. Orbital symmetry requirements for N2 capture by I and pertinent calculations are given. [ABSTRACT FROM AUTHOR]

Mechanistic studies for the palladium-catalyzed decarboxylative cyclization reactions of γ-methylidene-δ-valerolactones 1 with isocyanates 2 are described. The reactions can be effectively catalyzed by palladium triarylphosphine complexes to give piperidones 3 and/or azaspiro[2.4]heptanones 4. Through kinetic studies using NMR spectroscopy, it has been determined that the oxidative addition of lactones 1 to palladium(O) is the turnover-limiting step of the catalytic cycle. By changes in the electronic properties of the triarylphosphine ligands, the product distribution between 3 and 4 can be easily controlled, and an explanation for the origin of this selectivity is provided. The selectivity between 3 and 4 is also influenced by the nature of the nitrogen substituent on isocyanates 2, and more electron-rich substituents tend to give higher selectivity toward azaspiro[2.4]heptanones 4. These studies represent the first systematic investigation into the selectivity between terminal attack and central attack at (π-allyl)palladium species by nitrogenbased nucleophiles. [ABSTRACT FROM AUTHOR]

Stable isotope ratios of carbon (∂13C) and nitrogen (∂15N) and total mercury (T-Hg) concentrations were measured in red meat samples from 11 odontocete species (toothed whales, dolphins, and porpoises) sold in Japan (n = 96) and in muscle samples from stranded killer whales (n = 6) and melon-headed whales (n = 15), and the analytical data for these species were classified into three regions (northern, central, and southern Japan) depending on the locations in which they were caught or stranded. The ∂15N in the samples from southern Japan tended to be lower than that in samples from the north, whereas both ∂13C and T-Hg concentrations in samples from the south tended to higher than those in samples from northern Japan. Negative correlations were found between the ∂13C and ∂15N values and between the ∂15N value and T-Hg concentrations in the combined samples all three regions (γ= -0.238, n = 117, P < 0.01). The ∂13C, ∂15N, and T-Hg concentrations in the samples varied more by habitat than by species. Spatial variations in ∂13C, ∂15N, and T-Hg concentrations in the ocean may be the cause of these phenomena. [ABSTRACT FROM AUTHOR]

The Changjiang River is the largest freshwater river in China. Here, the sources and variability in nitrate of the Changjiang River are assessed for the first time using dual isotopic approach. Water samples were collected once in August 2006 from the main channel of the Changjiang and its major tributaries. The concentrations and isotopic composition of nitrate were then analyzed for the waters in the Changjiang River. The δ15N and δ18O of NO3- ranges from 7.3‰ to 12.9‰ and 2.4‰ to 11.2‰ in the Changjiang River waters, respectively. The ranges of isotopic compositions of nitrate suggested that nitrification (including "modified fertilizer") and urban sewage effluent are the major sources of nitrate in the Changjiang River. The high δ18O-NO3- values were observed in the water of the upper reaches, indicated that the current drought might be one important reason for shifting of isotopes in the special sampling period. In addition, there was a strong positive relationship between δ15N-NO3- and δ18O-NO3-, which indicated that denitrification added to the enrichment of heavy isotopes of nitrate. [ABSTRACT FROM AUTHOR]

A comparative CASSCF/6-31 G*-leveI computational study of the concerted [3,3] sigmatropic rearrangements of cis-1 -iminyl-2-ketenylcyclopropane (15), cis-1 -iminyl-2-propadienylcyclopropane (17), and cis-1-iminyl-2-keteniminylcyclopropane (19)to give products 16, 18, and 20, respectively, was conducted. Analysis of the active space MOs of TS15→16, TS17→18, and TS19→20 suggests that the 17→ 18 and 19→ 20 rearrangements are classically pericyclic, whereas the 15 16 rearrangement is pseudopericyclic with two orbital disconnections-one involving the nitrogen lone-pair orbital and the other the carbonyl carbon of the ketene moiety. The novel TS1→16 was also found to have a highly planar, tight, geometry, whereas TS17→18 and TS19→20 were both shown to have the boat-shaped geometry expected for classically pericyclic [3,3] sigmatropic rearrangements. Results of calculations on the [3,3] sigmatropic rearrangements involving additional transition structures, TS21→22, TS23→24, TS25→26, TS27→28, TS29→30, and TS31→32, demonstrate the relative uniqueness of the pseudopericyclic one, TS15→16. [ABSTRACT FROM AUTHOR]

Direct or indirect anthropogenic effects on ecosystem nitrogen cycles are important components of global change. Recent research has shown that N isotopes in tree rings reflect changes in ecosystem nitrogen sources or cycles and can be used to study past changes. We analyzed trends in two tree species from a remote and pristine tropical rainforest in Brazil, using trees of different ages to distinguish between the effect of tree age and long-term trends. Because sapwood differed from heartwood in δ15N and N concentration and N can be translocated between living sapwood cells, long-term trends are best seen in dead heartwood. Heartwood δ15N in Spanish cedar (Cedrela odorata) and big-leaf mahogany (Swietenia macrophylla) increased with tree age, and N concentrations increased with age in Cedrela. Controlling for tree age, δ15N increased significantly during the past century even when analyzing only heartwood and after removing labile N compounds. In contrast to northern temperate and boreal forests where wood δ15N often decreased, the δ15N increase in a remote rainforest is unlikely to be a direct signal of changed N deposition. More plausibly, the change in N isotopic composition indicates a more open N cycle, i.e., higher N losses relative to internal N cycling in the forest, which could be the result of changed forest dynamics. [ABSTRACT FROM AUTHOR]

Stable nitrogen isotope (δ15N) analysis has proven an effective "fingerprint" of sewage contamination in coral reef environments; however, short-term variability in nitrogen cycling and isotopic fractionation may obscure long-term trends. Here, we examine δ15N signatures in the organic endoskeletons of long-lived (20-40 years) gorgonian corals. Specimens were collected from relatively pristine reefs off Green Turtle Cay, Bahamas, and from reefs off southeast Florida heavily impacted by multiple sources of anthropogenic nitrogen. The δ15N of the most recently grown skeleton (branch tips) ranged from +2 to +3 ‰ at Green Turtle Cay, and +4.5 to +10 ‰ off Florida. These values closely match the δ15N of macroalgae collected from the same locations, indicating that gorgonian corals are isotopically similar to primary producers, and therefore suitable for assessing sources of dissolved inorganic nitrogen. Differences in the δ15N between younger and older skeleton indicated an overall decline of -0.34 ± 0.06 ‰ (1 s.e) over the last 20 - 40 years at Green Turtle Cay, reflecting a possible increase in nitrogen fixation and/or atmospheric deposition of anthropogenic nitrogen. Off southeast Florida, there was an overall increase in δ15N over the same time period, reflecting increasing wastewater discharges from the rapidly growing population. These results highlight the usefulness of δ15N recorded in gorgonians and other long-lived organisms in assessing spatiotemporal patterns of nitrogen sources to coastal marine environments. [ABSTRACT FROM AUTHOR]

We have developed an analytical method to determine the compound-specific nitrogen isotope compositions of individual amino acid enantiomers using gas chromatography/combustion/isotope ratio mass spectrometry. A novel derivatization of amino acid diastereomers by optically active (R)-(-)-2-butanol or (S)-(+)-2-butanol offers two advantages for nitrogen isotope analysis. First, chromatographic chiral separation can be achieved without the use of chiral stationary-phase columns. Second, the elution order of these compounds on the chromatogram can be switched by a designated esterification reaction. We applied the method to the compound-specific nitrogen isotope analysis of D- and L-alanine in a peptidoglycan derived from the cell walls of cultured bacteria (Firmicutes and Actinobacteria; Enterococcus faecalis, Staphylococcus aureus, Staphylococcus staphylolyticus, Lactobacillus acidophilus, Bacillus subtilis, Micrococcus luteus, and Streptomyces sp.), natural whole bacterial cells (Bacillus subtilis var. natto), (pseudo)-peptidoglycan from archaea (Methanobacterium sp.), and cell wall from eukaryota (Saccharomyces cerevisiae). We observed statistically significant differences in nitrogen isotopic compositions; e.g., δ15N (‰ vs air) in Staphylococcus staphylolyticus for D-alanine (19.2 ± 0.5‰, n = 4) and L-alanine (21.3 ± 0.8‰, n = 4) and in Bacillus subtilis for D-alanine (6.2 ± 0.2‰, n = 3) and L-alanine (8.2 ± 0.4‰, n = 3). These results suggest that enzymatic reaction pathways, including the alanine racemase reaction, produce a nitrogen isotopic difference in amino acid enantiomers, resulting in 15N-depleted D-alanine. This method is expected to facilitate compound-specific nitrogen isotope studies of amino acid stereoisomers. [ABSTRACT FROM AUTHOR]

We describe a new method for analysis of the nitrogen isotopic composition of sedimentary porphyrins. This method involves separation and purification of geoporphyrins from sediment samples using liquid chromatography and HPLC, oxidation of the nitrogen within porphyrin-enriched fractions using a two-step process, and isotopic analysis of the resulting nitrate using the denitrifier method. By analysis of these degradation products of chlorophylls, we are able to measure an isotopic signature that reflects the nitrogen utilized by primary producers. The high sensitivity of the denitrifier method allows measurement of small samples that contain low concentrations of porphyrins. Extraction of only 50 nmol of nitrogen (nmol N) allows the following five analyses to be made (each on ∼10 nmol N): nitrogen concentration, an assessment of potential contamination by nonporphyrin N, and three replicate isotopic measurements. The measured values of δ15N have an average analytical precision of ±0.5‰ (1σ) and an average contribution from Rayleigh fractionation of 0.7‰ from incomplete oxidation of porphyrin N to nitrate. The overall method will enable high-resolution records of δ15N values to be obtained for geological and ecological applications. [ABSTRACT FROM AUTHOR]