The calibration of the intramolecular nitrogen isotope distribution in nitrous oxide measured by isotope ratio mass spectrometry

Two alternative approaches for the calibration of the intramolecular nitrogen isotope distribution in nitrous oxide using isotope ratio mass spectrometry have yielded a difference in the 15N site preference (defined as the difference between the delta15N of the central and end position nitrogen in NNO) of tropospheric N2O of almost 30 per thousand. One approach is based on adding small amounts of labeled 15N2O to the N2O reference gas and tracking the subsequent changes in m/z 30, 31, 44, 45 and 46, and this yields a 15N site preference of 46.3 +/- 1.4 per thousand for tropospheric N2O. The other involves the synthesis of N2O by thermal decomposition of isotopically characterized ammonium nitrate and yields a 15N site preference of 18.7 +/- 2.2 per thousand for tropospheric N2O. Both approaches neglect to fully account for isotope effects associated with the formation of NO+ fragment ions from the different isotopic species of N2O in the ion source of a mass spectrometer. These effects vary with conditions in the ion source and make it impossible to reproduce a calibration based on the addition of isotopically enriched N2O on mass spectrometers with different ion source configurations. These effects have a much smaller impact on the comparison of a laboratory reference gas with N2O synthesized from isotopically characterized ammonium nitrate. This second approach was successfully replicated and leads us to advocate the acceptance of the site preference value 18.7 +/- 2.2 per thousand for tropospheric N2O as the provisional community standard until further independent calibrations are developed and validated. We present a technique for evaluating the isotope effects associated with fragment ion formation and revised equations for converting ion signal ratios into isotopomer ratios.     

Isotopomeric characterization of N2O produced, consumed, and emitted by automobiles

Fossil fuel combustion is the second largest anthropogenic source of nitrous oxide (N2O) after agriculture. The estimated global N2O flux from combustion sources, as well as from other sources, still has a large uncertainty. Herein, we characterize automobile sources using N2O isotopomer ratios (nitrogen and oxygen isotope ratios and intramolecular site preference of 15N, SP) to assess their contributions to total global sources and to deconvolute complex production/consumption processes during combustion and subsequent catalytic treatments of exhaust. Car exhaust gases were sampled under running and idling state, and N2O isotopomer ratios were measured by mass spectrometry. The N2O directly emitted from an engine of a vehicle running at constant velocity had almost constant isotopomer ratios (delta15Nbulk = -28.7 +/- 1.2 per thousand, delta18O = 28.6 +/- 3.3 per thousand, and SP = 4.2 +/- 0.8 per thousand) irrespective of the velocity. After passing through catalytic converters, the isotopomer ratios showed an increase which varied with the temperature and the aging of the catalysts. The increase suggests that both production and consumption of N2O occur on the catalyst and that their rates can be comparable. It was noticed that in the idling state, the N2O emitted from a brand new car has higher isotopomer ratios than that from used cars, which indicate that technical improvements in catalytic converters can reduce the N2O from mobile combustion sources. On average, the isotopomeric signatures of N2O finally emitted from automobiles are not sensitive to running/idling states or to aging of the catalysts. Characteristic average isotopomer ratios of N2O from automobile sources are estimated at -4.9 +/- 8.2 per thousand, 43.5 +/- 13.9 per thousand, and 12.2 +/- 9.1 per thousand for delta15Nbulk, delta18O, and SP, respectively.     

NO reductase activity of the tetraheme cytochrome C554 of Nitrosomonas europaea

The tetraheme cytochrome c(554) (cyt c(554)) from Nitrosomonas europaea is believed to function as an electron-transfer protein from hydroxylamine oxidoreductase (HAO). We show here that cyt c(554) also has significant NO reductase activity. The protein contains one high-spin and three low-spin c-type hemes. HAO catalyzed reduction of the cyt c(554), ligand binding, intermolecular electron transfer, and kinetics of NO reduction by cyt c(554) have been investigated. We detect the formation of a NO-bound ferrous heme species in cyt c(554) by EPR and M?ssbauer spectroscopies during the HAO catalyzed oxidation of hydroxylamine, indicating that N-oxide intermediates produced from HAO readily bind to cyt c(554). In the half-reduced state of cyt c(554), we detect a spin interaction between the [FeNO](7) state of heme 2 and the low-spin ferric state of heme 4. We find that ferrous cyt c(554) will reduce NO at a rate greater than 16 s(-1), which is comparable to rates of other known NO reductases. Carbon monoxide or nitrite are shown not to bind to the reduced protein, and previous results indicate the reactions with O(2) are slow and that a variety of ligands will not bind in the oxidized state. Thus, the enzymatic site is highly selective for NO. The NO reductase activity of cyt c(554) may be important during ammonia oxidation in N. europaea at low oxygen concentrations to detoxify NO produced by reduction of nitrite or incomplete oxidation of hydroxylamine.     

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.     

Isotope fractionation factors of N2O diffusion

Isotopic signatures of N2O are increasingly used to constrain the total global flux and the relative contribution of nitrification and denitrification to N2O emissions. Interpretation of isotopic signatures of soil-emitted N2O can be complicated by the isotopic effects of gas diffusion. The aim of our study was to measure the isotopic fractionation factors of diffusion for the isotopologues of N2O and to estimate the potential effect of diffusive fractionation during N2O fluxes from soils using simple simulations. Diffusion experiments were conducted to monitor isotopic signatures of N2O in reservoirs that lost N2O by defined diffusive fluxes. Two different mathematical approaches were used to derive diffusive isotope fractionation factors for 18O (epsilon18O), average 15N (epsilonbulk) and 15N of the central (alpha(-)) and peripheral (beta(-)) position within the linear N2O molecule (epsilon15Nalpha, epsilon15Nbeta). The measured epsilon18O was -7.79 +/- 0.27 per thousand and thus higher than the theoretical value of -8.7 per thousand. Conversely, the measured epsilonbulk (-5.23 +/- 0.27 per thousand) was lower than the theoretical value (-4.4 per thousand). The measured site-specific 15N fractionation factors were not equal, giving a difference between epsilon15Nalpha and epsilon15Nbeta (epsilonSP) of 1.55 +/- 0.28 per thousand. Diffusive fluxes of the N2O isotopologues from the soil pore space to the atmosphere were simulated, showing that isotopic signatures of N2O source pools and emitted N2O can be substantially different during periods of non-steady state fluxes. Our results show that diffusive isotope fractionation should be taken into account when interpreting natural abundance isotopic signatures of N2O fluxes from soils.     

Fractionation factors for stable isotopes of N and O during N2O reduction in soil depend on reaction rate constant

Nitrous oxide (N(2)O) is a major greenhouse gas that is mainly produced but also reduced by microorganisms in soils. We determined factors for N and O isotope fractionation during the reduction of N(2)O to N(2) in soil in a flow-through incubation experiment. The absolute value of the fractionation factors decreased with increasing reaction rate constant. Reaction rates constants ranged from 1.7 10(-4) s(-1) to 4.5 10(-3) s(-1). The minimum, maximum and median of the observed fractionation factors were for N -36.0 per thousand, -1.0 per thousand and -9.3 per thousand and for O -74.0 per thousand, -6.9 per thousand and -26.3 per thousand, respectively. The ratio of O isotope fractionation to N isotope fractionation was 2.4 +/- 0.3 and it was independent from the reaction rate constants. This leads us to conclude that fractionation factors are variables while their ratio in this particular reaction might be a constant.     

Stable carbon and oxygen isotopic analysis of carbon monoxide in natural waters

Techniques have been developed to allow on-line simultaneous analysis of concentration and stable isotopic compositions ((13)C and (18)O) of dissolved carbon monoxide (CO) in natural water, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted sequentially of a He-sparging bottle of water, a gas dryer, CO(2)-trapping stage using both Ascarite trap and silica-gel packed gas chromatography (GC), on-line oxidation to CO(2) using the Schütze reagent, cryofocusing, GC purification using a capillary column and measurement by CF-IRMS. Each sample analysis takes about 40 minutes. The detection limit with delta(13)C standard deviation of 0.5 per thousand is 300 pmol and that with delta(18)O deviation of 1.0 per thousand is 750 pmol. Analytical blanks associated with these methods are 21+/-9 pmol. The procedures are evaluated through analyses of temporally varying concentration and isotopic compositions of CO in an artificial lake on the university campus. The delta(13)C and delta(18)O values of CO showed wide variation in accordance with diurnal variation of CO concentration, probably due to significant isotopic effects during photochemical production and microbial oxidation of CO in the aquatic environment. The delta(13)C and delta(18)O values of CO should be a useful tool in studies of the mechanism and pathways of CO production and consumption in natural waters.     

Deuterium and oxygen-18 determination of microliter quantities of a water sample using an automated equilibrator

We describe a modified version of the equilibration method and a correction algorithm for isotope ratio measurements of small quantities of water samples. The deltaD and the delta(18)O of the same water sample can both be analyzed using an automated equilibrator with sample sizes as small as 50 microL. Conventional equilibration techniques generally require water samples of several microL. That limitation is attributable mainly to changes in the isotope ratio ((18)O/(16)O or D/H) of water samples during isotopic exchange between the equilibration gas (CO(2) or H(2)) and water, and therefore the technique for microL quantities of water requires mass-balance correction using the water/gas (CO(2) or H(2)) mole ratio to correct this isotopic effect. We quantitatively evaluate factors controlling the variability of the isotopic effect due to sample size. Theoretical consideration shows that a simple linear equation corrects for the effects without determining parameters such as isotope fractionation factors and water/gas mole ratios. Precisions (1-sigma) of 50-microL meteoric water samples whose isotopic compositions of -1.4 to -396.2 per thousand for deltaD are +/-0.5 to +/-0.6 per thousand, and of -0.37 to -51.37 per thousand for delta(18)O are +/-0.01 to +/-0.11 per thousand.     

Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid delta13C values determined by gas chromatography-combustion-isotope ratio mass spectrometry

Litter-dwelling (epigeic) Lumbricus rubellus and soil-dwelling (endogeic) Allolobophora chlorotica earthworms were observed aggregating under C(3) (delta(13)C = -31.3 per thousand; delta(15)N = 10.7 per thousand) and C(4) (delta(13)C = -12.6 per thousand; delta(15)N = 7.5 per thousand) synthetic dung pats applied to a temperate grassland (delta(13)C = -30.3 per thousand; delta(15)N = 5.7 per thousand) in an experiment carried out for 372 days. Bulk delta(13)C values of earthworms collected from beneath either C(3) or C(4) dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C(4) dung were significantly (13)C-enriched after 56 days (delta(13)C = -23.8 per thousand) and 112 days (delta(13)C = -22.4 per thousand) compared with those from C(3) dung treatments (56 days, delta(13)C = -26.5 per thousand; 112 days, delta(13)C = -27.0 per thousand), and (ii) A. chlorotica were 2.1 per thousand (13)C-enriched (delta(13)C = -24.2 per thousand) relative to those from C(3) dung (delta(13)C = -26.3 per thousand) treatments after 372 days. Bulk delta(15)N values did not suggest significant uptake of dung N by either species beneath C(3) or C(4) dung, but showed that the endogeic species (total mean delta(15)N = 3.3 per thousand) had higher delta(15)N values than the epigeic species (total mean delta(15)N = 5.4 per thousand). Although the two species exhibited similar fatty acid profiles, individual fatty acid delta(13)C values revealed extensive routing of dietary C into body tissue of L. rubellus, but minor incorporation into A. chlorotica. In particular, the direct incorporation of microbial biomarker fatty acids (iC(17:0), aC(17:0)) from (13)C-labelled dung in situ, the routing of dung C into de novo synthesised compounds (iC(20:4)(omega)(6),C(20:5)(omega)(3), and the assimilation of essential fatty acids ((C(18:1)(omega)(9), C(18:1)(omega(7), C(18:2)(omega(6), C(18:3)(omega)(3)) derived from dung, were determined.     

Component B binding to the soluble methane monooxygenase hydroxylase by saturation-recovery EPR spectroscopy of spin-labeled MMOB

Spin-labeled Cys89 of the soluble methane monooxygenase regulatory protein (MMOB) from Methylococcus capsulatus (Bath) binds within 15 +/- 4 A of the hydroxylase (MMOH) diiron center, placing the MMOB docking site in the MMOH "canyon" region on iron-coordinating helices E and F of the alpha-subunit.     

Dimethyl ether oxidation at elevated temperatures (295-600 K)

Dimethyl ether (DME) has been proposed for use as an alternative fuel or additive in diesel engines and as a potential fuel in solid oxide fuel cells. The oxidation chemistry of DME is a key element in understanding its role in these applications. The reaction between methoxymethyl radicals and O(2) has been examined over the temperature range 295-600 K and at pressures of 20-200 Torr. This reaction has two product pathways. The first produces methoxymethyl peroxy radicals, while the second produces OH radicals and formaldehyde molecules. Real-time kinetic measurements are made by transient infrared spectroscopy to monitor the yield of three main products-formaldehyde, methyl formate, and formic acid-to determine the branching ratio for the CH(3)OCH(2) + O(2) reaction pathways. The temperature and pressure dependence of this reaction is described by a Lindemann and Arrhenius mechanism. The branching ratio is described by f = 1/(1 + A(T)[M]), where A(T) = (1.6(+2.4)(-1.0) x 10(-20)) exp((1800 +/- 400)/T) cm(3) molecule(-1). The temperature dependent rate constant of the methoxymethyl peroxy radical self-reaction is calculated from the kinetics of the formaldehyde and methyl formate product yields, k(4) = (3.0 +/- 2.1) x 10(-13) exp((700 +/- 250)/T) cm(3) molecule(-1) s(-1). The experimental and kinetics modeling results support a strong preference for the thermal decomposition of alkoxy radicals versus their reaction with O(2) under our laboratory conditions. These characteristics of DME oxidation with respect to temperature and pressure might provide insight into optimizing solid oxide fuel cell operating conditions with DME in the presence of O(2) to maximize power outputs.     

Preparation and properties of the full series of cuboidal clusters [Mo(x)W4-xSe4(H2O)12]n+ (n = 4-6) and their derivatives

Hydrothermal reactions between incomplete cuboidal cluster aqua complexes [M3Q4(H2O)9]4+ and M(CO)6 (M = Mo, W; Q = S, Se) offer easy access to the corresponding cuboidal clusters M4Q4. The complete series of homometal and mixed Mo/W clusters [Mo(x)W4-xQ4(H2O)12]n+ (x = 0-4, n = 4-6) has been prepared. Upon oxidation of the mixed-metal clusters, it is the W atom which is lost, allowing selective preparation of new trinuclear clusters [Mo2WSe4(H2O)9]4+ and [MoW2Se4(H2O)9]4+. The aqua complexes were converted by ligand exchange reactions into dithiophosphato and thiocyanato complexes, and crystal structures of [W4S4((EtO)2PS2)6], [MoW3S4((EtO)2PS2)6], [Mo4Se4((EtO)2PS2)6], [W4Se4((i-PrO)2PS2)6], and (NH4)6[W4Se4(NCS)12]-4H20 were determined. Cyclic voltammetry was performed on [Mo(x)W4-xCO4(H2O)12]n+, showing reversible redox waves 6+/5+ and 5+/4+. The lower oxidation states are more difficult to access as the number of W atoms increases. The [Mo2WSe4(H2O)9]4+ and [MoW2Se4(H2O)9]4+ species were derivatized into [Mo2WSe4(acac)3(py)3]+ and [MoW2Se4(acac)3(py)3]+, which were also studied by CV. When appropriate, the products were also characterized by FAB-MS and NMR (31P, 1H) data.     

Kinetics and mechanisms of the oxidation of iodide and bromide in aqueous solutions by a trans-dioxoruthenium(VI) complex

The kinetics and mechanisms of the oxidation of I (-) and Br (-) by trans-[Ru (VI)(N 2O 2)(O) 2] (2+) have been investigated in aqueous solutions. The reactions have the following stoichiometry: trans-[Ru (VI)(N 2O 2)(O) 2] (2+) + 3X (-) + 2H (+) --> trans-[Ru (IV)(N 2O 2)(O)(OH 2)] (2+) + X 3 (-) (X = Br, I). In the oxidation of I (-) the I 3 (-)is produced in two distinct phases. The first phase produces 45% of I 3 (-) with the rate law d[I 3 (-)]/dt = ( k a + k b[H (+)])[Ru (VI)][I (-)]. The remaining I 3 (-) is produced in the second phase which is much slower, and it follows first-order kinetics but the rate constant is independent of [I (-)], [H (+)], and ionic strength. In the proposed mechanism the first phase involves formation of a charge-transfer complex between Ru (VI) and I (-), which then undergoes a parallel acid-catalyzed oxygen atom transfer to produce [Ru (IV)(N 2O 2)(O)(OHI)] (2+), and a one electron transfer to give [Ru (V)(N 2O 2)(O)(OH)] (2+) and I (*). [Ru (V)(N 2O 2)(O)(OH)] (2+) is a stronger oxidant than [Ru (VI)(N 2O 2)(O) 2] (2+) and will rapidly oxidize another I (-) to I (*). In the second phase the [Ru (IV)(N 2O 2)(O)(OHI)] (2+) undergoes rate-limiting aquation to produce HOI which reacts rapidly with I (-) to produce I 2. In the oxidation of Br (-) the rate law is -d[Ru (VI)]/d t = {( k a2 + k b2[H (+)]) + ( k a3 + k b3[H (+)]) [Br (-)]}[Ru (VI)][Br (-)]. At 298.0 K and I = 0.1 M, k a2 = (2.03 +/- 0.03) x 10 (-2) M (-1) s (-1), k b2 = (1.50 +/- 0.07) x 10 (-1) M (-2) s (-1), k a3 = (7.22 +/- 2.19) x 10 (-1) M (-2) s (-1) and k b3 = (4.85 +/- 0.04) x 10 (2) M (-3) s (-1). The proposed mechanism involves initial oxygen atom transfer from trans-[Ru (VI)(N 2O 2)(O) 2] (2+) to Br (-) to give trans-[Ru (IV)(N 2O 2)(O)(OBr)] (+), which then undergoes parallel aquation and oxidation of Br (-), and both reactions are acid-catalyzed.     

Nitrosation of N-methylhydroxylamine by nitroprusside. A kinetic and mechanistic study

The kinetics of the reaction between aqueous solutions of Na(2)[Fe(CN)(5)NO].2H(2)O (sodium pentacyanonitrosylferrate(ii), nitroprusside, SNP) and MeN(H)OH (N-methylhydroxylamine, MeHA) has been studied by means of UV-vis spectroscopy, using complementary solution techniques: FTIR/ATR, EPR, mass spectrometry and isotopic labeling ((15)NO), in the pH range 7.1-9.3, I = 1 M (NaCl). The main products were N-methyl-N-nitrosohydroxylamine (MeN(NO)OH) and [Fe(CN)(5)H(2)O](3-), characterized as the [Fe(CN)(5)(pyCONH(2))](3-) complex (pyCONH(2) = isonicotinamide). No reaction occurred with Me(2)NOH (N,N-dimethylhydroxylamine, Me(2)HA) as nucleophile. The rate law was: R = k(exp) [Fe(CN)(5)NO(2-)] x [MeN(H)OH] x [OH(-)], with k(exp) = 1.6 +/- 0.2 x 10(5) M(-2) s(-1), at 25.0 degrees C, and DeltaH(#) = 34 +/- 3 kJ mol(-1), DeltaS(#) = -32 +/- 11 J K(-1) mol(-1), at pH 8.0. The proposed mechanism involves the formation of a precursor associative complex between SNP and MeHA, followed by an OH(-)-assisted reversible formation of a deprotonated adduct, [Fe(CN)(5)(N(O)NMeOH)](3-), and rapid dissociation of MeN(NO)OH. In excess SNP, the precursor complex reacts through a competitive one-electron-transfer path, forming the [Fe(CN)(5)NO](3-) ion with slow production of small quantities of N(2)O. The stoichiometry and mechanism of the main adduct-formation path are similar to those previously reported for hydroxylamine (HA) and related nucleophiles. The nitrosated product, MeN(NO)OH, decomposes thermally at physiological temperatures, slowly yielding NO.     

Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II)

Two pro-ligands ((R)LH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole ((Bz)LH) or a 4,5-di-p-methoxyphenyl substituted imidazole ((PhOMe)LH), have been structurally characterised. Each possesses an intramolecular O-H[dot dot dot]N hydrogen bond between the phenolic O-H group and an imidazole nitrogen atom and (1)H NMR studies show that this bond is retained in solution. Each (R)LH undergoes an electrochemically reversible, one-electron, oxidation to form the [(R)LH] (+) radical cation that is considered to be stabilised by an intramolecular O...H-N hydrogen bond. The (R)LH pro-ligands react with M(BF(4))(2).H(2)O (M = Cu or Zn) in the presence of Et(3)N to form the corresponding [M((R)L)(2)] compound. [Cu((Bz)L)(2)] (), [Cu((PhOMe)L)(2)] (), [Zn((Bz)L)(2)] and [Zn((PhOMe)L)(2)] have been isolated and the structures of .4MeCN, .2MeOH, .2MeCN and .2MeCN determined by X-ray crystallography. In each compound the metal possesses an N(2)O(2)-coordination sphere: in .4MeCN and .2MeOH the {CuN(2)O(2)} centre has a distorted square planar geometry; in .2MeCN and .2MeCN the {ZnN(2)O(2)} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both and , in CH(2)Cl(2)-DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(ii) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)((R)L)((R)L )](+), comprising a M(ii) centre bound to a phenoxide ((R)L) and a phenoxyl radical ((R)L ) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF(4)]. The second oxidation produces [M(II)((R)L )(2)](2+). The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [(R)LH] (+) (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)((R)L)((R)L )](+) (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(ii) is slightly more effective than Zn(II) in this respect.     

One-electron oxidation of acetohydroxamic acid: the intermediacy of nitroxyl and peroxynitrite

The pharmacological effects of hydroxamate derivatives have been attributed not only to metal chelation or enzyme inhibition but also to their ability to serve as nitroxyl (HNO/NO(-)) and nitric oxide (NO) donors. However, the mechanism underlying the formation of these reactive nitrogen species is not clear and requires further elucidation. In the present study, one-electron oxidation of acetohydroxamic acid (aceto-HX) by (?)OH, (?)N(3), (?)NO(2), CO(3)(?-), and O(2)(?-) radicals was investigated using pulse radiolysis. It is demonstrated that only (?)OH, (?)N(3), and CO(3)(?-) radicals attack effectively and selectively the deprotonated form of the hydroxamate moiety, yielding the respective transient nitroxide radical. This nitroxide radical is a weak acid (CH(3)C(O)NHO(?), pK(a) = 9.1), which decays via a pH-dependent second-order reaction, 2k(2CH(3)C(O)NO(?-)) = (5.6 ± 0.4) × 10(7) M(-1) s(-1) (I = 0.002 M), 2k(CH(3)C(O)NO(?-) + CH(3)C(O)NHO(?)) = (8.3 ± 0.5) × 10(8) M(-1) s(-1)), and 2k(2CH(3)C(O)NHO(?)) = (8.7 ± 1.3) × 10(7) M(-1) s(-1). The second-order decomposition of the nitroxide yields transient species, one of which decomposes via a first-order reaction whose rate increases linearly upon increasing [CH(3)C(O)NHO(-)] or [OH(-)]. One-electron oxidation of aceto-HX under anoxia does not give rise to nitrite even after exposure to O(2), indicating that NO is not formed during the decomposition of the nitroxide radical. The presence of oxidants such as Tempol or O(2) during CH(3)C(O)NO(?-) decomposition had no effect on the reaction kinetics. Nevertheless, in the presence of Temopl, which does not react with NO but does with HNO, the formation of the hydroxylamine Tempol-H was observed. In the presence of O(2), about 60% of CH(3)C(O)NO(?-) yields ONOO(-), indicating that 30% NO(-) is formed in this system. It is concluded that under pulse radiolysis conditions, the transient nitroxide radicals derived from one-electron oxidation of aceto-HX decompose bimoleculary via a complex mechanism forming nitroxyl rather than NO.     

One-electron reduction of aqueous nitric oxide: a mechanistic revision

The pulse radiolysis of aqueous NO has been reinvestigated, the variances with the prior studies are discussed, and a mechanistic revision is suggested. Both the hydrated electron and the hydrogen atom reduce NO to yield the ground-state triplet (3)NO(-) and singlet (1)HNO, respectively, which further react with NO to produce the N(2)O(2)(-) radical, albeit with the very different specific rates, k((3)NO(-) + NO) = (3.0 +/- 0.8) x 10(9) and k((1)HNO + NO) = (5.8 +/- 0.2) x 10(6) M(-)(1) s(-)(1). These reactions occur much more rapidly than the spin-forbidden acid-base equilibration of (3)NO(-) and (1)HNO under all experimentally accessible conditions. As a result, (3)NO(-) and (1)HNO give rise to two reaction pathways that are well separated in time but lead to the same intermediates and products. The N(2)O(2)(-) radical extremely rapidly acquires another NO, k(N(2)O(2)(-) + NO) = (5.4 +/- 1.4) x 10(9) M(-)(1) s(-)(1), producing the closed-shell N(3)O(3)(-) anion, which unimolecularly decays to the final N(2)O + NO(2)(-) products with a rate constant of approximately 300 s(-)(1). Contrary to the previous belief, N(2)O(2)(-) is stable with respect to NO elimination, and so is N(3)O(3)(-). The optical spectra of all intermediates have also been reevaluated. The only intermediate whose spectrum can be cleanly observed in the pulse radiolysis experiments is the N(3)O(3)(-) anion (lambda(max) = 380 nm, epsilon(max) = 3.76 x 10(3) M(-)(1) cm(-)(1)). The spectra previously assigned to the NO(-) anion and to the N(2)O(2)(-) radical are due, in fact, to a mixture of species (mainly N(2)O(2)(-) and N(3)O(3)(-)) and to the N(3)O(3)(-) anion, respectively. Spectral and kinetic evidence suggests that the same reactions occur when (3)NO(-) and (1)HNO are generated by photolysis of the monoprotonated anion of Angeli's salt, HN(2)O(3)(-), in NO-containing solutions.     

Disproportionation pathways of aqueous hyponitrite radicals (HN2O2(*)/N2O2(*-))

Pulse radiolysis and flash photolysis are used to generate the hyponitrite radicals (HN2O2(*)/N2O2(*-)) by one-electron oxidation of the hyponitrite in aqueous solution. Although the radical decay conforms to simple second-order kinetics, its mechanism is complex, comprising a short chain of NO release-consumption steps. In the first, rate-determining step, two N2O2(*-) radicals disproportionate with the rate constant 2k = (8.2 +/- 0.5) x 10(7) M(-1) s(-1) (at zero ionic strength) effectively in a redox reaction regenerating N2O2(2-) and releasing two NO. This occurs either by electron transfer or, more likely, through radical recombination-dissociation. Each NO so-produced rapidly adds to another N2O2(*-), yielding the N3O3(-) ion, which slowly decomposes at 300 s(-1) to the final N2O + NO2(-) products. The N2O2(*-) radical protonates with pKa = 5.6 +/- 0.3. The neutral HN2O2(*) radical decays by an analogous mechanism but much more rapidly with the apparent second-order rate constant 2k = (1.1 +/- 0.1) x 10(9) M(-1) s(-1). The N2O2(*-) radical shows surprisingly low reactivity toward O2 and O2(*-), with the corresponding rate constants below 1 x 10(6) and 5 x 10(7) M(-1) s(-1). The previously reported rapid dissociation of N2O2(*-) into N2O and O(*-) does not occur. The thermochemistry of HN2O2(*)/N2O2(*-) is discussed in the context of these new kinetic and mechanistic results.     

Oxygen isotopes in nitrate: new reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibration

Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO(3)(-) in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying delta(18)O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO(3)) with low delta(18)O and USGS35 (NaNO(3)) with high delta(18)O and 'mass-independent' delta(17)O. The procedure used to produce USGS34 involved equilibration of HNO(3) with (18)O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO(3)(-) reference materials with a range of delta(18)O values and normal (mass-dependent) (18)O:(17)O:(16)O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (alpha) between [NO(3)(-)] and H(2)O decreases with increasing temperature from 1.0215 at 22 degrees C to 1.0131 at 100 degrees C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high (17)O:(18)O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO(3) (-) isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO(3)(-) samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of +/-0.2-0.3 per thousand, 1sigma): IAEA-N3 has delta(18)O = +25.6 per thousand and delta(17)O = +13.2 per thousand; USGS32 has delta(18)O = +25.7 per thousand; USGS34 has delta(18)O = -27.9 per thousand and delta(17)O = -14.8 per thousand; and USGS35 has delta(18)O = +57.5 per thousand and delta(17)O = +51.5 per thousand.     

Sensitive determinations of stable nitrogen isotopic composition of organic nitrogen through chemical conversion into N2O

We present a method for high-sensitivity nitrogen isotopic analysis of particulate organic nitrogen (PON) in seawater and freshwater, for the purpose of determining the aquatic nitrogen fixation rate through the 15N2 tracer technique for samples that contain a low abundance of organisms. The method is composed of the traditional oxidation/reduction methods, such as the oxidation of PON to nitrate (NO3*) using persulfate, the reduction of NO3* to nitrite (NO2*) using spongy cadmium, and further reduction of NO2* to nitrous oxide (N2O) using sodium azide. Then, N2O is purged from the water and trapped cryogenically with subsequent release into a gas chromatography column to analyze the stable nitrogen isotopic composition using continuous-flow isotope ratio mass spectrometry (CF-IRMS) by simultaneously monitoring the NO+ ion currents at masses 30, 31, and 32. The nitrogen isotopic fractionation was consistent within each batch of analysis. The standard deviation of sample measurements was less than 0.3 per thousand for samples containing PON of more than 50 nmolN, and 0.5 per thousand for those of more than 20 nmolN, by subtracting the contribution of blank nitrogen, 8 +/- 2 nmol at final N2O. By using this method, we can determine delta15N for lower quantities of PON better than by other methods, so we can reduce the quantities of water samples needed for incubation to determine the nitrogen fixation rate. In addition, we can expand the method to determine the nitrogen isotopic composition of organic nitrogen in general, such as that of total dissolved nitrogen (TDN; sum of NO3*, NO2*, ammonium, and DON), by applying the method to filtrates.