Structural and vibrational characteristics of the tetrasulfatodimolybdenum ions with MoMo bond orders of 3.5 and 4.0

The compound K₄[Mo₂(SO₄)₄]Br·4H₂O has been made and its crystal structure determined. Space group P4/mnc; unit cell dimensions, a = 11.903(2), c = 8.021(1) Å, V = 1136(1) ų. The compound is isomorphous with the analogous chloride whose structure has been reported. The MoMo and MoBr distances are 2.169(2) and 2.926(1) Å, respectively and the [Mo₂(SO₄)₄] ³⁻ ions reside on crystallographic special positions with 4/m symmetry. The Raman spectra of both the bromo and chloro compounds have been measured and the MoMo stretching frequency is 370 ± 1.5 cm⁻¹ in each, for the compounds containing the natural isotopic distribution of molybdenum. The chloro compound has been prepared containing the pure isotope ⁹²Mo as well, and the Raman spectra recorded. The v(MoMo) band is shifted by 6.8 ± 0.5 cm⁻¹. The compound K₄[Mo₂(SO₄)₄]·2H₂O has also been prepared with Mo at natural abundance and with the pure isotope ¹⁰⁰Mo, whereby a shift of 8.5 ± 0.5 cm⁻¹ was found. These and other results will be discussed with regard to the similarity of the Raman spectra of the Mo₂(S0₄)₄³⁻ and M0₂(S0₄)₄⁴⁻ species.     

Growth of Methanobacterium thermoautotrophicum on H2CO2: High CH4 productivities in continuous culture

The thermophilic methanogenic bacterium, Methanobacterium thermoautotrophicum, was grown on H₂CO₂. In continuous culture, high CH₄ productivities were obtained (288 litres litre⁻¹ day⁻¹) with 96% CH₄ in the effluent gas, i.e. the productivity was twice as high as that obtained previously by other authors, with pure or mixed cultures; the biomass was 3·6 g dry wt litre⁻¹.     

Synthesis and characterization of triaminecobalt(III) complexes and acid hydrolysis kinetics of fac-[Codpt(H2O)nX3−n]n+ (X = Cl,Br; n = 0,1,2)

The complexes CodptX₃ and [Codpt(H₂O)X₂]ClO₄ (X = Cl, Br; dpt = dipropylenetriamine = NH(CH₂CH₂CH₂NH₂)₂) have been prepared and characterized. Rate constants (s⁻¹) for aqueous solution at 25 °C and μ = 0.5 M (NaClO₄), for the acid-independent sequential ractions.

have been measured spectrophotometrically. For X = Cl: k₁ ⋍ 2 × 10⁻², k₂ = 1.7 × 10⁻⁴ and k₃ = 4.8 × 10⁻⁶, and for X = Br: k₁ ⋍ 2 × 10⁻², k₂ = 5.25 × 10⁻⁴ and k₃ = 2.5 × 10⁻⁵ The primary equation was found to be acid independent, while the secondary and tertiary aquations were acid-inhibited reactions. For the second step, the rate of the reaction was given by the rate equation

where C_t is the complex concentration in the aqua-and hydroxodihalo species, k′₂ is the rate constant for the acid-dependent pathway and K_a is the equilibrium constant between the hydroxo and aqua complex ions. The activation parameters were evaluated, for X = Cl: ΔH^≠₂ = 106.3 ± 0.4 kJ mol⁻¹ and ΔS^≠₂ = 40.2 ± 1.7 J K⁻¹ mol⁻, and for X = Br: ΔH^≠₂ = 91.6 ± 0.4 kJ mol⁻¹ and ΔS^≠₂ = 0.4 ± 1.7 J K⁻¹ mol⁻¹. The results are discussed and detailed comparisons of the reactivities of these complexes with other haloaminecobalt(III) species are presented.     

Kinetic studies on partial oxidation of methane over samarium oxides

Kinetic studies on the oxidative coupling of methane over Sm₂O₃ have been carried out. The experimental rate equation observed could be well explained in terms of the reaction mechanism proposed. The reaction is initiated by abstracting hydrogen atom from the methane adsorbed by the diatomic oxygen on the surface. The coupling of two CH₃· radicals leads to C₂H₆. Deep oxidation of CH₃· produces CO and CO₂. The large activation energy (149 kJ mol⁻¹) needed for the formation of CH₃· explains the sharp increase in the selectivity to C₂-compounds (C₂H₆ + C₂H₄) as raising temperatures. The oxygen species responsible for initiating the reaction was suggested to be O₂²⁻ or O₂⁻ on the surface.     

Stabilities,solubility, and kinetics and mechanism for formation and hydrolysis of some palladium(II)and platinum(II) iodo complexes in aqueous solution

Complex formation between Pd(II), Pt(II) and iodide has been studied at 25 °C for an aqueous 1.00 M perchloric acid medium. Measurements of the solubility of PdI₂(s) in aqueous mercury(II) perchlorate and of AgI(s) and PdI₂(s) in aqueous solutions of Pd²⁺(aq) and Ag⁺(aq) gave the solubility product of PdI₂(s) as K_s^o=(7±3) × 10⁻³² M³, which is much smaller than previous literature values.The stability constants β₁=[MI(H₂O)₃⁺]/([M(H₂O)₄²⁺][I⁻]) for the two systems were obtained as the ratio between rate constants for the forward and reverse reactions of (i).

The following values of k₁ (s⁻¹ M⁻¹), k₋₁ (s⁻¹) and β₁ (M⁻¹) were obtained at 25 °C: (1.14±0.11) × 10⁶, (0.92±0.18), (12±4) × 10⁵ for MPd, and (7.7±0.4), (8.0±0.7) × 10⁻⁵, (9.6±1.3) × 10⁴ for MPt. Combination with previous literature data gives the following values of log(β₁ (M⁻¹)) to log(β₄ (M⁻⁴)): 6.08, ∼22, 25.8 and 28.3 for MPd, and 4.98, ∼25, ∼28, and ∼30 for MPt. The present results show that the large overall stability constants β₄ observed for the M²⁺I⁻ systems are most likely due to a very large stability of the second complex MI₂(H₂O)₂, which is probably a cis-isomer. A distinct plateau in the formation curve for mean ligand number 2 is obtained both for MPd and Pt. The other iodo complexes are not especially stable compared to those of chloride and bromide.ΔH^≠ (kJ mol⁻¹) and ΔS^≠ (JK⁻¹ mol⁻¹) for the forward reaction of (i), MPd, are (17.3±1.7) and (−71±5), and for the reverse reaction of (i) MPd, (45±3) and (−95±6), respectively. The kinetics are compatible with associative activation (I_a). The contribution from bond-breaking in the formation of the transition state seems to be less important for Pd than for Pt.     

Equilibrium constants under physiological conditions for the condensation of acetaldehyde with tetrahydrofolic acid

The nonenzymatic reaction of ethanol-derived CH₃CHO with tissue constituents continues to be of interest as a potential mechanism underlying the toxicity of alcohol. The current study has focused on the spontaneous condensation of CH₃CHO with H₄folate under physiological conditions (38 °C, pH 7.0, I = 0.25 M). Computer analysis of uv spectral changes with increasing CH₃CHO concentrations demonstrated the presence of at least two different adducts. The observed equilibrium constant (K_obs) for the formation of the first adduct is 91 ± 2 m^?1 (121 ± 2 m^?1 at 25 °C), a value which is unaffected by variations in ionic strength (0.06–1.0 m) or by free [Mg²⁺] up to 5 mm. The NMR spectrum is compatible with the structure: 5,10-CH₃CH-H₄folate analogous to the naturally occurring 5,10-CH₂-H₄folate. The formation of the latter compound from HCHO and H₄folate, however, is much more favorable under the same conditions [K_obs = 3.0 ± 0.2 × 10⁴ M^?1 (38 °C), 3.6 ± 0.1 × 10⁴ M^?1 (25 °C)]. At the levels of CH₃CHO which accumulate during ethanol metabolism in vivo only a small fraction of the H₄folate will exist as the CH₃CHO derivative, yet it may ultimately be the ratio of free CH₃CHO to free HCHO in tissue which determines the physiological importance of the CH₃CHO adduct. Other adduct(s) of CH₃CHO with H₄folate are observed at very high levels of CH₃CHO but are unlikely to be of physiological significance.     

The kinetics and mechanisms of the reactions of chromium(III) with pentane-2,4-dione in aqueous solution

The equilibrium, kinetics and mechanism of the reaction of chromium(III) with pentane-2,4-dione (Hpd) have been investigated in aqueous solution at 55°C and ionic strength 0.5 mol dm⁻³ NaClO₄. The equilibrium constant (log β₁) is 10.08(±0.01) while the pK of Hpd is 8.69(±0.01). The kinetics are consistent with a mechanism in which [Cr(H₂0)₆]³⁺ and [Cr(H₂0)₅(OH)]²⁺ react with the enol tautomer of Hpd with rate constants of 1.05(±0.26) × 10⁻² and 2.78(±0.08) × 10⁻¹ dm³ mol⁻¹ s⁻¹ respectively. These rate constants are considerably more rapid than those predicted by the Eigen-Wilkins mechanism. These data are compared with literature values.     

Stimulation of CO2 reduction to methane by methylcoenzyme M in extracts Methanobacterium.

Addition of methyl-coenzyme M (CH₃SCH₂CH₂SO₃^?) to undialized, anaerobic, cell-extracts of $\text{Methanobacterium thermoautotrophicum}$ under an atmosphere of H₂ and CO₂ (80:20 v/v) stimulates 30-fold the rate of CO₂ reduction to methane. For each mol of CH₃SCH₂CH₂SO₃^? added 12 mol of methane is produced. This stimulation phenomenon requires magnesium ion, ATP, H₂, and CH₃SCH₂CH₂SO₃^?. Neither the reduced form of the cofactor, HSCH₂CH₂SO₃^?, nor the oxidized, disulfide form will replace the methylated coenzyme.     

Self-diffusion of water-soluble fullerene derivatives in mouse erythrocytes

Self-diffusion of water-soluble fullerene derivative (WSFD) C₆₀[S(CH₂)₃SO₃Na]₅H in mouse red blood cells (RBC) was characterized by ¹H pulsed field gradient NMR technique. It was found that a fraction of fullerene molecules (~13% of the fullerene derivative added in aqueous RBC suspension) shows a self-diffusion coefficient of (5.5 ± 0.8)·10⁻¹² m²/s, which is matching the coefficient of the lateral diffusion of lipids in the erythrocyte membrane (D_L = (5.4 ± 0.8)·10⁻¹² m²/s). This experimental finding evidences the absorption of the fullerene derivative by RBC. Fullerene derivative molecules are also absorbed by RBC ghosts and phosphatidylcholine liposomes as manifested in self-diffusion coefficients of (7.9 ± 1.2)·10⁻¹² m²/s and (7.7 ± 1.2)·10⁻¹² m²/s, which are also close to the lateral diffusion coefficients of (6.5 ± 1.0)·10⁻¹² m²/s and (8.5 ± 1.3)·10⁻¹² m²/s, respectively. The obtained results suggest that fullerene derivative molecules are, probably, fixed on the RBC surface. The average residence time of the fullerene derivative molecule on RBC was estimated as 440 ± 70 ms. Thus, the pulsed field gradient NMR was shown to be a versatile technique for investigation of the interactions of the fullerene derivatives with blood cells providing essential information, which can be projected on their behavior in-vivo after intravenous administration while screening as potential drug candidates.     

Antitumor activity of some organomettalic bismuth(III)thiolates

The neutral organobismuth(III)bis(thiolates) CH₃Bi(SCH₃)₂ and CH₃Bi(p-SC₆H₄NH₂)₂ and the ionic mercaptoanilinium derivative [CH₃Bi(p-SC₆H₄NH₂CH₃)₂]²⁺2I⁻ were tested for antitumor properties in the fluid Ehrlich ascites tumor systems of mice. They all effected an optimum cure rate of 100% and were characterized by values of the therapeutic index ranging between 3.2 and 5.0.     

Synthesis and characterization of palladium(II) complexes with aryl tellurols and diaryl ditellurides

Polymeric complexes of formula [PdCl(TeAr)]_n (I) and [Pd(TeAr)₂]_n (II) are readily obtained by the reaction between Na₂[PdCl₄] and NaTeAr (ArC₆H₅, C₆H₄OCH₃−4 and C₆H₄OCH₂CH₃−4) in ethanol at room temperature. Chemical and far infrared spectral evidences support alternating chloride and tellurol bridges in I and tellurol bridges in II. While the reaction of I (AtC₆H₄OCH₃−4) with PPh₃ in stoichiometric amount results in splitting of chloride bridges and formation of a tellurol bridged dimeric complex [PdCl(TeC₆H₄OCH₃−4)(PPh₃)]₂ (III), with excess of PPh₃, cleavage of both chloride and tellurol bridges leads to the formation of a monomeric compound [PdCl(TeC₆H₄OCH₃−4)(PPh₃)₂] (IV). Furthermore, the reaction of I (Ar C₆H₄OCH₂CH₃−4) with 1,2-bis(diphenyl phosphino)ethane in equimolar ratio also resulted in a monomeric compound [(PdCl(TeC₆H₄OCH₂CH₃−4)(diphos)] (V). The complex III (ArC₆H₄OCH₂CH₃−4) is also prepared by the reaction between Pd(PPh₃)₂Cl₂ and Ph₃SnTeC₆H₄OCH₂CH₃−4 in 1:1 molar ratio or between Pd₂Cl₄(PPh₃)₂ and Ph₃SnTeC₆H₄OCH₂CH₃−4 in 1:2 molar ratio in benzene at room temperature. Sodium tetrachloropalladate reacts readily with diarylditellurides in ethanol at 0 °C to form dimeric complexes [PdCl₂(ArTeTeAr)]₂ (VI). However, at 40 °C or above the same ditellurides form polymeric complexes I with Na₂[PdCl₄] in ethanol. The complex VI is also obtained by the reaction of Pd(PhCN)₂Cl₂ with Te₂Ar₂ in benzene at room temperature. The complexes were characterized by elemental analysis, IR, Raman and ¹H NMR spectra and, where possible, by conductivity measurements and molecular weight determinations.     

Reaction mechanisms of peroxyl and c-centered radicals with sulfhydryls

Rate constants for reactions of a peroxyl (CCl₃OO·) and C-centered radicals, that is, phenyl (·C₆H₄CH₂COO⁻) and vinyl (uracil-5-yl), with an aromatic thiol (p-CH₃OC₆H₄SH) were measured over a pH range (3–12) to include ArSH and ArS⁻ forms. The pH dependence of these rate constants indicates that peroxyl radicals react by a redox mechanism while the C-centered radicals react by an H-atom transfer process. The different mechanisms encountered in the repair of various radicals suggest design features to be incorporated into antiagents, such as radioprotectors and anticarcinogens.     

Resolution,structures, and vibrational circular dichroism of helicoidal trinickel and tricobalt paddlewheel complexes

It has been recently shown that enantiomers of the helicoidal paddlewheel complex [Co₃(dpa)₄(CH₃CN)₂]²⁺ (dpa = the anion of 2,2′-dipyridylamine) can be resolved using the chiral [As₂(tartrate)₂]²⁻ anion (AsT) and that these complexes demonstrate a strong chiroptical response in the ultraviolet-visible and X-ray energy regions. Here we report that the nickel congener, [Ni₃(dpa)₄(CH₃CN)₂]²⁺, can likewise be resolved using AsT. Depending on the stereochemistry of the enantiopure AsT anion, one or the other of the trinickel enantiomers crystallize from CH₃CN and diethyl ether in space group P42₁2 as the (NBu₄)₂[Ni₃(dpa)₄(CH₃CN)₂](AsT)₂·[solvent] salt. After resolution, the AsT salts were converted into the PF₆⁻ salts by anion exchange, with retention of the chirality of the trinickel complex. The enantiopure [Ni₃(dpa)₄(CH₃CN)₂](PF₆)₂·2CH₃CN and [Co₃(dpa)₄(CH₃CN)₂](PF₆)₂·CH₃CN·C₄H₁₀O compounds crystallize in space groups C2 and P2₁, respectively. Both the Ni(II) and Co(II) complex cations are stable towards racemization in CH₃CN. Vibrational circular dichroism (VCD) data obtained in CD₃CN demonstrate the expected mirror image spectra for the enantiomers, the observed peaks arising from the dpa ligand. The VCD response is significant, with Δε values up to 6 Lmol⁻¹ cm⁻¹ and vibrational dissymmetry factors on the order of 10⁻³. Density functional theory calculations well reproduce the experimental spectra, showing little difference between the peak position, sign, and intensity in the VCD for the cobalt and nickel complexes. These results suggest that VCD enhancement of these peaks is unlikely, and their remarkable intensity may be due to their rigid helicoidal structure.     

Using radon to quantify groundwater discharge and methane fluxes to a shallow,tundra lake on the Yukon-Kuskokwim Delta,Alaska

Northern lakes are a source of greenhouse gases to the atmosphere and contribute substantially to the global carbon budget. However, the sources of methane (CH₄) to northern lakes are poorly constrained limiting our ability to the assess impacts of future Arctic change. Here we present measurements of the natural groundwater tracer, radon, and CH₄ in a shallow lake on the Yukon-Kuskokwim Delta, AK and quantify groundwater discharge rates and fluxes of groundwater-derived CH₄. We found that groundwater was significantly enriched (2000%) in radon and CH₄ relative to lake water. Using a mass balance approach, we calculated average groundwater fluxes of 1.2 ± 0.6 and 4.3 ± 2.0 cm day⁻¹, respectively as conservative and upper limit estimates. Groundwater CH₄ fluxes were 7—24 mmol m⁻² day⁻¹ and significantly exceeded diffusive air–water CH₄ fluxes (1.3–2.3 mmol m⁻² day⁻¹) from the lake to the atmosphere, suggesting that groundwater is an important source of CH₄ to Arctic lakes and may drive observed CH₄ emissions. Isotopic signatures of CH₄ were depleted in groundwaters, consistent with microbial production. Higher methane concentrations in groundwater compared to other high latitude lakes were likely the source of the comparatively higher CH₄ diffusive fluxes, as compared to those reported previously in high latitude lakes. These findings indicate that deltaic lakes across warmer permafrost regions may act as important hotspots for CH₄ release across Arctic landscapes.

     

Temperature and pressure dependent equilibrium studies of 1, 4, 8, 11-tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(ii) in coordinating solvents

In coordinating solvents, the complex 1, 4, 8, 11- tetramethyl-1, 4, 8, 11-tetraazacyclotetradecane nickel(II) bisperchlorate exists as an equilibrium mixture involving four coordinate R,S,R,S-[Ni(tmc)]²⁺ and five coordinate R,S,R,S-[Ni(tmc)(solvent)]²⁺ species. Spectrophotometric measurements of this equilibrium in a number of solvents have been conducted over a range of temperatures and pressures. The stability order for the five coordinate complex in the solvents investigated is CH₃CN>DMF>DMSO>C₆H₅CN> H₂O>ClCH₂CN at 25 °C. Differences in stability are considered in terms of the measured thermodynamic parameters ΔH° and ΔS°. Both steric and electronic factors were found to influence solvent coordination with the macrocyclic complex.For the equilibrium in CH₃CN, C₆H₅CN, DMF and H₂O, reaction volumes, ΔV°, of −3.2±0.5, −4.2±0.5, −0.2±0.5 and −0.5±0.5 cm³ mol⁻¹ respectively have been determined. Each is significantly smaller than the corresponding solvent molar volume. The ΔV° for the equilibrium in CH₃CN is comparable with the previously determined activation volume for exchange of this solvent on R, S, R, S- [Ni(tmc)(CH₃CN)]²⁺. The equilibrium and measured volume parameters are discussed in relation to the mechanism for solvent exchange.     

N2(C2H2)ASE ACTIVITY BY ALNUS INCANA SSP. RUGOSA (BETULACEAE) IN THE NORTHERN HARDWOOD FOREST

Field assays of N₂(C₂H₂)ase activity were performed with intact nodules from a pure alder site (alder) and a mixed alder-aspen site (aspen). Assays were performed between 12 June and 12 August 1980 and in May 1981. N₂(C₂H₂)ase rates are expressed as g N g nodule oven-dry wt⁻¹ hr⁻¹ (g N g⁻¹ hr⁻¹). Diurnal N₂(C₂H₂)ase activity showed an increase in both sites between 0600 and midday, then decreased to a low by 1800. Nighttime activity in the May 1981 assay was approximately 25% of the daytime peak. Mean (±SE) 1200 hr N₂(C₂H₂)ase activity (μg N g⁻¹ hr⁻¹) for all sizes in the alder stand rose from 24.56 ± 6.56 on 12 June to 73.96 ± 28.37 on 26 June and declined to 9.20 ± 2.56 by 12 August. In the aspen stand activity decreased from the 12 June rate of 21.81 ± 4.59 to 3.64 ± 1.87 on 24 July but then increased to 30.00 ± 7.39 by 12 August. Based on diurnal assays, the seasonal mean N influx (μg N g⁻¹ hr⁻¹) is statistically higher (P 0.05) in the alder stand with a value of 26.70 compared to 14.63 in the aspen stand. Small size class shrubs had significantly higher (P < 0.05) N₂(C₂H₂)ase activity (μg N g⁻¹ hr⁻¹) in diurnal assays than medium or large class shrubs. The estimated mean (±SE) N₂(C₂H₂)ase activity (mg N g⁻¹ season⁻¹) for all sizes was 44.4 ± 18.6 in the alder stand compared to 16.2 ± 5.2 in the aspen stand. Nodule excavations showed the g shrub⁻¹ in the alder stand to be 16.48 ± 10.29, 38.57 ± 12.34 and 29.11 ± 7.15 for small, medium and large size shrubs and 12.73 ± 3.23, 28.21 ± 4.36 and 56.45 ± 16.23 for respective sizes in the aspen stand. Seasonal N influx was 4.69 kg ha⁻¹ in the alder stand and 0.84 kg ha⁻¹ in the aspen stand, representing 17.9% of the alder stand. Nitrogen feedback inhibition from uric acid-N influx and allelochemic interference from aspen are discussed as explanations for the differences in N influx in the two stands.     

Contemporary carbon balance and late Holocene carbon accumulation in a northern peatland

Northern peatlands contain up to 25% of the world's soil carbon (C) and have an estimated annual exchange of CO₂‐C with the atmosphere of 0.1–0.5 Pg yr⁻¹ and of CH₄‐C of 10–25 Tg yr⁻¹. Despite this overall importance to the global C cycle, there have been few, if any, complete multiyear annual C balances for these ecosystems. We report a 6‐year balance computed from continuous net ecosystem CO₂ exchange (NEE), regular instantaneous measurements of methane (CH₄) emissions, and export of dissolved organic C (DOC) from a northern ombrotrophic bog. From these observations, we have constructed complete seasonal and annual C balances, examined their seasonal and interannual variability, and compared the mean 6‐year contemporary C exchange with the apparent C accumulation for the last 3000 years obtained from C density and age‐depth profiles from two peat cores. The 6‐year mean NEE‐C and CH₄‐C exchange, and net DOC loss are −40.2±40.5 (±1 SD), 3.7±0.5, and 14.9±3.1 g m⁻² yr⁻¹, giving a 6‐year mean balance of −21.5±39.0 g m⁻² yr⁻¹ (where positive exchange is a loss of C from the ecosystem). NEE had the largest magnitude and variability of the components of the C balance, but DOC and CH₄ had similar proportional variabilities and their inclusion is essential to resolve the C balance. There are large interseasonal and interannual ranges to the exchanges due to variations in climatic conditions. We estimate from the largest and smallest seasonal exchanges, quasi‐maximum limits of the annual C balance between 50 and −105 g m⁻² yr⁻¹. The net C accumulation rate obtained from the two peatland cores for the interval 400–3000 bp (samples from the anoxic layer only) were 21.9±2.8 and 14.0±37.6 g m⁻² yr⁻¹, which are not significantly different from the 6‐year mean contemporary exchange.     

A comparison of the 8-hydroxydeoxyguanosine,chromosome aberrations and micronucleus techniques for the assessment of the genotoxicity of mercury compounds in human blood lymphocytes

We compared the mechanism of action of micronuclei (MN), unstable chromosome aberrations, and 8-hydroxydeoxyguanosine (8-OHdG) levels to evaluate the genotoxicity of methyl mercuric chloride (CH₃HgCl) and mercuric chloride (HgCl₂) in human peripheral lymphocytes. The chromosome aberrations in human peripheral lymphocytes exposed to various concentrations of CH₃HgCl or HgCl₂ increased in a concentration-dependent manner and were significantly higher than the control when the cells were incubated with 1 × 10⁻⁵ M (HgCl₂) or 2 × 10⁻⁶ M (CH₃HgCl). The increase in the incidence of micronucleated lymphocytes was significant among the exposed groups, being 2 × 10⁻⁵ M (HgCl₂) and 5 × 10⁻⁶ M (CH₃HgCl) compared with the control. CH₃HgCl was about 4-fold more potent than HgCl₂. We determined the 8-OHdG levels in human peripheral blood mononuclear cells(PBMC) and found that they were significantly higher in the exposed groups at 1 × 10⁻⁵ M (HgCl₂) and 5 × 10⁻⁶ M (CH₃HgCl) compared with the control. A detectable (p < 0.05) increase in the level of 8-OHdG was induced by CH₃HgCl at a concentration that was about 50% of the amount of HgCl₂ required to produce a similar response. The data confirmed the value of the MN and/or chromosome aberration assays for assessing of HgCl₂- and/or CH₃HgCl-induced genotoxicity, and indicated that they are about the same concentration as the 8-OHdG assay. The presence of genotoxic effects in peripheral blood lymphocytes exposed to the mercuric compounds indicated by the chromosome aberrations and the MN assays could be partly due either to the disturbance of the spindle mechanism, or to the elevated level of 8-OHdG brought by the generation of reactive oxygen species.     

CH4 and N2O emissions from smallholder agricultural systems on tropical peatlands in Southeast Asia

There are limited data for greenhouse gas (GHG) emissions from smallholder agricultural systems in tropical peatlands, with data for non-CO₂ emissions from human-influenced tropical peatlands particularly scarce. The aim of this study was to quantify soil CH₄ and N₂O fluxes from smallholder agricultural systems on tropical peatlands in Southeast Asia and assess their environmental controls. The study was carried out in four regions in Malaysia and Indonesia. CH₄ and N₂O fluxes and environmental parameters were measured in cropland, oil palm plantation, tree plantation and forest. Annual CH₄ emissions (in kg CH₄ ha⁻¹ year⁻¹) were: 70.7 ± 29.5, 2.1 ± 1.2, 2.1 ± 0.6 and 6.2 ± 1.9 at the forest, tree plantation, oil palm and cropland land-use classes, respectively. Annual N₂O emissions (in kg N₂O ha⁻¹ year⁻¹) were: 6.5 ± 2.8, 3.2 ± 1.2, 21.9 ± 11.4 and 33.6 ± 7.3 in the same order as above, respectively. Annual CH₄ emissions were strongly determined by water table depth (WTD) and increased exponentially when annual WTD was above −25 cm. In contrast, annual N₂O emissions were strongly correlated with mean total dissolved nitrogen (TDN) in soil water, following a sigmoidal relationship, up to an apparent threshold of 10 mg N L⁻¹ beyond which TDN seemingly ceased to be limiting for N₂O production. The new emissions data for CH₄ and N₂O presented here should help to develop more robust country level ‘emission factors’ for the quantification of national GHG inventory reporting. The impact of TDN on N₂O emissions suggests that soil nutrient status strongly impacts emissions, and therefore, policies which reduce N-fertilisation inputs might contribute to emissions mitigation from agricultural peat landscapes. However, the most important policy intervention for reducing emissions is one that reduces the conversion of peat swamp forest to agriculture on peatlands in the first place.     

Kinetics and mechanism of aluminum(III)/siderophore ligand exchange: mono(deferriferrioxamine B)aluminum(III) formation and dissociation in aqueous acid solution

The kinetics and mechanism of a linear trihydroxamic acid siderophore (deferriferrioxamine B, H₄DFB⁺) ligand exchange with Al(H₂O)₆³⁺ to form mono(deferriferrioxamine B)aluminum(III) (Al(H₂O)₄H₃DFB)³⁺ have been investigated at 25 °C over the [H⁺] range 0.001−1.0 M and I = 2.0 M (HClO₄/NaClO₄) by ²⁷Al NMR. Kinetic results are consistent with Al(H₂O)₄(H₃DFB)³⁺ formation and dissociation proceeding through a parallel path mechanistic scheme involving Al(H₂O)₆³⁺(k₂/k₋₁) and Al(H₂O)₅(OH)²⁺(k₂/k₋₂) where k₁ = 0.13 M⁻¹ s⁻¹, k₋₁ = 8.7 × 10⁻³ M⁻¹ s⁻¹, k₂ = 2.7 × 10³ M⁻¹ s⁻¹, and k₋₂ = 9.6 × 10⁻⁴ s⁻¹. Relative complex formation rates at Al(H₂O)₆³⁺ and Al(H₂O)₅OH²⁺, and comparison with kinetic data for a series of synthetic hydroxamic acids, suggest that an interchange mechanism is operative. These results are also discussed in relation to kinetic data for the corresponding iron(III)-deferriferrioxamine B system.