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1.
The observed equilibrium constants (Kobs) for the reactions of d-2-phosphoglycerate phosphatase, d-2-Phosphoglycerate3? + H2O → d-glycerate? + HPO42?; d-glycerate dehydrogenase (EC 1.1.1.29), d-Glycerate? + NAD+ → NADH + hydroxypyruvate? + H+; and l-serine:pyruvate aminotransferase (EC 2.6.1.51), Hydroxypyruvate? + l-H · alanine± → pyruvate? + l-H · serine±; have been determined, directly and indirectly, at 38 °C and under conditions of physiological ionic strength (0.25 m) and physiological ranges of pH and magnesium concentrations. From these observed constants and the acid dissociation and metal-binding constants of the substrates, an ionic equilibrium constant (K) also has been calculated for each reaction. The value of K for the d-2-phosphoglycerate phosphatase reaction is 4.00 × 103m [ΔG0 = ?21.4 kJ/mol (?5.12 kcal/mol)]([H20] = 1). Values of Kobs for this reaction at 38 °C, [K+] = 0.2 m, I = 0.25 M, and pH 7.0 include 3.39 × 103m (free [Mg2+] = 0), 3.23 × 103m (free [Mg2+] = 10?3m), and 2.32 × 103m (free [Mg2+] = 10?2m). The value of K for the d-glycerate dehydrogenase reaction has been determined to be 4.36 ± 0.13 × 10?13m (38 °C, I = 0.25 M) [ΔG0 = 73.6 kJ/mol (17.6 kcal/mol)]. This constant is relatively insensitive to free magnesium concentrations but is affected by changes in temperature [ΔH0 = 46.9 kJ/mol (11.2 kcal/mol)]. The value of K for the serine:pyruvate aminotransferase reaction is 5.41 ± 0.11 [ΔG0 = ?4.37 kJ/mol (?1.04 kcal/mol)] at 38 °C (I = 0.25 M) and shows a small temperature effect [ΔH0 = 16.3 kJ/ mol (3.9 kcal/mol)]. The constant showed no significant effect of ionic strength (0.06–1.0 m) and a response to the hydrogen ion concentration only above pH 8.5. The value of Kobs is 5.50 ± 0.11 at pH 7.0 (38 °C, [K+] = 0.2 m, [Mg2+] = 0, I = 0.25 M). The results have also allowed the value of K for the d-glycerate kinase reaction (EC 2.7.1.31), d-Glycerate? + ATP4? → d-2-phosphoglycerate3? + ADP3? + H+, to be calculated to be 32.5 m (38 °C, I = 0.25 M). Values for Kobs for this reaction under these conditions and at pH 7.0 include 236 (free [Mg2+] = 0) and 50.8 (free [Mg2+] = 10?3m).  相似文献   

2.
The tissue distribution of folate in its numerous coenzyme forms may influence the development of disease at different sites. For instance, the susceptibility of human colonic mucosa to localized folate deficiency may predispose to the development of colorectal cancer. We report a sensitive and robust ultra high-performance liquid chromatography (UHPLC) tandem mass spectrometry method for quantifying tissue H4folate, 5-CH3-H4folate, 5-CHO-H4folate, folic acid, and 5,10-CH+-H4folate concentration. Human colonic mucosa (20–100 mg) was extracted using lipase and conjugase enzyme digestion. Rapid separation of analytes was achieved on a UHPLC 1.9-μm C18 column over 7 min. Accurate quantitation was performed using stable isotopically labeled (13C5) internal standards. The instrument response was linear over physiological concentrations of tissue folate (R2 > 0.99). Limits of detection and quantitation were less than 20 and 30 fmol on column, respectively, and within- and between-run imprecision values were 6–16%. In colonic mucosal samples from 73 individuals, the average molar distribution of folate coenzymes was 58% 5-CH3-H4folate, 20% H4folate, 18% formyl-H4folate (sum of 5-CHO-H4folate and 5,10-CH+-H4folate), and 4% folic acid. This assay would be useful in characterizing folate distribution in human and animal tissues as well as the role of deregulated folate homeostasis on disease pathogenesis.  相似文献   

3.
The observed equilibrium constants (Kobs) for the l-phosphoserine phosphatase reaction [EC 3.1.3.3] have been determined under physiological conditions of temperature (38 °C) and ionic strength (0.25 m) and physiological ranges of pH and free [Mg2+]. Using Σ and square brackets to indicate total concentrations Kobs = Σ L-serine][Σ Pi]Σ L-phosphoserine]H2O], K = L-H · serine±]HPO42?][L-H · phosphoserine2?]H2O]. The value of Kobs has been found to be relatively sensitive to pH. At 38 °C, K+] = 0.2 m and free [Mg2+] = 0; Kobs = 80.6 m at pH 6.5, 52.7 m at pH 7.0 [ΔGobs0 = ?10.2 kJ/mol (?2.45 kcal/mol)], and 44.0 m at pH 8.0 ([H2O] = 1). The effect of the free [Mg2+] on Kobs was relatively slight; at pH 7.0 ([K+] = 0.2 m) Kobs = 52.0 m at free [Mg2+] = 10?3, m and 47.8 m at free [Mg2+] = 10?2, m. Kobs was insignificantly affected by variations in ionic strength (0.12–1.0 m) or temperature (4–43 °C) at pH 7.0. The value of K at 38 °C and I = 0.25 m has been calculated to be 34.2 ± 0.5 m [ΔGobs0 = ?9.12 kJ/mol (?2.18 kcal/ mol)]([H2O] = 1). The K for the phosphoserine phosphatase reaction has been combined with the K for the reaction of inorganic pyrophosphatase [EC 3.6.1.1] previously estimated under the same physiological conditions to calculate a value of 2.04 × 104, m [ΔGobs0 = ?28.0 kJ/mol (?6.69 kcal/mol)] for the K of the pyrophosphate:l-serine phosphotransferase [EC 2.7.1.80] reaction. Kobs = [Σ L-serine][Σ Pi][Σ L-phosphoserine][H2O], K = [L-H · serine±]HPO42?][L-H · phosphoserine2?]H2O. Values of Kobs for this reaction at 38 °C, pH 7.0, and I = 0.25 m are very sensitive to the free [Mg2+], being calculated to be 668 [ΔGobs0 = ?16.8 kJ/mol (?4.02 kcal/mol)] at free [Mg2+] = 0; 111 [ΔGobs0 = ?12.2 kJ/mol (?2.91 kcal/mol)] at free [Mg2+] = 10?3, m; and 9.1 [ΔGobs0 = ?5.7 kJ/mol (?1.4 kcal/mol) at free [Mg2+] = 10?2, m). Kobs for this reaction is also sensitive to pH. At pH 8.0 the corresponding values of Kobs are 4000 [ΔGobs0 = ?21.4 kJ/mol (?5.12 kcal/mol)] at free [Mg2+] = 0; and 97.4 [ΔGobs0 = ?11.8 kJ/ mol (?2.83 kcal/mol)] at free [Mg2+] = 10?3, m. Combining Kobs for the l-phosphoserine phosphatase reaction with Kobs for the reactions of d-3-phosphoglycerate dehydrogenase [EC 1.1.1.95] and l-phosphoserine aminotransferase [EC 2.6.1.52] previously determined under the same physiological conditions has allowed the calculation of Kobs for the overall biosynthesis of l-serine from d-3-phosphoglycerate. Kobs = [Σ L-serine][Σ NADH][Σ Pi][Σ α-ketoglutarate][Σ d-3-phosphoglycerate][Σ NAD+][Σ L-glutamat0] The value of Kobs for these combined reactions at 38 °C, pH 7.0, and I = 0.25 m (K+ as the monovalent cation) is 1.34 × 10?2, m at free [Mg2+] = 0 and 1.27 × 10?2, m at free [Mg2+] = 10?3, m.  相似文献   

4.
5,10-Methylenetetrahydrofolate (5,10-CH2-H4PteGlu) was identified as a major active reduced folate in rat bile using high-performance liquid chromatography with electrochemical detection (HPLC—ED). The identification of the folate derivative was based on the similarities in the retention-time profiles, electrochemical properties, UV absorption characteristics and demethylenation profiles of the bile folate and the synthetic standard. An HPLC—ED method was developed for the simultaneous determination of reduced folates including 5,10-CH2-H4PteGlu, tetrahydrofolate (H4PteGlu), 10-formyltetrahydrofolate (10-HCO-H4PteGlu) and 5-methyltetrahydrofolate (5-CH3-H4PteGlu) in rat bile. All peaks of the reduced folates in bile were separated using this method with a total retention time of less than 15 min. The detection limit was 0.01 ng/injection for H4PteGlu, 10-HCO-H4PteGlu and 5-CH3-H4PteGlu, and 0.02 ng/injection for 5,10-CH2-H4PteGlu at a signal-to-noise ratio of 3 and an injection volume of 100 μl. Recoveries of synthetic folates from rat bile were higher than 90%. The distribution percentages of 5,10-CH2-H4PteGlu, H4PteGlu, 10-HCO-H4PteGlu and 5-CH3-H4PteGlu in rat bile were 29.6 ± 7.2, 17.7 ± 3.5, 24.4 ± 6.5 and 28.2 ± 7.1%, respectively, and total secretion rate of the bile reduced folates was 1514 ± 663 ng/h (mean ± S.D., n = 9).  相似文献   

5.
The reaction of Ru(XTPP)(DMF)2, where XTPP is the dianion of para substituted tetraphenylporphyrins and X is MeO, Me, H, Cl, Br, I, F, with O2 and CO were studied in DMF. The process was found to be first-order in metalloporphyrin, first-order in molecular oxygen and carbon monoxide, and second-order overall. Second-order rate constants for the CO reaction ranged from 0.170 to 0.665 M?1 s?1 at 25°C, those for the O2 reaction from 0.132 to 0.840 M?1 s?1 at 25°C. Similar activation parameters (ΔHCO± = 87 ± 1 kJ mol?1, ΔSCO± = 22 ± 4 JK?1 mol?1; ΔHO2± = 81 ± 1 kJ mol?1, and ΔSO2± = 11 ± 5 JK?1 mol?1) were found within each series. Reactivities of X substituted metalloporphyrins were found to follow different Hammett σ functions. The CO reactions correlated with σ? following normal behavior; the O2 reactions correlated with σ8° indicating O2 is π-bonded in the transition states. A dissociative mechanism is postulated for the process.  相似文献   

6.
《Inorganica chimica acta》1988,142(2):291-299
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.  相似文献   

7.
The stopped flow technique has been used to study the kinetics of complex formation of iron(III) with pyridoxal-5-phosphate (PLP) in the pH range 1.00–2.50, and in the temperature range 18 °C– 30 °C, at an ionic strength of. 0.50 M (NaCl). From the initial concentration dependence of PLP (TPLP,) of the reaction rate it can be shown that two kinetic steps can be represented as: kobs′ = mi + miPLP where mi and mi′ are pH-dependent parameters. The calculated activation data are δE* = 23.2 ± 1.8 kcal mol?1 and 10.98 ± 0.53 kcal mol?1 for the first and second kinetic steps, respectively and δS* are ?20.50 ± 5.96 e.u. and 24.62 ± 1.81 e.u., respeetively.  相似文献   

8.
Global warming is associated with the continued increase in the atmospheric concentrations of greenhouse gases; carbon dioxide, methane (CH4) and nitrous oxide. Wetlands constitute the largest single natural source of atmospheric CH4 in the world contributing between 100 and 231 Tg year?1 to the total budget of 503–610 Tg year?1, approximately 60 % of which is emitted from tropical wetlands. We conducted diffusive CH4 emission measurements using static chambers in river channels, floodplains and lagoons in permanent and seasonal swamps in the Okavango Delta, Botswana. Diffusive CH4 emission rates varied between 0.24 and 293 mg CH4 m?2 h?1, with a mean (±SE) emission of 23.2 ± 2.2 mg CH4 m?2 h?1 or 558 ± 53 mg CH4 m?2 day?1. These emission rates lie within the range reported for other tropical wetlands. The emission rates were significantly higher (P < 0.007) in permanent than in seasonal swamps. River channels exhibited the highest average fluxes at 31.3 ± 5.4 mg CH4 m?2 h?1 than in floodplains (20.4 ± 2.5 mg CH4 m?2 h?1) and lagoons (16.9 ± 2.6 mg CH4 m?2 h?1). Diffusive CH4 emissions in the Delta were probably regulated by temperature since emissions were highest (20–300 mg CH4 m?2 h?1) and lowest (0.2–3.0 mg m?2 h?1) during the warmer-rainy and cooler winter seasons, respectively. Surface water temperatures between December 2010 and January 2012 varied from 15.3 °C in winter to 33 °C in summer. Assuming mean inundation of 9,000 km2, the Delta’s annual diffusive emission was estimated at 1.8 ± 0.2 Tg, accounting for 2.8 ± 0.3 % of the total CH4 emission from global tropical wetlands.  相似文献   

9.
The oxidation enthalpy of reduced flavin mononucleotide at pH 7.0 in 0.2 m phosphate buffer has been studied by determining the heat associated with the reaction: FMNH2 + 2 Fe(CN)?36 ? FMN + 2 Fe(CN)?46 + 2 H+. (a) (The quinone, semiquinone, and hydroquinone forms of FMN are represented as FMN, FMNH, and FMNH2, respectively.) Calorimetric experiments were performed in a flow microcalorimeter which was modified to prevent sample contamination by oxygen. The enthalpy observed for reaction (a), after correction for dilution and buffer effects, was ?39.2 ± 0.4 kcal (mole FMNH2)?1 at 25 °C. The potential difference, ΔE′, developed by reaction (a) was determined potentiometrically and corresponded to a free energy change, ΔG′, of ?30.3 kcal (mole FMNH2)?1. The resulting entropy change, ΔS′, was thus calculated to be ?29.8 e.u. Reaction (a) was also studied at temperatures of 7 °C and 35.5 °C. ΔCp′ for the reaction was calculated as ?155 ± 18 cal deg?1 (mole FMNH2)?1 at 20 °C. ΔH′ for the reaction (b), FMNH2 ? FMN + H2, (b) was calculated as +14.2 ± 0.7 kcal mole?1 at 25 °C, relative to the enthalpy of the hydrogen electrode being identically equal to zero at all values of pH and temperature. The free energy at pH 7.0 for reaction (b), calculated from the potential was found to be ?9.7 kcal mole?1, which resulted in an entropy for reaction (b) of 80.2 e.u. A thermal titration of reaction (a) was used to calculate the thermodynamic parameters for the formation of semiquinone dimer according to the reaction FMNH2 + FMN ? (·FMNH)2. (c) The free energy, enthalpy, and entropy changes for reaction (c) were estimated to be ?6.1 kcal mole?1, ?7 kcal mole?1, and ?3 e.u., respectively.  相似文献   

10.
The synthesis is reported of a series of ternary cationic complexes of general form [Co(R,Rpicchxn)(ARMA)+ (where picchxn is the N4 tetradentate N,N′-di(2-picolyl)-1,2-diaminocyclohexane and ARMA is a bidentate α-substituted-α-aminomalonate dianion). The aminomalonic acid (NH2· C(COOH)2·R) derivatives investigated have R = -CH3 (AMMAH2), -CH2·CH3 (AEMAH2), -CH2· CH2·CH3 (APMAH2), -CH2·(CH2)2·CH3 (ABuTMAH2, -CH2·C6H5 (ABMAH2), -CH2·(p-C6H4)· C(CH3)3 (ABuBMAH2) and -CH2·C10H7 (ANMAH2). The isomeric species in the complex products have been separated using cation exchange chromatography and each isomer has been characterized using NMR and circular dichroism techniques. In each synthesis the major isomeric product obtained has a Λ-β1 topology. However, where ARMAH2 possesses a lengthy alkyl sidechain trace amounts of Δ-α-[Co(R,R-picchxn)(ARMA)]+ isomers have been observed during the synthetic reactions. This unusual isomeric form readily undergoes inversion of its absolute configuration in DMSO solution to yield the more thermodynamically stable Λ-β1-[Co(R,R-picchxn()R-ARMA)]+ species stereospecifically.In the case of Λ-β1-[Co(R,R,-pichxn)(S-APMA)]ClO4·2NaClO4·5H2O the crystal structure has been determined. The compound crystallises in the orthorhombic space group P212121, with a = 10.056(3),b = 16.475(7),c = 23.370(7)Å and Z = 4. The structure was refined to R = 0.079 for 4460 non-zero reflexions, and confirms the absolute configuration of each chiral centre to be consistent with the NMR and circular dichroism interpretations.The decar☐ylation of these chelate ARMAH2 derivatives under acid conditions leads to corresponding complexes containing mixtures of coordinated R-andS-α-aminoacids in various ratios. This ratio has been determined in each case, and factors which may influence the degree of chiral induction observed are discussed.  相似文献   

11.
The sec, rac-CH3Co(H2O)L2+ (L=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) was prepared successfully via meso-CH3Co(H2O)L2+ in aqueous solution. The isomerizations from meso-RCo(H2O)L2+ (R=CH3, C2H5 and C3H7) and sec, rac-CH3Co(H2O)L2+ to pri, rac-RCo(H2O)L2+ were both base catalyzed in aqueous solution. The kinetic results showed the reaction to be first order in both organocobalt complex and hydroxide ion with the reactivity order for the alkyl group being C3H7 ∼ C2H5 ? CH3. However, the conversion from the most steric hindered isomer form of sec, rac- was slow. The ratio of the isomerization rate constants between meso-CH3Co(H2O)L2+ and sec, rac-CH3Co(H2O)L2+ to pri, rac-CH3Co(H2O)L2+ is almost a factor of 100. The thermodynamic activation parameters for these isomerization reactions were investigated.  相似文献   

12.
The aim of this study was to evaluate the daily variations in the thermoregulatory behavior of 4- to 6-week-old naked neck broilers (Label Rouge) in an equatorial semi-arid environment. A total of 220 birds were monitored for 5 days starting at 0600 hours and ending at 1800 hours. The period of observation was divided into classes of hours (C H). The observed behaviors were as follows: feed and water intake, wing-spreading, sitting or lying, and beak-opening. A total of 14,300 behavioral data values were registered. In C H 2 (0900 hours to 1100 hours) and 3 (1200 hours to 1500 hours), the greatest average body surface temperature was recorded (34.67?±?0.25 °C and 35.12?±?0.22 °C, respectively). The C H had an effect on the exhibition of all behaviors with the exception of the water intake behavior. Feed intake was more frequent in C H 1 (0600 hours to 0800 hours) and 4 (1600 hours to 1800 hours). In C H 2 and 3, the highest frequency of sitting or lying behavior was observed. Beak-opening and wing-spreading behaviors occurred more frequently in C H 3 where the body surface temperature (35.12?±?0.22 °C), radiant heat load (519.38?±?2.22 W m?2), and enthalpy (82.74?±?0.36 kJ kg?1 of dry air) reached maximum recorded averages. Thus, it can be concluded that naked neck broilers adjust their behavior in response to daily variations in the thermal environment. Wing-spreading and beak-opening behaviors are important adaptive responses to the thermal challenges posed by the equatorial semi-arid environment.  相似文献   

13.
A 14-membered tetraaza macrocycle, 2,13-bis(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.1.1807.12]docosane (L2) bearing two N-CH2CH2COOMe groups, and its nickel(II) and copper(II) complexes have been prepared and characterized. The nickel(II) and copper(II) complexes of 2-(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.1.1807.12]docosane (L3) containing one N-CH2CH2COOMe group have also been prepared. The crystal structure of [NiL2](ClO4)2 shows that the complex has a slightly distorted trans-octahedral coordination geometry with two relatively short axial Ni-O (N-CH2CH2COOMe group) bonds (2.136(3) Å). In various solvents, however, a considerable proportion of [NiL2]2+ exists as a square-planar form, in which the functional pendant arms are not involved in coordination. The proportion of the square-planar isomer varies with solvents in the order of nitromethane ? acetonitrile < H2O < DMF ? DMSO. In the case of [CuL2](ClO4)2, only one N-CH2CH2COOMe group is involved in coordination. The N-CH2CH2COOMe group of [NiL3](ClO4)2 is not directly involved in coordination even in the solid state, though the functional group of [CuL3](ClO4)2 is coordinated to the metal ion.  相似文献   

14.
5-CH3[G-3H]H4PteGlu (sp act 5.0 Ci/nmol) was synthesized by direct enzymatic reduction of [G-3H]PteGlu by dihydropteroylglutamate reductase to H4PteGlu. The latter was reacted with formaldehyde to give 5,10-CH2-H4-PteGlu which was reduced to the final product by sodium borohydride. A single chromatographic step on TEAE-cellulose afforded 5-CH3[G-3H]H4PteGlu in high yield at a radiochemical purity of about 99%.  相似文献   

15.
Chimeric oligodeoxyribonucleotides where the phosphodiester linkage -C3′-O-PO2? -O-CH2-C4′- of DNA is substituted by the amide linkage -C3′-CH2-CH*(CH3)-CO-NH-CH2-C4′ (*either R or S stereochemistry) have been prepared and their binding to RNA targets have been investigated. Incorporation of a single amide unit increases the Tm by approximately 1.4–1.9°C. Circular dichroic spectra of these modified duplexes are similar to the wildtype DNA/RNA.  相似文献   

16.
The binding of[Co(CN)6]3?, and that of[Fe(CN)6]3? and [Ru(CN)6]4? using a competitive method, to horse cytochrome c has been studied by 59 Co NMR spectroscopy. At I = 0.07 M, without added salt and in 2H2O at ph* 7.3 (measured in 2H2O) and 25°C, there are at least two binding sites on ferricytochrome c and ferrocytochrome c for [Co(CN)6]3?. Association constants were determined to be 2.0 ± 0.6 × 103M?1 and 1.5 ± 0.5 × 102M?1 respectively. with no effect of the oxidation state of the cytochrome. At higher ionic strength (I = 0.12 M adjusted with KCl the binding markedly decreased, and, although it was not possible to determine the precise binding stoichiometry and magnitude of association constants, it is clear that the association constants are ≤ 1.5 × 10tM?1 The binding of [Ru(CN)6]4? at I = 0.07, without added salt and in 2H2O at pH 1.3 and 23°C, was not precisely defined, but its binding strength relative to that of [Fe(CN)6]3? was determined. Extrapolating this to I = 0.12 (KCl) suggests that under these conditions the association constant for [Ru(CN)6]4? binding to ferricytochrome c is ≤ 3 × 102M?1.  相似文献   

17.
Forest soils and canopies are major components of ecosystem CO2 and CH4 fluxes. In contrast, less is known about coarse woody debris and living tree stems, both of which function as active surfaces for CO2 and CH4 fluxes. We measured CO2 and CH4 fluxes from soils, coarse woody debris, and tree stems over the growing season in an upland temperate forest. Soils were CO2 sources (4.58 ± 2.46 µmol m?2 s?1, mean ± 1 SD) and net sinks of CH4 (?2.17 ± 1.60 nmol m?2 s?1). Coarse woody debris was a CO2 source (4.23 ± 3.42 µmol m?2 s?1) and net CH4 sink, but with large uncertainty (?0.27 ± 1.04 nmol m?2 s?1) and with substantial differences depending on wood decay status. Stems were CO2 sources (1.93 ± 1.63 µmol m?2 s?1), but also net CH4 sources (up to 0.98 nmol m?2 s?1), with a mean of 0.11 ± 0.21 nmol m?2 s?1 and significant differences depending on tree species. Stems of N. sylvatica, F. grandifolia, and L. tulipifera consistently emitted CH4, whereas stems of A. rubrum, B. lenta, and Q. spp. were intermittent sources. Coarse woody debris and stems accounted for 35% of total measured CO2 fluxes, whereas CH4 emissions from living stems offset net soil and CWD CH4 uptake by 3.5%. Our results demonstrate the importance of CH4 emissions from living stems in upland forests and the need to consider multiple forest components to understand and interpret ecosystem CO2 and CH4 dynamics.  相似文献   

18.
Kinetic studies of the reduction of ferrioxamine B (Fe(Hdesf)+) by Cr(H2O)62+, V(H2O)62+, and dithionite have been performed. For Cr(H2O)62+ and V(H2O)62+, the rate is ?d[Fe(Hdesf)+]/dt = k[Fe(Hdesf)+][M2+]. For Cr(H2O)62+, k = 1.19 × 104 M?1 sec?1 at 25°C and μ = 0.4 M, and k is independent of pH from 2.6 to 3.5. For V(H2O)62+, k = 6.30 × 102 M?1 sec?1 at 25°C, μ = 1.0 M, and pH = 2.2. The rate is nearly independent of pH from 2.2 to 4.0. For Cr(H2O)62+ and V(H2O)62+, the activation parameters are ΔH = 8.2 kcal mol?1, ΔS ?12 eu and ΔH = 1.7 kcal mol?1, ΔS = ?40 eu (at pH 2.2) respectively. Reduction by Cr(H2O)62+ is inner-sphere, while reduction by V(H2O)62+ is outer-sphere. Reduction by dithionite follows the rate law ?d[Fe(Hdesf)+]/dt =kK12[Fe(Hdesf)+][S2O42?]12 where K is the equilibrium constant for dissociation of S2O42? into SO2? radicals. The value of k at 25°C and μ = 0.5 is 2.7 × 103 M?1 sec?1 at pH 5.8, 3.5 × 103 M?1 sec?1 at pH 6.8, and 4.6 × 103 M?1 sec?1 at pH 7.8, and ΔH = 6.8 kcal mol?1 and ΔS = ?19 eu at pH 7.8.  相似文献   

19.
ABSTRACT

Monomethylmercury (CH3Hg +) is both the most ecologically significant and the least well characterized species of mercury in environmental settings. Our understanding of the environmental speciation behavior of this compound is limited both as the result of lesser available laboratory data (when compared to inorganic mercury) as well as the uncertainties associated with our understanding of the properties of environmental ligands. A careful examination and synthesis of data reported in the technical literature led to the following findings: (1) a 25°C, zero ionic strength bicarbonate ion complexation constant estimate is remarkably close to an earlier reported value at 0.4 M: CH3Hg+ + HCo3-?CH3HgHCO3,log10K = 2.6 (±0.22, 1 SD), (2) three 25°C zero ionic strength reaction constants reported by DeRobertis et al.(1998) were confirmed to within ~±0.1 log10K units: CH3Hg ++ OH-?CH3HgOH, log10K = 9.47; 2CH3Hg + + H2O?(CH3Hg)2OH + + H+, log10K =?2.15; CH3Hg ++ Cl-?CH3HgCl, log10K = 5.45, (3) “best estimate” literature complexation constants corrected to zero ionic strength include: CH3Hg + + F-?CH3HgF, log10K = 1.75 (20°C corr. Schwart-zenbach and Schellenberg, 1965); CH3Hg + + Br-?CH3HgBr, log10K = 6.87 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg + +1-?CH3HgI, log10K = 8.85 (20°C corr. Schwartzenbach and Schellenberg, 1965); and CH3Hg ++ SO42-?CH3HgSO4-,log10K = 2.64 (25°C, DeRobertis et al., 1998), (4) literature reported values for simulating monomethylmercury complexation with the carbonate ion may be too low: CH3Hg ++ CO32-?CH3HgCO3-, log10K = 6.1 (Rabenstein et al., 1976; Erni, 1981), and (5) ‘‘best estimate’’ constants for simulating methyl mercury complexation with reduced environmental sulfur species include: CH3Hg + + S2-?CH3HgS -, log10K = 21.1; CH3Hg ++ SH -? CH3HgSH, log10K = 14.5 (H + + SH-?CH2S, log10K = 6.88; Dyrssen and Wedborg, 1991); CH3Hg + + RS-?CH3HgSR, log10K = 16.5 (H + + RS-?RSH, log10K = 9.96; Qian et al., 2002); and CH3Hg ++ CH3HgS1 -?(CH3Hg)2S, log10K = 16.32 (Schwartzenbach and Schellenberg, 1965; Rabenstein et al., 1978; and Erni, 1981).  相似文献   

20.
The endogenous levels of the various folate compounds in rat liver were determined using high-pressure liquid chromatography for the rapid separation of folate monoglutamate forms with specific quantitation of the folates by microbiological analysis of eluted fractions. The eight folate derivatives that were assayable were tetrahydrofolic acid (H4PteGlu), 5-methyl-H4PteGlu, 10-formyl-H4PteGlu, 5-formyl-H4PteGlu, 5,10-methenyl-H4PteGlu, 5,10-methylene-H4PteGlu, H2PteGlu, and PteGlu. New techniques for the preparation of tissues were developed in order to reduce the degradation of the folates. Tissue folates were converted to the monoglutamate form by a partially purified hog kidney polyglutamate hydrolase preparation and incubations were carried out at pH 6.0. This minimized folate degradation but still allowed for maximal polyglutamate hydrolase activity. Rapid removal of tissues was compared with freeze-clamping techniques. The major folates in rat liver were H4PteGlu and 5-methyl-H4PteGlu, comprising 42 and 39%, respectively, of the total liver folate pool of 27.30 nmol/g liver (about 13 μg/g liver). In addition, 10-formyl-H4PteGlu and 5-formyl-H4PteGlu each comprised 10% of the total folate pool. No endogenous PteGlu, H2PteGlu, or 5,10-methylene-H4PteGlu was detected in rat liver samples under our conditions. Distribution of 14C derived from a previous [14C]folic acid injection paralleled the distribution of folate as determined microbiologically after high-pressure liquid chromatography separation. The importance of these methods for the direct determination and estimation of flux of H4PteGlu, 5-methyl-H4PteGlu, and 10-formyl-H4PteGlu in studies dealing with the folate system was emphasized.  相似文献   

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