H2 oxidation,O2 uptake and CO2 fixation in hydrogen treated soils

In many legume nodules, the H₂ produced as a byproduct of N₂ fixation diffuses out of the nodule and is consumed by the soil. To study the fate of this H₂ in soil, a H₂ treatment system was developed that provided a 300 cm³ sample of a soil:silica sand (2:1) mixture with a H₂ exposure rate (147 nmol H₂ cm^–3hr^–1) similar to that calculated exist in soils located within 1–4 cm of nodules (30–254 nmol H₂ cm^–3hr^–1). After 3 weeks of H₂ pretreatment, the treated soils had a K_m and V_max for H₂ uptake (1028 ppm and 836 nmol cm^–3 hr^–1, respectively) much greater than that of control, air-treated soil (40.2 ppm and 4.35 nmol cm^–3 hr^–1, respectively). In the H₂ treated soils, O₂, CO₂ and H₂ exchange rates were measured simultaneously in the presence of various pH₂. With increasing pH₂, a 5-fold increase was observed in O₂ uptake, and CO₂ evolution declined such that net CO₂ fixation was observed in treatments of 680 ppm H₂ or more. At the H₂ exposure rate used to pretreat the soil, 60% of the electrons from H₂ were passed to O₂, and 40% were used to support CO₂ fixation. The effect of H₂ on the energy and C metabolism of soil may account for the well-known effect of legumes in promoting soil C deposition.     

Contribution of plants to N 2 O emissions in soil-winter wheat ecosystem: pot and field experiments

Outdoor pot and field experiments were conducted to assess the role of growing plants in agricultural ecosystem N₂O emissions. N₂O emissions from plants were quantified as the difference in soil-crop system N₂O emissions before and immediately after cutting plants during the main growth stages in 2001–02 and 2002–03 winter wheat seasons. Emissions of N₂O from plants depended on biomass within the same plant developmental status. Field results indicated that the seasonal contribution of N₂O emissions from plants to ecosystem fluxes averaged 25%, ranging from 10% at wheat tillering to 62% at the heading stage. The fluxes of N₂O emissions from plants varied between 0.3 and 3.9 mg N₂O-N m⁻² day⁻¹ and its seasonal amount was equivalent to 0.23% of plant N released as N₂O. A N₂O emission coefficient (N₂O_E, mg N₂O-N g⁻¹ C day⁻¹), defined as N₂O-N emission in milligrams from per gram carbon of plant dry matter within a day, was represented by a 5-fold variation ranging from 0.021 to 0.004 mg N₂O-N g C⁻¹ day⁻¹. A linear relationship (y=0.4611x+0.0015, r ²=0.9352, p < 0.001) between N₂O_E (y) and plant dark respiration rate (x, mg CO₂-C g C⁻¹ day⁻¹) suggested that in the absence of photosynthesis, some N₂O production in plant N assimilation was associated with plant respiration. Although this study could not show whether N₂O was produced or transferred by winter wheat plants, these results indicated an important role for higher plant in N₂O exchange. Identifying its potential contribution is critical for understanding agricultural ecosystem N₂O sources.     

Effect of lime, dicyandiamide and soil water content on ammonia and nitrous oxide emissions following application of liquid hog manure to a marshland soil

The loss of nitrogen (N) from field-applied animal manure through ammonia (NH₃) volatilisation and nitrous oxide (N₂O) emission is of major environmental concern. Both lime and dicyandiamide (DCD) have been suggested as amendments that can mitigate N₂O emissions, but simultaneously increase the risk of NH₃ volatilisation. This study evaluated the impact of lime and DCD on NH₃ and N₂O emissions following application of liquid hog manure. Hydrated lime (Ca(OH)₂) was added to an acidic soil to achieve three pH levels (4.7, 6.3 and 7.4). Soil samples (100 g) were then placed in 500 ml screw-top Mason-jars and de-ionised water was added to bring the samples to 50, 70 and 90% water-filled pore space (WFPS). Slurry was applied at a rate equivalent to 116,000 l ha⁻¹, while DCD was applied at 30% of the NH₄-N rate applied. Jars were sealed and incubated at 21°C for 21 d. Ammonia volatilisation was quantified using boric acid traps, while N₂O gas concentration was analysed using gas chromatography. Dicyandiamide had no effect (P>0.05) on either NH₃ or N₂O emissions. Both NH₃ and N₂O emissions increased (P<0.05) as WFPS increased, with emissions ranging from 0.9 to 1.4 kg NH₃-N ha⁻¹ and 123 to 353 g N₂O-N ha⁻¹, respectively. Liming decreased (P<0.01) N₂O emissions from 547 to 46 g N₂O-N ha⁻¹, but increased (p<0.01) NH₃ volatilisation from 0.36 to 1.92 kg NH₃-N ha⁻¹. Results suggest that liming to a pH ≥6.3 can reduce N₂O emissions, however, this reduction will be accompanied by a substantial loss of NH₃. Section Editor: H. Lambers     

Outdoor mass culture ofAzolla spp.: yields and efficiencies of nitrogen fixation

Summary The productivity of three species of Azolla (A. pinnata, A. filiculoides andA. caroliniana) in outdoor culture has been evaluated at different planting densities. The highest yields were obtained with biomass concentration ranging from 40 to 70g d.w. m^–2. The mean productivity over a 90 days period (from May 10th to August 10th) ranged from 10g d.w. m^–2 day^–1 forA. filiculoides up to 11.5 g d.w. m^–2 day^–1 forA. caroliniana. The nitrogen content of the dried biomasses was 48.3 mg (g d.w.)^–1 forA. pinnata, 51.5mg (g d.w.)^–1 forA. filiculoides and 52.3 mg (g d.w.)^–1 forA. caroliniana. Very little variations of the nitrogen content of the ferns during the experimental period were observed.The nitrogen-fixing efficiency of the Azolla-Anabaena azollae symbiosis grown in outdoor conditions was evaluated both by direct measurement of the amount of N₂ fixed by the culture and by the C₂H₂-reduction and H₂-evolution tests in an air atmosphere. These tests were performed outdoor under the same environmental conditions as the growing cultures. For all the species the ratios of C₂H₂-reduced to N₂-fixed were unexpectedly low, ranging from 2.04 (A. pinnata) to 1.50 (A. caroliniana).The results suggest that the reliability of the C₂H₂-reduction assay, particularly when applied to complex biological N₂-fixing systems, must be re-examined.     

Acetylene reduction (N2-fixation) by Enterobacteriaceae isolated from industrial wastewaters and biological treatment systems

Summary Acetylene reducing (N₂-fixing) Entero-bacteriaceae have been isolated from activated sludge plants treating waste from the paper and food industries (10³ to 10⁶ cells per ml) and from composting plants handling forest waste (10⁵ to 10⁶ cells per g wet weight). Detailed studies on se-lected strains of all taxa showed that: (1) pure cul-tures were able to utilize a range of carbohy-drates, polyols, amino acids and carboxylic acids as sole sources of carbon (2) high levels of nitro-genase were attained during growth with a range of carbon substrates: highest levels (12—66 n mole C₂H₄.min⁻¹.mg protein⁻¹) were found for glucose and sucrose, variable levels for polyols, and lower levels for citrate and fumarate (1—23 n mole C₂H₄.min⁻¹.mg protein⁻¹) (3) organic ni-trogen compounds which were utilized as sole sources of nitrogen did not generally repress the synthesis of nitrogenase, although low levels were found for some strains during growth with glu-cosamine. Samples from a laboratory model acti-vated sludge system showed a mean rate of acety-lene reduction corresponding to the fixation of 26 μg N.h⁻¹.1⁻¹, and direct analysis of the in-fluent and effluent waters and sludge showed a net increase in nitrogen. These observations corre-lated with the presence of a population of N₂-fix-ing Enterobacteriaceae of ca. 10⁵ cells per ml and pure strains isolated from the system had a mean nitrogenase specific activity of 88 n mole C₂H₄.min⁻¹.mg protein⁻¹. It is therefore con-cluded that endogenous N₂-fixing Enterobacteria-ceae contained in some kinds of industrial waste-waters could successfully be used to diminish the addition of combined nitrogen to activated sludge treatment plants.     

Role of Microorganisms in Emission of Nitrous Oxide and Methane in Pulse Cultivated Soil Under Laboratory Incubation Condition

Soil from a pulse cultivated farmers land of Odisha, India, have been subjected to incubation studies for 40 consecutive days, to establish the impact of various nitrogenous fertilizers and water filled pore space (WFPS) on green house gas emission (N₂O & CH₄). C₂H₂ inhibition technique was followed to have a comprehensive understanding about the individual contribution of nitrifiers and denitrifiers towards the emission of N₂O. Nevertheless, low concentration of C₂H₂ (5 ml: flow rate 0.1 kg/cm²) is hypothesized to partially impede the metabolic pathways of denitrifying bacterial population, thus reducing the overall N₂O emission rate. Different soil parameters of the experimental soil such as moisture, total organic carbon, ammonium content and nitrate–nitrogen contents were measured at regular intervals. Application of external N-sources under different WFPS conditions revealed the diverse role played by the indigenous soil microorganism towards green house gas emission. Isolation of heterotrophic microorganisms (Pseudomonas) from the soil samples, further supported the fact that denitrification might be prevailing during specific conditions thus contributing to N₂O emission. Statistical analysis showed that WFPS was the most influential parameter affecting N₂O formation in soil in absence of an inhibitor like C₂H₂.     

Production of N2O in soil during decomposition of dead yeast cells with different spatial distributions

Production and sources of N₂O were determined in soil columns amended with autoclaved yeast cells either mixed into or added as 0.5 cm³ lumps to the soil in combination with no or 200 g NO₃ ^--N g^-1. At four occasions over a two-week study period, subsets of cores were measured for N₂O production during 4-hour incubations under atmospheres of ambient air, 10 Pa of C₂H₂, and N₂, respectively. Denitrification enzyme activity (DEA) was assessed in subsamples of cores that had been incubated continuously under air.Autoclaved yeast provided a C-source readily available for denitrifying bacteria in the soil. Nitrous oxide production was negligible in unamended columns whereas accumulated N₂O losses in the presence of yeast material were substantial, varying between 15 to 49 ng N₂O-N g^-1 h^-1. Mixing yeast into the soil caused the highest production of N₂O followed by the yeast lump and no yeast treatments. Incubation in the presence of 10 Pa C₂H₂ indicated that denitrification was the sole source of N₂O, in accordance with an increase in DEA. Nitrous oxide production and DEA peaked after 4–7 days of incubation, and both were unaffected by additional NO₃ ^-. Two-to four-fold responses to anaerobiosis and accumulation of NO₃ ^- and NH₄ ⁺ in proximity of the lumps indicated that N₂O production here was limited by relatively low C-availability. In contrast, 10- to 12-fold responses to anaerobiosis and no accumulation of inorganic N suggested a higher C-availability where yeast was mixed into the soil.     

Soil core method for direct simultaneous determination of N2 and N2O emissions from forest soils

In order to quantify N₂-emissions from a spruce and a beech site at the Höglwald Forest, a new measuring system was developed, that allowed simultaneous, direct determination of N₂- and N₂O-emission with high accuracy (detection limit approx. 10 g N m^–2 h^–1 for N₂ and <1 g for N₂O) using a gas-flow core method. This method requires exchange of the soil atmosphere with an artificial atmosphere, that differs only in that N₂ is substituted by He. The measuring system, the methodology of measurements and validation experiments are described in detail. Due to the huge heterogeneity of denitrification activity in different soil cores taken from our forest sites, no general trends of N₂ and N₂O production in relation to soil moisture and temperature could be demonstrated. Based on reasonable number of measurements, this work gives for the first time an estimate of the magnitude of N₂-losses from temperate forest soils. Both the magnitude of N₂-emissions (spruce: 7.2±0.7 kg N₂-N ha^–1 yr^–1; beech: 12.4±3.1 kg N₂-N ha^–1 yr^–1), as well as the N₂O–N₂ ratio (spruce: 0.136±0.04; beech: 0.52±0.19) were significantly higher for soils from the beech sites as compared to soils from the spruce site. The results suggests that N₂-emissions from N-saturated forest soils, still receiving high loads of atmospheric N-deposition, are approx. 30% of atmospheric N-input at the spruce site, and approx. 50% at the beech site. Our results demonstrate that losses of nitrogen in the form of N₂ cannot be neglected in the context of calculating N-balances for given forest sites.     

Gas phase H<Subscript>2</Subscript>S product recovery in a packed bed bioreactor with immobilized sulfate-reducing bacteria

An N₂ strip gas was used in a packed bed sulfate-reducing bioreactor to recover the dissolved sulfide product and improve sulfate conversion. The highest volumetric productivity obtained was 261 mol H₂S m⁻³ d⁻¹. Lowering the initial pH of the medium from 7 to 6 increased the H₂S content of the strip gas from 3.6 to 5.8 mol%. The ratio of strip gas to liquid flow rates (G/L) was found be to a suitable basis for scaling the process. Calculations indicated that modest G/L values (<10²) were required to recover the residual dissolved sulfide in a downstream stripping column.     

Paramagnetic spin catalysis of a radical recombination reaction

Recombination of the triplet state radical pair consisting of two hydrogen atoms catalysed by molecular oxygen is considered as a simulating example of a paramagnetic-exchange catalytic process. Intermolecular exchange interaction in the collision complex between the H₂ and O₂ molecules is calculatedab initio in STO-6G and 6–31 G^* basis sets with complete active space configuration interaction. Calculations are done at a fixed O–H distance (3 Å), scanning the H–H bond length from 0.6 till 12 Å at the linear geometry of collision. The mixture of the triplet (T)³ _u ⁺ and singlet (S)¹ _g ⁺ states of the hydrogen moiety is possible because both states have the same triplet symmetry in the collision complex with O₂ (³ _g ^– ). A strong mixture of the S (¹ _g ⁺ , H₂ +³ _g ^– , O₂) and T) and T (³ _u ⁺ , H₂ +³ _g ^– , O₂) states is actually obtained even at large H–H distances. The quintet and singlet states^5,1(³ _u ⁺ , H₂ +³ _g ^– , O₂) are also considered for comparison of the exchange potentials. Atr(H–H)4.4 Å the S-T splitting is approximately constant (12 cm^-1 in the STO-6G basis set; 55.5 cm^-1 in the 6–31 G^* basis set) and is determined by the exchange interaction between O₂ and the nearest hydrogen atom in the O–O...H fragment. The paramagnetic catalyst can accelerate radical recombination through the triplet-singlet nonadiabatic transition to the lowest S reactive state when the radical encounter takes place in the vicinity of the catalyst. Though we do not consider the radical dynamics in a real solvent, which modulates the exchange potentials and the T-S transitions, the nature of this mechanism of spin catalysis is obvious. The electric polarization and charge transfer are important in the analysis of the exchange interaction and radical recombination potentials for all multiplets. In accordance with the concept of spin catalysis, the electronic spin-uncoupling mechanism, induced by O₂ perturbation, has the same nature as other known catalytic processes of paramagnetic-exchange type.     

1H-NMR study of GM2 ganglioside: evidence that an interresidue amide-carboxyl hydrogen bond contributes to stabilization of a preferred conformation

Several properties of the exchangeable amide protons of the ganglioside G_M2 were studied in detail by¹H-NMR spectroscopy in fully deuterated dimethylsulfoxide [²H₆]DMSO/2% H₂O, and compared with data obtained for the simpler constituent glycosphingolipids G_A2 and G_M3. In addition to chemical shifts,³ J _2,HN coupling constants, and temperature shift coefficients, the kinetics of NH/²H chemical exchange were examined by following the disappearance of the amide resonances in [²H₆]DMSO/2%²H₂O. The results included observation of an increase in half-life of theN-acetylgalactosamine acetamido HN by more than an order of magnitude in G_M2 compared to G_A2, attributable to the presence of the additionalN-acetylneuraminic acid residue. Additional one-dimensional dipolar cross relaxation experiments were also performed on nonexchangeable protons of G_M2. The results of all of these experiments support a three-dimensional model for the terminal trisaccharide in which a hydrogen bond is formed between theN-acetylgalactosamine acetamido NH and theN-acetylneuraminic acid carboxyl group. The interaction is proposed to be of the -acceptor type, a possibility which has not yet been explored in the literature on carbohydrates. The proposed model is discussed in comparison with that of Sabesanet al. (1984,Can J Chem 62: 1034–45), and the models of G_M1 proposed more recently by Acquottiet al. (1990,J Am Chem Soc 112:7772–8) and Scarsdaleet al. (1990,Biochemistry 29:9843–55).     

Enzymes and coenzymes of the carbon monoxide dehydrogenase pathway for autotrophic CO2 fixation in Archaeoglobus lithotrophicus and the lack of carbon monoxide dehydrogenase in the heterotrophic A. profundus

Archaeoglobus lithotrophicus is a hyperthermophilic Archaeon that grows on H₂ and sulfate as energy sources and CO₂ as sole carbon source. The autotrophic sulfate reducer was shown to contain all the enzyme activities and coenzymes of the reductive carbon monoxide dehydrogenase pathway for autotrophic CO₂ fixation as operative in methanogenic Archaea. With the exception of carbon monoxide dehydrogenase these enzymes and coenzymes were also found in A. profundus. This organism grows lithotrophically on H₂ and sulfate, but differs from A. lithotrophicus in that it cannot grow autotrophically: A. profundus requires acetate and CO₂ for biosynthesis. The absence of carbon monoxide dehydrogenase in A. profundus is substantiated by the observation that this organism, in contrast to A. lithotrophicus, is not mini-methanogenic and contains only relatively low concentrations of corrinoids.Abbreviations F ₄₂₀ coenzyme F₄₂₀ - MFR methanofuran - CHO-MFR formylmethanofuran - H ₄MPT 5,6,7,8-tetrahydromethanopterin - CHO–H ₄MPT N⁵ formyl-H₄MPT - CHH₄MPT⁺N⁵ methenyl-H₄MPT - CH ₂=H₄MPT N⁵, N¹⁰ methylene-H₄MPT - CH ₃–H₄MPT N⁵ methyl-H₄MPT - H ₄F tetrahydrofolate - I U 1 mol/min - t _d doubling time     

Multiple reactions in vanadyl-V(IV) oxidation by H2O2

Oxidation of vanadyl sulfate by H₂O₂ involves multiple reactions at neutral pH conditions. The primary reaction was found to be oxidation of V(IV) to V(V) using 0.5 equivalent of H₂O₂, based on the loss of blue color and the visible spectrum. The loss of V(IV) and formation V(V) compounds were confirmed by ESR and⁵¹V-NMR spectra, respectively. In the presence of excess H₂O₂ (more than two equivalents), the V(V) was converted into diperoxovanadate, the major end-product of these reactions, identified by changes in absorbance in ultraviolet region and by the specific chemical shift in NMR spectrum. The stoichiometric studies on the H₂O₂ consumed in this reaction support the occurrence of reactions of two-electron oxidation followed by complexing two molecules of H₂O₂. Addition of a variety of compounds—Tris, ethanol, mannitol, benzoate, formate (hydroxyl radical quenching), histidine, imidazole (singlet oxygen quenching), and citrate—stimulated a secondary reaction of oxygen-consumption that also used V(IV) as the reducing source. This reaction requires concomitant oxidation of vanadyl by H₂O₂, favoured at low H₂O₂:V(IV) ratio. Another secondary reaction of oxygen release was found to occur during vanadyl oxidation by H₂O₂ in acidic medium in which the end-product was not diperoxovanadate but appears to be a mixture of VO ₃ ⁺ (–546 ppm), VO³⁺ (–531 ppm) and VO ₂ ⁺ (–512 ppm), as shown by the⁵¹V-NMR spectrum. This reaction also occurred in phosphate-buffered medium but only on second addition of vanadyl. The compounds that stimulated the oxygen-consumption reaction were found to inhibit the oxygen-release reaction. A combination of these reactions occur depending on the proportion of the reactants (vanadyl and H₂O₂), the pH of the medium and the presence of some compounds that affect the secondary reactions.     

Thermophilic H2 production from a cellulose-containing wastewater

Cellulose in wastewater was converted into H₂ by a mixed culture in batch experiments at 55°C with various wastewaters pH (5.5–8.5) and cellulose concentrations (10–40 g l^–1). At the optimal pH of 6.5, the maximum H₂ yield was 102 ml g^–1 cellulose and the maximum production rate was 287 ml d^–1 for each gram of volatile suspended solids (VSS). Analysis of 16S rDNA sequences showed that the cellulose-degrading mixed culture was composed of microbes closely affiliated to genus Thermoanaerobacterium.     

Hormonal effects on tube elongation, 14CO2 fixation and phosphoenolpyruvate carboxylase activity in Amaryllis pollen: Promotion by abscisic acid

Abscisic acid promoted tube elongation, ¹⁴CO₂ fixation and phosphoenolpyruvate carboxylase activity during in vitro germination of Amaryllis vittata Ait. pollen. Other growth regulators tested viz. indol-3yl-acetic acid, gibberellic acid, N⁶-benzyladenine and ethephon did not affect pollen tube elongation at 1 g/ml concentration but inhibited it at 10 g/ml. The case for abscisic acid as a growth promoter is argued.     

Purification,properties and primary structure of H2-formingN 5,N10-methylenetetrahydromethanopterin dehydrogenase fromMethanococcus thermolithotrophicus

H₂-FormingN ⁵,N¹⁰-methylenetetrahydromethanopterin dehydrogenase (Hmd) is a novel type of hydrogenase found in methanogenic Achaea that contains neither nickel nor iron-sulfur clusters. The enzyme has previously been characterized fromMethanobacterium thermoautotrophicum and fromMethanopyrus kandleri. We report here on the purification and properties of the enzyme fromMethanococcus thermolithotrophicus. Thehmd gene was cloned and sequenced. The results indicate that the enzyme fromMc. thermolithotrophicus is functionally and structurally closely related to the H₂-forming methylene tetrahydromethanopterin dehydrogenase fromMb. thermoautotrophicum andMp. kandleri. From amino acid sequence comparisons of the three enzymes, a phylogenetic tree was deduced that shows branching orders similar to those derived from sequence comparisons of the 16S rRNA of the orders Methanococcales, Methanobacteriales, and Methanopyrales.Abbreviations H ₂ Forming dehydrogenase orHmd - H₂-FormingN ⁵,N¹⁰ methylene tetrahydromethanopterin dehydrogenase - H ₄MPT Tetrahydromethanopterin - CH ₂=H₄MPT N⁵,N¹⁰ Methylene tetrahydromethanopterin - CHH ₄MPT⁺ N⁵,N¹⁰ Methenyltetrahydromethanopterin - MALDI-TOF-MS Matrix-assisted laser desorption     

Oligochitosan induced Brassica napus L. production of NO and H2O2 and their physiological function

NO (nitric oxide) and H₂O₂ (hydrogen peroxide) are important signaling molecule in plants. Brassica napus L. was used to understand oligochitosan inducing production of NO (nitric oxide) and H₂O₂ (hydrogen peroxide) and their physiological function. The result showed that the production of NO and H₂O₂ in epidermal cells of B. napus L. was induced with oligochitosan by fluorescence microscope. And it was proved that there was an interaction between NO and H₂O₂ with L-NAME (N^G-nitro-l-arg-methyl eater), which is an inhibitor of NOS (NO synthase) in mammalian cells that also inhibits plant NO synthesis, and CAT (catalase), which is an important H₂O₂ scavenger, respectively. It was found that NO and H₂O₂ induced by oligochitosan took part in inducing reduction in stomatal aperture and LEA protein gene expression of leaves of B. napus L. All these results showed that oligochitosan have potential activities of improving resistance to water stress.     

UV resonance Raman studies on the activation mechanism of human hematopoietic prostaglandin D2 synthase by a divalent cation, Mg

The Mg²⁺ ion-assisted activation mechanism of the active site Tyr8 of a human hematopoietic prostaglandin D₂ synthase (H-PGDS) was studied by ultraviolet resonance Raman (UVRR) spectroscopy. Addition of Mg²⁺ to the native H-PGDS at pH 8.0 resulted in the Y8a Raman band of Tyr8 shifting from 1615 cm⁻¹ to 1600 cm⁻¹. This large shift to lower energy of the tyrosine Y8a vibrational mode is caused by the deprotonation of the tyrosine phenol group promoted by binding of Mg²⁺. Upon subsequent addition of glutathione (GSH), the Mg²⁺/H-PGDS solution showed the Tyr8 Raman band shifted to 1611 cm⁻¹, which is 11 cm⁻¹ higher than the frequency of the Mg²⁺ complex of H-PGDS, but 4 cm⁻¹ lower than the Mg²⁺ free enzyme. These UVRR observations suggest that the deprotonated Tyr8 in the presence of Mg²⁺ is re-protonated by the abstraction of H⁺ from the thiol group of GSH, and that the re-protonated Tyr8 species forms a hydrogen bond with the thiolate anion of GSH. Density functional theory calculations on several model complexes of p-cresol were also performed, which suggested that the pK_a and vibrational frequencies of the Tyr8 phenol group are affected by the degree and structure of hydration of the Tyr8 residue.     

Methyl-coenzyme M reductase and other enzymes involved in methanogenesis from CO2 and H2 in the extreme thermophile Methanopyrus kandleri

Methanopyrus kandleri belongs to a novel group of abyssal methanogenic archaebacteria that can grow at 110°C on H₂ and CO₂ and that shows no close phylogenetic relationship to any methanogen known so far. Methyl-coenzyme M reductase, the enzyme catalyzing the methane forming step in the energy metabolism of methanogens, was purified from this hyperthermophile. The yellow protein with an absorption maximum at 425 nm was found to be similar to the methyl-coenzyme M reductase from other methanogenic bacteria in that it was composed each of two -, - and -subunits and that it contained the nickel porphinoid coenzyme F₄₃₀ as prosthetic group. The purified reductase was inactive. The N-terminal amino acid sequence of the -subunit was determined. A comparison with the N-terminal sequences of the -subunit of methyl-coenzyme M reductases from other methanogenic bacteria revealed a high degree of similarity.Besides methyl-coenzyme M reductase cell extracts of M. kandleri were shown to contain the following enzyme activities involved in methanogenesis from CO₂ (apparent V_max at 65°C): formylmethanofuran dehydrogenase, 0.3 U/mg protein; formyl-methanofuran: tetrahydromethanopterin formyltransferase, 13 U/mg; N ⁵,N¹⁰-methenyltetrahydromethanopterin cyclohydrolase, 14 U/mg; N ⁵,N¹⁰-methylenetetrahydromethanopterin dehydrogenase (H₂-forming), 33 U/mg; N ⁵,N¹⁰-methylenetetrahydromethanopterin reductase (coenzyme F₄₂₀ dependent), 4 U/mg; heterodisulfide reductase, 2 U/mg; coenzyme F₄₂₀-reducing hydrogenase, 0.01 U/mg; and methylviologen-reducing hydrogenase, 2.5 U/mg. Apparent K_m values for these enzymes and the effect of salts on their activities were determined.The coenzyme F₄₂₀ present in M. kandleri was identified as coenzyme F₄₂₀-2 with 2 -glutamyl residues.Abbreviations H–S-CoM coenzyme M - CH₃–S-CoM methylcoenzyme M - H–S-HTP 7-mercaptoheptanoylthreonine phosphate - MFR methanofuran - CHO-MFR formyl-MFR - H₄MPT tetrahydromethanopterin - CHO–H₄MPT N ⁵-formyl-H₄MPT - CH=H₄MPT⁺ N ⁵,N¹⁰-methenyl-H₄MPT - CH₂=H₄MPT N ⁵,N¹⁰-methylene-H₄MPT - CH₃–H₄MPT N ⁵-methyl-H₄MPT - F₄₂₀ coenzyme F₄₂₀ - 1 U= 1 mol/min     

Simulation of separation of C2H6 from CH4 using zeolitic imidazolate frameworks

Separation of important chemical feedstocks, such as C₂H₆ from natural gas, can greatly benefit the petrochemical industry. In this paper, the grand canonical Monte Carlo method has been used to study the adsorption and separation of CH₄ and C₂H₆ in zeolites, isoreticular metal-organic framework-1 (IRMOF-1) and zeolitic imidazolate frameworks (ZIFs) with different topology, including soadlite, gmelinite and RHO topologies. Compared with mordenite zeolite and IRMOF-1, ZIFs and mordenite framework inverted (MFI) zeolite have better separation performance for C₂H₆/CH₄ mixtures at different mole fractions of C₂H₆. From the study of equilibrium snapshots and density distribution profiles, adsorption sites could be grouped as (1) sites with strong interactions with adsorbent and (2) sites with strong interactions with surrounding adsorbates. The gas molecules occupied the first site and then went on to occupy the second site. In CH₄/C₂H₆ mixture adsorption/separation, the adsorption of CH₄ was confined by the existence of C₂H₆. Due to energetic effect, C₂H₆ selectivity was affected by temperature at a low-pressure range, but did not change as much in a high-pressure range because of packing effect in micropore. In binary adsorption, large C₂H₆ molecules favour sites with strong adsorbent interactions. At high pressures, packing effects played an important role and it became easy for small CH₄ molecules to access the sites with strong adsorbate interactions.