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At least 703 records · Page 39Linked to original sources
PubMed2003

The membrane-associated form of methane mono-oxygenase from Methylococcus capsulatus (Bath) is a copper/iron protein.

A protocol has been developed which permits the purification of a membrane-associated methane-oxidizing complex from Methylococcus capsulatus (Bath). This complex has approximately 5 fold higher specific activity than any purified particulate methane mono-oxygenase (pMMO) previously reported from M. capsulatus (Bath). This efficiently functioning methane-oxidizing complex consists of the pMMO hydroxylase (pMMOH) and an unidentified component we have assigned as a potential pMMO reductase (pMMOR). The complex was isolated by solubilizing intracytoplasmic membrane preparations containing the high yields of active membrane-bound pMMO (pMMO(m)), using the non-ionic detergent dodecyl-beta-D-maltoside, to yield solubilized enzyme (pMMO(s)). Further purification gave rise to an active complex (pMMO(c)) that could be resolved (at low levels) by ion-exchange chromatography into two components, the pMMOH (47, 27 and 24 kDa subunits) and the pMMOR (63 and 8 kDa subunits). The purified complex contains two copper atoms and one non-haem iron atom/mol of enzyme. EPR spectra of preparations grown with (63)Cu indicated that the copper ion interacted with three or four nitrogenic ligands. These EPR data, in conjunction with other experimental results, including the oxidation by ferricyanide, EDTA treatment to remove copper and re-addition of copper to the depleted protein, verified the essential role of copper in enzyme catalysis and indicated the implausibility of copper existing as a trinuclear cluster. The EPR measurements also demonstrated the presence of a tightly bound mononuclear Fe(3+) ion in an octahedral environment that may well be exchange-coupled to another paramagnetic species.

Ascorbic Acid↗
PubMed2006

Applied reaction dynamics: efficient synthesis gas production via single collision partial oxidation of methane to CO on Rh111.

Supersonic molecular beams have been used to determine the yield of CO from the partial oxidation of CH4 on a Rh111 catalytic substrate, CH4+12O2-->CO+2H2, as a function of beam kinetic energy. These experiments were done under ultrahigh vacuum conditions with concurrent molecular beams of O2 and CH4, ensuring that there was only a single collision for the CH4 to react with the surface. The fraction of CH4 converted is strongly dependent on the normal component of the incident beam's translational energy, and approaches unity for energies greater than approximately 1.3 eV. Comparison with a simplified model of the methane-Rh111 reactive potential gives insight into the barrier for methane dissociation. These results demonstrate the efficient conversion of methane to synthesis gas, CO+2H2, are of interest in hydrogen generation, and have the optimal stoichiometry for subsequent utilization in synthetic fuel production (Fischer-Tropsch or methanol synthesis). Moreover, under the reaction conditions explored, no CO2 was detected, i.e., the reaction proceeded with the production of very little, if any, unwanted greenhouse gas by-products. These findings demonstrate the efficacy of overcoming the limitations of purely thermal reaction mechanisms by coupling nonthermal mechanistic steps, leading to efficient C-H bond activation with subsequent thermal heterogeneous reactions.

Journal Article↗
PubMed2003

Purified particulate methane monooxygenase from Methylococcus capsulatus (Bath) is a dimer with both mononuclear copper and a copper-containing cluster.

Particulate methane monooxygenase (pMMO) is a membrane-bound enzyme that catalyzes the oxidation of methane to methanol in methanotropic bacteria. Understanding how this enzyme hydroxylates methane at ambient temperature and pressure is of fundamental chemical and potential commercial importance. Difficulties in solubilizing and purifying active pMMO have led to conflicting reports regarding its biochemical and biophysical properties, however. We have purified pMMO from Methylococcus capsulatus (Bath) and detected activity. The purified enzyme has a molecular mass of approximately 200 kDa, probably corresponding to an alpha(2)beta(2)gamma(2) polypeptide arrangement. Each 200-kDa pMMO complex contains 4.8 +/- 0.8 copper ions and 1.5 +/- 0.7 iron ions. Electron paramagnetic resonance spectroscopic parameters corresponding to 40-60% of the total copper are consistent with the presence of a mononuclear type 2 copper site. X-ray absorption near edge spectra indicate that purified pMMO is a mixture of Cu(I) and Cu(II) oxidation states. Finally, extended x-ray absorption fine structure data are best fit with oxygennitrogen ligands and a 2.57-A Cu-Cu interaction, providing direct evidence for a copper-containing cluster in pMMO.

Cell Fractionation↗
PubMed2000

Transformations in methane hydrates.

Detailed study of pure methane hydrate in a diamond cell with in situ optical, Raman, and x-ray microprobe techniques reveals two previously unknown structures, structure II and structure H, at high pressures. The structure II methane hydrate at 250 MPa has a cubic unit cell of a = 17.158(2) A and volume V = 5051.3(13) A(3); structure H at 600 MPa has a hexagonal unit cell of a = 11.980(2) A, c = 9.992(3) A, and V = 1241.9(5) A(3). The compositions of these two investigated phases are still not known. With the effects of pressure and the presence of other gases in the structure, the structure II phase is likely to dominate over the known structure I methane hydrate within deep hydrate-bearing sediments underlying continental margins.

Journal Article↗
PubMed1994

Ion transport and methane production in Methanobacterium thermoautotrophicum.

In Methanobacterium thermoautotrophicum, the protonmotive force for the H+-translocating ATPase consists mainly of a transmembrane electrical gradient (Deltapsi). These cells do not establish a significant transmembrane pH gradient (inside alkaline) and, in fact, if the suspending medium is of pH >/= 7.0, the pH gradient may be reversed-i.e., inside acid with respect to the extracellular pH. These studies show by both 23Na NMR and 22Na+ distribution that Na+ extrusion with the generation of Deltapsi precedes methanogenesis in Mb. thermoautotrophicum. It is calculated that the newly established Na+ gradients increase Deltapsi by approximately 50 mV (inside negative). There is no detectable H+ extrusion during methane synthesis; instead there is a high rate of H+ consumption for methane synthesis and an increase in internal pH. This was supported by 31P NMR experiments, which showed an internal pH shift from 6.8 to 7.6. With the cells maintained at an external pH of 7.2, the initial transmembrane pH gradient of -0.4 (inside acid) at 60 degrees C is equivalent to Deltapsi of + 27 mV (inside positive); after 20 min of incubation, the transmembrane pH gradient is + 0.4 (inside alkaline), which at 60 degrees C is equivalent to Deltapsi of -27 mV (inside negative). Actively respiring cells generated a protonmotive force of -198 mV. It is proposed that energy for CO2 reduction to the level of formaldehyde (the first step in methane synthesis) in Mb. thermoautotrophicum is derived from the Deltapsi generated by electrogenic Na+ extrusion. The protonmotive force required for ATP synthesis consists primarily of Deltapsi and appears to be the result of both an electrogenic Na+ extrusion and a pH gradient (inside alkaline) which develops during methanogenesis.

Journal Article↗
PubMed1999

Structure of the soluble methane monooxygenase regulatory protein B.

The soluble methane monooxygenase (sMMO; EC 1.14.13.25) from the pseudothermophile Methylococcus capsulatus (Bath) is a three-component enzyme system that catalyzes the selective oxidation of methane to methanol. We have used NMR spectroscopy to produce a highly refined structure of MMOB, the 16-kDa regulatory protein of this system. This structure has a unique and intricate fold containing seven beta-strands forming two beta-sheets oriented perpendicular to each other and bridged by three alpha-helices. The rate and efficiency of the methane hydroxylation by sMMO depend on dynamic binding interactions of the hydroxylase with the reductase and regulatory protein components during catalysis. We have monitored by NMR the binding of MMOB to the hydroxylase in the presence and absence of the reductase. The results of these studies provide structural insight into how the regulatory protein interacts with the hydroxylase.

Amino Acid Sequence↗
PubMed1999

Constraints on the Space Density of Methane Dwarfs and the Substellar Mass Function from a Deep Near-Infrared Survey.

We report preliminary results of a deep near-infrared search for methane-absorbing brown dwarfs; almost 5 yr after the discovery of Gl 229b, there are only a few confirmed examples of this type of object. New J-band, wide-field images, combined with preexisting R-band observations, allow efficient identification of candidates by their extreme (R-J) colors. Follow-up measurements with custom filters can then confirm objects with methane absorption. To date, we have surveyed a total of 11.4 deg2 to J approximately 20.5 and R approximately 25. Follow-up CH4 filter observations of promising candidates in one-fourth of these fields have turned up no methane-absorbing brown dwarfs. With 90% confidence, this implies that the space density of objects similar to Gl 229b is less than 0.012 pc-3. These calculations account for the vertical structure of the Galaxy, which can be important for sensitive measurements. Combining published theoretical atmospheric models with our observations sets an upper limit of alpha</=0.8 for the exponent of the initial mass function power law in this domain.

Journal Article↗
PubMed2000

The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital Sky Survey Commissioning Data.

We report the discovery of a second field methane brown dwarf from the commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS J134646.45-003150.4 (hereafter SDSS 1346-00), was selected because of its very red color and stellar appearance. Its spectrum between 0.8 and 2.5 µm is dominated by strong absorption bands of H2O and CH4 and closely mimics those of Gliese 229B and SDSS 162414.37+002915.6 (hereafter SDSS 1624+00), two other known methane brown dwarfs. SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that it lies about 11 pc from the Sun. The ratio of flux at 2.1 µm to that at 1.27 µm is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which suggests that SDSS 1346-00 has a slightly higher effective temperature than the others. Based on a search area of 130 deg2 and a detection limit of z*=19.8, we estimate a space density of 0.05 pc-3 for methane brown dwarfs with Teff approximately 1000 K in the 40 pc3 volume of our search. This estimate is based on small-sample statistics and should be treated with appropriate caution.

Journal Article↗
PubMed2004

Bis(2-chloroethoxy)methane-induced mitochondrial and myofibrillar damage: short-term time-course study.

Cardiotoxicity induced by 2-, 3-, 5-, and 12-day dermal administration of 400 and 600 mg/kg/day of bis(2-chloroethoxy)methane to F344/N male and female rats was characterized. The severity and incidence of lesions were similar among males and females and in all three regions of the heart examined (atrium, ventricle, interventricular septum). Damage induced by bis(2-chloroethoxy)methane consisted of time-related development of myofiber vacuolation, necrosis, mononuclear-cell infiltration, fibrosis, and atrial thrombosis. Changes were pronounced at day 2, increased in severity at day 3, appeared to decrease at day 5, and resolved by study-day 16 that corresponded to 12 dosings. Ultrastructural analysis of 2- and 5-day 600 mg/kg/day-treated females elucidated the primary site of damage, the mitochondrion, and two types of vacuolation, one that formed as damaged mitochondria became devoid of cristae and their bounding double membranes became reduced to singleness, and the other manifested as distention of the sarcoplasmic reticulum. After the initial damage induced by bis(2-chloroethoxy)methane, or its metabolite, thiodiglycolic acid, protective mechanisms within the heart were apparently initiated, enabling it to cope with the continued exposure to the toxicant while eliminating some damaged myofibers.

Animals↗
PubMed1992

Functional expression in Escherichia coli of proteins B and C from soluble methane monooxygenase of Methylococcus capsulatus (Bath).

Methylococcus capsulatus (Bath) uses a soluble methane monooxygenase (sMMO) to catalyse the oxidation of methane to methanol. sMMO is comprised of three components; A, B and C. Protein C (the reductase) transfers electrons from NADH to protein A (the hydroxylase) which contains the active site, and protein B regulates this electron flow. The five genes encoding the sMMO proteins and their subunits are clustered and have been cloned in Escherichia coli. A DNA fragment containing mmoB, the gene encoding protein B, was subcloned into pT7-5, a plasmid of the T7 RNA polymerase promoter expression system. Upon induction, E. coli expressed protein B which was fully functional after purification. The gene encoding protein C, mmoC, was amplified with unique restriction sites at each end using the polymerase chain reaction and then subcloned into pT7-7 (a plasmid similar to pT7-5 but containing its own ribosome-binding site and ATG start codon). Protein C expressed in E. coli was also found to be functional. This is the first report of the functional expression of methanotroph methane monooxygenase genes in a heterologous host and represents a significant step forward in our analysis of the assembly and catalysis of sMMO.

Base Sequence↗
PubMed2003

Genes involved in the copper-dependent regulation of soluble methane monooxygenase of Methylococcus capsulatus (Bath): cloning, sequencing and mutational analysis.

The key enzyme in methane metabolism is methane monooxygenase (MMO), which catalyses the oxidation of methane to methanol. Some methanotrophs, including Methylococcus capsulatus (Bath), possess two distinct MMOs. The level of copper in the environment regulates the biosynthesis of the MMO enzymes in these methanotrophs. Under low-copper conditions, soluble MMO (sMMO) is expressed and regulation takes place at the level of transcription. The structural genes of sMMO were previously identified as mmoXYBZ, mmoD and mmoC. Putative transcriptional start sites, containing a sigma(70)- and a sigma(N)-dependent motif, were identified in the 5' region of mmoX. The promoter region of mmoX was mapped using truncated 5' end regions fused to a promoterless green fluorescent protein gene. A 9.5 kb region, adjacent to the sMMO structural gene cluster, was analysed. Downstream (3') from the last gene of the operon, mmoC, four ORFs were found, mmoG, mmoQ, mmoS and mmoR. mmoG shows significant identity to the large subunit of the bacterial chaperonin gene, groEL. In the opposite orientation, two genes, mmoQ and mmoS, showed significant identity to two-component sensor-regulator system genes. Next to mmoS, a gene encoding a putative sigma(N)-dependent transcriptional activator, mmoR was identified. The mmoG and mmoR genes were mutated by marker-exchange mutagenesis and the effects of these mutations on the expression of sMMO was investigated. sMMO transcription was impaired in both mutants. These results indicate that mmoG and mmoR are essential for the expression of sMMO in Mc. capsulatus (Bath).

Bacterial Proteins↗
PubMed2001

Transition from cage clathrate to filled ice: the structure of methane hydrate III.

The structure of a new methane hydrate has been solved at 3 GPa from neutron and x-ray powder diffraction data. It is a dihydrate in which a 3D H-bonded network of water molecules forms channels surrounding the methane molecules. The network is closely related to that of ice-Ih and the methane-water system appears to be the first in which a cage clathrate hydrate is transformed into an ice-related hydrate (a "filled ice").

Journal Article↗
PubMed2001

Characterization of silica-supported Ni catalysts effective for methane decomposition by Ni K-edge XAFS.

The structural change of Ni species during the methane decomposition into hydrogen and carbon over Ni/SiO2 catalyst was investigated by Ni K-edge XANES/EXAFS. Before the contact of methane with the Ni/SiO2 catalyst, Ni species were present as Ni metal mainly. The structure of the Ni metal did not change appreciably when the Ni/SiO2 was actively decomposing methane. In contrast, the formation of nickel carbide species was observed at the deactivation stage of the catalyst.

Journal Article↗
PubMed1986

Contact allergy to 4,4'-dihydroxy-(hydroxymethyl)-diphenyl methanes: sensitizers in a phenol-formaldehyde resin.

A particular phenol-formaldehyde resin of the resol type (P-F-R-2) was shown in a previous study to be a good tracer of patients with contact allergy to phenol-formaldehyde resins. Eleven patients with sensitivity to P-F-R-2 participated in the study in which chromatographically separated fractions of P-F-R-2 were patch tested. This procedure was repeated until a positive reaction to a single substance was noted. In this study 3 new sensitizers are reported. Investigations by mass spectrometry and nuclear magnetic resonance spectrometry identified the compounds to be 4,4'-dihydroxy-3-(hydroxymethyl)-diphenyl methane (4,4'-H-3-HPM), 4,4'-dihydroxy-3,3'-di-(hydroxymethyl)-diphenyl methane (4,4'-H-3,3'-HPM), and 4,4'-dihydroxy-3,5-di-(hydroxymethyl)-diphenyl methane (4,4'-H-3,5-HPM). Six of the 11 patients reacted to at least one of these compounds. All 6 patients reacted to 4,4'-H-3,3'-HPM and 5 reacted simultaneously to 4,4'-H-3-HPM. Four patients reacted to all 3 compounds tested. By patch testing with serial dilutions (ethanol 99.5%) of the three 4,4'-H-HPM, positive reactions were noted down to 0.5 X 10(-6)%, 1.0 X 10(-6)%, and 1.3 X 10(-3)% w/v for 4,4'-H-3-HPM, 4,4'-H-3,3'-HPM, and 4,4'-H-3,5-HPM, respectively.

Allergens↗
PubMed1986

Sensitizing capacity of dihydroxydiphenyl methane (bisphenol F) in the guinea pig.

The sensitizing capacity of 2,2(1)-dihydroxydiphenyl methane (2,2(1)-HPM), 4,4(1)-dihydroxydiphenyl methane (4,4(1)-HPM) and 2,4(1)-dihydroxydiphenyl methane (2,4(1)-HPM) was investigated with the guinea pig maximization test. These compounds are known contact sensitizers in phenol-formaldehyde resins. The study was performed to assess and compare their sensitizing capacities. A weak sensitizing capacity of 2,2(1)-HPM was demonstrated when the results for 2 separate series were added, while no sensitizing capacity was demonstrated for 4,4(1)-HPM or 2,4(1)-HPM. The compounds tested were chromatographically pure.

Animals↗
PubMed2003

Vibrational mode-specific reaction of methane on a nickel surface.

The dissociation of methane on a nickel catalyst is a key step in steam reforming of natural gas for hydrogen production. Despite substantial effort in both experiment and theory, there is still no atomic-scale description of this important gas-surface reaction. We report quantum state-resolved studies, using pulsed laser and molecular beam techniques, of vibrationally excited methane reacting on the nickel (100) surface. For doubly deuterated methane (CD2H2), we observed that the reaction probability with two quanta of excitation in one C-H bond was greater (by as much as a factor of 5) than with one quantum in each of two C-H bonds. These results clearly exclude the possibility of statistical models correctly describing the mechanism of this process and attest to the importance of full-dimensional calculations of the reaction dynamics.

Journal Article↗
PubMed1997

Low-Temperature Nonoxidative Activation of Methane over H-Galloaluminosilicate (MFI) Zeolite

Conversion of methane to higher hydrocarbons by its low-temperature activation without forming undesirable carbon oxides is of great scientific and practical importance. Methane can be highly activated, yielding high rates of conversion to higher hydrocarbons and aromatics (10 to 45 percent) at low temperatures (400&deg; to 600&deg;C), by its reaction over H-galloaluminosilicate ZSM-5 type (MFI) zeolite in the presence of alkenes or higher alkanes. The methane activation results from its hydrogen-transfer reaction with alkenes.

Journal Article↗
PubMed2000

Carbon isotopic evidence for methane hydrate instability during quaternary interstadials

Large (about 5 per mil) millennial-scale benthic foraminiferal carbon isotopic oscillations in the Santa Barbara Basin during the last 60,000 years reflect widespread shoaling of sedimentary methane gradients and increased outgassing from gas hydrate dissociation during interstadials. Furthermore, several large, brief, negative excursions (up to -6 per mil) coinciding with smaller shifts (up to -3 per mil) in depth-stratified planktonic foraminiferal species indicate massive releases of methane from basin sediments. Gas hydrate stability was modulated by intermediate-water temperature changes induced by switches in thermohaline circulation. These oscillations were likely widespread along the California margin and elsewhere, affecting gas hydrate instability and contributing to millennial-scale atmospheric methane oscillations.

Journal Article↗