81 research outputs found
Comparison of VLBI, TV and traveling clock techniques for time transfer
A three part experiment was conducted to develop and compare time transfer techniques. The experiment consisted of (1) a very long baseline interferometer (VLBI), (2) a high precision portable clock time transfer system between the two sites, and (3) a television time transfer. A comparison of the VLBI and traveling clock shows each technique can perform satisfactorily at the five nsec level. There was a systematic offset of 59 nsec between the two methods, which we attributed to a difference in epochs between VLBI formatter and station clock. The VLBI method had an internal random error of one nsec at the three sigma level for a two day period. Thus, the Mark II system performed well, and VLBI shows promise of being an accurate method of time transfer. The TV system, which had technical problems during the experiment, transferred time with a random error of about 50 nsec
Observation of superoxide production during catalysis of Bacillus subtilis oxalate decarboxylase at pH 4
This contribution describes the trapping of the hydroperoxyl radical at a pH of 4 during turnover of wild-type oxalate decarboxylase and its T165V mutant using the spin trap BMPO. Radicals were detected and identified by a combination of EPR and mass spectrometry. Superoxide, or its conjugate acid, the hydroperoxyl radical, is expected as an intermediate in the decarboxylation and oxidation reactions of the oxalate monoanion both of which are promoted by oxalate decarboxylase. Another intermediate, the carbon dioxide radical anion was also observed. The quantitative yields of superoxide trapping is similar in the wild type and the mutant while it is significantly different for the trapping of the carbon dioxide radical anion. This suggests that the two radicals are released from different sites of the protein
Assigning the EPR fine structure parameters of the Mn(II) centers in bacillus subtilis oxalate decarboxylase by site-directed mutagenesis and DFT/MM calculations
Oxalate decarboxylase (OxDC) catalyzes the Mn-dependent conversion of the oxalate monoanion into CO2 and formate. EPR-based strategies for investigating the catalytic mechanism of decarboxylation are complicated by the difficulty of assigning the signals associated with the two Mn(II) centers located in the N- and C-terminal cupin domains of the enzyme. We now report a mutational strategy that has established the assignment of EPR fine structure parameters to each of these Mn(II) centers at pH 8.5. These experimental findings are also used to assess the performance of a multistep strategy for calculating the zero-field splitting parameters of protein-bound Mn(II) ions. Despite the known sensitivity of calculated D and E values to the computational approach, we demonstrate that good estimates of these parameters can be obtained using cluster models taken from carefully optimized DFT/MM structures. Overall, our results provide new insights into the strengths and limitations of theoretical methods for understanding electronic properties of protein-bound Mn(II) ions, thereby setting the stage for future EPR studies on the electronic properties of the Mn(II) centers in OxDC and site-specific variants
New Studies of the Pulsar Wind Nebula in the Supernova Remnant CTB 80
We investigated the kinematics of the pulsar wind nebula (PWN) associated
with PSR B1951+32 in the old supernova remnant CTB 80 using the Fabry-Perot
interferometer of the 6m Special Astrophysical Observatory telescope. In
addition to the previously known expansion of the system of bright filaments
with a velocity of 100-200km/s, we detected weak high-velocity features in the
H-alpha line at least up to velocities of 400-450km/s. We analyzed the
morphology of the PWN in the H-alpha, [SII], and [OIII] lines using HST data
and discuss its nature. The shape of the central filamentary shell, which is
determined by the emission in the [OIII] line and in the radio continuum, is
shown to be consistent with the bow-shock model for a significant (about 60
degrees) inclination of the pulsar's velocity vector to the plane of the sky.
In this case, the space velocity of the pulsar is twice higher than its
tangential velocity, i.e., it reaches ~500 km/s, and PSR B1951+32 is the first
pulsar whose line-of-sight velocity (of about 400 km/s) has been estimated from
the PWN observations. The shell-like H-alpha-structures outside the bow shock
front in the east and the west may be associated with both the pulsar's jets
and the pulsar-wind breakthrough due to the layered structure of the extended
CTB 80 shell.Comment: to appear in Astronomy Letters, 12 pages, 6 postscript figures, two
in colour; for a version with high resolution figures see
http://www.sao.ru/hq/grb/team/vkom/CTB80_fine.pd
Si and Fe depletion in Galactic star-forming regions observed by the Spitzer Space Telescope
We report the results of the mid-infrared spectroscopy of 14 Galactic
star-forming regions with the high-resolution modules of the Infrared
Spectrograph (IRS) on board the Spitzer Space Telescope. We detected [SiII]
35um, [FeII] 26um, and [FeIII] 23um as well as [SIII] 33um and H2 S(0) 28um
emission lines. Using the intensity of [NII] 122um or 205um and [OI] 146um or
63um reported by previous observations in four regions, we derived the ionic
abundance Si+/N+ and Fe+/N+ in the ionized gas and Si+/O0 and Fe+/O0 in the
photodissociation gas. For all the targets, we derived the ionic abundance of
Si+/S2+ and Fe2+/S2+ for the ionized gas. Based on photodissociation and HII
region models the gas-phase Si and Fe abundance are suggested to be 3-100% and
<8% of the solar abundance, respectively, for the ionized gas and 16-100% and
2-22% of the solar abundance, respectively, for the photodissociation region
gas. Since the [FeII] 26um and [FeIII] 23um emissions are weak, the high
sensitivity of the IRS enables to derive the gas-phase Fe abundance widely in
star-forming regions. The derived gas-phase Si abundance is much larger than
that in cool interstellar clouds and that of Fe. The present study indicates
that 3-100% of Si atoms and <22% of Fe atoms are included in dust grains which
are destroyed easily in HII regions, probably by the UV radiation. We discuss
possible mechanisms to account for the observed trend; mantles which are
photodesorbed by UV photons, organometallic complexes, or small grains.Comment: 43 pages with 7 figures, accepted in Astrophysical Journa
Long-lived photoexcited states in polydiacetylenes with different molecular and supramolecular organization
With the aim of determining the importance of the molecular and supramolecular organization on the excited states of polydiacetylenes, we have studied the photoinduced absorption spectra of the red form of poly[1,6-bis(3,6-didodecyl-N-carbazolyl)-2,4-hexadiyne] (polyDCHD-S) and the results compared with those of the blue form of the same polymer. An interpretation of the data is given in terms of both the conjugation length and the interbackbone separation also in relation to the photoinduced absorption spectra of both blue and red forms of poly[1,6-bis(N-carbazolyl)-2,4-hexadiyne] (polyDCHD), which does not carry the alkyl substituents on the carbazolyl side groups. Information on the above properties is derived from the analysis of the absorption and Raman spectra of this class of polydiacetylenes
Plant-derived antimalarial agents: new leads and efficient phythomedicines. Part I. Alkaloids
Terpene metabolites from the tropical marine sponge Axinyssa sp. nov
A new sesquiterpene isothiocyanate, (-)-9-isothiocyanatopupukeanane (4), has been isolated along with the known sesquiterpene metabolites (-)-9-isocyanopupukeanane (1), (-)-2-thiocyanatoneopupukeanane (2) and (-)-epipolasin-A (3) from the sponge Axinyssa sp. nov. Metabolites (1), (2) and (4) showed modest in vitro antimalarial activity
Structural element that controls the redox preferences of the active site manganese in oxalate decarboxylase
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