3,657 research outputs found
The Reduction of Flavins by Borohydride: 3,4-Dihydroflavin
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66296/1/j.1432-1033.1969.tb00621.x.pd
A versatile disulfide-driven recycling system for NADP+ with high cofactor turnover number
NADP+-dependent enzymes are important in many biocatalytic processes to generate high-value chemicals for the pharmaceutical and food industry; hence, a costeffective, efficient, and environmentally friendly recycling system for the relatively expensive and only marginally stable enzyme cofactor NADP+ offers significant benefits. NADP+ regeneration schemes have previously been described, but their application is severely limited by the low total turnover numbers (TTN) for the cofactor. Here, we report a glutathione-based recycling system that combines glutaredoxin from E. coli (EcGRX) and the glutathione reductase from S. cerevisiae (ScGR) for NADP+ regeneration. This system employs inexpensive latent organic disulfides such as oxidized cysteine or 2-hydroxyethyl disulfide (HED) as oxidizing agents and allows NADP+ recycling under both aerobic and anaerobic conditions with a TTN in excess of 5 × 105, indicating that each regeneration cycle is 99.9998% selective toward forming the cofactor. Accordingly, for each 1 mol of product generated, less than $0.05 of cofactor is needed. Finally, the EcGRX/ScGR pair is compatible with eight enzymes in the guanosine monophosphate (GMP) biosynthetic pathway, giving the corresponding isotopically labeled nucleotide in high yield. The glutathione-based NADP+ recycling system has potential for biocatalytic applications in academic and industrial settings
Transnational social capital: the socio‐spatialities of civil society
Civil society remains a contested concept, but one that is widely embedded in global development processes. Transnationalism within civil society scholarship is often described dichotomously, either through hierarchical dependency relations or as a more amorphous networked global civil society. These two contrasting spatial imaginaries produce very particular ideas about how transnational relations contribute to civil society. Drawing on empirical material from research with civil society organizations in Barbados and Grenada, in this article I contend that civil society groups use forms of transnational social capital in their work. This does not, however, resonate with the horizontal relations associated with grassroots globalization or vertical chains of dependence. These social relations are imbued with power and agency and are entangled in situated historical, geographical and personal contexts. I conclude that the diverse transnational social relations that are part of civil society activity offer hope and possibilities for continued civil society action in these unexpected spatial arrangements
Mesoscopic Behavior Near a Two-Dimensional Metal-Insulator Transition
We study conductance fluctuations in a two-dimensional electron gas as a
function of chemical potential (or gate voltage) from the strongly insulating
to the metallic regime. Power spectra of the fluctuations decay with two
distinct exponents (1/v_l and 1/v_h). For conductivity , we find a third exponent (1/v_i) in the shortest samples, and
non-monotonic dependence of v_i and v_l on \sigma. We study the dependence of
v_i, v_l, v_h, and the variances of corresponding fluctuations on \sigma,
sample size, and temperature. The anomalies near
indicate that the dielectric response and screening length are critically
behaved, i.e. that Coulomb correlations dominate the physics.Comment: Revised according to referee remark
A Region of Violent Star Formation in the Irr Galaxy IC 10: Structure and Kinematics of Ionized and Neutral Gas
We have used observations of the galaxy IC 10 at the 6-m telescope of the
Special Astrophysical Observatory with the SCORPIO focal reducer in the
Fabry-Perot interferometer mode and with the MPFS spectrograph to study the
structure and kinematics of ionized gas in the central region of current
intense star formation. Archive VLA 21-cm observations are used to analyze the
structure and kinematics of neutral gas in this region. High-velocity wings of
the H-alpha and [SII] emission lines were revealed in the inner cavity of the
nebula HL 111 and in other parts of the complex of violent star formation. We
have discovered local expanding neutral-gas shells around the nebulae HL 111
and HL 106.Comment: 22 pages, 10 figures; accepted in Astronomy Report
Polycyclic aromatic hydrocarbons in the dwarf galaxy IC 10
Infrared observations from the Spitzer Space Telescope archive are used to
study the dust component of the interstellar medium in the IC~10 irregular
galaxy. Dust distribution in the galaxy is compared to the distributions of
H and [SII] emission, neutral hydrogen and CO clouds, and ionizing
radiation sources. The distribution of polycyclic aromatic hydrocarbons (PAH)
in the galaxy is shown to be highly non-uniform with the mass fraction of these
particles in the total dust mass reaching 4%. PAHs tend to avoid bright HII
regions and correlate well with atomic and molecular gas. This pattern suggests
that PAHs form in the dense interstellar gas. We propose that the significant
decrease of the PAH abundance at low metallicity is observed not only globally
(at the level of entire galaxies), but also locally (at least, at the level of
individual HII regions). We compare the distribution of the PAH mass fraction
to the distribution of high-velocity features, that we have detected earlier in
wings of H and SII lines, over the entire available galaxy area. No
conclusive evidence for shock destruction of PAHs in the IC~10 galaxy could be
found.Comment: Accepted for publication in Astronomy Report
A Model for the Voltage Steps in the Breakdown of the Integer Quantum Hall Effect
In samples used to maintain the US resistance standard the breakdown of the
dissipationless integer quantum Hall effect occurs as a series of dissipative
voltage steps. A mechanism for this type of breakdown is proposed, based on the
generation of magneto-excitons when the quantum Hall fluid flows past an
ionised impurity above a critical velocity. The calculated generation rate
gives a voltage step height in good agreement with measurements on both
electron and hole gases. We also compare this model to a hydrodynamic
description of breakdown.Comment: 4 pages including 3 figure
Gas Emission Spectrum in the Irr Galaxy IC 10
Spectroscopic long-slit observations of the dwarf Irr galaxy IC 10 were
conducted at the 6-m Special Astrophysical Observatory telescope with the
SCORPIO focal reducer. The ionized-gas emission spectra in the regions of
intense current star formation were obtained for a large number of regions in
IC 10. The relative abundances of oxygen, N+, and S+ in about twenty HII
regions and in the synchrotron superbubble were estimated. We found that the
galaxy-averaged oxygen abundance is 12 + log(O/H) = 8.17 +- 0.35 and the
metallicity is Z = 0.18 +- 0.14 Z_sun. Our abundances estimated from the strong
emission lines are found to be more reliable than those obtained by comparing
diagnostic diagrams with photoionization models.Comment: Abridged; accepted in Astronomy Letter
HV 11423: The Coolest Supergiant in the SMC
We call attention to the fact that one of the brightest red supergiants in
the SMC has recently changed its spectral type from K0-1 I (December 2004) to
M4 I (December 2005) and back to K0-1 I (September 2006). An archival spectrum
from the Very Large Telescope reveals that the star was even cooler (M4.5-M5 I)
in December 2001. By contrast, the star was observed to be an M0 I in both
October 1978 and October 1979. The M4-5 I spectral types is by far the latest
type seen for an SMC supergiant, and its temperature in that state places it
well beyond the Hayashi limit into a region of the H-R diagram where the star
should not be in hydrostatic equilibrium. The star is variable by nearly 2 mag
in V, but essentially constant in K. Our modeling of its spectral energy
distribution shows that the visual extinction has varied during this time, but
that the star has remained essentially constant in bolometric luminosity. We
suggest that the star is currently undergoing a period of intense instability,
with its effective temperature changing from 4300 K to 3300 K on the time-scale
of months. It has one of the highest 12-micron fluxes of any RSG in the SMC,
and we suggest that the variability at V is due primarily to changes in
effective temperature, and secondly, due to changes in the local extinction due
to creation and dissipation of circumstellar dust. We speculate that the star
may be nearing the end of its life.Comment: Accepted by the Astrophysical Journa
The Massive Star Content of NGC 3603
We investigate the massive star content of NGC 3603, the closest known giant
H II region. We have obtained spectra of 26 stars in the central cluster using
the Baade 6.5-m telescope (Magellan I). Of these 26 stars, 16 had no previous
spectroscopy. We also obtained photometry of all of the stars with previous or
new spectroscopy, primarily using archival HST ACS/HRC images. We use these
data to derive an improved distance to the cluster, and to construct an H-R
diagram for discussing the masses and ages of the massive star content of this
cluster.Comment: Accepted by the Astronomical Journal. This revision updates the
coordinates in Table 1 by (-0.18sec, +0.2") to place them on the UCAC2 syste
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