599 research outputs found
Fluorescence-detected magnetic field effects on radical pair reactions from femtolitre volumes
We show that the effects of applied magnetic fields on radical pair reactions can be sensitively measured from sample volumes as low as ~100 femtolitres using total internal reflection fluorescence microscopy. Development of a fluorescence-based microscope method is likely to be a key step in further miniaturisation that will allow detection of magnetic field effects on single molecules
Two-sided combinatorial volume bounds for non-obtuse hyperbolic polyhedra
We give a method for computing upper and lower bounds for the volume of a
non-obtuse hyperbolic polyhedron in terms of the combinatorics of the
1-skeleton. We introduce an algorithm that detects the geometric decomposition
of good 3-orbifolds with planar singular locus and underlying manifold the
3-sphere. The volume bounds follow from techniques related to the proof of
Thurston's Orbifold Theorem, Schl\"afli's formula, and previous results of the
author giving volume bounds for right-angled hyperbolic polyhedra.Comment: 36 pages, 19 figure
A Hubble Space Telescope Study of Lyman Limit Systems: Census and Evolution
We present a survey for optically thick Lyman limit absorbers at z<2.6 using
archival Hubble Space Telescope observations with the Faint Object Spectrograph
and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems
(LLSs) increasing the number of catalogued LLSs at z<2.6 by a factor of ~10. We
compile a statistical sample of 50 tau_LLS > 2 LLSs drawn from 249 QSO sight
lines that avoid known targeting biases. The incidence of such LLSs per unit
redshift, l(z)=dn/dz, at these redshifts is well described by a single power
law, l(z) = C1 (1+z)^gamma, with gamma=1.33 +/- 0.61 at z<2.6, or with
gamma=1.83 +/- 0.21 over the redshift range 0.2 < z < 4.9. The incidence of
LLSs per absorption distance, l(X), decreases by a factor of ~1.5 over the ~0.6
Gyr from z=4.9 to 3.5; l(X) evolves much more slowly at low redshifts,
decreasing by a similar factor over the ~8 Gyr from z=2.6 to 0.25. We show that
the column density distribution function, f(N(HI)), at low redshift is not well
fitted by a single power law index (f(N(HI)) = C2 N(HI)^(-beta)) over the
column density range 13 17.2. While low and high
redshift f(N(HI)) distributions are consistent for log N(HI)>19.0, there is
some evidence that f(N(HI)) evolves with z for log N(HI) < 17.7, possibly due
to the evolution of the UV background and galactic feedback. Assuming LLSs are
associated with individual galaxies, we show that the physical cross section of
the optically thick envelopes of galaxies decreased by a factor of ~9 from z~5
to 2 and has remained relatively constant since that time. We argue that a
significant fraction of the observed population of LLSs arises in the
circumgalactic gas of sub-L* galaxies.Comment: Accepted by Ap
Composition and Acidification of the Culture Medium Influences Chronological Aging Similarly in Vineyard and Laboratory Yeast
Chronological aging has been studied extensively in laboratory yeast by culturing cells into stationary phase in synthetic complete medium with 2% glucose as the carbon source. During this process, acidification of the culture medium occurs due to secretion of organic acids, including acetic acid, which limits survival of yeast cells. Dietary restriction or buffering the medium to pH 6 prevents acidification and increases chronological life span. Here we set out to determine whether these effects are specific to laboratory-derived yeast by testing the chronological aging properties of the vineyard yeast strain RM11. Similar to the laboratory strain BY4743 and its haploid derivatives, RM11 and its haploid derivatives displayed increased chronological life span from dietary restriction, buffering the pH of the culture medium, or aging in rich medium. RM11 and BY4743 also displayed generally similar aging and growth characteristics when cultured in a variety of different carbon sources. These data support the idea that mechanisms of chronological aging are similar in both the laboratory and vineyard strains
Lipoprotein lipase is active as a monomer
Lipoprotein lipase (LPL), the enzyme that hydrolyzes triglycerides in plasma lipoproteins, is assumed to be active only as a homodimer. In support of this idea, several groups have reported that the size of LPL, as measured by density gradient ultracentrifugation, is ∼110 kDa, twice the size of LPL monomers (∼55 kDa). Of note, however, in those studies the LPL had been incubated with heparin, a polyanionic substance that binds and stabilizes LPL. Here we revisited the assumption that LPL is active only as a homodimer. When freshly secreted human LPL (or purified preparations of LPL) was subjected to density gradient ultracentrifugation (in the absence of heparin), LPL mass and activity peaks exhibited the size expected of monomers (near the 66-kDa albumin standard). GPIHBP1-bound LPL also exhibited the size expected for a monomer. In the presence of heparin, LPL size increased, overlapping with a 97.2-kDa standard. We also used density gradient ultracentrifugation to characterize the LPL within the high-salt and low-salt peaks from a heparin-Sepharose column. The catalytically active LPL within the high-salt peak exhibited the size of monomers, whereas most of the inactive LPL in the low-salt peak was at the bottom of the tube (in aggregates). Consistent with those findings, the LPL in the low-salt peak, but not that in the high-salt peak, was easily detectable with single mAb sandwich ELISAs, in which LPL is captured and detected with the same antibody. We conclude that catalytically active LPL can exist in a monomeric state
Traces of past activity in the Galactic Centre
The Milky Way centre hosts a supermassive Black Hole (BH) with a mass of
~4*10^6 M_Sun. Sgr A*, its electromagnetic counterpart, currently appears as an
extremely weak source with a luminosity L~10^-9 L_Edd. The lowest known
Eddington ratio BH. However, it was not always so; traces of "glorious" active
periods can be found in the surrounding medium. We review here our current view
of the X-ray emission from the Galactic Center (GC) and its environment, and
the expected signatures (e.g. X-ray reflection) of a past flare. We discuss the
history of Sgr A*'s past activity and its impact on the surrounding medium. The
structure of the Central Molecular Zone (CMZ) has not changed significantly
since the last active phase of Sgr A*. This relic torus provides us with the
opportunity to image the structure of an AGN torus in exquisite detail.Comment: Invited refereed review. Chapter of the book: "Cosmic ray induced
phenomenology in star forming environments" (eds. Olaf Reimer and Diego F.
Torres
Maturation-Induced Cloaking of Neutralization Epitopes on HIV-1 Particles
To become infectious, HIV-1 particles undergo a maturation process involving proteolytic cleavage of the Gag and Gag-Pol polyproteins. Immature particles contain a highly stable spherical Gag lattice and are impaired for fusion with target cells. The fusion impairment is relieved by truncation of the gp41 cytoplasmic tail (CT), indicating that an interaction between the immature viral core and gp41 within the particle represses HIV-1 fusion by an unknown mechanism. We hypothesized that the conformation of Env on the viral surface is regulated allosterically by interactions with the HIV-1 core during particle maturation. To test this, we quantified the binding of a panel of monoclonal antibodies to mature and immature HIV-1 particles by immunofluorescence imaging. Surprisingly, immature particles exhibited markedly enhanced binding of several gp41-specific antibodies, including two that recognize the membrane proximal external region (MPER) and neutralize diverse HIV-1 strains. Several of the differences in epitope exposure on mature and immature particles were abolished by truncation of the gp41 CT, thus linking the immature HIV-1 fusion defect with altered Env conformation. Our results suggest that perturbation of fusion-dependent Env conformational changes contributes to the impaired fusion of immature particles. Masking of neutralization-sensitive epitopes during particle maturation may contribute to HIV-1 immune evasion and has practical implications for vaccine strategies targeting the gp41 MPER
Atomic Hydrogen Gas in Dark-Matter Minihalos and the Compact High Velocity Clouds
We calculate the coupled hydrostatic and ionization structures of
pressure-supported gas clouds that are confined by gravitationally dominant
dark-matter (DM) mini-halos and by an external bounding pressure provided by a
hot medium. We focus on clouds that are photoionized and heated by the
present-day background metagalactic field and determine the conditions for the
formation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen
(HI) cores in the DM-dominated clouds. We consider LCDM dark-matter halos, and
we compute models for a wide range of halo masses, total cloud gas masses, and
external bounding pressures. We present models for the pressure-supported HI
structures observed in the Local Group dwarf galaxies Leo A and Sag DIG. We
then construct minihalo models for the multi-phased (and low-metallicity)
compact high-velocity HI clouds (CHVCs). If the CHVCs are drawn from the same
family of halos that successfully reproduce the dwarf galaxy observations, then
the CHVCs must be "circumgalactic objects" with characteristic distances of 150
kpc. For such systems we find that multi-phased behavior occurs for peak WNM HI
column densities between 2e19 and 1e20 cm^-2, consistent with observations. If
the large population of CHVCs represent "missing low-mass satellites" of the
Galaxy, then these clouds must be pressure-confined to keep the gas neutral.
For an implied CHVC minihalo scale velocity of v_s=12 km s^-1, the confining
pressure must exceed ~50 cm^-3 K. A hot (2e6 K) Galactic corona could provide
the required pressure at 150 kpc.Comment: 82 pages, 17 figures. To appear in Astrophysical Journal Supplement.
(Abridged abstract
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