480 research outputs found
Electronic ground states of Fe and Co as determined by x-ray absorption and x-ray magnetic circular dichroism spectroscopy
The electronic ground state of the Co diatomic molecular cation
has been assigned experimentally by x-ray absorption and x-ray magnetic
circular dichroism spectroscopy in a cryogenic ion trap. Three candidates,
, , and , for the electronic ground state of Fe
have been identified. These states carry sizable orbital angular momenta that
disagree with theoretical predictions from multireference configuration
interaction and density functional theory. Our results show that the ground
states of neutral and cationic diatomic molecules of transition elements
cannot generally be assumed to be connected by a one-electron process
Direct Observation of High-Spin States in Manganese Dimer and Trimer Cations by X-ray Magnetic Circular Dichroism Spectroscopy in an Ion Trap
The electronic structure and magnetic moments of free Mn and Mn
are characterized by x-ray absorption and x-ray magnetic circular
dichroism spectroscopy in a cryogenic ion trap that is coupled to a synchrotron
radiation beamline. Our results show directly that localized magnetic moments
of 5 are created by states at each ionic core,
which are coupled in parallel to form molecular high-spin states via indirect
exchange that is mediated in both cases by a delocalized valence electron in a
singly-occupied derived orbital with an unpaired spin. This leads to total
magnetic moments of 11 for Mn and 16 for Mn, with
no contribution of orbital angular momentum
Coordination-driven magnetic-to-nonmagnetic transition in manganese doped silicon clusters
The interaction of a single manganese impurity with silicon is analyzed in a
combined experimental and theoretical study of the electronic, magnetic, and
structural properties of manganese-doped silicon clusters. The structural
transition from exohedral to endohedral doping coincides with a quenching of
high-spin states. For all geometric structures investigated, we find a similar
dependence of the magnetic moment on the manganese coordination number and
nearest neighbor distance. This observation can be generalized to manganese
point defects in bulk silicon, whose magnetic moments fall within the observed
magnetic-to-nonmagnetic transition, and which therefore react very sensitively
to changes in the local geometry. The results indicate that high spin states in
manganese-doped silicon could be stabilized by an appropriate lattice
expansion
Fighting a losing battle: Vigorous immune response countered by pathogen suppression of host defenses in the chytridiomycosis-susceptible frog Atelopus zeteki
The emergence of the disease chytridiomycosis caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd) has been implicated in dramatic global amphibian declines. Although many species have undergone catastrophic declines and/or extinctions, others appear to be unaffected or persist
at reduced frequencies after Bd outbreaks. The reasons behind this variance in disease outcomes are poorly
understood: differences in host immune responses have been proposed, yet previous studies suggest a lack
of robust immune responses to Bd in susceptible species. Here, we sequenced transcriptomes from clutchmates
of a highly susceptible amphibian, Atelopus zeteki, with different infection histories. We found
significant changes in expression of numerous genes involved in innate and inflammatory responses in
infected frogs despite high susceptibility to chytridiomycosis. We show evidence of acquired immune
responses generated against Bd, including increased expression of immunoglobulins and major histocompatibility
complex genes. In addition, fungal-killing genes had significantly greater expression in frogs
previously exposed to Bd compared with Bd-naïve frogs, including chitinase and serine-type proteases.
However, our results appear to confirm recent in vitro evidence of immune suppression by Bd, demonstrated
by decreased expression of lymphocyte genes in the spleen of infected compared with control frogs. We propose susceptibility to chytridiomycosis is not due to lack of Bd-specific immune responses but instead is caused by failure of those responses to be effective. Ineffective immune pathway activation and timing of antibody production are discussed as potential mechanisms. However, in light of our findings,suppression of key immune responses by Bd is likely an important factor in the lethality of this fungus
Spin and orbital magnetic moments of size-selected iron, cobalt, and nickel clusters and their link to the bulk phase diagrams
Spin and orbital magnetic moments of cationic iron, cobalt, and nickel
clusters have been determined from x-ray magnetic circular dichroism
spectroscopy. In the size regime of atoms, these clusters show
strong ferromagnetism with maximized spin magnetic moments of 1~ per
empty state because of completely filled majority spin bands. The
only exception is where an unusually low average spin
magnetic moment of ~ per unoccupied state is
detected; an effect, which is neither observed for nor
.\@ This distinct behavior can be linked to the existence
and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in
the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the
experimental data, available density functional theory calculations generally
seem to underestimate the spin magnetic moments significantly. In all clusters
investigated, the orbital magnetic moment is quenched to \,\% of the
atomic value by the reduced symmetry of the crystal field. The magnetic
anisotropy energy is well below 65 eV per atom
More than skin deep: Functional genomic basis for resistance to Amphibian Chytridiomycosis
The amphibian-killing chytrid fungus Batrachochytriumdendrobatidis (Bd) is one of themost generalist pathogens known, capable of
infecting hundreds of species globally and causing widespread population declines and extinctions. However, some host species are
seemingly unaffected by Bd, tolerating or clearing infections without clinical signs of disease. Variation in host immune responses is
commonly evoked for these resistant or tolerant species, yet to date,we have nodirect comparisonof amphibian species responses to
infection at the level of gene expression. In this study,we challenged four CentralAmerican frog species that vary in Bd susceptibility,
with a sympatric virulent strain of the pathogen. We compared skin and spleen orthologous gene expression using differential
expression tests and coexpression gene network analyses.Wefound that resistant species have reduced skin inflammatory responses
andincreased expressionofgenes involved inskin integrity. Incontrast, onlyhighly susceptible species exhibited suppressionof splenic
T-cell genes. We conclude that resistance to chytridiomycosis may be related to a species’ ability to escape the immunosuppressive
activity of the fungus. Moreover, our results indicate that within-species differences in splenic proteolytic enzyme gene expression
may contribute to intraspecific variation in survival. This first comparison of amphibian functional immunogenomic architecture in
response to Bd provides insights into key genetic mechanisms underlying variation in disease outcomes among amphibian species
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