1,066 research outputs found
The importance of N2 leptogenesis
We argue that fast interactions of the lightest singlet neutrino would
project part of a preexisting lepton asymmetry onto a direction that is
protected from washout effects, thus preventing it from being erased. In
particular, we consider an asymmetry generated in decays, assuming that
interactions are fast enough to bring into full thermal
equilibrium. If decays occur at T\gsim 10^9 GeV, that is, before the
muon Yukawa interactions enter into thermal equilibrium, then generically part
of survives. In this case some of the constraints implied by the standard
leptogenesis scenario hold only if . For T\lsim 10^9
GeV, is generally erased, unless special alignment/orthogonality
conditions in flavor space are realized.Comment: 5 pages. A few clarifications added, conclusions unchanged. Version
published in Phys. Rev. Lett. (Title changed in journal
Blue-light reception through quaternary transitions
Sensory photoreceptors absorb light via their photosensor modules and trigger
downstream physiological adaptations via their effector modules. Light
reception accordingly depends on precisely orchestrated interactions between
these modules, the molecular details of which often remain elusive. Using
electron-electron double resonance (ELDOR) spectroscopy and site-directed spin
labelling, we chart the structural transitions facilitating blue-light
reception in the engineered light-oxygen-voltage (LOV) histidine kinase YF1
which represents a paradigm for numerous natural signal receptors. Structural
modelling based on pair-wise distance constraints derived from ELDOR pinpoint
light-induced rotation and splaying apart of the two LOV photosensors in the
dimeric photoreceptor. Resultant molecular strain likely relaxes as left-
handed supercoiling of the coiled-coil linker connecting sensor and effector
units. ELDOR data on a photoreceptor variant with an inverted signal response
indicate a drastically altered dimer interface but light-induced structural
transitions in the linker that are similar to those in YF1. Taken together, we
provide mechanistic insight into the signal trajectories of LOV photoreceptors
and histidine kinases that inform molecular simulations and the engineering of
novel receptors
Comparison of 20nm silver nanoparticles synthesized with and without a gold core: Structure, dissolution in cell culture media, and biological impact on macrophages
Widespread use of silver nanoparticles raises questions of environmental and biological impact. Many synthesis approaches are used to produce pure silver and silver-shell gold-core particles optimized for specific applications. Since both nanoparticles and silver dissolved from the particles may impact the biological response, it is important to understand the physicochemical characteristics along with the biological impact of nanoparticles produced by different processes. The authors have examined the structure, dissolution, and impact of particle exposure to macrophage cells of two 20 nm silver particles synthesized in different ways, which have different internal structures. The structures were examined by electron microscopy and dissolution measured in Rosewell Park Memorial Institute media with 10% fetal bovine serum. Cytotoxicity and oxidative stress were used to measure biological impact on RAW 264.7 macrophage cells. The particles were polycrystalline, but 20 nm particles grown on gold seed particles had smaller crystallite size with many high-energy grain boundaries and defects, and an apparent higher solubility than 20 nm pure silver particles. Greater oxidative stress and cytotoxicity were observed for 20 nm particles containing the Au core than for 20 nm pure silver particles. A simple dissolution model described the time variation of particle size and dissolved silver for particle loadings larger than 9 μg/ml for the 24-h period characteristic of many in-vitro studies
A structural model for the full-length blue light-sensing protein YtvA from Bacillus subtilis, based on EPR spectroscopy
A model for the full-length structure of the blue light-sensing protein YtvA
from Bacillus subtilis has been determined by EPR spectroscopy, performed on
spin labels selectively inserted at amino acid positions 54, 80, 117 and 179.
Our data indicate that YtvA forms a dimer in solution and enable us, based on
the known structures of the individual domains and modelling, to propose a
three-dimensional model for the full length protein. Most importantly, this
includes the YtvA N-terminus that has so far not been identified in any
structural model. We show that our data are in agreement with the crystal
structure of an engineered LOV-domain protein, YF1, that shows the N-terminus
of the protein to be helical and to fold back in between the β-sheets of the
two LOV domains, and argue for an identical arrangement in YtvA. While we
could not detect any structural changes upon blue-light activation of the
protein, this structural model now forms an ideal basis for identifying
residues as targets for further spin labelling studies to detect potential
conformational changes upon irradiation of the protein
CP violation in scatterings, three body processes and the Boltzmann equations for leptogenesis
We obtain the Boltzmann equations for leptogenesis including decay and
scattering processes with two and three body initial or final states. We
present an explicit computation of the CP violating scattering asymmetries. We
analyze their possible impact in leptogenesis, and we discuss the validity of
their approximate expressions in terms of the decay asymmetry. In scenarios in
which the initial heavy neutrino density vanishes, the inclusion of CP
asymmetries in scatterings can enforce a cancellation between the lepton
asymmetry generated at early times and the asymmetry produced at later times.
We argue that a sizeable amount of washout is crucial for spoiling this
cancellation, and we show that in the regimes in which the washouts are
particularly weak, the inclusion of CP violation in scatterings yields a
reduction in the final value of the lepton asymmetry. In the strong washout
regimes the inclusion of CP violation in scatterings still leads to a
significant enhancement of the lepton asymmetry at high temperatures; however,
due to the independence from the early conditions that is characteristic of
these regimes, the final value of the lepton asymmetry remains approximately
unchanged.Comment: 24 pages, 6 figures. One appendix added. Some numerical results and
corresponding figures (mainly fig. 3) corrected. Final version to be
published in JHE
Signal transduction in light-oxygen-voltage receptors lacking the adduct- forming cysteine residue
Light–oxygen–voltage (LOV) receptors sense blue light through the
photochemical generation of a covalent adduct between a flavin-nucleotide
chromophore and a strictly conserved cysteine residue. Here we show that,
after cysteine removal, the circadian-clock LOV-protein Vivid still undergoes
light-induced dimerization and signalling because of flavin photoreduction to
the neutral semiquinone (NSQ). Similarly, photoreduction of the engineered LOV
histidine kinase YF1 to the NSQ modulates activity and downstream effects on
gene expression. Signal transduction in both proteins hence hinges on flavin
protonation, which is common to both the cysteinyl adduct and the NSQ. This
general mechanism is also conserved by natural cysteine-less, LOV-like
regulators that respond to chemical or photoreduction of their flavin
cofactors. As LOV proteins can react to light even when devoid of the adduct-
forming cysteine, modern LOV photoreceptors may have arisen from ancestral
redox-active flavoproteins. The ability to tune LOV reactivity through
photoreduction may have important implications for LOV mechanism and
optogenetic applications
Surface and Interface Properties of 10–12 Unit Cells Thick Sputter Deposited Epitaxial CeO 2
Ultrathin and continuous epitaxial films with relaxed lattice strain can potentially maintain more of its bulk physical and chemical properties and are useful as buffer layers. We study surface, interface, and microstructural properties of ultrathin (∼10–12 unit cells thick) epitaxial ceria films grown on single crystal YSZ substrates. The out-of -plane and in-plane lattice parameters indicate relaxation in the continuous film due to misfit dislocations seen by high-resolution transmission electron microscopy (HRTEM) and substrate roughness of ∼1-2 unit cells, confirmed by atomic force microscopy and HRTEM. A combination of secondary sputtering, lattice mismatch, substrate roughness, and surface reduction creating secondary phase was likely the cause of surface roughness which should be reduced to a minimum level for effective use of it as buffer layers
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