4,405 research outputs found
Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect
Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through
partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe
valence and the degree of B-site order were probed by 57Fe Mossbauer
spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms
occupying wrong sites, i.e. antisite disorder. It had very little effect on the
Fe valence: a small but visible increase in the isomer shift was seen for the
mixed-valent FeII/III atoms occupying the right site indicating a slight
movement towards divalency of these atoms, which was more than counterbalanced
by the increase in the fraction of antisite Fe atoms with III valence state. It
is therefore argued that the bulk of the electron doping is received by
antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the
lower IV/V valence state. Thus under Nd substitution, the charge-neutrality
requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.Comment: 15 pages, 6 figures, to appear in Solid State Commu
Frequency dependence of signal power and spatial reach of the local field potential
The first recording of electrical potential from brain activity was reported
already in 1875, but still the interpretation of the signal is debated. To take
full advantage of the new generation of microelectrodes with hundreds or even
thousands of electrode contacts, an accurate quantitative link between what is
measured and the underlying neural circuit activity is needed. Here we address
the question of how the observed frequency dependence of recorded local field
potentials (LFPs) should be interpreted. By use of a well-established
biophysical modeling scheme, combined with detailed reconstructed neuronal
morphologies, we find that correlations in the synaptic inputs onto a
population of pyramidal cells may significantly boost the low-frequency
components of the generated LFP. We further find that these low-frequency
components may be less `local' than the high-frequency LFP components in the
sense that (1) the size of signal-generation region of the LFP recorded at an
electrode is larger and (2) that the LFP generated by a synaptically activated
population spreads further outside the population edge due to volume
conduction
Iron valence in double-perovskite (Ba,Sr,Ca)2FeMoO6: Isovalent substitution effect
In the Fe-Mo based B-site ordered double-perovskite, A2FeMoO6.0, with iron in
the mixed-valence II/III state, the valence value of Fe is not precisely fixed
at 2.5 but may be fine-tuned by means of applying chemical pressure at the
A-cation site. This is shown through a systematic 57Fe Mossbauer spectroscopy
study using a series of A2FeMoO6.0 [A = (Ba,Sr) or (Sr,Ca)] samples with high
degree of Fe/Mo order, the same stoichiometric oxygen content and also almost
the same grain size. The isomer shift values and other hyperfine parameters
obtained from the Mossbauer spectra confirm that Fe remains in the
mixed-valence state within the whole range of A constituents. However, upon
increasing the average cation size at the A site the precise valence of Fe is
found to decrease such that within the A = (Ba,Sr) regime the valence of Fe is
closer to II, while within the A = (Sr,Ca) regime it is closer to the actual
mixed-valence II/III state. As the valence of Fe approaches II, the difference
in charges between Fe and Mo increases, and parallel with this the degree of
Fe/Mo order increases. Additionally, for the less-ordered samples an increased
tendency of clustering of the anti-site Fe atoms is deduced from the Mossbauer
data.Comment: 19 pages, 6 figures, submitted to Phys. Rev.
Computing the local field potential (LFP) from integrate-and-fire network models
Leaky integrate-and-fire (LIF) network models are commonly used to study how the spiking dynamics of neural networks changes with stimuli, tasks or dynamic network states. However, neurophysiological studies in vivo often rather measure the mass activity of neuronal microcircuits with the local field potential (LFP). Given that LFPs are generated by spatially separated currents across the neuronal membrane, they cannot be computed directly from quantities defined in models of point-like LIF neurons. Here, we explore the best approximation for predicting the LFP based on standard output from point-neuron LIF networks. To search for this best "LFP proxy", we compared LFP predictions from candidate proxies based on LIF network output (e.g, firing rates, membrane potentials, synaptic currents) with "ground-truth" LFP obtained when the LIF network synaptic input currents were injected into an analogous three-dimensional (3D) network model of multi-compartmental neurons with realistic morphology, spatial distributions of somata and synapses. We found that a specific fixed linear combination of the LIF synaptic currents provided an accurate LFP proxy, accounting for most of the variance of the LFP time course observed in the 3D network for all recording locations. This proxy performed well over a broad set of conditions, including substantial variations of the neuronal morphologies. Our results provide a simple formula for estimating the time course of the LFP from LIF network simulations in cases where a single pyramidal population dominates the LFP generation, and thereby facilitate quantitative comparison between computational models and experimental LFP recordings in vivo
Generalized Elliptic Integrals and the Legendre M-function
We study monotonicity and convexity properties of functions arising in the
theory of elliptic integrals, and in particular in the case of a
Schwarz-Christoffel conformal mapping from a half-plane to a trapezoid. We
obtain sharp monotonicity and convexity results for combinations of these
functions, as well as functional inequalities and a linearization property.Comment: 28 page
Decay times in turnover statistics of single enzymes
The first passage times for enzymatic turnovers in non-equilibrium steady
state display a statistical symmetry property related to non-equilibrium
fluctuation theorems, that makes it possible to extract the chemical driving
force from single molecule trajectories in non-equilibrium steady state. Below,
we show that this system violates the general expectation that the number of
decay constants needed to fit a first passage time distribution reflects the
number of states in the escape problem. In fact, the structure of the kinetic
mechanism makes half of the decay times vanish identically from the turnover
time distribution. The terms that cancel out correspond to the eigenvalues of a
certain sub-matrix of the master equation matrix for the first exit time
problem. We discuss how these results make modeling and data analysis easier
for such systems, and how the turnovers can be measured.Comment: 4 pages, 1 figure v2: Published version, minor corrections in
response to referee comment
Multiple Lac-mediated loops revealed by Bayesian statistics and tethered particle motion
The bacterial transcription factor LacI loops DNA by binding to two separate
locations on the DNA simultaneously. Despite being one of the best-studied
model systems for transcriptional regulation, the number and conformations of
loop structures accessible to LacI remain unclear, though the importance of
multiple co-existing loops has been implicated in interactions between LacI and
other cellular regulators of gene expression. To probe this issue, we have
developed a new analysis method for tethered particle motion, a versatile and
commonly-used in vitro single-molecule technique. Our method, vbTPM, performs
variational Bayesian inference in hidden Markov models. It learns the number of
distinct states (i.e., DNA-protein conformations) directly from tethered
particle motion data with better resolution than existing methods, while easily
correcting for common experimental artifacts. Studying short (roughly 100 bp)
LacI-mediated loops, we provide evidence for three distinct loop structures,
more than previously reported in single-molecule studies. Moreover, our results
confirm that changes in LacI conformation and DNA binding topology both
contribute to the repertoire of LacI-mediated loops formed in vitro, and
provide qualitatively new input for models of looping and transcriptional
regulation. We expect vbTPM to be broadly useful for probing complex
protein-nucleic acid interactions.Comment: 34 pages, 25 figures, including Supporting information. To appear in
Nucleic Acids Research. Accompanying open-source software:
http://sourceforge.net/projects/vbtpm
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