561,474 research outputs found

### Energetics of ion competition in the DEKA selectivity filter of neuronal sodium channels

The energetics of ionic selectivity in the neuronal sodium channels is
studied. A simple model constructed for the selectivity filter of the channel
is used. The selectivity filter of this channel type contains aspartate (D),
glutamate (E), lysine (K), and alanine (A) residues (the DEKA locus). We use
Grand Canonical Monte Carlo simulations to compute equilibrium binding
selectivity in the selectivity filter and to obtain various terms of the excess
chemical potential from a particle insertion procedure based on Widom's method.
We show that K$^{+}$ ions in competition with Na$^{+}$ are efficiently excluded
from the selectivity filter due to entropic hard sphere exclusion. The
dielectric constant of protein has no effect on this selectivity. Ca$^{2+}$
ions, on the other hand, are excluded from the filter due to a free energetic
penalty which is enhanced by the low dielectric constant of protein.Comment: 14 pages, 7 figure

### The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1 : contributions of the pore domains

We have investigated the contribution to ionic
selectivity of residues in the selectivity filter and pore
helices of the P1 and P2 domains in the acid sensitive
potassium channel TASK-1. We used site directed mutagenesis
and electrophysiological studies, assisted by structural
models built through computational methods. We have
measured selectivity in channels expressed in Xenopus
oocytes, using voltage clamp to measure shifts in reversal
potential and current amplitudes when Rb+ or Na+ replaced
extracellular K+. Both P1 and P2 contribute to selectivity,
and most mutations, including mutation of residues in the
triplets GYG and GFG in P1 and P2, made channels nonselective.
We interpret the effects of these—and of other
mutations—in terms of the way the pore is likely to be
stabilised structurally. We show also that residues in the
outer pore mouth contribute to selectivity in TASK-1.
Mutations resulting in loss of selectivity (e.g. I94S, G95A)
were associated with slowing of the response of channels to
depolarisation. More important physiologically, pH sensitivity
is also lost or altered by such mutations. Mutations
that retained selectivity (e.g. I94L, I94V) also retained their
response to acidification. It is likely that responses both to
voltage and pH changes involve gating at the selectivity filter

### Metamaterial Broadband Angular Selectivity

We demonstrate how broadband angular selectivity can be achieved with stacks
of one-dimensionally periodic photonic crystals, each consisting of alternating
isotropic layers and effective anisotropic layers, where each effective
anisotropic layer is constructed from a multilayered metamaterial. We show that
by simply changing the structure of the metamaterials, the selective angle can
be tuned to a broad range of angles; and, by increasing the number of stacks,
the angular transmission window can be made as narrow as desired. As a proof of
principle, we realize the idea experimentally in the microwave regime. The
angular selectivity and tunability we report here can have various applications
such as in directional control of electromagnetic emitters and detectors.Comment: 5 pages, 5 figure

### Selectivity in Quaternion Algebras

We prove an integral version of the classical Albert-Brauer-Hasse-Noether
theorem regarding quaternion algebras over number fields. Let $\mathfrak A$ be
a quaternion algebra over a number field $K$ and assume that $\mathfrak A$
satisfies the Eichler condition; that is, there exists an archimedean prime of
$K$ which does not ramify in $\mathfrak A$. Let $\Omega$ be a commutative,
quadratic $\mathcal{O}_K$-order and let $\mathcal{R}\subset \mathfrak A$ be an
order of full rank. Assume that there exists an embedding of $\Omega$ into
$\mathcal R$. We describe a number of criteria which, if satisfied, imply that
every order in the genus of $\mathcal R$ admits an embedding of $\Omega$. In
the case that the relative discriminant ideal of $\Omega$ is coprime to the
level of $\mathcal R$ and the level of $\mathcal R$ is coprime to the
discriminant of $\mathfrak A$, we give necessary and sufficient conditions for
an order in the genus of $\mathcal R$ to admit an embedding of $\Omega$. We
explicitly parameterize the isomorphism classes of orders in the genus of
$\mathcal R$ which admit an embedding of $\Omega$. In particular, we show that
the proportion of the genus of $\mathcal{R}$ admitting an embedding of $\Omega$
is either 0, 1/2 or 1. Analogous statements are proven for optimal embeddings.Comment: Final version; to appear in the Journal of Number Theor

### Isotopic and spin selectivity of H_2 adsorbed in bundles of carbon nanotubes

Due to its large surface area and strongly attractive potential, a bundle of
carbon nanotubes is an ideal substrate material for gas storage. In addition,
adsorption in nanotubes can be exploited in order to separate the components of
a mixture. In this paper, we investigate the preferential adsorption of D_2
versus H_2(isotope selectivity) and of ortho versus para(spin selectivity)
molecules confined in the one-dimensional grooves and interstitial channels of
carbon nanotube bundles. We perform selectivity calculations in the low
coverage regime, neglecting interactions between adsorbate molecules. We find
substantial spin selectivity for a range of temperatures up to 100 K, and even
greater isotope selectivity for an extended range of temperatures,up to 300 K.
This isotope selectivity is consistent with recent experimental data, which
exhibit a large difference between the isosteric heats of D_2 and H_2 adsorbed
in these bundles.Comment: Paper submitted to Phys.Rev. B; 17 pages, 2 tables, 6 figure

### Dehydration as a Universal Mechanism for Ion Selectivity in Graphene and Other Atomically Thin Pores

Ion channels play a key role in regulating cell behavior and in electrical
signaling. In these settings, polar and charged functional groups -- as well as
protein response -- compensate for dehydration in an ion-dependent way, giving
rise to the ion selective transport critical to the operation of cells.
Dehydration, though, yields ion-dependent free-energy barriers and thus is
predicted to give rise to selectivity by itself. However, these barriers are
typically so large that they will suppress the ion currents to undetectable
levels. Here, we establish that graphene displays a measurable dehydration-only
mechanism for selectivity of $\mathrm{K}^+$ over $\mathrm{Cl}^-$. This
fundamental mechanism -- one that depends only on the geometry and hydration --
is the starting point for selectivity for all channels and pores. Moreover,
while we study selectivity of $\mathrm{K}^+$ over $\mathrm{Cl}^-$, we find that
dehydration-based selectivity functions for all ions, i.e., cation over cation
selectivity (e.g., $\mathrm{K}^+$ over $\mathrm{Na}^+$). Its likely detection
in graphene pores resolves conflicting experimental results, as well as
presents a new paradigm for characterizing the operation of ion channels and
engineering molecular/ionic selectivity in filtration and other applications.Comment: 27 page

### Investigating the selectivity of weed harrowing with new methods

In six field experiments it was investigated whether row spacing, timing, direction and orientation of post-emergence weed harrowing in spring barley influenced the selectivity and whether it is important that increasing intensities of harrowing are generated either by increasing number of passes or increasing driving speed. Selectivity was defined as the relationship between crop burial in soil immediately after treatment and weed control. To estimate crop burial, digital image analysis was used in order to make the estimations objective. The study showed that narrow row spacing decreased selectivity in a late growth stage (21) whereas row spacing in the range of 5.3 cm to 24 cm had no effects in an early growth stage (12). Harrowing across rows decreased selectivity in one out of two experiments. Whether repeated passes with the harrowing were carried out in the same orientation along the rows or in alternative orientations forth and back was unimportant. There were indications that high driving speed decreases selectivity and that repeated passes with low driving speed are better than single treatments with high driving speed. Impacts on selectivity, however, were small and only significant at high degrees of weed control. Timing had no significant impact on selectivity

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