The conduction pathway of potassium channels is water free under physiological conditions.

Ion conduction through potassium channels is a fundamental process of life. On the basis of crystallographic data, it was originally proposed that potassium ions and water molecules are transported through the selectivity filter in an alternating arrangement, suggesting a "water-mediated" knock-on mechanism. Later on, this view was challenged by results from molecular dynamics simulations that revealed a "direct" knock-on mechanism where ions are in direct contact. Using solid-state nuclear magnetic resonance techniques tailored to characterize the interaction between water molecules and the ion channel, we show here that the selectivity filter of a potassium channel is free of water under physiological conditions. Our results are fully consistent with the direct knock-on mechanism of ion conduction but contradict the previously proposed water-mediated knock-on mechanism

The adjoint problem in the presence of a deformed surface: the example of the Rosensweig instability on magnetic fluids

The Rosensweig instability is the phenomenon that above a certain threshold of a vertical magnetic field peaks appear on the free surface of a horizontal layer of magnetic fluid. In contrast to almost all classical hydrodynamical systems, the nonlinearities of the Rosensweig instability are entirely triggered by the properties of a deformed and a priori unknown surface. The resulting problems in defining an adjoint operator for such nonlinearities are illustrated. The implications concerning amplitude equations for pattern forming systems with a deformed surface are discussed.Comment: 11 pages, 1 figur

Field-dependent Vortex Pinning Strength in a Periodic Array of Antidots

We explore the dynamic response of vortex lines in a Pb thin film with a periodic array of antidots by means of ac-susceptibility measurements. For low drive field amplitudes, within the Campbell regime, vortex motion is of intra-valley type and the penetration depth is related to the curvature of the pinning potential well, $\alpha$. For dc-fields below the first matching field $H_1$, $\alpha$ reaches its highest value associated with a Mott Insulator-like phase where vortex lines are strongly localized at the pinning sites. For $H_1<H_{dc}<H_2$, the response is mainly due to the interstitial vortices and $\alpha$ drops to smaller values as expected for this metallic-like regime. Strikingly, for $H_2<H_{dc}<H_3$, we observe that $\alpha$ reduces further down. However, for $H_3<H_{dc}<H_4$, a reentrance in the pinning strength is observed, due to a specific configuration of the flux line lattice which strongly restricts the mobility of vortices. We present a possible explanation for the measured $\alpha(H_{dc})$ dependence based on the different flux line lattice configurations.Comment: 5 pages, to be published in Phys. Rev.

Exclusión y desigualdad espacial: retrato desde la movilidad cotidiana

Daily urban mobility is one of the most complex and relevant phenomena of contemporary spatial configurations and organizations. However, ignoring contributions from theories of urbanism, its spatiality has been disarticulated by planners who are unawate of the anthropological dimensions of spaces of human beings. The following article tries to develop and problematise urban social exclusion from an urban daily mobility point of view. This approach is important as it sets out the theoretical-conceptual contributions developed recently on both concepts; it analyses the links and relationships between both concepts and their importance to the understanding of urban space; and it sets urban subjects as key figures in the concepts in discussion.La movilidad cotidiana urbana es uno de los fenómenos más complejos y relevantes de las configuraciones y organizaciones espaciales contemporáneas. Sin embargo –desconociendo los aportes de las teorías de urbanismo– su espacialidad ha sido desarticulada por planificaciones que desconocen la dimensión antropológica de los espacios de los grupos humanos. El presente artículo tiene por objetivo desarrollar y fundamentar una problematización de la exclusión social urbana desde la perspectiva de la movilidad cotidiana urbana. Lo anterior adquiere relevancia en la medida que permite poner sobre el tapete los alcances teórico-conceptuales desarrollados recientemente sobre ambos conceptos, permite analizar los vínculos y relaciones existentes entre ambos y su importancia para la comprensión del espacio urbano, y por último, permite poner en el centro de la discusión a los sujetos urbanos como principales protagonistas de los conceptos en discusión

The costs of 'free': Experiences of facility-based childbirth after Benin's caesarean section exemption policy.

: As one of many similar policies in the region, in 2009 Benin launched a free c-section policy in publicly funded hospitals intended to decrease the barriers to facility delivery and the heavy financial burdens on women and their families. We conducted a qualitative study for eight months between 2012 and 2014 to understand women's experiences of care in maternity wards. We carried out semi-structured interviews with 30 women who had delivered via c-section at five hospitals. Two of these hospitals became case study sites where in-depth research was undertaken that consisted of participant observation in each maternity ward and 32 further interviews with women who had complicated, vaginal and c-section deliveries. Overall, women continue to pay for care, both in the form of under-the-table payments to health workers and prescribed payments for services not covered by the policy, though they consider the costs reasonable compared to what the charges were before. Lifting the fees has facilitated conditions for midwives to alert doctors that the procedure might be needed. Partly because c-sections are still feared by most women, in one hospital this led to some women perceiving them as a threat if their labour was progressing more slowly. Implementation of the policy differed greatly between the two case study hospitals. We conclude that some burdens on women's access to care have been addressed but deterrents remain to the improved perception of quality of care on the part of women. Findings detail how important context is to the implementation of the policy, and suggest that similar user-fee removal policies should be accompanied by other measures addressing staff management and quality of care.<br/

Via Hexagons to Squares in Ferrofluids: Experiments on Hysteretic Surface Transformations under Variation of the Normal Magnetic Field

We report on different surface patterns on magnetic liquids following the Rosensweig instability. We compare the bifurcation from the flat surface to a hexagonal array of spikes with the transition to squares at higher fields. From a radioscopic mapping of the surface topography we extract amplitudes and wavelengths. For the hexagon--square transition, which is complex because of coexisting domains, we tailor a set of order parameters like peak--to--peak distance, circularity, angular correlation function and pattern specific amplitudes from Fourier space. These measures enable us to quantify the smooth hysteretic transition. Voronoi diagrams indicate a pinning of the domains. Thus the smoothness of the transition is roughness on a small scale.Comment: 17 pages, 14 figure

A Millimeter-wave Galactic Plane Survey with the BICEP Polarimeter

In order to study inflationary cosmology and the Milky Way Galaxy's composition and magnetic field structure, Stokes I, Q, and U maps of the Galactic plane covering the Galactic longitude range 260° < ℓ < 340° in three atmospheric transmission windows centered on 100, 150, and 220 GHz are presented. The maps sample an optical depth 1 ≾ AV ≾ 30, and are consistent with previous characterizations of the Galactic millimeter-wave frequency spectrum and the large-scale magnetic field structure permeating the interstellar medium. The polarization angles in all three bands are generally perpendicular to those measured by starlight polarimetry as expected and show changes in the structure of the Galactic magnetic field on the scale of 60°. The frequency spectrum of degree-scale Galactic emission is plotted between 23 and 220 GHz (including WMAP data) and is fit to a two-component (synchrotron and dust) model showing that the higher frequency BICEP data are necessary to tightly constrain the amplitude and spectral index of Galactic dust. Polarized emission is detected over the entire region within two degrees of the Galactic plane, indicating the large-scale magnetic field is oriented parallel to the plane of the Galaxy. A trend of decreasing polarization fraction with increasing total intensity is observed, ruling out the simplest model of a constant Galactic magnetic field orientation along the line of sight in the Galactic plane. A generally increasing trend of polarization fraction with electromagnetic frequency is found, varying from 0.5%-1.5% at frequencies below 50 GHz to 2.5%-3.5% above 90 GHz. The effort to extend the capabilities of BICEP by installing 220 GHz band hardware is described along with analysis of the new band

The Epstein-Glaser approach to pQFT: graphs and Hopf algebras

The paper aims at investigating perturbative quantum field theory (pQFT) in the approach of Epstein and Glaser (EG) and, in particular, its formulation in the language of graphs and Hopf algebras (HAs). Various HAs are encountered, each one associated with a special combination of physical concepts such as normalization, localization, pseudo-unitarity, causality and an associated regularization, and renormalization. The algebraic structures, representing the perturbative expansion of the S-matrix, are imposed on the operator-valued distributions which are equipped with appropriate graph indices. Translation invariance ensures the algebras to be analytically well-defined and graded total symmetry allows to formulate bialgebras. The algebraic results are given embedded in the physical framework, which covers the two recent EG versions by Fredenhagen and Scharf that differ with respect to the concrete recursive implementation of causality. Besides, the ultraviolet divergences occuring in Feynman's representation are mathematically reasoned. As a final result, the change of the renormalization scheme in the EG framework is modeled via a HA which can be seen as the EG-analog of Kreimer's HA.Comment: 52 pages, 5 figure

Atomic structure and handedness of the building block of a biological assembly.

Noncovalent supramolecular assemblies possess in general several unique subunit subunit interfaces.The basic building block of such an assembly consists of several subunits and contains all unique interfaces. Atomic-resolution structures of monomeric subunits are typically accessed by crystallography or solution NMR and fitted into electron microscopy density maps. However, the structure of the intact building block in the assembled state remains unknown with this hybrid approach. Here, we present the solid-state NMR atomic structure of the building block of the type III secretion system needle. The building block structure consists of a homotetrameric subunit complex with three unique supramolecular interfaces. Side-chain positions at the interfaces were solved at atomic detail. The high-resolution structure reveals unambiguously the helical handedness of the assembly, determined to be right-handed for the type III secretion system needle.Additionally, the axial rise per subunit could be extracted from the tetramer structure and independently validated by mass-per-length measurements