Synthesis and ion conductance behavior of a tetrameric alamethicin ion channel

[structure: see text] A porphyrin-tethered construct, containing four full-length alamethicin monomers, has been synthesized and characterized. The ion conductance data of the assembly in 1 M HCl display long-lived, albeit noisy, channels that appear to be voltage-independent multiples of only one conductance state. The noise in the data is consistent with the molecular modeling studies, which indicate that the side chain of glutamine 7 of alamethicin does not fit well into the narrow pore of a parallel four-helix bundle.     

Electronic structure and conductance of large molecules and DNA

Using a new technique based on embedding in a local orbital formalism, the electronic structure and electron transmission properties of long biological molecules are calculated, in particular DNA. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present, an extended Hückel scheme is used for the Hamiltonian. The transmission is also calculated within the embedding scheme, taking the molecule–metal contacts into account. The results for transmission depend greatly on the orbitals to which contact is made, and also on energy. The implications of these calculations for conductance are discussed.     

Ion channels of biological membranes: prediction of single channel conductance

The Poisson-Boltzmann equation was solved numerically for models of the pore regions of the Shaker K⁺ channel and of two glycoporins (LamB and ScrY) to yield electrostatic potential profiles along the pore axes. From these potential profiles, single-channel current-voltage (I–V) relations were calculated. The importance of a proper treatment of the ionisation state of two rings of aspartate sidechains at the mouth of the K⁺ channel pore emerged from such calculations. The calculated most likely state, in which only two of the eight aspartate sidechains were deprotonated, yielded better agreement with experimental conductance data. An approximate calculation of single-channel conductances based simply on pore geometry yielded very similar conductance values for the two glycoporins. This differed from an␣experimentally determined conductance ratio of ScrY:LamB=10:1. Preliminary electrostatics calculations appeared to reproduce the observed difference in conductance between the two glycoporins, confirming that single-channel conductance is determined by electrostatic as well as geometric considerations. Received: 25 May 1998 / Accepted: 4 August 1998 / Published online: 2 November 1998     

TOF-2: a large 1D channel thorium organic framework

A new neutral 1D channel thorium organic framework material (TOF-2) has been synthesized under hydrothermal conditions. TOF-2 exhibits a hexagonal channel structure consisting of eight-coordinate ThO6F2 polyhedra and 1,3,5-benzentricarboxylate ligands. The channels run along the c-axis and are approximately 13 A in diameter. The single-crystal X-ray structure suggests that the amount of void space is 41%. The structure is stable to ca. 400 degrees C. Gas adsorption measurements show deferential gas uptake behavior.     

Routine charged particle activation analysis of light elements

In order to develop a highly reliable method of routine charged particle activation analysis on a commercial basis by request of an outside company, (1) a new target holder was developed for brittle or low melting-point samples, (2) some modifications and improvements were introduced in the chemical separation, and (3) the reliability of the results was further examined. Oxygen, nitrogen, carbon, and boron in a greater variety of matrices have thus become analyzable with a higher reliability and efficiency.     

Simulating ion channel activation mechanisms using swarms of trajectories

Atomic-level studies of protein activity represent a significant challenge as a result of the complexity of conformational changes occurring on wide-ranging timescales, often greatly exceeding that of even the longest simulations. A prime example is the elucidation of protein allosteric mechanisms, where localized perturbations transmit throughout a large macromolecule to generate a response signal. For example, the conversion of chemical to electrical signals during synaptic neurotransmission in the brain is achieved by specialized membrane proteins called pentameric ligand-gated ion channels. Here, the binding of a neurotransmitter results in a global conformational change to open an ion-conducting pore across the nerve cell membrane. X-ray crystallography has produced static structures of the open and closed states of the proton-gated GLIC pentameric ligand-gated ion channel protein, allowing for atomistic simulations that can uncover changes related to activation. We discuss a range of enhanced sampling approaches that could be used to explore activation mechanisms. In particular, we describe recent application of an atomistic string method, based on Roux's “swarms of trajectories” approach, to elucidate the sequence and interdependence of conformational changes during activation. We illustrate how this can be combined with transition analysis and Brownian dynamics to extract thermodynamic and kinetic information, leading to understanding of what controls ion channel function. © 2019 Wiley Periodicals, Inc.     

Numerical methods for light propagation in large LC cells: a new approach

Accurate numerical methods for the propagation of light in large 3D samples with strong lateral variation of the director field require prohibitive amounts of time. We consider and compare a standard spectral method and the Finite Difference in Frequency Domain method, showing that the CPU time can be reduced by one or two orders of magnitude using a perturbation approach or a recently developed Reduced Order Method. The equations obtained are applied to liquid crystal cells with in-plane switching, illuminated by a large incoherent source. The developed formalism, based on numerically exact equations, is particularly suitable for treating magnetic or optically active media and for extending to such media the well known approximations based on the 4 ×4 (Berreman) or 2 ×2 (Jones) matrices.     

Sequence-independent activation of photocycloadditions using two colours of light

We exploit two reactive chromophores to establish sequence-independent photochemical activation, employing ortho-methyl benzaldehyde (oMBA) and N,N-(dimethylamino)pyrene aryl tetrazole (APAT) with N-(2-hydroxy)ethyl maleimide (NHEM), without any additives. Critically, the order of the irradiation sequence is irrelevant, as the shorter wavelength does not activate the higher wavelength activated species. Therefore, full sequence-independent λ-orthogonality is achieved through differences in both the reaction quantum yields (Φ_r,oMBA and Φ_r,APAT) and wavelength-dependent reactivity profiles of the employed chromophores.

We exploit two reactive chromophores to establish sequence-independent photochemical activation without any additives.     

Numerical methods for light propagation in large LC cells: a new approach

Accurate numerical methods for the propagation of light in large 3D samples with strong lateral variation of the director field require prohibitive amounts of time. We consider and compare a standard spectral method and the Finite Difference in Frequency Domain method, showing that the CPU time can be reduced by one or two orders of magnitude using a perturbation approach or a recently developed Reduced Order Method. The equations obtained are applied to liquid crystal cells with in-plane switching, illuminated by a large incoherent source. The developed formalism, based on numerically exact equations, is particularly suitable for treating magnetic or optically active media and for extending to such media the well known approximations based on the 4 × 4 (Berreman) or 2 × 2 (Jones) matrices.     

Quantitative determination of large structures by small-angle light scattering

A new small-angle light scattering camera has been developed. In contrast to conventional detection the present system is based on a recently developed two-dimensional charge-coupled-device chip made by Thomson (France). The advantage of this chip is its excellent linear response and very low dark signal even at room temperature. The best linearity was obtained by leading each pixel signal through the same amplifying system. The optical system covered a diffraction angular region from about 1° to 15° (q = 0.2–2.6 μm⁻¹). The camera was calibrated with grids and pinholes and was tested with polymer latex particles in solution and with spherulites from polymer films. Received: 06 December 1999 Accepted: 04 February 2000     

Mechanosynthesis of a Copper complex for redox initiating systems with a unique near infrared light activation

The first use of a new mechanosynthesized copper complex (Cu(acac)(2dppba)) as a initiator for the redox and redox photoactivated polymerization of methacrylates under air is proposed. This paper (i) describes the mechanosynthesis of this complex, (ii) outlines the relative efficiency of the complex for redox polymerization (mechanosynthesized product vs. solvent synthesized product), (iii) follows the polymerization enhancement under a 405 nm light, and (iv) demonstrates the high performance of this complex in near infrared photoactivated redox polymerization where a completely colorless polymer is obtained (unprecedented under NIR irradiations, 785 nm, here). The light activated polymerization exhibit higher conversions, better time controls (activation control) and higher surface conversions than redox polymerization. The mechanosynthesis is well characterized by two solvent‐free methods (visual color change and Electron Spin Resonance) and two solvent‐based methods (high resolution‐electrospray ionization‐mass spectrometry (HR‐ESI‐MS) and UV–vis spectrometry). The involved mechanisms are discussed. Mechanosynthesis of copper complexes opens new perspectives for copper (photo)redox polymerization catalysts as the environmental impact and economical costs of the complex synthesis are significantly reduced. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 3646–3655     

Routine determination of light elements by charged-particle activation analysis

Bombardment conditions, chemical separation, and counting method were examined for routine determination of carbon, nitrogen, and oxygen in various matrices by charged-particle activation analysis with respect to reliability, reproducibility, simplicity, and rapidity. A system was then set up for trustworthy measurement of these elements.     

Mathematical modeling of the myosin light chain kinase activation

The mathematical model presented here describes the interactions among Ca2+, calmodulin (CaM), and myosin light chain kinase (MLCK) and consists of a kinetic scheme taking into account 7 reactions instead of 12 as proposed previously. We derive a system of 5 nonlinear ordinary differential equations. Solving it yields the prediction of active MLCK as a function of [Ca2+] whereby the active MLCK is defined to be proportional to the Ca4CaM.MLCK complex concentration. The model predictions are compared with other theoretical and experimental predictions of active MLCK as well as with the results of our previously proposed complex model.     

Charged particle activation analysis of light elements at sub-ppb level

We have developed charged particle activation analysis to determine light elements at a sub-ppb level. This analytical method is characterized by sample bombardment with charged particles at a few tens of mA and substoichiometric separation for¹³N,¹¹C and¹⁸F within two half-life times and with decontamination factors of more than 10⁸. Nuclear reactor interference is also estimated with this method. This analytical method is confirmed to be useful for characterizing highly purified materials from analytical results for boron, carbon, nitrogen and oxygen in Nb refined by the floating zone melting method.     

Voltage dependence of a stochastic model of activation of an alpha helical S4 sensor in a K channel membrane

The voltage dependence of the ionic and gating currents of a K channel is dependent on the activation barriers of a voltage sensor with a potential function which may be derived from the principal electrostatic forces on an S4 segment in an inhomogeneous dielectric medium. By variation of the parameters of a voltage-sensing domain model, consistent with x-ray structures and biophysical data, the lowest frequency of the survival probability of each stationary state derived from a solution of the Smoluchowski equation provides a good fit to the voltage dependence of the slowest time constant of the ionic current in a depolarized membrane, and the gating current exhibits a rising phase that precedes an exponential relaxation. For each depolarizing potential, the calculated time dependence of the survival probabilities of the closed states of an alpha helical S4 sensor are in accord with an empirical model of the ionic and gating currents recorded during the activation process.     

Effect of chemical quenching on background counting rate in tritium channel of a large volume liquid scintillation counter

Background counting rate in tritium channel of a large volume liquid scintillation counter increased with water content ranged 0 to 50% of liquid scintillator. This phenomenon can be explained as follows: The height of scintillation pulses of Compton electrons induced by background gamma radiation is lowered by chemical quenching and shifts to tritium channel. The background counting rate in tritium channel showed a linear relationship with external standard channel ratio of background samples. This relationship is applicable to determine the correct background counting rate for quenching samples and to achieve higher precision of tritium measurement.     

Contribution of static light scattering to the textural characterization of large aggregates

The industrial processes of water clarification often imply flocs of millimeter length. The principal motivation of this work relates to the characterization of these large flocs with laser diffractometry, for which the authors propose particular experimental approaches. In addition, a reformulation of the various properties of the flocs accessible by laser diffractometry is presented, in particular for the determination of the size, density, porosity, volume fraction, and fractal dimension. By way of illustration, these experimental and theoretical developments are applied to the characterization of flocs obtained by flocculation of a commercial kaolin. The size, fractal dimension, and density of kaolin floc were examined under various flocculant concentrations. Measurements reveal important variations of the granulometric and textural properties of large flocs in response to flocculation, opening ways of optimization for the associated industrial processes.     

Thermal activation parameters for large strain deformation of polyethylene and polycarbonate

Experiments were performed to determine the effects of strain rate, temperature, and pressure on the flow stress of polyethylene and Lexan polycarbonate deformed in shear. The results were analyzed to determine the activation enthalpy and the shear and dilatation activation volumes of the rate-limiting mechanism of the deformation process. Results show that the activation event involves a volume containing several monomer units and that this volume must dilate by as much as 7% during the activation event. The activation enthalpy was approximately 2.5 × 10^?12 erg for polyethylene and 1.1 × 10^?12 erg for polycarbonate. The rate-limiting mechanism for polyethylene seemed to be unchanged by plastic strains of up to 250%.     

Instrumental neutron activation analysis of large samples: A pilot experiment

A large sample INAA (LS-INAA) was conducted based on available experimental conditions. Four different materials, e.g., a quartz-sand and three wastes from an incineration plant were analyzed on sample size of 1 kg. The neutron flux spatial distribution was determined by irradiation of flux monitors in the sample. The gamma-ray apparent counting efficiency was evaluated based on the effective-solid angle concept, using linear attenuation coefficients calculated from an iteration process. The k ₀- and the modified monostandard methods were modified for the LS-INAA. To check the LS-INAA performance, a conventional small sample INAA was carried out for the quartz sand and a waste. All detected elements in the LS-INAA are presented and the discussion is conducted to explore the sources of errors in the LS-INAA. As a pilot experiment, the information from this work will be used in constructing irradiation and counting devices for the new FRM-II neutron source in Garching, Germany.