Thermodynamics of ion binding and occupancy in potassium channels

Potassium channels modulate various cellular functions through efficient and selective conduction of K⁺ ions. The mechanism of ion conduction in potassium channels has recently emerged as a topic of debate. Crystal structures of potassium channels show four K⁺ ions bound to adjacent binding sites in the selectivity filter, while chemical intuition and molecular modeling suggest that the direct ion contacts are unstable. Molecular dynamics (MD) simulations have been instrumental in the study of conduction and gating mechanisms of ion channels. Based on MD simulations, two hypotheses have been proposed, in which the four-ion configuration is an artifact due to either averaged structures or low temperature in crystallographic experiments. The two hypotheses have been supported or challenged by different experiments. Here, MD simulations with polarizable force fields validated by ab initio calculations were used to investigate the ion binding thermodynamics. Contrary to previous beliefs, the four-ion configuration was predicted to be thermodynamically stable after accounting for the complex electrostatic interactions and dielectric screening. Polarization plays a critical role in the thermodynamic stabilities. As a result, the ion conduction likely operates through a simple single-vacancy and water-free mechanism. The simulations explained crystal structures, ion binding experiments and recent controversial mutagenesis experiments. This work provides a clear view of the mechanism underlying the efficient ion conduction and demonstrates the importance of polarization in ion channel simulations.

Polarization shapes the energy landscape of ion conduction in potassium channels.     

Effect of surface conduction–induced electromigration on current monitoring method for electroosmotic flow measurement

Current monitoring method for measurement of EOF in microchannels involves measurement of time-varying current while an electrolyte displaces another electrolyte having different conductivity due to EOF. The basic premise of the current monitoring method is that an axial gradient in conductivity of a binary electrolyte in a microchannel advects only due to EOF. In the current work, using theory and experiments, we show that this assumption is not valid for low concentration electrolytes and narrow microchannels wherein surface conduction is comparable with bulk conduction. We show that in presence of surface conduction, a gradient in conductivity of binary electrolyte not only advects with EOF but also undergoes electromigration. This electromigration phenomenon is nonlinear and is characterized by propagation of shock and rarefaction waves in ion concentrations. Consequently, in presence of surface conduction, the current–time relationships for forward and reverse displacement in the current monitoring method are asymmetric and the displacement time is also direction dependent. To quantify the effect of surface conduction, we present analytical expressions for current–time relationship in the regime when surface conduction is comparable to bulk conduction. We validate these relations with experimental data by performing a series of current monitoring experiments in a glass microfluidic chip at low electrolyte concentrations. The experimentally validated analytical expressions for current–time relationships presented in this work can be used to correctly estimate EOF using the current monitoring method when surface conduction is not negligible.     

On surface conduction and its role in electrokinetics

A review is given of the role of surface conduction in electrokinetics. Experimental evidence is considered to indicate when such conduction has to be considered and to what extent this conduction is carried by charges within the stagnant layer. For non-penetrable surfaces, lateral mobilities of monovalent ions in the stern layer are not much lower than those in bulk and under certain conditions conduction behind the slip plane may be of the same order of magnitude as that beyond it. New experiments on the conductivity of polystyrene latex plugs illustrate these phenomena.     

Ultrafast Carrier Dynamics in CdSe/CdS/ZnS Quantum Dots

The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell (about 130 fs) than that in the conduction band of the CdSe core (about 400 fs). From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating thatultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.     

All-valence-electron band structures of infinite stacked poly(TCNQ) and poly(TTF) chains

We have computed the energy band structures of the inifite poly(TCNQ), poly(TCNQ^2?), poly(TTF) and poly(TTF²⁺) chains using the CNDO/2 and where possible the MINDO/2 crystal orbital approximation schemes. The results show a broad conduction band for poly(TCNQ) and a broad valence band for poly(TTF). The bandwidths within the MINDO/2 CO scheme are found to be smaller by roughly a factor of $\text{1}\text{2}$ as compared with those found within the CNDO/2 CO scheme. Our findings are in agreement with a bandwidth of 0.4–0.5 eV for the conduction band of TCNQ—TTF crystals as found by experiments. A brief discussion is given of the k-dependence of the physically interesting bands which is not always simple.     

Segmental Dynamics and Ionic Conduction in Poly(vinyl methyl ether)-Lithium Perchlorate Complexes

Broadband dielectric spectroscopy was used to investigate the segmental dynamics and ionic conduction in LiClO4/PVME complexes with Li/O from 0.1/100 to 10/100, at temperatures from Tg to approximately Tg + 80 degrees C. Although no microphase separation is observed via DSC, dielectric experiments reveal two segmental relaxations and one localized ion motion process. The fastest process is attributed to relaxations of segments in ion-depleted domains and it slows down with increasing salt content, as does the ion motion. The segmental relaxation of PVME chains in ion-rich domains is even slower than the ion motion process, and about 104 times slower than the fast segmental process in the 0.5/100 complex. This process becomes faster with increasing LiClO4 content, despite the concurrent increase in the bulk Tg. Maximum molar conductivity is obtained in the 2/100 complex and the ionic conduction is about 10-9 S/cm at 30 degrees C. By using the dynamic bond percolation model, it was found that the ions move about 0.8 nm for the 0.5/100 complex at 25 degrees C at the time scale of the slow segmental relaxation, assuming that structural renewal is realized by the latter. This size, together with the strong correlation between the ionic conduction and the slow segmental relaxation, supports the idea that hopping from one segment to another one is probably the effective fundamental step giving rise to macroscopic conduction.     

An electroacoustic and high-frequency dielectric response study of stagnant layer conduction in emulsion systems

Stagnant layer conduction (or anomalous surface conduction) in perfluoromethyldecalin (PFMD) and n-hexadecane emulsions has been measured by electroacoustics and verified by high-frequency dielectric response experiments. The electroacoustic technique can detect the presence of stagnant layer conduction from the salt dependence of the dynamic mobility. As the indifferent electrolyte concentration is increased from low values (<5 mM), the zeta-potential and droplet size, estimated from the dynamic mobility by the normal procedures, gradually increase in magnitude until the size plateaus and the zeta-potential begins to decrease with added salt in the usual fashion. When stagnant layer conduction is taken into account, the dynamic mobility can be fitted to a constant size distribution and more realistic zeta-potential values with varying electrolyte concentration. High-frequency dielectric response has been used to measure the total conduction in a PFMD emulsion system. Very good agreement between these two independent techniques verifies the existence of conduction behind the shear plane and demonstrates that electroacoustics alone can detect and quantify its extent. This is possible because of the unique character of the AcoustoSizer procedure, which estimates both particle size and zeta-potential from the same signal.     

Langmuir-Blodgett film characteristics and phospholipid membrane ion conduction : Part 2. Ethylenic Acyl Chain Oxidation

The influence of polar moieties located in the non-polar hydrocarbon zone in bilayer lipid membranes on ion conduction is described. Natural egg-derived phosphatidyl choline combined with cholesterol produces membranes containing several ethylenic residues in the hydrocarbon interior. Catalytic oxidation of these residues by ultraviolet radiation provides a significant density of permanently-bound polar species within the membrane. The extent of such oxidation is correlated with the Arrhenius energy barrier to ion conduction for bilayer membranes and molecular packing characteristics obtained from Langmuir-Blodgett monolayer compression experiments. The results confirm that membrane ion current is almost entirely controlled by molecular packing.     

γ-二氧化锰/K3[Fe(CN)6]溶液界面的电子转移反转区

Marcus电子转移理论的主要成果之一是预言了电子转移反应反转区的存在.从实验上 验证Marcus反转区仍是一个热点问题.通过对γ-MnO2/K3[Fe(CN)6]溶液界面平带电位、极 化曲线的测量,计算了在极化条件下,γ-MnO2导带的电子直接转移到溶液中氧化态物种[F e(CN)6]3-空电子能级上的速率常数ket,logket对外加电位作图,观察到了Marcus反转区. 从理论上也论证了在半导体/溶液界面上的直接电子转移反转区的存在.     

Interference-induced electron- and hole-conduction asymmetry

Principles established by Shephard and Paddon-Row for optimizing and controlling intramolecular electron transport through the modulation of interfering pathways are employed to design new molecules for steady-state conduction experiments aimed at manifesting electron?Chole conduction asymmetry in a unique way. First, a review of the basic principles is presented through application to a pertinent model system in which a molecule containing donor and acceptor terminal linking groups with an internal multiple-pathway bridge is used to span two metal electrodes. Different interference patterns are produced depending on whether the through-molecule coupling pathways are symmetric or antisymmetric with respect to a topological bisecting plane, giving rise to asymmetric electron and hole conductances at the tight-binding (Hückel) level; this process is also described from a complementary molecular-orbital viewpoint. Subsequently, a new molecular system based on organic polyradicals is designed to allow such asymmetry to be realized in single-molecule conduction experiments. These polyradicals are analyzed using analogous simple models, density-functional theory (DFT) calculations of steady-state transmission, and intermediate neglect of differential overlap (INDO) calculations of intramolecular connectivity, verifying that polyradicals at low temperatures should show experimentally measureable electron?Chole conduction asymmetry. A key feature of this system is that the polyradicals form a narrow partially occupied band of orbitals that lie within and well separated from the HOMO and LUMO orbitals of the surrounding molecular scaffold, allowing for holes and electrons to be transported through the same molecular band.     

Design and testing of a calorimeter for biological uses

A rotatory microcalorimeter of the conduction type has been designed for the study of microbial metabolic processes under both aerobic and anaerobic conditions. The instrument can be performed in either batch mode or flow mode by changing the calorimeter vessels and the tube connections. The sensitivity and the time constant were determined by electrical calibrations. The heat sensitivity was 0.12 mV/mW with both yeast and other fungi. Because of the sufficient aeration and agitation, the calorimeter is available for studies on the fungi growth experiments of biotechnical interest.     

d.c. conductivity and structure of nylon 6-LiCl mixtures

d.c. Conductivity experiments have been performed for a wide range of temperature under a constant voltage application for nylon 6 and its mixture with 4% w/w LiCl. The data confirm previous indications of decrease of crystallinity and increase of glass transition due to lithium halides. Support is also given to previous interpretations of the conduction mechanism for polyamides, i.e. essentially electronic below 80° but essentially ionic above 100°.     

Device physics of polymer light-emitting diodes

This article reviews a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole-only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space-charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron-only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 10¹⁸ cm⁻³. Combining the results of electron- and hole-only devices a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias-dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency (photon/carrier) of a PLED is temperature independent. © 1998 John Wiley & Sons, Ltd.     

Research on fast solid-state chemical reactions at the Siberian Synchrotron and Terahertz Radiation Center

The potent equipment of the shared-use center “Siberian Synchrotron and Terahertz Radiation Center” at Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences (BINP SB RAS) enables conduction of time-resolved X-ray diffraction experiments using synchrotron radiation of the VEPP-3/VEPP-4 storage rings. These experiments are performed mostly for institutions of the Siberian Branch of the Russian Academy of Sciences (SB RAS). The experiments yielded novel techniques, which have been implemented for investigation into the dynamics of the nucleation and formation of nanoparticles under explosion and shock wave impact with a nanosecond-scale time resolution (the X-ray exposure time is 73 ps), dynamics of structural transformations in chemical reactions, and kinetics of chemical reactions during gasless combustion (SHS) with a millisecond-scale time resolution, obtaining structural information about the state of catalysts and so on.     

Electrokinetic characterization of porous plugs from streaming potential coupled with electrical resistance measurements

The zeta potential of mixed nickel-iron oxide particles is evaluated by a new laboratory instrument. This latter allows the measurement of streaming potential together with the electrical resistance of porous plugs. The conductivity of electrolyte inside plug (pore conductivity) is deduced from electrical resistance measurements and is used together with streaming potential to evaluate the zeta potential by accounting for the surface conduction phenomenon. It is shown that neglecting the surface conduction phenomenon leads to a substantial underestimation of the zeta potential. The coupled measurements of streaming potential and plug electrical resistance yield zeta potential values that are in very good agreement with those obtained by electrophoresis. The densification of the porous plug with increasing pressure increments is put in evidence by the decrease in measured streaming potentials. Electrical resistance measurements make it possible to account for the increase in surface conductivity resulting from the more compacted structure of the plug. By doing so, the calculated zeta potential is found to be virtually independent of the pressure difference involved in streaming potential experiments, whereas the negligence of surface conduction phenomenon leads to a decrease in the apparent zeta potential with increasing pressure level.     

Combined use of adiabatic calorimetry and heat conduction calorimetry for quantifying propellant cook-off hazards

Recent work performed at DERA (now QinetiQ) has shown how accelerating rate calorimetry (ARC) can be used to obtain time to maximum rate curves using larger samples of energetic materials. The use of larger samples reduces the influence of thermal inertia, permitting experimental data to be gathered at temperatures closer to those likely to be encountered during manufacture, transportation or storage of an explosive device. However, in many cases, extrapolation of the time to maximum rate curve will still be necessary. Because of its low detection limit compared to the ARC, heat conduction calorimetry can be used to obtain data points at, or below, the region where an explosive system might exceed its temperature of no return and undergo a thermal explosion.Paired ARC and heat conduction calorimetry experiments have been conducted on some energetic material samples to explore this possibility further. Examples of where both agreement and disagreement are found between the two techniques are reported and the significance of these discussed. Ways in which combining ARC and heat conduction calorimetry experiments can enhance, complement and validate the results obtained from each technique are examined.     

Resonance charge transfer in atom-surface scattering

We present a review of theoretical work on resonance charge transfer in atom-surface scattering. We consider resonance coupling of the atomic level to a wide conduction band, and to low-lying core states. In each case the models, and the methods available for the calculation of the probability of electron transfer are summarised and their applicability demonstrated by selected applications to scattering and sputtering experiments.     

Molecular electrodes at the exposed edge of metal/insulator/metal trilayer structures

Producing reliable electrical contacts of molecular dimensions has been a critical challenge in the field of molecule-based electronics. Conventional thin film deposition and photolithography techniques have been utilized to construct novel nanometer-sized electrodes on the exposed vertical plane on the edge of a thin film multilayer structure (metal/insulator/metal). Via thiol surface attachment to metal leads, an array of paramagnetic, cyanide-bridged octametal complexes, [(pzTp)FeIII(CN)3]4[NiII(L)]4[O3SCF3]4 (1) [(pzTp) = tetra(pyrazol-1-yl)borate; L = 1-S(acetyl)tris(pyrazolyl)decane], were covalently linked onto the electrodes forming a dominant conduction pathway. A series of molecule-based devices were fabricated using Ni, NiFe, Ta, and Au as metal electrodes separated by insulating Al2O3 spacers, followed by treatment with 1. A series of control experiments were also performed to demonstrate that the conduction path was through tethered metal clusters. The molecular current was analyzed via the Simmons tunnel model, and calculations are consistent with electron tunneling through the alkane ethers to the central metal core. With a Ni/Al2O3/Au molecular electrode, the tether binding was found to be reversible to the top Au layer, allowing for a new class of chemical detection based on the steric bulk of coordinating analytes to disconnect the molecular current path. Simple and economical photolithography/liftoff/self-assembly fabrication techniques afford robust molecular junctions with high reproducibility (>90%) and long operational lifetimes (>1 year).     

Determining the Zeta Potential of Porous Membranes Using Electrolyte Conductivity inside Pores

The zeta potential is an important and reliable indicator of the surface charge of membranes, and knowledge of it is essential for the design and operation of membrane processes. The zeta potential cannot be measured directly, but must be deduced from experiments by means of a model. The possibility of determining the zeta potential of porous membranes from measurements of the electrolyte conductivity inside pores (lambda(pore)) is investigated in the case of a ceramic microfiltration membrane. To this end, experimental measurements of the electrical resistance in pores are performed with the membrane filled with KCl solutions of various pHs and concentrations. lambda(pore) is deduced from these experiments. The farther the pH is from the isoelectric point and/or the lower the salt concentration is, the higher the ratio of the electrolyte conductivity inside pores to the bulk conductivity is, due to a more important contribution of the surface conduction. Zeta potentials are calculated from lambda(pore) values by means of a space charge model and compared to those calculated from streaming potential measurements. It is found that the isoelectric points are very close and that zeta potential values for both methods are in quite good agreement. The differences observed in zeta potentials could be due to the fact that the space charge model does not consider the surface conductivity in the inner part of the double layer. Measurements of the electrolyte conductivity within the membrane pores are proved to be a well-adapted procedure for the determination of the zeta potential in situations where the contribution of the surface conduction is significant, i.e., for small and charged pores. Copyright 2001 Academic Press.     

Ion conduction and polymer dynamics of poly(2-vinylpyridine)-lithium perchlorate mixtures

Ion conduction and polymer dynamics of homogeneous mixtures of poly(2-vinylpyridine) (P2VPy) with 0.1 to 10 mol % lithium perchlorate (LiClO(4)) were investigated using broadband dielectric spectroscopy. Interpretation of the relaxation behavior was assisted by findings from differential scanning calorimetry, Fourier transform infrared spectroscopy, dynamic mechanical analysis, and wide-angle and small-angle X-ray scattering experiments. Five dielectric relaxations were observed: a local beta-process in the glassy state, a segmental relaxation, a slow segmental process, an ion-mode relaxation, and electrode polarization. The local P2VPy beta-relaxation was strongly suppressed with increasing LiClO(4) content arising from the formation of transient crosslinks, which lead to a subsequent decrease in the number of free pyridine groups and/or a reduction in the local free volume in the presence of LiClO(4). Ion conduction at low LiClO(4) concentrations (<10 mol %) is governed by the diffusion of anions through the matrix, which is strongly coupled with the segmental relaxation. At relatively high LiClO(4) concentration (10 mol %), partial decoupling between ion motion and the segmental relaxation was observed, leading to increased conductivity.