Gramicidin channels

Gramicidin channels are mini-proteins composed of two tryptophan-rich subunits. The conducting channels are formed by the transbilayer dimerization of nonconducting subunits, which are tied to the bilayer/solution interface through hydrogen bonds between the indole NH groups and the phospholipid backbone and water. The channel structure is known at atomic resolution and the channel's permeability characteristics are particularly well defined: gramicidin channels are selective for monovalent cations, with no measurable permeability to anions or polyvalent cations; ions and water move through a pore whose wall is formed by the peptide backbone; and the single-channel conductance and cation selectivity vary when the amino acid sequence is varied, even though the permeating ions make no contact with the amino acid side chains. Given the amount of experimental information that is available-for both the wild-type channels and for channels formed by amino acid-substituted gramicidin analogues-gramicidin channels provide important insights into the microphysics of ion permeation through bilayer-spanning channels. For the same reason, gramicidin channels constitute the system of choice for evaluating computational strategies for obtaining mechanistic insights into ion permeation through the complex channels formed by integral membrane proteins.     

Potassium channels

Potassium channels are integral membrane proteins that selectively transport K/sup +/ across the cell membrane. They are present in all mammalian cells and have a wide variety of roles in both excitable and nonexcitable cells. The phenotypic diversity required to accomplish their various roles is created by differences in conductance, the timecourse and mechanisms of different gating events, and the interaction of channels with a variety of accessory proteins. Through the integration of biophysical, molecular, structural, and theoretical studies, significant progress has been made toward understanding the structural basis of K/sup +/ channel function, and diseases associated with K/sup +/ channel dysfunction.     

Voltage-gated ion channels

Voltage-dependent ion channels are membrane proteins that conduct ions at high rates regulated by the voltage across the membrane. They play a fundamental role in the generation and propagation of the nerve impulse and in cell homeostasis. The voltage sensor is a region of the protein bearing charged amino acids that relocate upon changes in the membrane electric field. The movement of the sensor initiates a conformational change in the gate of the conducting pathway thus controlling the flow of ions. Major advances in molecular biology, spectroscopy, and structural techniques are delineating the main features and possible structural changes that account for the function of voltage-dependent channels.     

Use of opaque sales channels in addition to traditional channels by service providers

Motivated by the recent opaque selling trend promoted by Priceline and Hotwire, this study examines a game in which two collaborative service providers may use an opaque selling channel to satisfy demand from both leisure and business customers. Further, the service providers must make a strategic decision as to which opaque selling channel is more profitable: posted price (PP) or name your own price (NYOP). By comparing the profits from the three cases (traditional single channel, traditional and PP dual channel, and traditional and NYOP dual channel), we find some interesting results driven by the strategic interaction between two service providers and by the heterogeneity of customers. Firstly, a dual channel offers advantages over the single traditional channel, as opaque selling allows service providers to utilise customers’ heterogeneity, and thus facilitates price discrimination and customer segmentation. Secondly, choosing the traditional and NYOP mechanism enables service providers to optimise profits when the proportion and valuation of business customers, the opacity of the service, and the leisure customers’ degree of pessimism are all relatively high. Lastly, the traditional and NYOP combination outperforms the traditional and PP mechanism as a result of the relatively large pessimistic state of leisure customers.     

Laser-induced mixing in microfluidic channels

We demonstrate a novel strategy for mixing solutions and initiating chemical reactions in microfluidic systems. This method utilizes highly focused nanosecond laser pulses from a Q-switched Nd:YAG laser at lambda = 532 nm to generate cavitation bubbles within 100- and 200-microm-wide microfluidic channels containing the parallel laminar flow of two fluids. The bubble expansion and subsequent collapse within the channel disrupts the laminar flow of the parallel fluid streams and produces a localized region of mixed fluid. We use time-resolved imaging and fluorescence detection methods to visualize the mixing process and to estimate both the volume of mixed fluid and the time scale for the re-establishment of laminar flow. The results show that mixing is initiated by liquid jets that form upon cavitation bubble collapse and occurs approximately 20 micros following the delivery of the laser pulse. The images also reveal that mixing occurs on the millisecond time scale and that laminar flow is re-established on a 50-ms time scale. This process results in a locally mixed fluid volume in the range of 0.5-1.5 nL that is convected downstream with the main flow in the microchannel. We demonstrate the use of this mixing technique by initiating the horseradish peroxidase-catalyzed reaction between hydrogen peroxide and nonfluorescent N-acetyl-3,7-dihydroxyphenoxazine (Amplex Red) to yield fluorescent resorufin. This approach to generate the mixing of adjacent fluids may prove advantageous in many microfluidic applications as it requires neither tailored channel geometries nor the fabrication of specialized on-chip instrumentation.     

Dynamics of parallel boiling channels

The stability of parallel-connected boiling channels is examined in the case of slight deviations from stationary equilibrium conditions. The conditions for stability are deduced and the influence of the parameters on the location of the stability limits is defined.     

Attoliter-scale dispensing in nanofluidic channels

As fabrication techniques improve, functional fluidic devices with nanometer scale dimensions are rapidly being developed for chemical analysis. Here, we present fluid dispensing in nanochannels with injection volumes ranging from 42 aL to 4.1 fL. Devices with hybrid poly(dimethylsiloxane) and glass nanochannels, 130 nm deep and 580 nm wide or 130 nm deep and 670 nm wide, were used to evaluate two sample dispensing schemes, modified pinched and gated injections. Electrokinetic transport was achieved by applying up to 10 V directly from an analog output board without amplification, producing modest electric field strengths in the nanochannels (0.2-2 kV/cm) and enabling rapid dispensing and analysis (10-100 ms).     

New channels for mechanical twinning

The creation of twins at twinning wedge boundaries in bismuth single crystals whose surface is deformed by a concentrated load, has been observed and explained for the first time. Pis’ma Zh. Tekh. Fiz. 24, 43–49 (May 12, 1998)     

Flow boiling in small diameter channels

Many studies have been carried out on two-phase flow heat transfer in channels with hydraulic diameter bigger than 6 mm, but relatively little work has been done for small diameter channels in the meso and compact range (diameter from 0.1 to 3 mm). The use of exchangers with small channels in refrigeration units, which are very numerous besides, could bring a significant reduction of the internal volume of the exchanger, and therefore diminish the refrigerant charge of the whole refrigerating system. One can imagine the interest to widen knowledge on the flow and the heat transfer in small-diameter tubes. This paper examine the thermal behavior of refrigerants boiling in small pipes. The correlations available for in-tube evaporation heat transfer coefficient are discussed and evaluated, when possible, and new research areas of relevance than can contribute to expand the use of small-diameter channels evaporators in refrigeration units are suggested.     

Critical two-phase flow in long channels

Results are presented from experimental studies of the critical efflux of a two-phase flow through long channels.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 43, No. 5, pp. 715–718, November, 1982.     

Electrohydrodynamic flow between parallel dielectric channels

Steady, accelerated, and pulsating electro-dynamic flows in a plane dielectric channel are considered, along with Couette flow. It is shown that for these types of electrohydrodynamic flows the effect is concentrated in a thin layer near the walls, which can considerably change the friction stress on the walls. Some exact solutions of the energy equation are obtained.     

Detection for channels with transition noise

Transition noise is known to be a major cause of errors for high density magnetic recording. This noise is signal dependent and can be modeled as multiplicative noise in a linear channel model. The maximum-likelihood method was not considered for detection of signals in such noise in the past. In this study, a detector model for an asymptotic maximum-likelihood (AML) detection is developed for systems with such noise. Based on a linear partial response channel model, a recursive procedure is obtained as a tree search algorithm, leading to the maximum likelihood detection asymptotically, as the tree-search depth is increased. Performance estimation will be discussed in a separate paper     

Transients in reconfigurable signal processing channels

System reconfiguration at run time may cause unacceptable transients. In this paper, a new design methodology is proposed for reducing transients due to reconfiguration in recursive digital signal processing (DSP) systems. The technique utilizes the fact that, 1) transfer functions can be realized by different processing structures, and 2) these alternative realizations show different transient properties when reconfigured in one step. By selecting processing structures that are less prone to reconfiguration transients, i.e., generate smaller transients due to the abrupt change of coefficients, transients can be reduced for a wide-range of input-output mappings. Selection of the preferable structures is based on the evaluation and control of the dynamic range of the internal variables     

Hydrodynamics in channels with porous walls

An experimental study has been made concerning the hydrodynamics of turbulent air flow through porous channels of uniform cross section with drain or feed along the path. Formulas are proposed for calculating the variations in pressure and flow rate along such a channel.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 23, No. 4, pp. 589–596, October, 1972.     

Solid-state nanopore channels with DNA selectivity

Solid-state nanopores have emerged as possible candidates for next-generation DNA sequencing devices. In such a device, the DNA sequence would be determined by measuring how the forces on the DNA molecules, and also the ion currents through the nanopore, change as the molecules pass through the nanopore. Unlike their biological counterparts, solid-state nanopores have the advantage that they can withstand a wide range of analyte solutions and environments. Here we report solid-state nanopore channels that are selective towards single-stranded DNA (ssDNA). Nanopores functionalized with a 'probe' of hair-pin loop DNA can, under an applied electrical field, selectively transport short lengths of 'target' ssDNA that are complementary to the probe. Even a single base mismatch between the probe and the target results in longer translocation pulses and a significantly reduced number of translocation events. Our single-molecule measurements allow us to measure separately the molecular flux and the pulse duration, providing a tool to gain fundamental insight into the channel-molecule interactions. The results can be explained in the conceptual framework of diffusive molecular transport with particle-channel interactions.     

Hot channels engineer enhanced water transport

Designing the high-flux nanofluidic devices is still a challenge. In this work, we show by molecular dynamics simulations that the permeation of single-file water molecules through a carbon nanotube (CNT) can be significantly enhanced by means of heating up the CNT. Specifically, with the increase in channel temperature, the water flow exhibits a remarkable maximum behavior, corresponding to the decrease in water occupancy. The maximum flow is clearly caused by the channel vibration at high temperatures that leads to the breakdown of single-file water chain, suggesting a new mechanism for fast water conduction. Furthermore, with the increase in channel temperature, the water translocation time decreases monotonously and the flipping frequency of water dipole orientation increases as a whole. The distributions of occupancy, hydrogen bond number, dipole angle and axial density profiles also demonstrate unique behaviors and suggest the breakdown of single-file water chain. Our results provide a significant new method to breakdown the collective motion of single-file water chain and achieve the fast water transport, which is helpful for the design of high-flux nanofluidic devices.