Activity of synthetic ion channels is influenced by cation-pi interactions with phospholipid headgroups

A suite of synthetic hydraphile ion channels has been used to probe the possibility of cation-pi interactions between the channel and the phospholipid bilayer. The hydraphiles selected for this study contained either no sidearm, aliphatic sidearms or aromatic sidearms that varied in electron-richness. An ion selective electrode (ISE) method was used to evaluate the ion transport ability of these hydraphiles across synthetic bilayers. Transport was dependent on sidearm identity. Ion transport activity for the aromatic sidechained compounds was greatest when the sidearms were electron rich and vesicles were prepared from 100% DOPC (trimethylammonium cation headgroup, overall neutral). When the lipid headgroups were made more negative by changing the composition from DOPC to 70 : 30 (w/w) DOPC : DOPA, transport by the aromatic-sidechained channels was reduced. Fluorescence studies showed that when the lipid composition changed, the headgroups experienced a different polarity, suggesting reorientation. The data are in accord with a stabilizing cation-pi interaction between the aromatic sidearm of the hydraphile channel and the ammonium phospholipid headgroup.     

Synthetic ion channel activity documented by electrophysiological methods in living cells

Hydraphiles are synthetic ion channels that use crown ethers as entry portals and that span phospholipid bilayer membranes. Proton and sodium cation transport by these compounds has been demonstrated in liposomes and planar bilayers. In the present work, whole cell patch clamp experiments show that hydraphiles integrate into the membranes of human embryonic kidney (HEK 293) cells and significantly increase membrane conductance. The altered membrane permeability is reversible, and the cells under study remain vital during the experiment. Control compounds that are too short (C(8)-benzyl channel) to span the bilayer or are inactive owing to a deficiency in the central relay do not induce similar conductance increases. Control experiments confirm that the inactive channel analogues do not show nonspecific effects such as activation of native channels. These studies show that the combination of structural features that have been designed into the hydraphiles afford true, albeit simple, channel function in live cells.     

In vivo cell death mediated by synthetic ion channels

Synthetic ion channel hydraphiles, which are known to infiltrate membranes and disrupt ion homeostasis, were tested as direct injection toxins in live mice as potential schlerotic agents. The study uses a near-IR dye to image and evaluate the success of the approach.     

Electrorheology in nanopores via lattice Boltzmann simulation

The conductance of ionic species through a cylindrical nanochannel is explored by means of a mesoscopic lattice Boltzmann numerical method. It is shown that in the absence of any external voltage, the ionic profiles develop a considerable amount of structure within the channel. The details of this structure depend on the ionic density, channel length, and Debye length. In the presence of a trans-membrane electrostatic potential, less structure is observed and Ohm's law is found to hold.     

Estimation of zeta potential by electrokinetic analysis of ionic fluid flows through a divergent microchannel

The streaming potential is generated by the electrokinetic flow effect within the electrical double layer of a charged solid surface. Surface charge properties are commonly quantified in terms of the zeta potential obtained by computation with the Helmholtz-Smoluchowski (H-S) equation following experimental measurement of streaming potential. In order to estimate a rigorous zeta potential for cone-shaped microchannel, the correct H-S equation is derived by applying the Debye-Hückel approximation and the fluid velocity of diverging flow on the specified position. The present computation provides a correction ratio relative to the H-S equation for straight cylindrical channel and enables us to interpret the effects of the channel geometry and the electrostatic interaction. The correction ratio decreases with increasing of diverging angle, which implies that smaller zeta potential is generated for larger diverging angle. The increase of Debye length also reduces the correction ratio due to the overlapping of the Debye length inside of the channel. It is evident that as the diverging angle of the channel goes to nearly zero, the correction ratio converges to the previous results for straight cylindrical channel.     

Large scale lithography-free nano channel array on polystyrene

This paper reports a new fabrication method of lithography-free nanochannel array. It is based on the cracking process on the surface of a polystyrene (PS) Petri-dish, one type of thermoplastic that is composed of uni-axial macromolecular chains. Under proper conditions, parallel nanochannels with equal interspaces are obtained. Control over the channel depth from 20 nm to 200 nm is achieved, with the channel length reaching tens of millimetres. The PDMS replication based on PS nanochannel array has been successfully carried out. In combination with the microstructure, both an ion enrichment device and a current rectification device are fabricated, and their quantified characters manifested the applicability of the channel array structure in nanofluidics.     

Condensation of plasmid DNA by benzyl hydraphiles and lariat ethers: dependence on pH and chain length

Lariat ethers and hydraphiles have been studied by the use of gel electrophoresis to evaluate the complexation of a 10 kilobase plasmid DNA. Both receptor systems show behaviour that reflects differences in side or spacer chain lengths. In addition, the formation of complexes or aggregates (identified by transmission electron microscopy, TEM) is pH dependent. Some detergents and polyamines can affect the formation and/or deaggregation of the complexes which TEM shows form particles of a relatively uniform size (~100–150 nm) and density for a specific receptor.     

Single-file diffusion near channel boundaries

Molecular transport under the conditions of single-file diffusion was investigated near the channel boundaries by using dynamic Monte Carlo and molecular dynamics simulations of tracer exchange between single-file channels and their surroundings. The boundary effect reported in our recent papers (Vasenkov S.; K?rger, J. Phys. Rev. E 2002, 66, 052601. Schüring, A.; Vasenkov S.; Fritzsche, S. J. Phys. Chem. B 2005, 109, 16711) was studied in detail. This boundary effect is characterized by deviations of the intrachannel concentration profiles of tracer molecules observed in the case of single-file diffusion near the channel boundaries from the corresponding profiles typical for normal diffusion. It has been shown in our previous studies that these deviations occur under the conditions when the potential-energy difference inside and outside of single-file channels was both comparable and much larger than the activation energy of intrachannel diffusion. Here, we report a quantitative model describing the boundary effect. According to this model, an occurrence of the boundary effect is related to a complex character of diffusion in finite single-file systems. Such diffusion can be described by the following two types of movements occurring in parallel: (i) correlated displacements of all molecules in any particular channel and (ii) fast displacements of single molecules, which are uncorrelated with the displacements of all other molecules in the same channel. The latter displacements are restricted to a certain length interval that depends on the channel length and the channel occupancy. This length interval is shown to determine the extensions of the channel margins where the boundary effect is observed.     

Performance of hollow-fiber flow field-flow fractionation in protein separation

Since hollow-fiber flow field-flow fractionation (HF FIFFF) utilizes a cylindrical channel made of a hollow-fiber membrane, which is inexpensive and simple in channel assembly and thus disposable, interests are increasing as a potential separation device in cells, proteins, and macromolecules. In this study, performance of HF FIFFF of proteins is described by examining the influence of flow rate conditions and length of fiber (polyacrylonitrile or PAN in this work) on sample recovery as well as experimental plate heights. The interfiber reproducibility in terms of separation time and recovery was also studied. Experiments showed that sample recovery was consistent regardless of the length of fiber when the effective field strength (equivalent to the mean flow velocity at the fiber wall) and the channel void time were adjusted to be equivalent for channels of various fiber lengths. This supported that the majority of sample loss in HF FIFFF separation of apoferritin and their aggregates may occur before the migration process. It is finally demonstrated that HF FIFFF can be applied for characterizing the reduction in Stokes' size of low density lipoproteins from blood plasma samples obtained from patients having coronary artery disease and from healthy donors.     

Surface Architectures Built around Perylenediimide Stacks

Simple stacks of perylenediimides (PDIs) grown directly on solid surfaces are an intriguing starting point for the construction of multicomponent architectures because their intrinsic activity is already very high. The ability of PDI stacks to efficiently generate photocurrent originates from the strong absorption of visible light and the efficient transport of both electrons and holes after generation with light. The objective of this study was to explore whether or not the excellent performance of these remarkably simple single‐channel photosystems could be further improved in more sophisticated multicomponent architectures. We report that the directional construction of strings of anions or cations along the PDI stacks does not significantly improve their activity; that is, the intrinsic activity of PDI stacks is too high to yield ion‐gated photosystems. The directional construction of electron‐ and hole‐transporting stacks of naphthalenediimides (NDIs) and oligothiophenes along the central PDI stack did not improve photocurrent generation under standard conditions either. However, the activity of double‐channel photosystems increased with increasing thickness, whereas increasing charge recombination with single‐channel PDI stacks resulted in decreasing activity with increasing length. Most efficient long‐distance charge transport was found with double‐channel photosystems composed of PDIs and NDIs. This finding suggests that over long distances, PDI stacks transport holes better than electrons, at least under the present conditions. Triple‐channel photosystems built around PDI stacks with oligothiophenes and triphenylamines were less active, presumably because hole mobility in the added channels was inferior to that in the original PDI stacks, thus promoting charge recombination.     

Synthetic cation transporters incorporating crown ethers and calixarenes as headgroups and central relays: a comparison of sodium and chloride selectivity

An earlier study showed that a calix[4]arene could function as a central relay unit to form an ion conductance pathway through a phospholipid bilayer membrane. The present study expands the range of compounds from calix[4]arene to calix[6]arene and incorporates them either as central units or as headgroups, substituting one or more diaza-18-crown-6 residues in functioning hydraphiles. Ion release was assayed by detecting either Na(+) or Cl(-) release from phospholipid vesicles. The ion transport activity for calix[4]arenes in either position is modest, but is almost non-existent when calix[6] residues were incorporated either as head groups or central relay units. The poor activity of the calix[6]arenes may result from an inability to penetrate to the midplane of the bilayer or pass entirely through it to form a conductance pathway. The transmembrane "flip-flop" may result from high polarity or steric bulk, or both. A hydraphile incorporating a single -NHCOC(6)H(4)OCH(2)CONH- as a central relay proved to be an excellent Na(+) conductor, but less selective for Cl(-). The fact that this new hydraphile molecule shows selectivity for Na (+) over Cl(-) transport and possesses two secondary amide residues in the central relay suggests a means to control ion selectivity in synthetic ion transporters.     

Comments on the separation efficiency of asymmetrical flow field-flow fractionation in channels of constant channel and crossflow velocities leading to constant separation efficiency

It is shown theoretically that a claim in the literature about the overall separation efficiency of asymmetrical flow FFF channels being improved by geometries that permit a uniform channel flow velocity throughout the channel length is untrue.     

Analysis of channel-geometry effects on separation efficiency in rectangular-capillary electrochromatography columns

A three-dimensional random walk model was developed to evaluate the impact of column geometry on separation efficiency in chromatography systems driven by electroosmotic flow. Contributions of injection plug length, cross-sectional area of channels, and aspect ratio of rectangular channels were examined in these simulation studies. Sample plug length had no impact on efficiency until it exceeded roughly 0.4% of the channel length. Plate height increased rapidly with increasing k' as expected, almost doubling in going from k'=0.25 to 0.35. Channel geometry also had a major effect on efficiency. Plate height increased sharply in rectangular channel columns until the channel aspect ratio reached 4-8. But the effect of channel depth was even more dramatic. Minimum plate height (Hmin) was roughly half that of the channel depth in ideal cases. Hmin in a 10x2 microm channel was at 1.6 mm s(-1). Rectangular channels comparable to those obtained by microfabrication are equivalent to packed column capillary electrochromatography columns in all cases.     

On the no-field method for void time determination in flow field-flow fractionation

Elution time measurements of colloidal particles injected in a symmetrical flow field-flow fractionation (flow FFF) system when the inlet and outlet cross-flow connections are closed have been performed. This no-field method has been proposed earlier for void time (and void volume) determination in flow FFF Giddings et al. (1977). The elution times observed were much larger than expected on the basis of the channel geometrical volume and the flow rate. In order to explain these discrepancies, a flow model allowing the carrier liquid to flow through the porous walls toward the reservoirs located behind the porous elements and along these reservoirs was developed. The ratio between the observed elution time and expected one is found to depend only on a parameter which is a function of the effective permeability and thickness of the porous elements and of the channel thickness and length. The permeabilities of the frits used in the system were measured. Their values lead to predicted elution times in reasonable agreement with experimental ones, taking into account likely membrane protrusion inside the channel on system assembly. They comfort the basic feature of the flow model, in the no-field case. The carrier liquid mostly bypasses the channel to flow along the system mainly in the reservoir. It flows through the porous walls toward the reservoirs near channel inlet and again through the porous walls from the reservoirs to the channel near channel outlet before exiting the system. In order to estimate the extent of this bypassing process, it is desirable that the hydrodynamic characteristics of the permeable elements (permeability and thickness) are provided by flow FFF manufacturers. The model applies to symmetrical as well as asymmetrical flow FFF systems.     

Selective filling for patterning in microfluidic channels

The ability to pattern different polymers in microfluidic channels is indispensable for the development of multifunctional, "lab-on-a-chip" devices. A simple method, based on the concept of selective filling, is described for introducing different polymers at defined locations in microfluidic channels. Selective filling is based on the difference in the free energy of filling between an open and a covered part of the channel. It is implemented by covering part of the channel, along its length, with a temporary poly(dimethylsiloxane) (PDMS) slab. Preferential filling is related to the contact angle of the liquid solution on the chip surface. An expression for the critical contact angle is derived, and its dependence on the geometry of the channel is established. It is further shown that a trapezoidal geometry of the cross-section of the channel is optimal for selective filling. Experimental verification of the applicability of the critical contact angle in predicting selective filling is demonstrated by introducing liquid prepolymer solutions of different contact angles in the glass channel that was etched using photolithography and wet etching. Finally, patterning of two different polymers along the axial direction of the microfluidic channel is demonstrated using this selective filling technique.     

Application of an intensified narrow channel reactor to the aqueous phase precipitation of barium sulphate

A homogeneous liquid phase reaction between barium chloride (BaCl(2)) and sodium sulphate (Na(2)SO(4)) was conducted in a narrow channel reactor to produce barium sulphate (BaSO(4)) precipitate. The effects of channel dimensions and channel residence times on crystal size, crystal size distribution, nucleation rates, crystal morphology and conversion of reactants were investigated at different levels of reactant supersaturation ratio. Our results indicate that the smallest particle sizes are favoured when supersaturation ratios and channel velocities are high. The minimum average particle diameter observed was approximately 0.2 microm in a channel of hydraulic diameter 0.5 mm and length 20 cm at an initial supersaturation ratio of 4483 (0.1 M), which correspond to conditions giving rise to the highest nucleation rates. It has also been observed that particle size depends on the conversion to product, the smallest particles being formed when conversion lies within the range of 30 to 40%. Conversions in excess of 60% have been reached but there is a detectable limiting effect with increased supersaturation and reduced residence times. Experiments conducted at similar levels of supersaturation under stirred tank conditions showed that particle size was consistently larger and particle size distribution was much broader than that achieved in the narrow channel reactor. Scanning electron microscopy (SEM) images of the crystals formed in the narrow channels show that spherical particles dominate in the smallest channels at high velocities whilst coarse, tabular crystals are obtained in the larger channels. Greater tendency to agglomerate is also observed at high supersaturation ratios, after one minute of reaction.     

Focusing particles by induced charge electrokinetic flow in a microchannel

A novel method of sheathless particle focusing by induced charge electrokinetic flow in a microchannel is presented in this paper. By placing a pair of metal plates on the opposite walls of the channel and applying an electrical field, particle focusing is achieved due to the two pairs of vortex that constrain the flow of the particle solution. As an example, the trajectories of particles under different electrical fields with only one metal plate on one side channel wall were numerically simulated and experimentally validated. Other flow focusing effects, such as the focused width ratio (focused width/channel width) and length ratio (focused length/half‐length of metal plate) of the sample solution, were also numerically studied. The results show that the particle firstly passes through the gaps between the upstream vortices and the channel walls. Afterwards, the particle is focused to pass through the gap between the two downstream vortices that determine the focused particle position. Numerical simulations show that the focused particle stream becomes thin with the increases in the applied electrical field and the length of the metal plates. As regards to the focused length ratio of the focused stream, however, it slightly increases with the increase in the applied electrical field and almost keeps constant with the increase in the length of the metal plate. The size of the focused sample solution, therefore, can be easily adjusted by controlling the applied electrical field and the sizes of the metal plates.     

Local Mass Transfer during Electrodialysis with Ion-Exchange Membranes and Spacers

Mass transfer during electrodialysis with ion-conducting spacers in the intermembrane gap is modeled experimentally using a local-distribution analysis of solutions. Due to emergence of back-flow regions near the spacers, local quantities (diffusion layer thicknesses, surface concentrations, Sherwood numbers) are distributed nonuniformly over the channel length. In a smooth channel, due to concentration polarization, local mass transfer rates steadily decrease along the solution supply coordinate, but introducing ion-conducting spacers into the channel intensifies mass transfer because of periodic interruption of diffusion layers and formation of recirculation zones. Effect of geometrical size of spacers on the mass transfer is studied. It is found that the mass transfer rate is maximum for ion-conducting spacers with the ratio 3.5 < l/h< 4.0.     

介孔分子筛SBA-15中α-胰凝乳蛋白酶组装及催化活性研究

介孔分子筛由于规则孔道或笼的存在 ,使其具有择形催化作用、高比表面积和强吸附性能 ,其孔道中可组装多种物质而改变其理化性能 .Thomas等 [1] 在介孔 Si O2 上接枝金属茂复合物 .白妮 [2 ] 和张雪峥 [3 ] 等分别将脂溶性金属酞菁衍生物和杂多酸封装在介孔分子筛中 ,得到的组装体催化性能优良 .近年来由于不断合成出 MCM- 41 [4 ] 和 SBA- 1 5 [5] 等孔径较大的介孔分子筛 ,使在介孔材料孔道中组装生物大分子成为可能 .Yen等 [6] 将细胞色素 c组装到孔道中 ,并使酶的稳定性得到提高 ,而α-胰凝乳蛋白酶 (Mr=2 5 0 0 0 ,分子动力学直径…