Thermodynamic and kinetic stability of synthetic multifunctional rigid-rod beta-barrel pores: evidence for supramolecular catalysis

The lessons learned from p-octiphenyl beta-barrel pores are applied to the rational design of synthetic multifunctional pore 1 that is unstable but inert, two characteristics proposed to be ideal for practical applications. Nonlinear dependence on monomer concentration provided direct evidence that pore 1 is tetrameric (n = 4.0), unstable, and "invisible," i.e., incompatible with structural studies by conventional methods. The long lifetime of high-conductance single pores in planar bilayers demonstrated that rigid-rod beta-barrel 1 is inert and large (d approximately 12 A). Multifunctionality of rigid-rod beta-barrel 1 was confirmed by adaptable blockage of pore host 1 with representative guests in planar (8-hydroxy-1,3,6-pyrenetrisulfonate, KD = 190 microM, n = 4.9) and spherical bilayers (poly-L-glutamate, KD < or = 105 nM, n = 1.0; adenosine triphosphate, KD = 240 microM, n = 2.0) and saturation kinetics for the esterolysis of a representative substrate (8-acetoxy-1,3,6-pyrenetrisulfonate, KM = 0.6 microM). The thermodynamic instability of rigid-rod beta-barrel 1 provided unprecedented access to experimental evidence for supramolecular catalysis (n = 3.7). Comparison of the obtained kcat = 0.03 min(-1) with the kcat approximately 0.18 min(-1) for stable analogues gave a global KD approximately 39 microM3 for supramolecular catalyst 1 with a monomer/barrel ratio approximately 20 under experimental conditions. The demonstrated "invisibility" of supramolecular multifunctionality identified molecular modeling as an attractive method to secure otherwise elusive insights into structure. The first molecular mechanics modeling (MacroModel, MMFF94) of multifunctional rigid-rod beta-barrel pore hosts 1 with internal 1,3,6-pyrenetrisulfonate guests is reported.     

Synthetic multifunctional pores: lessons from rigid-rod beta-barrels

In this account, studies on synthetic multifunctional pores formed by rigid-rod beta-barrels are summarized comprehensively. The first section outlines the evolution of synthetic multifunctional pores from the introduction of rigid-rod molecules in bioorganic chemistry and the discovery of synthetic beta-barrels in comparison with pertinent developments in related areas of research. Design strategies to position active sites at the inner surface of rigid-rod beta-barrel pores are described in the second section. The third section focuses on the characteristics of transmembrane barrel-stave pores, emphasizing the dynamic nature of supramolecular oligomers with the aid of notional phase and energy diagrams. Section four introduces multifunctionality with the use of synthetic pores as hosts of a rich collection of guests, reaching from inorganic cations to organic macromolecules like peptides, oligonucleotides, polysaccharides and polyacetylenes. In section five, practical applicability of molecular recognition by synthetic multifunctional pores is documented with non-invasive fluorometric enzyme sensing. The application of host-guest chemistry within synthetic pores to couple molecular recognition and translocation with molecular transformation is the topic of section six. The last section mentions some perspectives and challenges with synthetic multifunctional pores.     

Chemical modification of the internal surfaces of cylindrical pores of submicrometer size in poly(ethylene terephthalate)

Chemical modification of the internal surfaces of cylindrical pores with submicrometer pore diameter in a poly(ethylene terephthalate) (PET) film was examined. The modification involved the alkylation of the carboxylic acid on the surfaces with the alkylation reagent containing a fluorescent probe, and it was monitored by observing the change in fluorescent emission intensity. When the N,N-dimethylformamide solution of 4-(bromomethyl)-6,7-dimethoxycoumarin (BrCU), which bore a coumarin fluorophore, was introduced into the pores, the emission and excitation intensities of the membranes increased proportionally with increases of the pore surface areas. Fluorescent spots about 300 nm in diameter, which were located at the positions of the pores, can be observed in the fluorescence microscope image of the membranes, indicating that highly concentrated fluorescent probes are chemically incorporated on the internal surfaces of the cylindrical pores with 210 nm diameter in the membranes. In the reactions of the PET surfaces with BrCU, the fluorescent intensities increased with increases of the contact angles. This result indicates that the hydrophilicity of the internal pore surfaces can be qualitatively modified by controlling the change in the fluorescent intensities.     

Protein adsorption equilibrium and kinetics in multimodal cation exchange resins

Protein adsorption equilibria and kinetics are obtained experimentally for two multimodal cation exchange resins—Nuvia cPrime, which is based on a polymeric matrix, and Capto MMC, which is based on an agarose matrix. In both resins, the ligand contains a phenyl group, a carboxyl group, and a peptide bond but with a different arrangement. Transmission electron microscopy and inverse size exclusion chromatography indicate a bimodal distribution of pores in Nuvia cPrime, including small pores with 10 nm radius and pores larger than 400 nm, and a monodispersed distribution of pores in Capto MMC, averaging 32 nm in radius. Potentiometric titration curves show similar buffering ranges and pK _a values for the ligands in both resins and a slightly higher ligand density for Nuvia cPrime. Equilibrium binding capacities for lysozyme and a monoclonal antibody (mAb) are also similar for both resins at comparable pH and salt concentrations, although Capto MMC shows a weaker dependence on salt concentration as a result of its more hydrophobic character. The main difference is the adsorption kinetics of the mAb, which is the larger of the two proteins studied. For both resins, as shown by means of confocal laser scanning miscopy, the adsorption kinetics is controlled by pore diffusion. Capto MMC with its smaller pores has a slower rate of mass transfer than Nuvia cPrime. As a result, for the mAb, much higher column dynamic binding capacities are obtained for Nuvia cPrime than for Capto MMC.     

Screening of pi-basic naphthalene and anthracene amplifiers for pi-acidic synthetic pore sensors

Synthetic ion channels and pores attract current attention as multicomponent sensors in complex matrixes. This application requires the availability of reactive signal amplifiers that covalently capture analytes and drag them into the pore. pi-Basic 1,5-dialkoxynaphthalenes (1,5-DAN) are attractive amplifiers because aromatic electron donor-acceptor (AEDA) interactions account for their recognition within pi-acidic naphthalenediimide (NDI) rich synthetic pores. Focusing on amplifier design, we report here the synthesis of a complete collection of DAN and dialkoxyanthracene amplifiers, determine their oxidation potentials by cyclic voltammetry, and calculate their quadrupole moments. Blockage experiments reveal that subtle structural changes in regioisomeric DAN amplifiers can be registered within NDI pores. Frontier orbital overlap in AEDA complexes, oxidation potentials, and, to a lesser extent, quadrupole moments are shown to contribute to isomer recognition by synthetic pores. Particularly important with regard to practical applications of synthetic pores as multianalyte sensors, we further demonstrate that application of the lessons learned with DAN regioisomers to the expansion to dialkoxyanthracenes provides access to privileged amplifiers with submicromolar activity.     

Boronic acid converters for reactive hydrazide amplifiers: polyphenol sensing in green tea with synthetic pores

Multicomponent sensing in complex matrices with synthetic pores became feasible with the introduction of amplifiers. Amplifiers are defined as molecules that can covalently capture undetectable analytes after enzymatic signal generation and drag them into the pore for transduction. Here, we introduce converters as molecules that can shift the reactivity of amplifiers in situ to capture chemoorthogonal analytes. For this purpose, a series of dialkoxynaphthalene (DAN) and dialkoxyanthracene (DAA) hydrazinoboronic acids was prepared in situ from DAN and DAA hydrazides and formylphenylboronic acids. These converted amplifiers efficiently inactivate synthetic pores with internal naphthalenediimide clamps. This pore inactivation by DAN and DAA hydrazinoboronic acids vanishes in the presence of catechols such as (+)-catechin, presumably because the obtained boronate esters are too large to bind within the synthetic beta-barrel pore or because they prefer to partition into the bilayer membrane. The resulting increase in pore activity with increasing catechol concentration at constant amplifier concentration is shown to be compatible with the sensing of polyphenols in green tea.     

Solvation force, structure and thermodynamics of fluids confined in geometrically rough pores

The effect of periodic surface roughness on the behavior of confined soft sphere fluids is investigated using grand canonical Monte Carlo simulations. Rough pores are constructed by taking the prototypical slit-shaped pore and introducing unidirectional sinusoidal undulations on one wall. For the above geometry our study reveals that the solvation force response can be phase shifted in a controlled manner by varying the amplitude of roughness. At a fixed amplitude of roughness, a, the solvation force for pores with structured walls was relatively insensitive to the wavelength of the undulation, lambda for 2.3/=0.5. The predictions of the superposition approximation, where the solvation force response for the rough pores is deduced from the solvation force response of the slit-shaped pores, was in excellent agreement with simulation results for the structured pores and for lambda/sigma(ff)>/=7 in the case of smooth walled pores. Grand potential computations illustrate that interactions between the walls of the pore can alter the pore width corresponding to the thermodynamically stable state, with wall-wall interactions playing an important role at smaller pore widths and higher amplitudes of roughness.     

Development of a molecular recognition ion gating membrane and estimation of its pore size control

We have fabricated a molecular recognition ion gating membrane. This synthetic membrane spontaneously opens and closes its pores in response to specific solvated ions. In addition to this switching function, we found that this membrane could control its pore size in response to a known concentration of a specific ion. The membrane was prepared by plasma graft copolymerization, which filled the pores of porous polyethylene film with a copolymer of NIPAM (N-isopropylacrylamide) and BCAm (benzo[18]crown-6-acrylamide). NIPAM is well-known to have an LCST (lower critical solution temperature), at which its volume changes dramatically in water. The crown receptor of the BCAm traps a specific ion, and causes a shift in the LCST. Therefore, selectively responding to either K(+) or Ba(2+), the grafted copolymer swelled and shrank in the pores at a constant temperature between two LCSTs. The solution flux in the absence of Ba(2+) decreased by about 2 orders of magnitude over a solution flux containing Ba(2+). The pore size was estimated by the filtration of aqueous dextran solutions with various solute sizes. This revealed that the membrane changed its pore size between 5 and 27 nm in response to the Ba(2+) concentration changes. No such change was observed for Ca(2+) solutions. Furthermore, this pore size change occurred uniformly in all pores, as a clear cut-off value for a solute size that could pass through pores was always present. This membrane may be useful not only as a molecular recognition ion gate, but also as a device for spontaneously controlling the permeation flux and solute size.     

High Internal Phase Emulsion Templating – A Path To Hierarchically Porous Functional Polymers

Recently, a series of new monomers and polymerization mechanisms has been applied to the templating of high internal phase emulsions (HIPEs) providing a route to hierarchically porous materials with a range of functionalities and applications. The high degree of control over the pore size is another attractive feature of these materials. Usually, the continuous phase contains monomers, the droplet phase is used to template the large, primary pores, which are interconnected by secondary pores. The addition of nonpolymerizable components to the continuous phase can result in phase separation during polymerization and tertiary pores. Applications include polymer supports for catalysis and synthesis, separation and filtration, cell culture media, enzyme supports, and structural and isolation applications.     

Sorption kinetics of strontium in porous hydrous ferric oxide aggregates I. The Donnan diffusion model

Sorption of ions by hydrous ferric oxide (HFO) often shows a fast initial sorption reaction followed by a much slower sorption process. The second step is diffusion-controlled and can continue for days or months before equilibrium is reached. In this paper, we demonstrate that the diffusion rate may be explained by electrostatic interactions. The internal and external surfaces of HFO are generally positively charged and therefore repel cations. This can result in extremely low cation concentrations in pores, and therefore a significant reduction in pore diffusion rate. The theory is demonstrated here for sorption of Sr(2+) in HFO aggregates. The ion concentrations in the pore space are calculated using a Donnan model and diffusion is calculated from the Donnan concentration and potential gradients. This diffusion model is compared with nonelectrostatic pore diffusion, which does not take electrostatic interactions into account. The Donnan model predicts very low concentrations of Sr(2+) in the pores and diffusion rates that are up to 8000 times lower than predicted with a nonelectrostatic model.     

Interpretation of water isotherm hysteresis for an activated charcoal using stochastic pore networks

Water vapor adsorption equilibria on activated carbons typically exhibit hysteresis. The size and shape of the hysteresis loop which separates the adsorption and desorption branches is a strong function of the pore size and interconnectivity of the pores. Neither conventional pore filling models nor statistical thermodynamics approaches provide a means for predicting the extent of hysteresis from only adsorption measurements. This work uses the Kelvin Equation in conjunction with the structural concept of a stochastic pore network to describe measured water isotherms on BPL carbon. Using a pore segment distribution function determined from the adsorption branch, it is shown that totally random assemblies underestimate the extent of hysteresis. It is possible, however, to closely fit the measured BPL-water hysteresis loop using a patchy heterogeneity in which a proportion of the larger pores are preferentially located on the exterior, mid-range pores are concentrated in a sub-surface layer and some large pores form shielded voids behind much smaller pores.Nomenclature p vapor phase partial pressure of sorbate - p _sat saturation vapor pressure of sorbate - R gas constant - r pore radius - T absolute temperature - t adsorbed layer thickness - V _L molar volume of adsorbed phase - surface tension - contact angle     

CsxH3-xPW12O40的孔结构

Ｎ２静态吸附容量法的测定结果表明，磷钨酸铯盐（ＣｓｘＨ３－ｘＰＷ１２Ｏ４０）的孔窝和孔分布与ｘ值的大小相关。ｘ〈１．５的ＣｓｘＨ３－ｘＰＷ１２Ｏ４０孔容相近，孔分布近似；当ｘ〉１．５时，ＣｓｘＨ３－ｘＰＷ１２Ｏ４０的孔主要是孔径小于１０ｎｍ的中孔和微孔，平均孔径及孔容随ｘ的增加而增大。ＳＥＭ和ＴＥＭ的观测结果表明，ＣｓｘＨ３－ｘＰＷ１２Ｏ４０的孔是微细粒子堆积留下的空隙孔，可能不存在晶内孔。     

Nanoporous polymer networks based on <Emphasis Type="Italic">N</Emphasis>-vinylpyrrolidone

A method of preparing nanoporous polymer networks containing N-vinylpyrrolidone units via the crosslinking radical copolymerization in bulk performed in the presence of amphiphilic N-vinylpyrrolidone copolymers with the branched morphology and different physicochemical characteristics is developed. It is shown that macromolecular nanoobjects may be extracted from polymer composites using good solvents, such as chloroform and isopropyl alcohol. The physicomechanical, thermal, and diffusion–sorption properties of polymer composites before and after their extraction are compared. SEM and low-temperature nitrogen adsorption measurements reveal that nanosized pores are contained in the network copolymers after extraction of the polymer additives. The specific surface area, total pore volume, pore size, and pore-size distribution are determined. The maximum specific surface area of polymer networks attains ~26 m²/g, and mesopores compose the main type of pores.     

Electrokinetic study of etching latent tracks of accelerated heavy ions in poly(ethylene terephthalate) and polyimide

The etching of latent tracks and pore formation in track membranes are studied. It is shown that the incorporation of K⁺ and Ba²⁺ ions into alkaline solutions accelerates the etching of poly(ethylene terephthalate) (PET) and latent tracks in it. The etching is accelerated due to a decrease in the negative surface charge of PET surface and the pores in track membranes. Isoelectric points are determined for PET and polyimide track membranes and it is established that they depend on pore diameter. As the pore diameter is enlarged from 30 to 70 nm, the magnitude of the negative surface charge rises seemingly due to the increase in the concentration of carboxyl groups on the pore surface. It is assumed that this effect is due to either a high mobility of carboxyl groups in a gel, which is formed on pore walls as a result of etching latent tracks, or a displacement of the slipping plane caused by a decrease in the thickness of the gel layer.     

Modeling liquid porosimetry in modeled and imaged 3-D fibrous microstructures

In this paper, an analysis to distinguish the geometric and porosimetric pore size distributions of a fibrous material is presented. The work is based on simulating the intrusion of nonwetting fluid in a series of 3-D fibrous microstructures obtained from 3-D image reconstruction or virtual geometries mathematically generated according to the properties of the media. We start our study by computing the pore size distribution of two typical hydroentangled nonwoven materials and present a theoretical model for their geometric pore size distributions based on Poisson line network model of the fibrous media. It is shown that the probability density function of the geometric pore size distribution can be approximated by a two-parametric Gamma distribution. We also study connectivity of the pore space in fibrous media by computing and comparing the accessible and allowed pore volumes in the form access function graphs. It is shown that the so-called ink-bottle effect can significantly influence the fluid intrusion in a porous material. The pore space connectivity of a homogeneous fibrous media is observed to be a function of thickness, solid volume fraction (SVF), and fiber diameter. It is shown that increasing the materials' thickness or SVF, while other properties are kept constant, reduces the pore space connectivity. On the other hand, increasing the fiber diameter enhances the connectivity of the pores if all other parameters are fixed. Moreover, modeling layered fibrous microstructures; it is shown that the access function graphs can be used to detect the location of the bottle neck pores in a layered/composite porous material.     

Permanent microporosity and enantioselective sorption in a chiral open framework

A homochiral microporous material is presented. The phase has 47% permanently porous void volume and is shown to have >1 nm diameter pores with three-dimensional channels using probe molecule sorption. Enantioselective guest sorption is strongly dependent on guest size. The homochiral microporous phase was identified by reactive selection from a first-generation chiral but nonporous framework. Chiral permanent porosity is established by directional noncovalent interactions between framework-forming and nonframework forming components of the stable second-generation material, which become stronger upon loss of the guests from the pore system.     

Direct Current Conductometry of Track Membranes

The electrical conductivity measured for a KCl solution in pores of poly(ethylene terephthalate) track membranes has been studied as depending on electrolyte concentration and pore diameter with the use of a direct-current source. The difference between the experimentally determined conductivity and the standard value has been shown to decrease with increasing electrolyte concentration and pore diameter. At the same time, its value is significantly lower than that determined by impedance spectroscopy. This result is related to a decrease in the contribution of a gel layer formed on the pore surface upon coming into the contact with the electrolyte to the electrical resistance of a membrane.     

Molecular simulation of binary mixture adsorption in buckytubes and MCM-41

MCM-41 and buckytubes are novel porous materials with controllable pore sizes and narrow pore size distributions. Buckytubes are carbon tubes with internal diameters in the range 1–5 urn. The structure of each tube is thought to be similar to one or more graphite sheets rolled up in a helical manner. MCM-41 is one member of a new family of highly uniform mesoporous silicate materials produced by Mobil, whose pore size can be accurately controlled in the range 1.5–10 nm. We present grand canonical Monte Carlo (GCMC) simulations of single fluid and binary mixture adsorption in a model buckytube, and nonlocal density functional theory (DFT) calculations of trace pollutant separation in a range of buckytubes and MCM-41 pores. Three adsorbed fluids are considered; methane, nitrogen and propane. The GCMC studies show that the more strongly adsorbed pure fluid is adsorbed preferentially from an equimolar binary mixture. Ideal adsorbed solution theory (IAST) is shown to give good qualitative agreement with GCMC when predicting binary mixture separations. The DFT results demonstrate the very large increases in trace pollutant separation that can be achieved by tuning the pore size, structure, temperature and pressure of the MCM-41 and buckytube adsorbent systems to their optimal values.     

Visualisation of the fibrillar and pore morphology of cellulosic fibres applying transmission electron microscopy

Applying transmission electron microscopy (TEM) on ultra-thin cross-sections of fibres, the main characteristics of the internal morphology of cotton and the main man-made cellulosic fibres (modal, viscose and lyocell) could be visualised. To obtain an appropriate contrast for TEM, isoprene was polymerised into the swollen fibres after a stepwise solvent exchange from water to acetone. The included polymer is stainable with osmium tetraoxide. Significant differences in distribution of pore sizes and pore arrangements in the cellulosic fibres were seen. Cotton showed very small pores in the bulk of the fibre, but drying cracks and flat pores between the sheets of the secondary wall appear as larger pores. Lyocell contains only nanopores in the bulk of the fibre with a slight gradient in pore density, and a very porous skin layer. In viscose and modal, a very wide pore size distribution from nanometer to micrometer size can be seen.