Transport across artificial membranes–an analytical perspective

Biosensors that make use of transport processes across lipid membranes are very rare even though a stimulus, the binding of a single analyte molecule, can enhance the sensor response manifold if the analyte leads to the transport of more than one ion or molecule across the membrane. Prerequisite for a proper function of such membrane based biosensors is the formation of lipid bilayers attached to a support that allow for the insertion of membrane peptides and proteins in a functional manner. In this review, the current state of the art technologies to obtain lipid membranes on various supports are described. Solid supported membranes on transparent and electrically conducting surfaces, lipid bilayers on micromachined apertures and on porous materials are discussed. The focus lies on the applicability of such membranes for the investigation of transport phenomena across lipid bilayers facilitated by membrane embedded peptides, channel proteins and transporters. Carriers and channel forming peptides, which are easy to handle and rather robust, are used frequently to build up membrane based biosensors. However, channel forming proteins and transporters are more difficult to insert functionally and thus, there are yet only few examples that demonstrate the applicability of such systems as biosensor devices.      

How do P-type ATPases transport ions?

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins, the energy-providing ATP hydrolysis is coupled to ion transport of one or two ion species across the respective membrane. The pump function of the investigated pumps is described by a so-called Post-Albers cycle. Main features of the pumping process are (1) a Ping-Pong mechanism, i.e. both transported ion species are transferred successively and in opposite direction across the membrane, (2) the transport process for each ion species consists of a sequence of reaction steps, which are ion binding, ion occlusion, conformational transition of the protein, successive deocclusion of the ions and release to the other side of the membrane. (3) Recent experimental evidence shows that the ion-binding sites are placed in the transmembrane section of the proteins and that ion movements occur preferentially during the ion binding and release processes. The main features of the mechanism include narrow access channels from both sides, one gate per access channel, and an ion-binding moiety that is adapted specifically to the ions that are transported, and differently in both principal conformations.     

Functional Tethered Bilayer Lipid Membranes on Aluminum Oxide

Tethered bilayer lipid membranes are established as well‐suited model membrane systems adaptable to different surfaces, for example, gold and silicon. These solid supported membranes are highly flexible in their tethering and lipid parts and can thus be optimized for functional incorporation of membrane proteins. The excellent sealing properties of the tethered membranes allow incorporated ion‐channel proteins to be investigated. Preparation of ultrasmooth aluminum oxide by sputtering and synthesis of new tethering lipids with phosphonic acid anchor groups enable formation of an electrically sealing membrane on this surface. This process is monitored by electrochemical impedance spectroscopy and by surface plasmon resonance spectroscopy. High sealing performance of the membrane and functional incorporation of the ion carrier valinomycin are demonstrated.     

Cation‐Transporting Peptides: Scaffolds for Functionalized Pores?

Protein pores that selectively transport ions across membranes are among nature’s most efficient machines. The selectivity of these pores can be exploited for ion sensing and water purification. Since it is difficult to reconstitute membrane proteins in their active form for practical applications it is desirable to develop robust synthetic compounds that selectively transport ions across cell membranes. One can envision tuning the selectivity of pores by incorporating functional groups inside the pore. Readily accessible octapeptides containing (aminomethyl)benzoic acid and alanine are reported here that preferentially transport cations over halides across the lipid bilayer. Ion transport is hypothesized through pores formed by stable assemblies of the peptides. The aromatic ring(s) appear to be proximal to the pore and could be potentially utilized for functionalizing the pore interior.     

Membrane electrochemistry

Electrochemistry and biomembranes are interface science in that both are concerned with the phenomena at, as well as across, the interfaces. Membrane electrochemistry may be defined as the application of electrochemistry to biomembrane studies. Additionally, transport processes within the membrane are involved in biomembranes. Since biomembranes are diverse and are usually not amenable to probing by electrochemicophysical techniques, model membrane systems have been developed for their investigation.

The introduction of experimental bilayer lipid membranes (BLM) technique and its modifications have been instrumental in the development and testing of membrane transport concepts (carriers vs channels) and electronic processes in membranes. Instead merely viewing a biomembrane as a physical barrier containing carriers or channels to carry out ionic processes, an ultrathin lipid or biological membrane can also be considered as a complete ‘electrochemical cell’ with one membrane/solution interface reducing (as a cathode) and the other membrane/solution interface oxidizing (as an anode). It is now possible to understand energy transduction (charge generation, separation, and redox reactions) in terms of ultrathin lipid membranes separating two aqueous solutions.

In this paper, we shall discuss the basic principles of electrochemistry as they are applied to membrane studies. Emphasis will be on experimental bilayer lipid membranes (BLM) which have been extensively investigated as models of biomembranes.     

Correction of pH of diluted solutions of electrolytes by electrodialysis with bipolar membranes

The current efficiencies of the water dissociation water and the voltage-current characteristics of the bipolar (asymmetric bipolar) membranes were measured in a two-chamber electrochemical cell. The cell was formed of an MB-3 bipolar membrane or an asymmetric bipolar membrane, which is an MA-40 heterogeneous membrane with a thin surface layer in the form of a cation-selective homogeneous film and MA-40 and MA-41 heterogeneous monopolar membranes. The dissociation of water on MA-40 in 0.01 M sodium chloride decreased the current efficiency of the acid and alkali both in the channel with a bipolar membrane and in the channel with an asymmetric bipolar membrane. The effective ion transport numbers across MA-40 and MA-41 at different pH values were determined. The water dissociation rate on MA-40 decreased at pH > 9.5. A kinetic model of the electrodialysis of a dilute solution of sodium chloride in a two-chamber unit cell with a bipolar and anionite membranes was suggested.     

Structural studies of apoptosis and ion transport regulatory proteins in membranes

Solid-state NMR spectroscopy is being used to determine the structures of membrane proteins involved in the regulation of apoptosis and ion transport. The Bcl-2 family includes pro- and anti-apoptotic proteins that play a major regulatory role in mitochondrion-dependent apoptosis or programmed cell death. The NMR data obtained for (15)N-labeled anti-apoptotic Bcl-xL in lipid bilayers are consistent with membrane association through insertion of the two central hydrophobic alpha-helices that are also required for channel formation and cytoprotective activity. The FXYD family proteins regulate ion flux across membranes, through interaction with the Na(+), K(+)-ATPase, in tissues that perform fluid and solute transport or that are electrically excitable. We have expressed and purified three FXYD family members, Mat8 (mammary tumor protein), CHIF (channel-inducing factor) and PLM (phospholemman), for structure determination by NMR in lipids. The solid-state NMR spectra of Bcl-2 and FXYD proteins, in uniaxially oriented lipid bilayers, give the first view of their membrane-associated architectures.     

平板双分子层脂膜的制备及锌离子跨该膜的输运过程

制备了氧化胆固醇 卵磷脂(脑磷脂)平板双分子层脂膜,研究了膜配方对双分子层脂膜的稳定性和离子通透性的影响,得到了最佳制膜工艺,建立了锌离子跨卵 (脑 )磷脂膜的吸附 -扩散模型,其计算值与实验值基本吻合.     

Utilization of photoinduced charge-separated state of donor-acceptor-linked molecules for regulation of cell membrane potential and ion transport

The control of ion transport across cell membranes by light is an attractive strategy that allows targeted, fast control of precisely defined events in the biological membrane. Here we report a novel general strategy for the control of membrane potential and ion transport by using charge-separation molecules and light. Delivery of charge-separation molecules to the plasma membrane of PC12 cells by a membranous nanocarrier and subsequent light irradiation led to depolarization of the membrane potential as well as inhibition of the potassium ion flow across the membrane. Photoregulation of the cell membrane potential and ion transport by using charge-separation molecules is highly promising for control of cell functions.     

Development of urethane acrylate composite ion-exchange membranes and their electrochemical characterization

With the objective of introducing antifouling characteristics into interpolymer types of cation and anion exchange membranes, the surface of these membranes was coated with a 12-microm-thick urethane acrylate layer and was cured by UV radiation of wavelengths 308 and 172 nm under a complete inert atmosphere. Different urethane acrylate composite ion exchange membranes developed were characterized in NaCl solution by measuring their ion-exchange capacity, volume fraction of water, contact angle with water, membrane conductance, and membrane potential. It was found that the electrochemical transport properties of urethane acrylate composite cation-exchange membranes were increased due to resonance stabilization of the urethane group, which acts as a weak acid and dissociates as a negatively charged urethane ion and a positively charged proton. This contributes toward the net charge density of the membrane matrix responsible for enhanced selectivity and conductivity, while for urethane acrylate composite anion-exchange membranes reduction in net charge density was responsible for reduction in electrochemical transport properties. Counterion transport number, permselectivity, and counterion diffusion coefficient values for these membranes were also estimated. Experiments were also carried out in higher homologs of sodium carboxylate solutions in order to observe the fouling tendencies of these membranes. It was concluded that it is possible to obtain antifouling characteristics of ion-exchange membranes by coating and curing thin hydrophilic layers of urethane acrylate on their surfaces without sacrificing their electrochemical transport properties.     

Monitoring and Quantifying the Passive Transport of Molecules Through Patch–Clamp Suspended Real and Model Cell Membranes

Transport of active molecules across biological membranes is a central issue for the success of many pharmaceutical strategies. Herein, we combine the patch–clamp principle with amperometric detection for monitoring fluxes of redox‐tagged molecular species across a suspended membrane patched from a macrophage. Solvent‐ and protein‐free lipid bilayers (DPhPC, DOPC, DOPG) patched from single‐wall GUV have been thoroughly investigated and the corresponding fluxes measurements quantified. The quality of the patches and their proper sealing were successfully characterized by electrochemical impedance spectroscopy. This procedure appears versatile and perfectly adequate to allow the investigation of transport and quantification of the transport properties through direct measurement of the coefficients of partition and diffusion of the compound in the membrane, thus offering insight on such important biological and pharmacological issues.     

辐射接枝法制备离子交换膜的研究现状

聚合物离子交换膜有多种制备方法，其中高分子材料辐射引发接枝功能性单体是一种文献中屡见报道且简单可行的方法．通过在不同聚合物基体上接枝各种类型的单体，可以改变接枝膜的电化学性能、物理化学等性能．丈中详细介绍了不同的高分子基材辐射接枝各类单体制备聚合物离子交换膜的研究现状．     

Transport of NaNO3 solutions across an activated composite membrane: electrochemical and chemical surface characterizations

Electrochemical characterization of two different samples of an activated membrane, which consists of a polymeric support containing different amounts of Di-(2-ethylhexyl) phosphoric acid as a carrier, was made by measuring the electrical resistance, salt diffusion and membrane potential for the activated membranes and the polymeric support in contact with NaNO₃ solutions. Transport parameters such as the ion transport numbers and concentration of fixed charge in the membrane, salt and ionic permeabilities at different NaNO₃ concentrations were obtained. A comparison of the different electrochemical parameters obtained with both activated membranes and the polymeric support shows how the carrier affects the transport of NaNO₃ solutions across the activated membranes. On the other hand, chemical composition of the membrane surfaces as a function of the amount of carrier was determined by X-ray photoelectron spectroscopy technique, which also allows an envisagement of the chemical bonding between the carrier and the membrane top layer (polyamide).     

Theory of Ion Transport in Electrochemically Switchable Nanoporous Metallized Membranes

A physicomathematical model of ion transport through a synthetic electrochemically switchable membrane with nanometric metal‐plated pores is presented. Due to the extremely small size of the cylindrical pores, electrical double layers formed inside overlap, and thus, strong electrostatic fields whose intensities vary across the cross‐sections of the nanopores are created. Based on the proposed model a relationship between the relative electrostatic energies experienced by ions in the nanopores and the potential applied to the membrane is established. This allows the prediction of transference numbers and explains quantitatively the ion‐transport switching capability of such synthetic membranes. The predictions of this model agree satisfactorily with previous experimental data obtained for this type of devices by Martin and co‐workers.     

荷电膜的膜电位研究进展

膜电位的测定是表征荷电膜的传递现象的重要参数之一。本文简要介绍了膜电位理论基础,包括T. M. S.理论和不可逆热力学理论。分别阐述了关于离子交换膜、双极膜、两性膜以及复合膜的膜电位的最新进展,并提出今后的发展方向。     

Water‐mediated transport in ion‐containing polymers

Water‐mediated ion conduction enables high conductivity in hydrated polymer membranes commonly used in electrochemical devices. Understanding the coupling of the absorbed water with the polymer matrix and the dynamics of water inside the polymer network across the full range of length scales in the membrane is important for unraveling the structure–property relationships in these materials. By considering the water behavior in ion‐containing polymers, next‐generation fuel cell membranes are being designed that exceed the conductivity of the state‐of‐the‐art materials and have optimized conductivity and permeability that may be useful in other types of devices such as redox flow batteries. Water–polymer associations can be exploited to tune the transport and mechanical property tradeoffs in these polymers. Measurements of water motion provide important criteria for assessing the factors that control the performance of these types of materials. This review article discusses current understanding of water behavior in ion‐containing polymers and the relationship between water motion and ion and molecular transport. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Electrochemical impedance spectroscopy fingerprints the ion selectivity of microgel functionalized ion-exchange membranes

Surface modification methods are applied to alter interfacial phenomena and improve ion transport through membranes. In this work we present a novel method for tailoring the surface of cation-exchange membranes based on the deposition of thin microgel monolayers. The charge of such layers exerts a strong influence on the monovalent-ion-selectivity, and this is reflected in the electrochemical impedance responses. Membranes coated with uncharged microgels show similar behavior to that of unmodified ones, with impedance spectra dominated by low-frequency diffusional arcs. However, membranes modified with positively charged microgels exhibit an increased resistance due to the hindered transport of cations through the modification. An additional high-frequency capacitive arc is obtained with the monovalent-ion-selective membranes, which is attributed to concentration polarization effects at the membrane/modification interface. The characteristic frequency of this arc decreases with the valency of the ion, thus proving that multivalent ions pass through the modification layer at rates much slower than monovalent ones. Accordingly, electrochemical impedance spectroscopy has been used to feature monovalent-ion-selective properties of layered membranes.     

Effect of light-harvesting complex II on ion transport across model lipid membranes

The effect of the incorporation of the major light-harvesting complex of photosystem II (LHCII) to planar bilayer lipid membranes (BLMs) formed from soybean asolectin and unilamellar small liposomes formed from egg-yolk phosphatidylcholine on ion transport across the lipid bilayer has been studied. The specific conductivity of the BLM rises from 5.2 +/- 0.8 x 10(-9) up to 510 x 10(-9) O(-1) cm(-2) upon the incorporation of LHCII. The conductivity of the membrane with LHCII depends upon the ionic strength of the bathing solution and is higher by a factor of five when the KCl concentration increases from 0.02 to 0.22 M. Such a strong effect has not been observed in the same system without LHCII. The liposome model is also applied to analyse the effect of LHCII on the bilayer permeability to protons. Unilamellar liposomes with a diameter less than 50 nm have been prepared, containing (trapped inside) Neutral Red, a pigment sensitive to proton concentration. A gradient of protons on the membrane is generated by the acidification of the liposome suspension and spectral changes of Neutral Red are recorded in time, reflecting the penetration of protons into the internal space of liposomes. Two components of proton permeation across liposome membranes are observed: a fast one (proceeding within seconds) and a slow one (operating on the time scale of minutes). The rate of both components of proton transport across LHCII-containing membranes is higher than for liposomes alone. The enhancement effect of LHCII on the ion transport across the lipid membrane is discussed in terms of aggregation of the pigment-protein complexes. The possible physiological importance of such an effect in controlling ion permeability across the thylakoid membrane is discussed.     

Microscale Ionic Diodes: An Overview

Ionic rectifier membranes or devices generate uni-directional ion transport to convert an alternating current (AC) ion current input into stored energy or direct current (DC) in the form of ion/salt gradients. Electrochemical experiments 80 years ago were conducted on biological membrane rectifier systems, but today a plethora of artificial ionic rectifier types has been developed and electroanalytical tools are employed to explore mechanisms and performance. This overview focuses on microscale ionic rectifiers with a comparison to nano- and macroscale ionic rectifiers. The potential is surveyed for applications in electrochemical analysis, desalination, energy harvesting, electrochemical synthesis, and in selective ion extraction.