Tethered bilayer lipid membranes self-assembled on mercury electrodes

Tethered and Polymer Supported Bilayer Lipid Membranes: Structure and Function

Solid supported bilayer lipid membranes are model systems to mimic natural cell membranes in order to understand structural and functional properties of such systems. The use of a model system allows for the use of a wide variety of analytical tools including atomic force microscopy, impedance spectroscopy, neutron reflectometry, and surface plasmon resonance spectroscopy. Among the large number of different types of model membranes polymer-supported and tethered lipid bilayers have been shown to be versatile and useful systems. Both systems consist of a lipid bilayer, which is de-coupled from an underlying support by a spacer cushion. Both systems will be reviewed, with an emphasis on the effect that the spacer moiety has on the bilayer properties.

Voltage dependent closure of PorB class II porin from Neisseria meningitidis investigated using impedance spectroscopy in a tethered bilayer lipid membrane interface

Electrochemical impedance spectroscopy (EIS) was used to characterize voltage-dependent closure of PorB class II (PorBII) porin from Neisseria meningitidis incorporated in a tethered bilayer lipid membrane (tBLM). The tBLM's lower leaflet was fabricated by depositing a self assembled monolayer (SAM) of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (DPPTE) on a gold electrode, and the upper leaflet was formed by depositing1,2-dioleoyl-sn-glycero-3-phoshocholine (DOPC) liposomes. At 0mV bias DC potential, incorporation of PorBII decreased the membrane resistance (R(m)) from 2.5 MΩc m(2) to 0.6 MΩ cm(2), giving a ΔR(m) of 1.9 MΩ cm(2) and a normalized ΔR(m) (ΔR(m) divided by the R(m) of the tBLM without PorBII) of 76%. When the bias DC potential was increased to 200 mV, the normalized ΔR(m) value decreased to 20%. The effect of applied voltage on ΔR(m) was completely reversible, suggesting voltage-dependent closure of PorBII. The voltage dependence of PorBII was further studied in a planar bilayer lipid membrane made from 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhytPC). Following a single insertion event, PorBII exhibited multiple conductance states, with reversible, voltage-dependent closure of PorBII porin occurring at high transmembrane potentials. The trimetric porin closed in three discrete steps, each step corresponding to closure of one conducting monomer unit. The most probable single channel conductance was 4.2 nS. The agreement between results obtained with the tBLM and pBLM platforms demonstrates the utility of EIS to screen channel proteins immobilized in tBLM for voltage-gated behavior.

Stable insulating tethered bilayer lipid membranes

Tethered bilayer lipid membranes have been shown to be an excellent model system for biological membranes. Coupling of a membrane to a solid supports creates a stable system that is accessible for various surface analytical tools. Good electrical sealing properties also enable the use of the membranes in practical sensing applications. The authors have shown that tethered membranes have extended lifetimes up to several months. Air-stability of the bilayer can be achieved by coating the membrane with a hydrogel. The structure of a monolayer and its stability under applied dc potentials have been investigated by neutron scattering.

Constituent-dependent liposome structure and organization

We have used steady state and time-resolved fluorescence spectroscopy in concert with TEM to study organization and dynamics of molecules comprising liposomes, discoidal micelles, and spherical micelles. The lipid aggregates contained controlled amounts of lipids with headgroups modified with a thiol-terminated polyethylene glycol (thio-PEG lipids) and a small amount of 1-palmitoyl-2-(pyrene-1-yl)decanoyl-sn-glycero-3-phosphocholine (pyrene tethered DPPC), pyrene, or perylene as spectroscopic probes. The maximum diameter of the lipid aggregates was controlled by the polycarbonate filter pore size used in the extrusion process. The concentration of thio-PEG lipid in the aggregates determines not only the shape of the lipid assemblies but also the organization of the molecules within the assembly. Fluorescence lifetime and anisotropy decay data show that the immediate environment of pyrene tethered DPPC changes with the addition of thio-PEG lipid. In contrast, the dynamics of free chromophore (perylene) are insensitive to the addition of thio-PEG lipid. The addition of thio-PEG lipid to the lipid assembly produces changes in organization that are most pronounced in the lipid headgroup region. Reorientation dynamics of perylene show that the organization of the lipid bilayer acyl chain region is affected little by the addition of thio-PEG lipid and consequent macroscopic changes in the morphology of the lipid assemblies.

Measuring ion channels on solid supported membranes

Application of solid supported membranes (SSMs) for the functional investigation of ion channels is presented. SSM-based electrophysiology, which has been introduced previously for the investigation of active transport systems, is expanded for the analysis of ion channels. Membranes or liposomes containing ion channels are adsorbed to an SSM and a concentration gradient of a permeant ion is applied. Transient currents representing ion channel transport activity are recorded via capacitive coupling. We demonstrate the application of the technique to liposomes reconstituted with the peptide cation channel gramicidin, vesicles from native tissue containing the nicotinic acetylcholine receptor, and membranes from a recombinant cell line expressing the ionotropic P2X2 receptor. It is shown that stable ion gradients, both inside as well as outside directed, can be applied and currents are recorded with an excellent signal/noise ratio. For the nicotinic acetylcholine receptor and the P2X2 receptor excellent assay quality factors of Z' = 0.55 and Z' = 0.67, respectively, are obtained. This technique opens up new possibilities in cases where conventional electrophysiology fails like the functional characterization of ion channels from intracellular compartments. It also allows for robust fully automatic assays for drug screening.

Incorporation of the HERG potassium channel in a mercury supported lipid bilayer

The HERG potassium channel was incorporated in a mercury-supported tethered bilayer lipid membrane (tBLM) obtained by anchoring a thiolipid monolayer to the mercury surface and by self-assembling a lipid monolayer on top of it from a lipid film spread on the surface of an electrolyte solution. HERG was then incorporated in this tBLM from its micellar solution in Triton X-100, thus avoiding the use of vesicles in the preparation of the tBLM and of proteoliposomes in channel incorporation. The HERG "inward" current following a repolarization step was obtained by subtracting the current recorded upon addition of the specific inhibitor WAY from that recorded prior to this addition. This current was compared with that reported in the literature by the patch-clamp technique.

Gramicidin Conducting Dimers in Lipid Bilayers Are Stabilized by Single-File Ionic Flux along Them

Gramicidin D was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer, and its behavior was investigated in aqueous 0.1 M KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy. The impedance spectra, recorded from 0.1 to 1 x 10(5) Hz over a potential range of 0.7 V, were fitted to a series of RC meshes, which were related to the different substructural elements of the biomimetic membrane. These impedance spectra were compared with those obtained by incorporating valinomycin, under otherwise identical conditions. The potential dependence of the stationary currents reported on bilayer lipid membranes by Bamberg and Läuger (Bamberg, E.; Läuger, P. J. Membrane Biol. 1973, 11, 177-194) as well as those extracted from potential-step chronocoulometric measurements was interpreted by relating the increase in gramicidin dimerization to a progressive increase in single-file K+ flux along the dimeric channels. An analogous approach was adopted in explaining the difference between the impedance spectra obtained with gramicidin D and those obtained with valinomycin. It is concluded that gramicidin has a low tendency to form dimers in the absence of ionic flux.

An electrochemical investigation of sarcolipin reconstituted into a mercury-supported lipid bilayer

Sarcolipin was incorporated into a lipid bilayer anchored to a mercury electrode through a hydrophilic tetraethyleneoxy chain. The behavior of this tethered bilayer lipid membrane incorporating sarcolipin was investigated by electrochemical impedance spectroscopy and by recording charge versus time curves after potential jumps. When the transmembrane potential starts to become negative on the trans side, evidence is provided that sarcolipin aggregates into ion-conducting pores. Over the range of physiological transmembrane potentials, these pores are permeable to small inorganic anions such as chloride, phosphate, or sulfate but impermeable to inorganic cations such as Na+ and K+. Only at transmembrane potentials more negative than approximately -150 mV on the trans side do sarcolipin channels allow the translocation of the latter cations. A tentative relationship between this property of sarcolipin and its regulatory function on Ca-ATPase of sarcoplasmic reticulum is proposed.

Functional incorporation of the pore forming segment of AChR M2 into tethered bilayer lipid membranes

Tethered bilayer lipid membranes (tBLMs) are robust and flexible model platforms for the investigation of various membrane related processes. They are especially suited to study the incorporation and function of ion channel proteins, where a high background resistance of the membrane is essential. Synthetic M2 peptides, analogues of the transmembrane fragment of the acetylcholine receptor, could be incorporated into two different membrane architectures. The functional reconstitution and the formation of a conducting pore are shown by electrochemical impedance spectroscopy (EIS). The pore is selective for small monovalent cations, while bulky ions cannot pass. This is a significant step towards a novel biosensing approach. We envision a device, where a stable and insulating membrane would be attached to an electronic read-out unit and embedded proteins would serve as actual sensing units.

Insulating tethered bilayer lipid membranes to study membrane proteins

Tethered bilayer lipid membranes are stable and promising model systems that mimic several properties of biological membranes. They provide an electrically insulating platform for the incorporation and study of functional membrane proteins, especially ion channels. Covalently linked to a solid support, they also offer enhanced stability compared with other model architectures. If the support can be used as an electrode, electrical characterisation of the system is possible and biosensing applications can be envisioned.Here, we will review some tethered bilayer structures developed in the past and show some examples of functional protein incorporation, both on oxide and gold substrates.

Tethered Lipid Bilayers on Electrolessly Deposited Gold for Bioelectronic Applications

This paper presents the formation of a novel biomimetic interface consisting of an electrolessly deposited gold film overlaid with a tethered bilayer lipid membrane (tBLM). Self-assembly of colloidal gold particles was used to create an electrolessly deposited gold film on a glass slide. The properties of the film were characterized using field-effect scanning electron microscopy, energy dispersive spectroscopy, and atomic force microscopy. Bilayer lipid membranes were then tethered to the gold film by first depositing an inner molecular leaflet using a mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[3-(2-pyridyldithio)propionate], 1,2-di-O-phytanyl-sn-glycero-3-phosphoethanolamine (DPGP), and cystamine in ethanol onto a freshly prepared electrolessly deposited gold surface. The outer leaflet was then formed by the fusion of liposomes made from DPGP or 1,2-dioleoyl-sn-glycero-3-phosphocholine on the inner leaflet. To provide functionality, two membrane biomolecules were also incorporated into the tBLMs: the ionophore valinomycin and a segment of neuropathy target esterase containing the esterase domain. Electrochemical impedance spectroscopy, UV/visible spectroscopy, and fluorescence recovery after pattern photobleaching were used to characterize the resulting biomimetic interfaces and confirm the biomolecule activity of the membrane. Microcontact printing was used to form arrays of electrolessly deposited gold patterns on glass slides. Subsequent deposition of lipids yielded arrays of tBLMs. This approach can be extended to form functional biomimetic interfaces on a wide range of inexpensive materials, including plastics. Potential applications include high-throughput screening of drugs and chemicals that interact with cell membranes and for probing, and possibly controlling, interactions between living cells and synthetic membranes. In addition, the gold electrode provides the possibility of electrochemical applications, including biocatalysis, bio-fuel cells, and biosensors.

Arrays of lipid bilayers and liposomes on patterned polyelectrolyte templates

This paper presents novel methods to produce arrays of lipid bilayers and liposomes on patterned polyelectrolyte multilayers. We created the arrays by exposing patterns of poly(dimethyldiallylammonium chloride) (PDAC), polyethylene glycol (m-dPEG) acid, and poly(allylamine hydrochloride) (PAH) on polyelectrolyte multilayers (PEMs) to liposomes of various compositions. The resulting interfaces were characterized by total internal reflection fluorescence microscopy (TIRFM), fluorescence recovery after pattern photobleaching (FRAPP), quartz crystal microbalance (QCM), and fluorescence microscopy. Liposomes composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dioleoyl-sn-glycero-3-phosphate (monosodium salt) (DOPA) were found to preferentially adsorb on PDAC and PAH surfaces. On the other hand, liposome adsorption on sulfonated poly(styrene) (SPS) surfaces was minimal, due to electrostatic repulsion between the negatively charged liposomes and the SPS-coated surface. Surfaces coated with m-dPEG acid were also found to resist liposome adsorption. We exploited these results to create arrays of lipid bilayers by exposing PDAC, PAH and m-dPEG patterned substrates to DOPA/DOPC vesicles of various compositions. The patterned substrates were created by stamping PDAC (or PAH) on SPS-topped multilayers, and m-dPEG acid on PDAC-topped multilayers, respectively. This technique can be used to produce functional biomimetic interfaces for potential applications in biosensors and biocatalysis, for creating arrays that could be used for high-throughput screening of compounds that interact with cell membranes, and for probing, and possibly controlling, interactions between living cells and synthetic membranes.

Electrochemical investigation of melittin reconstituted into a mercury-supported lipid bilayer

The channel-forming peptide melittin was incorporated into a biomimetic membrane consisting of a mercury electrode coated with a thiolipid monolayer, with a lipid monolayer self-assembled on top of it. The thiolipid consisted of a hydrophilic tetraethyleneoxy chain terminated at one end with a disulfide group, for anchoring to the mercury surface, and covalently linked at the other end to two diphytanyl chains, which formed a lipid bilayer with the overhanging lipid monolayer. The conductance of the lipid bilayer in contact with aqueous 0.1 M KCl was measured by electrochemical impedance spectroscopy over a frequency range from 1 x 10(-2) to 1 x 10(5) Hz and a potential range of 0.7 V for different compositions of the outer lipid monolayer. The conductance increases abruptly above the background level at sufficiently negative applied potentials, attaining a maximum value that increases with the composition of the outer monolayer in the order PC/chol (60:40) < PC < PC/SM/chol (59:15:26) < PS, with PC = phosphatidylcholine, chol = cholesterol, SM = sphingomyelin, and PS = phosphatidylserine. The higher the maximum conductance, the less negative the applied potential at which it is attained. This behavior is also discussed using a model of the electrified interphase.

Impedance spectroscopy of OmpF porin reconstituted into a mercury-supported lipid bilayer

The channel-forming protein OmpF porin was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a thiolipid, and it was investigated in aqueous KCl by electrochemical impedance spectroscopy. The impedance spectra, recorded from 1 x 10(-2) to 1 x 10(5) Hz over a potential range of 0.7 V, were fitted to an equivalent circuit consisting of four RC meshes. The dependence of the resulting circuit elements upon the applied potential was interpreted on the basis of a general approximate approach based on a model of the electrified interphase and on the kinetics of the translocation of potassium and chloride ions across the lipid bilayer, assisted by the OmpF porin.

Tethered bilayer lipid membranes based on monolayers of thiolipids mixed with a complementary dilution molecule. 1. Incorporation of channel peptides

Tethered bilayer lipid membranes (tBLMs) are described based on the self-assembly of a monolayer on template stripped gold of an archea analogue thiolipid, 2,3-di-o-phytanyl-sn-glycerol-1-tetraethylene glycol-d,l-alpha-lipoic acid ester lipid (DPTL), and a newly designed dilution molecule, tetraethylene glycol-d,l-alpha-lipoic acid ester (TEGL). The tBLM is completed by fusion of liposomes made from a mixture of diphytanoylphosphatidyl choline (DPhyPC), cholesterol, and 1,2-diphytanoyl-sn-glycero-3-phosphate (DPhyPG) in a molar ratio of 6:3:1. Melittin and gramicidin are incorporated into these tBLMs as shown by surface plasmon resonance (SPR) and electrochemical impedance spectroscopy (EIS) studies. Ionic conductivity at 0 V vs Ag|AgCl, 3 M KCl, measured by EIS measurements are comparable to the results obtained by other research groups. Admittance plots as a function of potential are discussed on a qualitative basis in terms of the kinetics of ion transport through the channels.

Potassium Ion Transport by Valinomycin across a Hg-Supported Lipid Bilayer

A biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer was investigated in aqueous KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy, both in the absence and in the presence of the ionophore valinomycin. Impedance spectra, recorded from 1 x 10(-2) to 1 x 10(5) Hz over a potential range of 0.8 V, are satisfactorily fitted to a series of four RC meshes, which are straightforwardly related to the different substructural elements of the biomimetic membrane. The frequency-independent resistances and conductances of both the lipid bilayer and the hydrophilic spacer show a maximum when plotted against the applied potential. This behavior is interpreted on the basis of a general approximate approach that applies the concepts of impedance spectroscopy to a model of the electrified interphase and to the kinetics of potassium ion transport assisted by valinomycin across the lipid bilayer.