Electrochemical Impedance Spectroscopy of Tethered Bilayer Membranes: An Effect of Heterogeneous Distribution of Defects in Membranes

Membrane-Disruptive Effects of Fatty Acid and Monoglyceride Mitigants on E. coli Bacteria-Derived Tethered Lipid Bilayers

We report electrochemical impedance spectroscopy measurements to characterize the membrane-disruptive properties of medium-chain fatty acid and monoglyceride mitigants interacting with tethered bilayer lipid membrane (tBLM) platforms composed of E. coli bacterial lipid extracts. The tested mitigants included capric acid (CA) and monocaprin (MC) with 10-carbon long hydrocarbon chains, and lauric acid (LA) and glycerol monolaurate (GML) with 12-carbon long hydrocarbon chains. All four mitigants disrupted E. coli tBLM platforms above their respective critical micelle concentration (CMC) values; however, there were marked differences in the extent of membrane disruption. In general, CA and MC caused larger changes in ionic permeability and structural damage, whereas the membrane-disruptive effects of LA and GML were appreciably smaller. Importantly, the distinct magnitudes of permeability changes agreed well with the known antibacterial activity levels of the different mitigants against E. coli, whereby CA and MC are inhibitory and LA and GML are non-inhibitory. Mechanistic insights obtained from the EIS data help to rationalize why CA and MC are more effective than LA and GML at disrupting E. coli membranes, and these measurement capabilities support the potential of utilizing bacterial lipid-derived tethered lipid bilayers for predictive assessment of antibacterial drug candidates and mitigants.

Sparsely tethered bilayer lipid membranes formed by self-assembly of bicelles: Spectroelectrochemical characterization and incorporation of transmembrane protein

Many biochemical processes related to proper homeostasis take place in cell membranes. The key molecules involved in these processes are proteins, including transmembrane proteins. These macromolecules still challenge the understanding of their function within the membrane. Biomimetic models that mimic the properties of the cell membrane can help understand their functionality. Unfortunately, preserving the native protein structure in such systems is problematic. A possible solution to this problem involves the use of bicelles. Their unique properties make integrating bicelles with transmembrane proteins manageable while preserving their native structure. Hitherto, bicelles have not been used as precursors for protein-hosting lipid membranes deposited on solid substrates like pre-modified gold. Here, we demonstrated that bicelles can be self-assembled to form sparsely tethered bilayer lipid membranes and the properties of the resulting membrane satisfy the conditions suitable for transmembrane protein insertion. We showed that the incorporation of α-hemolysin toxin in the lipid membrane leads to a decrease in membrane resistance due to pore formation. Simultaneously, the insertion of the protein causes a drop in the capacitance of the membrane-modified electrode, which can be explained by the dehydration of the polar region of the lipid bilayer and the loss of water from the submembrane region.

Model Lipid Raft Membranes for Embedding Integral Membrane Proteins: Reconstitution of HMG-CoA Reductase and Its Inhibition by Statins

For the first time, HMG-CoA reductase, the membrane protein responsible for cholesterol synthesis, was incorporated into a lipid membrane consisting of DOPC:Chol:SM at a 1:1:1 molar ratio, which mimics the lipid rafts of cell membranes. The membrane containing the protein was generated in the form of either a proteoliposomes or a film obtained by spreading the proteoliposomes at the air-water interface to prepare a protein-rich and stable lipid layer over time. The lipid vesicle parameters were characterized using dynamic light scattering (DLS) and fluorescence microscopy. The incorporation of HMG-CoA reductase was reflected in the increased size of the proteoliposomes compared to that of the empty liposomes of model rafts. Enzyme reconstitution was confirmed by measuring the activity of NADPH, which participates in the catalytic process. The thin lipid raft films formed by spreading liposomes and proteoliposomes at the air-water interface were investigated using the Langmuir technique. The activities of the HMG-CoA reductase films were preserved over time, and the two lipid raft systems, nanoparticles and films, were exposed to solutions of fluvastatin, a HMG-CoA reductase inhibitor commonly used in the treatment of hypercholesterolemia. Both lipid raft systems constructed were useful membrane models for the determination of reductase activity and for monitoring the statin inhibitory effects and may be used for investigating other integral membrane proteins during exposure to inhibitors/activators considered to be potential drugs.

Biosensors Based on Bio-Functionalized Semiconducting Metal Oxides

Immobilization of biomaterials is a very important task in the development of biofuel cells and biosensors. Some semiconducting metal-oxide-based supporting materials can be used in these bioelectronics-based devices. In this article, we are reviewing some functionalization methods that are applied for the immobilization of biomaterials. The most significant attention is paid to the immobilization of biomolecules on the surface of semiconducting metal oxides. The improvement of biomaterials immobilization on metal oxides and analytical performance of biosensors by coatings based on conducting polymers, self-assembled monolayers and lipid membranes is discussed.

AI-based atomic force microscopy image analysis allows to predict electrochemical impedance spectra of defects in tethered bilayer membranes

Atomic force microscopy (AFM) image analysis of supported bilayers, such as tethered bilayer membranes (tBLMs) can reveal the nature of the membrane damage by pore-forming proteins and predict the electrochemical impedance spectroscopy (EIS) response of such objects. However, automated analysis involving pore detection in such images is often non-trivial and can require AI-based object detection techniques. The specific object-detection algorithm we used to determine the defect coordinates in real AFM images was a convolutional neural network (CNN). Defect coordinates allow to predict the EIS response of tBLMs populated by the pore-forming toxins using finite element analysis (FEA) modeling. We tested if the accuracy of the CNN algorithm affected the EIS spectral features sensitive to defect densities and other physical parameters of tBLMs. We found that the EIS spectra can be predicted sufficiently well, however, systematic errors of characteristic spectral points were observed and need to be taken into account. Importantly, the comparison of predicted EIS curves with experimental ones allowed to estimate important physical parameters of tBLMs such as the specific resistance of submembrane reservoir. This reservoir separates phospholipid bilayer from the solid support. We found that the specific resistance of the reservoir amounts to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10^{4.25 \pm 0.10}$$\end{document}104.25±0.10\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Omega \cdot cm$$\end{document}Ω·cm which is approximately two orders of a magnitude higher compared to the specific resistance of the buffer bathing tBLMs studied in this work. We hypothesize that such effect may be related in part due to decreased concentration of ionic carriers in the submembrane due to decreased relative dielectric permittivity in this region.

Electrochemical Impedance Spectroscopy as a Convenient Tool to Characterize Tethered Bilayer Membranes

In this paper, we describe the application of electrochemical impedance spectroscopy (EIS) to characterize process of formation and properties of solid-supported tethered bilayer membranes on solid conducting substrates. Along with the description of experimental procedures to prepare substrates and self-assembly of phospholipid bilayers onto gold-coated glass slides, we describe the detailed protocols of EIS measurements. We demonstrate the utility of EIS in the evaluation of the properties of both molecular anchor layers used to immobilize tBLMs as well as characterization of tBLMs. We show that the EIS methodology extends the applicability of this technique well beyond the mere evaluation of electric parameters. Specifically, we demonstrate how by using EIS one may evaluate both density and size of water-filled defects (ion-channels) in tBLMs, to determine the structural mode (homogeneous, heterogeneous, or clustered) of distribution of defects in tBLMs. Our methodology can be applied in both basic protein membrane interaction studies, as well as in the development of precision biosensoric systems with tBLMs as a sensing element.

Hybrid bilayer membranes on metallurgical polished aluminum

In this work we describe the functionalization of metallurgically polished aluminum surfaces yielding biomimetic electrodes suitable for probing protein/phospholipid interactions. The functionalization involves two simple steps: silanization of the aluminum and subsequent fusion of multilamellar vesicles which leads to the formation of a hybrid bilayer lipid membrane (hBLM). The vesicle fusion was followed in real-time by fast Fourier transform electrochemical impedance spectroscopy (FFT EIS). The impedance-derived complex capacitance of the hBLMs was approximately 0.61 µF cm⁻², a value typical for intact phospholipid bilayers. We found that the hBLMs can be readily disrupted if exposed to > 400 nM solutions of the pore-forming peptide melittin. However, the presence of cholesterol at 40% (mol) in hBLMs exhibited an inhibitory effect on the membrane-damaging capacity of the peptide. The melittin-membrane interaction was concentration dependent decreasing with concentration. The hBLMs on Al surface can be regenerated multiple times, retaining their dielectric and functional properties essentially intact.

Electrochemical Properties of Lipid Membranes Self-Assembled from Bicelles

Supported lipid membranes are widely used platforms which serve as simplified models of cell membranes. Among numerous methods used for preparation of planar lipid films, self-assembly of bicelles appears to be promising strategy. Therefore, in this paper we have examined the mechanism of formation and the electrochemical properties of lipid films deposited onto thioglucose-modified gold electrodes from bicellar mixtures. It was found that adsorption of the bicelles occurs by replacement of interfacial water and it leads to formation of a double bilayer structure on the electrode surface. The resulting lipid assembly contains numerous defects and pinholes which affect the permeability of the membrane for ions and water. Significant improvement in morphology and electrochemical characteristics is achieved upon freeze–thaw treatment of the deposited membrane. The lipid assembly is rearranged to single bilayer configuration with locally occurring patches of the second bilayer, and the number of pinholes is substantially decreased. Electrochemical characterization of the lipid membrane after freeze–thaw treatment demonstrated that its permeability for ions and water is significantly reduced, which was manifested by the relatively high value of the membrane resistance.

Alzheimer's disease-related amyloid β peptide causes structural disordering of lipids and changes the electric properties of a floating bilayer lipid membrane

Neurodegeneration in Alzheimer's disease is associated with disruption of the neuronal cell membrane by the amyloid β (Aβ) peptide. However, the disruption mechanism and the resulting changes in membrane properties remain to be elucidated. To address this issue, herein the interaction of amyloid β monomers (AβMs) and amyloid β oligomers (AβOs) with a floating bilayer lipid membrane (fBLM) was studied using electrochemical and IR spectroscopy techniques. IR measurements showed that both Aβ forms interacted similarly with the hydrophobic membrane core (lipid acyl chains), causing conformational and orientational changes of the lipid acyl chains, thus decreasing acyl chain mobility and altering the lipid packing unit cell. In the presence of AβOs, these changes were more significant than those in the presence of AβMs. However, respective interactions of AβMs and AβOs with the membrane hydrophilic exterior (lipid heads) were quite different. AβMs dehydrated lipid heads without affecting their orientation while AβOs changed the orientation of lipid heads keeping their hydration level intact. Electrochemical measurements showed that only AβOs porated the fBLM, thus significantly changing the fBLM electrical properties. The present results provide new molecular-level insight into the mechanism of membrane destruction by AβOs and changes in the membrane properties.

Electrochemical Biosensors Based on Membrane-Bound Enzymes in Biomimetic Configurations

In nature, many enzymes are attached or inserted into the cell membrane, having hydrophobic subunits or lipid chains for this purpose. Their reconstitution on electrodes maintaining their natural structural characteristics allows for optimizing their electrocatalytic properties and stability. Different biomimetic strategies have been developed for modifying electrodes surfaces to accommodate membrane-bound enzymes, including the formation of self-assembled monolayers of hydrophobic compounds, lipid bilayers, or liposomes deposition. An overview of the different strategies used for the formation of biomimetic membranes, the reconstitution of membrane enzymes on electrodes, and their applications as biosensors is presented.

The shape of lipid molecules affects potential-driven molecular-scale rearrangements in model cell membranes on electrodes

Planar asymmetric lipid bilayers composed of phosphatidylethanolamine and phosphatidylglycerol lipids are transferred onto a gold electrode surface. Lipids containing two saturated, one monounsaturated and two monounsaturated hydrocarbon chains compose the model membranes. Results of electrochemically controlled polarization modulation infrared reflection absorption spectroscopy and quartz crystal microbalance with energy dissipation studies reveal two different types of electric potential-dependent structural rearrangements in the bilayers. They are correlated with the geometry of the lipid molecule. Packing parameter correlates the cross-section area of the hydrophobic and hydrophilic parts of amphiphilic molecules. In bilayers composed of lipids with the packing parameter <1, the hydrocarbon chains are tilted with respect to the bilayer plane and the polar head groups are well hydrated. At a threshold potential an abrupt flow of water through the bilayer is connected with membrane dehydration and upward orientation of the chains. In bilayers composed of lipids with packing parameter ≥1, electric potentials have negligible effect on the membrane structure. A simple rule correlating the packing parameter with molecular scale changes occurring at electrified membranes has a large diagnostic implication for biomimetic studies and our understanding of molecular processes occurring in biological cell membranes.

Kalata B1 and Kalata B2 Have a Surfactant-Like Activity in Phosphatidylethanolomine-Containing Lipid Membranes

Cyclotides are cyclic disulfide-rich peptides that are chemically and thermally stable and possess pharmaceutical and insecticidal properties. The activities reported for cyclotides correlate with their ability to target phosphatidylethanolamine (PE)-phospholipids and disrupt cell membranes. However, the mechanism by which this disruption occurs remains unclear. In the current study we examine the effect of the prototypic cyclotides, kalata B1 (kB1) and kalata B2 (kB2), on tethered lipid bilayer membranes (tBLMs) using swept frequency electrical impedance spectroscopy. We confirmed that kB1 and kB2 bind to bilayers only if they contain PE-phospholipids. We hypothesize that the increase in membrane conduction and capacitance observed upon addition of kB1 or kB2 is unlikely to result from ion channel like pores but is consistent with the formation of lipidic toroidal pores. This hypothesis is supported by the concentration dependence of effects of kB1 and kB2 being suggestive of a critical micelle concentration event rather than a progressive increase in conduction arising from increased channel insertion. Additionally, conduction behavior is readily reversible when the peptide is rinsed from the bilayer. Our results support a mechanism by which kB1 and kB2 bind to and disrupt PE-containing membranes by decreasing the overall membrane critical packing parameter, as would a surfactant, which then opens or increases the size of existing membrane defects. The cyclotides need not participate directly in the conductive pore but might exert their effect indirectly through altering membrane packing constraints and inducing purely lipidic conductive pores.