EIS and PM-IRRAS studies of alamethicin ion channels in a tethered lipid bilayer

Native Function of the Bacterial Ion Channel SthK in a Sparsely Tethered Lipid Bilayer Membrane Architecture

The plasma membrane protects the interiors of cells from their surroundings and also plays a critical role in communication, sensing, and nutrient import. As a result, the cell membrane and its constituents are among the most important drug targets. Studying the cell membrane and the processes it facilitates is therefore crucial, but it is a highly complex environment that is difficult to access experimentally. Various model membrane systems have been developed to provide an environment in which membrane proteins can be studied in isolation. Among them, tethered bilayer lipid membranes (tBLMs) are a promising model system providing a solvent-free membrane environment which can be prepared by self-assembly, is resistant to mechanical disturbances and has a high electrical resistance. tBLMs are therefore uniquely suitable to study ion channels and charge transport processes. However, ion channels are often large, complex, multimeric structures and their function requires a particular lipid environment. In this paper, we show that SthK, a bacterial cyclic nucleotide gated (CNG) ion channel that is strongly dependent on the surrounding lipid composition, functions normally when embedded into a sparsely tethered lipid bilayer. As SthK has been very well characterized in terms of structure and function, it is well-suited to demonstrate the utility of tethered membrane systems. A model membrane system suitable for studying CNG ion channels would be useful, as this type of ion channel performs a wide range of physiological functions in bacteria, plants, and mammals and is therefore of fundamental scientific interest as well as being highly relevant to medicine.

Multimodal Investigation into the Interaction of Quinacrine with Microcavity-Supported Lipid Bilayers

Quinacrine is a versatile drug that is widely recognized for its antimalarial action through its inhibition of the phospholipase enzyme. It also has antianthelmintic and antiprotozoan activities and is a strong DNA binder that may be used to combat multidrug resistance in cancer. Despite extensive cell-based studies, a detailed understanding of quinacrine's influence on the cell membrane, including permeability, binding, and rearrangement at the molecular level, is lacking. Herein, we apply microcavity-suspended lipid bilayers (MSLBs) as in vitro models of the cell membrane comprising DOPC, DOPC:Chol(3:1), and DOPC:SM:Chol(2:2:1) to investigate the influence of cholesterol and intrinsic phase heterogeneity induced by mixed-lipid composition on the membrane interactions of quinacrine. Using electrochemical impedance spectroscopy (EIS) and surface-enhanced Raman spectroscopy (SERS) as label-free surface-sensitive techniques, we have studied quinacrine interaction and permeability across the different MSLBs. Our EIS data reveal that the drug is permeable through ternary DOPC:SM:Chol and DOPC-only bilayer compositions. In contrast, the binary cholesterol/DOPC membrane arrested permeation, yet the drug binds or intercalates at this membrane as reflected by an increase in membrane impedance. SERS supported the EIS data, which was utilized to gain structural insights into the drug-membrane interaction. Our SERS data also provides a simple but powerful label-free assessment of drug permeation because a significant SERS enhancement of the drug's Raman signature was observed only if the drug accessed the plasmonic interior of the pore cavity passing through the membrane. Fluorescent lifetime correlation spectroscopy (FLCS) provides further biophysical insight, revealing that quinacrine binding increases the lipid diffusivity of DOPC and the ternary membrane while remarkably decreasing the lipid diffusivity of the DOPC:Chol membrane. Overall, because of its adaptability to multimodal approaches, the MSLB platform provides rich and detailed insights into drug-membrane interactions, making it a powerful tool for in vitro drug screening.

Designing a Useful Lipid Raft Model Membrane for Electrochemical and Surface Analytical Studies

A model biomimetic system for the study of protein reconstitution or drug interactions should include lipid rafts in the mixed lipid monolayer, since they are usually the domains embedding membrane proteins and peptides. Four model lipid films composed of three components: 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), cholesterol (Chol) and sphingomyelin (SM) mixed in different molar ratios were proposed and investigated using surface pressure measurements and thermodynamic analysis of the monolayers at the air-water interface and imaged by Brewster angle microscopy. The ternary monolayers were transferred from the air-water onto the gold electrodes to form bilayer films and were studied for the first time by electrochemical methods: alternative current voltammetry and electrochemical impedance spectroscopy and imaged by atomic force microscopy. In excess of DOPC, the ternary systems remained too liquid for the raft region to be stable, while in the excess of cholesterol the layers were too solid. The layers with SM in excess lead to the formation of Chol:SM complexes but the amount of the fluid matrix was very low. The equimolar content of the three components lead to the formation of a stable and well-organized assembly with well-developed raft microdomains of larger thickness, surrounded by the more fluid part of the bilayer. The latter is proposed as a convenient raft model membrane for further physicochemical studies of interactions with drugs or pollutants or incorporation of membrane proteins.

Biomembranes in bioelectronic sensing

Cell membranes are integral to the functioning of the cell and are therefore key to drive fundamental understanding of biological processes for downstream applications. Here, we review the current state-of-the-art with respect to biomembrane systems and electronic substrates, with a view of how the field has evolved towards creating biomimetic conditions and improving detection sensitivity. Of particular interest are conducting polymers, a class of electroactive polymers, which have the potential to create the next step-change for bioelectronics devices. Lastly, we discuss the impact these types of devices could have for biomedical applications.

Interaction of LL-37 human cathelicidin peptide with a model microbial-like lipid membrane

The only representative of cathelicidin peptides in humans is LL-37, a multifunctional antimicrobial peptide (AMP) that is a part of the innate immune response. Details of the LL-37 direct activity against pathogens are not well understood at the molecular level. Here, we present research on the mechanism of interaction between LL-37 and a model multicomponent bilayer lipid membrane (BLM), mimicking microbial cell membrane. Electrochemical impedance spectroscopy (EIS), high-resolution atomic force microscopy (AFM) imaging, and polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) were applied to study the peptide influence on a model microbial-like membrane. We show that LL-37 causes changes in the phospholipid molecules conformation and orientation, leading to membrane disintegration, significantly affecting the membrane electrical parameters, such as capacitance and resistance. High-resolution AFM imaging shows topographical and mechanical effects of such disintegration, while PM-IRRAS data indicates that introduction of LL-37 causes changes in the phospholipid acyl chains from all-trans to gauche conformations. Moreover, the presence of LL-37 significantly alters the value of the phospholipid tilt angle. Altogether, our results suggest a "carpet" membrane dissolution followed by a detergent-like membrane disruption mechanism upon LL-37 activity. This research gives a novel insight into the understanding of LL-37 influence on multicomponent model membranes and a promising contribution to the development of LL-37-derived therapeutic agents against drug-resistant bacteria.

Toxicity of selected airborne nitrophenols on eukaryotic cell membrane models

Nitroaromatics belong to the group of toxic components of aerosol particles and atmospheric hydrometeors that enter the atmosphere through biomass burning and fuel combustion. In the present work, we report on the cytotoxic effects of a 2-, 3- and 4-nitrophenol mixture on a model eukaryotic-like cell membrane and compared it with in vitro cellular models BEAS-2B (immortalized bronchial epithelial cells) and A549 (cancerous alveolar epithelial cells). A selected model biomembrane comprised of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine), DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) was studied. The electrochemical-based method, combined with atomic force microscopy (AFM) and phase-contrast microscopy imaging, allowed to get insights into the mechanism of cellular function disruption caused by airborne nitrophenols. The efficacy of the method is supported by the data obtained from in vitro experiments performed on cell models. The nitrophenol mixture exhibited cytotoxic effects at concentrations above 100 μg mL^-1, as demonstrated by phase-contrast microscopy in real lung cell lines. Electrochemical impedance spectroscopy (EIS) revealed the formation of membrane defects at a nitrophenol concentration of 200 μg mL^-1. AFM imaging confirmed the model membrane disintegration and phospholipids rearrangement in the presence of nitrophenols. These observations indicate that particle-bound nitrophenols induce substantial changes in cell membranes and make them more permeable to aerosol, resulting in major cellular damage in the lungs when inhaled. The study provides initial evidence of cellular membrane damage induced by three important nitrated phenols present in the environment.

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.

Effects of valinomycin doping on the electrical and structural properties of planar lipid bilayers supported on polyelectrolyte multilayers

Supported Lipid Bilayers (SLBs) on Polyelectrolyte Multilayers (PEMs) have large potential as models for developing sensor devices. SLBs can be designed with receptors and channels, which benefit from the biological environment of the lipid layers, to create a sensing interface for ions and biomarkers. PEMs assembled by the Layer-by-Layer (LBL) technique and used as supports for a lipid bilayer enable an easy integration of the bilayer on almost any surface and device. For electrochemical sensors, LBL assembly enables nanoscale tunable separation of the lipid bilayer from the electrode surface, avoiding undesired effects of the electrode surface on the lipid bilayers. We study the fabrication of valinomycin-doped SLBs on PEMs as a model system for biophysical studies and for selective ion sensing. SLBs are fabricated from dioleoylphosphatidylcholine (DOPC) and dioleoylphosphatidylserine (DOPS) 50:50 vesicles doped with valinomycin, as a K⁺-selective carrier. SLBs were deposited on electrodes coated with poly(allyl amine hydrochloride) (PAH) and poly(styrene sodium sulfonate) (PSS) multilayers. Lipid bilayer formation was monitored by using Quartz Crystal Microbalance with Dissipation (QCMD) technique and Atomic Force Microscopy (AFM). Electrochemical impedance spectroscopy (EIS) and potentiometric measurements were performed to assess K⁺ selectivity over other ions and the potential of valinomycin-doped SLBs for K⁺-sensing.

Water Structure in the Submembrane Region of a Floating Lipid Bilayer: The Effect of an Ion Channel Formation and the Channel Blocker

The structure of water in the submembrane region of the bilayer of DPhPC floating (fBLM) on a monolayer of 1-thio-β-d-glucose (β-Tg)-modified gold nanoparticle film was studied by the surface-enhanced infrared absorption spectroscopy (SEIRAS). SEIRAS employs surface enhancement of the mean square electric field of the photon, which is acting on a few molecular layers above the film of gold nanoparticles. Therefore, it is uniquely suited to probe water molecules in the submembrane region and provides unique information concerning the structure of the hydrogen bond network of water surrounding the lipid bilayer. The IR spectra indicated that water with a strong hydrogen network is separating the membrane from the gold surface. This water is more ordered than the water in the bulk. When alamethicin, a peptide forming ion channels, is inserted into the membrane, the network is only slightly loosened. The addition of amiloride, an ion channel blocker, results in a significant decrease in the amount of water in the submembrane region. The remaining water has a significantly distorted hydrogen bond network. This study provides unique information about the effect of the ion channel on water transport across the bilayer. The electrode potential has a relatively small effect on water structure in the submembrane region. However, the IR studies demonstrated that water is less ordered at positive transmembrane potentials. The present results provide significant insight into the nature of hydration of a floating lipid bilayer on the gold electrode surface.

Mini Review on Antimicrobial Peptides, Sources, Mechanism and Recent Applications

Antimicrobial peptides in recent years have gained increased interest among scientists, health professionals and the pharmaceutical companies owing to their therapeutic potential. These are low molecular weight proteins with broad range antimicrobial and immuno modulatory activities against infectious bacteria (Gram positive and Gram negative), viruses and fungi. Inability of micro-organisms to develop resistance against most of the antimicrobial peptide has made them as an efficient product which can greatly impact the new era of antimicrobials. In addition to this these peptides also demonstrates increased efficacy, high specificity, decreased drug interaction, low toxicity, biological diversity and direct attacking properties. Pharmaceutical industries are therefore conducting appropriate clinical trials to develop these peptides as potential therapeutic drugs. More than 60 peptide drugs have already reached the market and several hundreds of novel therapeutic peptides are in preclinical and clinical development. Rational designing can be used further to modify the chemical and physical properties of existing peptides. This mini review will discuss the sources, mechanism and recent therapeutic applications of antimicrobial peptides in treatment of infectious diseases.

How Valinomycin Ionophores Enter and Transport K+ across Model Lipid Bilayer Membranes

Valinomycin, a cyclic peptide, was incorporated into a biomimetic lipid membrane tethered to the surface of a gold (111) electrode. Electrochemical impedance spectroscopy was used to study the ionophore properties of the peptide, and polarization modulation infrared reflection absorption spectroscopy was employed to determine the conformation and orientation of valinomycin in the membrane. The combination of these two techniques provided unique information about the ionophore mechanism where valinomycin transports ions across the membrane by creating a complex with potassium ions and forming an ion pair with a counter anion. The ion pair resides within the hydrophobic fragment of the membrane and adopts a small angle of ∼22° with respect to the surface normal. This novel study provides new insights explaining the valinomycin ion transport mechanism in model biological membranes.

Effects of Amiloride, an Ion Channel Blocker, on Alamethicin Pore Formation in Negatively Charged, Gold-Supported, Phospholipid Bilayers: A Molecular View

The effects of amiloride on the structure and conductivity of alamethicin ion pore formation within negatively charged, gold-supported, 1,2-dimyristoyl- sn-glycero-3-phosphocholine/Egg-PG membranes were investigated with the help of electrochemical impedance spectroscopy (EIS), photon polarization modulation-infrared reflection spectroscopy (PM-IRRAS), and atomic force microscopy (AFM). The EIS results indicate that ion conductivity across negatively charged phospholipid bilayers containing alamethicin decreases by an order of magnitude when amiloride is introduced to the system. Despite the reduction in ion conductivity, the PM-IRRAS data shows that amiloride does not inhibit ion channel formation by alamethicin peptides. High-resolution AFM images revealed that amiloride enlarges and distorts the shape of alamethicin ion pores when introduced to the system, indicating that it is inserting itself into the mouth of the alamethicin pores. This effect is driven by electrostatic interactions between positively charged amiloride molecules and the negative charge on the membrane.

Pore Forming Properties of Alamethicin in Negatively Charged Floating Bilayer Lipid Membranes Supported on Gold Electrodes

Electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and photon polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) were employed to investigate the formation of alamethicin pores in negatively charged bilayers composed of a mixture of 1,2-dimyristoyl- sn-glycero-3-phosphocholine (DMPC) and egg-PG floating at gold (111) electrode surfaces modified by self-assembled monolayers of 1-thio-β-d-glucose (β-Tg). The EIS data showed that the presence of alamethicin decreases the membrane resistivity by about 1 order of magnitude. PM-IRRAS measurements provided information about the tilt angles of peptide helical axis with respect to the bilayer normal. The small tilt angles obtained for the peptide helical axis prove that the alamethicin molecules were inserted into the DMPC/egg-PG membranes. The tilt angles decreased when negative potentials were applied, which correlates with the observed decrease in membrane resistivity, indicating that ion pore formation is assisted by the transmembrane potential. Molecular resolution AFM images provided visual evidence that alamethicin molecules aggregate forming hexagonal porous 2D lattices with periodicities of 2.0 ± 0.2 nm. The pore formation by alamethicin in the negatively charged membrane was compared with the interaction of this peptide with a bilayer formed by zwitterionic lipids. The comparison of these results showed that alamethicin preferentially forms ion translocating pores in negatively charged phospholipid membranes.

Role of Transmembrane Potential and Defects on the Permeabilization of Lipid Bilayers by Alamethicin, an Ion-Channel-Forming Peptide

The insertion and ion-conducting channel properties of alamethicin reconstituted into a 1,2-di- O-phytanyl- sn-glycero-3-phosphocholine bilayer floating on the surface of a gold (111) electrode modified with a 1-thio-β-d-glucose (β-Tg) self-assembled monolayer were investigated using a combination of electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The hydrophilic β-Tg monolayer separated the bilayer from the gold substrate and created a water-rich spacer region, which better represents natural cell membranes. The EIS measurements acquired information about the membrane resistivity (a measure of membrane porosity), and the PM-IRRAS experiments provided insight into the conformation and orientation of the membrane constituents as a function of the transmembrane potential. The results showed that the presence of alamethicin had a small effect on the conformation and orientation of phospholipid molecules within the bilayer for all studied potentials. In contrast, the alamethicin peptides assumed a surface state, where the helical axes adopted a large tilt angle with respect to the surface normal, at small transmembrane potentials, and inserted into the bilayer at sufficiently negative transmembrane potentials forming pores, which behaved as barrel-stave ion channels for ionic transport across the membrane. The results indicated that insertion of alamethincin peptides into the bilayer was driven by the dipole-field interactions and that the transitions between the inserted and surface states were electrochemically reversible. Additionally, the EIS measurements performed on phospholipid bilayers without alamethicin also showed that the application of negative transmembrane potentials introduces defects into the bilayer. The membrane resistances measured in both the absence and presence of alamethicin show similar dependencies on the electrode potential, suggesting that the insertion of the peptide may also be assisted by the electroporation of the membrane. The findings in this study provide new insights into the mechanism of alamethicin insertion into phospholipid bilayers.

Monitoring the Orientational Changes of Alamethicin during Incorporation into Bilayer Lipid Membranes

Antimicrobial peptides (AMPs) are the first line of defense after contact of an infectious invader, for example, bacterium or virus, with a host and an integral part of the innate immune system of humans. Their broad spectrum of biological functions ranges from cell membrane disruption over facilitation of chemotaxis to interaction with membrane-bound or intracellular receptors, thus providing novel strategies to overcome bacterial resistances. Especially, the clarification of the mechanisms and dynamics of AMP incorporation into bacterial membranes is of high interest, and different mechanistic models are still under discussion. In this work, we studied the incorporation of the peptaibol alamethicin (ALM) into tethered bilayer lipid membranes on electrodes in combination with surface-enhanced infrared absorption (SEIRA) spectroscopy. This approach allows monitoring the spontaneous and potential-induced ion channel formation of ALM in situ. The complex incorporation kinetics revealed a multistep mechanism that points to peptide-peptide interactions prior to penetrating the membrane and adopting the transmembrane configuration. On the basis of the anisotropy of the backbone amide I and II infrared absorptions determined by density functional theory calculations, we employed a mathematical model to evaluate ALM reorientations monitored by SEIRA spectroscopy. Accordingly, ALM was found to adopt inclination angles of ca. 69°-78° and 21° in its interfacially adsorbed and transmembrane incorporated states, respectively. These orientations can be stabilized efficiently by the dipolar interaction with lipid head groups or by the application of a potential gradient. The presented potential-controlled mechanistic study suggests an N-terminal integration of ALM into membranes as monomers or parallel oligomers to form ion channels composed of parallel-oriented helices, whereas antiparallel oligomers are barred from intrusion.