Formation and Characterization of Supported Lipid Bilayers Composed of Phosphatidylethanolamine and Phosphatidylglycerol by Vesicle Fusion, a Simple but Relevant Model for Bacterial Membranes

Phospholipid Epitaxial Assembly Behavior on a Hydrophobic Highly Ordered Pyrolytic Graphite Surface

Supported lipid bilayers (SLBs) are excellent models of cell membranes. However, most SLBs exist in the form of phospholipid molecules standing on a substrate, making it difficult to have a side view of the phospholipid membranes. In this study, the phospholipid striped lamella with the arrangement of their alkane tails lying on highly ordered pyrolytic graphite (HOPG) was constructed by a spin coating method. Atomic force microscopy and molecular dynamics simulations are utilized to study the self-assembly of phospholipids on HOPG. Results show that various phospholipids with different packing parameters and electrical property are able to epitaxially adsorb on HOPG. 0.1 mg/mL Plasm PC (0.1 mg/mL) could form a striped monolayer with a width of 5.93 ± 0.21 nm and form relatively stable four striped layers with the concentration increasing to 1 mg/mL. The width of the DOPS multilayer is more than that of electroneutral lipids due to the static electrical repulsion force. This universal strategy sheds light on direct observation of the membrane structure from the side view and modification of 2D materials with amphiphilic biomolecules.

Structural and functional insights into the delivery of a bacterial Rhs pore-forming toxin to the membrane

Bacterial competition is a significant driver of toxin polymorphism, which allows continual compensatory evolution between toxins and the resistance developed to overcome their activity. Bacterial Rearrangement hot spot (Rhs) proteins represent a widespread example of toxin polymorphism. Here, we present the 2.45 Å cryo-electron microscopy structure of Tse5, an Rhs protein central to Pseudomonas aeruginosa type VI secretion system-mediated bacterial competition. This structural insight, coupled with an extensive array of biophysical and genetic investigations, unravels the multifaceted functional mechanisms of Tse5. The data suggest that interfacial Tse5-membrane binding delivers its encapsulated pore-forming toxin fragment to the target bacterial membrane, where it assembles pores that cause cell depolarisation and, ultimately, bacterial death.

Latest Update on Outer Membrane Vesicles and Their Role in Horizontal Gene Transfer: A Mini-Review

Outer membrane vesicles (OMVs) are spherical, lipid-based nano-structures, which are released by Gram-negative bacteria in both in vitro and in vivo conditions. The size and composition of OMVs depend on not only the producer bacterial species but also cells belonging to the same strain. The mechanism of vesicles' biogenesis has a key role in determining their cargo and the pattern of macromolecules exposed on their surface. Thus, the content of proteins, lipids, nucleic acids, and other biomolecules defines the properties of OMVs and their beneficial or harmful effects on human health. Many studies have provided evidence that OMVs can be involved in a plethora of biological processes, including cell-to-cell communication and bacteria-host interactions. Moreover, there is a growing body of literature supporting their role in horizontal gene transfer (HGT). During this process, OMVs can facilitate the spreading of genes involved in metabolic pathways, virulence, and antibiotic resistance, guaranteeing bacterial proliferation and survival. For this reason, a deeper understanding of this new mechanism of genetic transfer could improve the development of more efficient strategies to counteract infections sustained by Gram-negative bacteria. In line with this, the main aim of this mini-review is to summarize the latest evidence concerning the involvement of OMVs in HGT.

Kinetics of Ganglioside-Rich Supported Lipid Bilayer Formation with Tracer Vesicle Fluorescence Imaging

Gangliosides, forming a class of lipids complemented by sugar chains, influence the lateral distribution of membrane proteins or membrane-binding proteins, act as receptors for viruses and bacterial toxins, and mediate several types of cellular signaling. Gangliosides incorporated into supported lipid bilayers (SLBs) have been widely applied as a model system to examine these biological processes. In this work, we explored how ganglioside composition affects the kinetics of SLB formation using the vesicle rupturing method on a solid surface. We imaged the attachment of vesicles and the subsequent SLB formation using the time-lapse total internal reflection fluorescence microscopy technique. In the early phase, the ganglioside type and concentration influence the adsorption kinetics of vesicles and their residence/lifetime on the surface before rupturing. Our data confirm that a simultaneous rupturing of neighboring surface-adsorbed vesicles forms microscopic lipid patches on the surface and it is triggered by a critical coverage of the vesicles independent of their composition. In the SLB growth phase, lipid patches merge, forming a continuous SLB. The propagation of patch edges catalyzes the process and depends on the ganglioside type. Our pH-dependent experiments confirm that the polar/charged head groups of the gangliosides have a critical role in these steps and phases of SLB formation kinetics.

Engineering phosphatidylinositol-4,5-bisphosphate model membranes enriched in endocytic cargo: A neutron reflectometry, AFM and QCM-D structural study

The combination of in vitro models of biological membranes based on solid-supported lipid bilayers (SLBs) and of surface sensitive techniques, such as neutron reflectometry (NR), atomic force microscopy (AFM) and quartz crystal microbalance with dissipation monitoring (QCM-D), is well suited to provide quantitative information about molecular level interactions and lipid spatial distributions. In this work, cellular plasma membranes have been mimicked by designing complex SLB, containing phosphatidylinositol 4,5-bisphosphate (PtdIns4,5P2) lipids as well as incorporating synthetic lipo-peptides that simulate the cytoplasmic tails of transmembrane proteins. The QCM-D results revealed that the adsorption and fusion kinetics of PtdIns4,5P2 are highly dependent of Mg2+. Additionally, it was shown that increasing concentrations of PtdIns4,5P2 leads to the formation of SLBs with higher homogeneity. The presence of PtdIns4,5P2 clusters was visualized by AFM. NR provided important insights about the structural organization of the various components within the SLB, highlighting that the leaflet symmetry of these SLBs is broken by the presence of CD4-derived cargo peptides. Finally, we foresee our study to be a starting point for more sophisticated in vitro models of biological membranes with the incorporation of inositol phospholipids and synthetic endocytic motifs.

Suspended phospholipid bilayers: A new biological membrane mimetic

HYPOTHESIS

The attractive interaction between a cationic surfactant monolayer at the air-water interface and vesicles, incorporating anionic lipids, is sufficient to drive the adsorption and deformation of the vesicles. Osmotic rupture of the vesicles produces a continuous lipid bilayer beneath the monolayer.

EXPERIMENTAL

Specular neutron reflectivity has been measured from the surface of a purpose-built laminar flow trough, which allows for rapid adsorption of vesicles, the changes in salt concentration required for osmotic rupture of the adsorbed vesicles into a bilayer, and for neutron contrast variation of the sub-phase without disturbing the monolayer.

FINDINGS

The neutron reflectivity profiles measured after vesicle addition are consistent with the adsorption and flattening of the vesicles beneath the monolayer. An increase in the buffer salt concentration results in further flattening and fusion of the adsorbed vesicles, which are ruptured by a subsequent decrease in the salt concentration. This process results in a continuous, high coverage, bilayer suspended 11 Åbeneath the monolayer. As the bilayer is not constrained by a solid substrate, this new mimetic is well-suited to studying the structure of lipid bilayers that include transmembrane proteins.

Facial amphiphilicity index correlating chemical structures with antimicrobial efficacy

Facial amphiphilicity is an extraordinary chemical structure feature of a variety of antimicrobial peptides and polymers. Vast efforts have been dedicated to small molecular, macromolecular and dendrimer-like systems to mimic this highly preferred structure or conformation, including local facial amphiphilicity and global amphiphilicity. This work conceptualizes Facial Amphiphilicity Index (FAI) as a numerical value to quantitatively characterize the measure of chemical compositions and structural features in dictating antimicrobial efficacy. FAI is a ratio of numbers of charges to rings, representing both compositions of hydrophilicity and hydrophobicity. Cationic derivatives of multicyclic compounds were evaluated as model systems for testing antimicrobial selectivity against Gram-negative and Gram-positive bacteria. Both monocyclic and bicyclic compounds are non-antimicrobial regardless of FAIs. Antimicrobial efficacy was observed with systems having larger cross-sectional areas including tricyclic abietic acid and tetracyclic bile acid. While low and high FAIs respectively lead to higher and lower antimicrobial efficacy, in consideration of cytotoxicity, the sweet spot is typically suited with intermediate FAIs for each specific system. This can be well explained by the synergistic hydrophobic-hydrophobic and electrostatic interactions with bacterial cell membranes and the difference between bacterial and mammalian cell membranes. The adoption of FAI would pave a new avenue toward the design of next-generation antimicrobial macromolecules and peptides.

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Established a numerical index to quantify the effect of facial amphiphilicity on antimicrobial efficacy.

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Evaluated the facial amphiphilicity index of multicyclic compounds possessing various rings and cationic charges.

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Provided this index a new tool toward more quantitative designs of AMP mimics.

Bacterial Membrane Mimetics: From Biosensing to Disease Prevention and Treatment

Plasma membrane mimetics can potentially play a vital role in drug discovery and immunotherapy owing to the versatility to assemble facilely cellular membranes on surfaces and/or nanoparticles, allowing for direct assessment of drug/membrane interactions. Recently, bacterial membranes (BMs) have found widespread applications in biomedical research as antibiotic resistance is on the rise, and bacteria-associated infections have become one of the major causes of death worldwide. Over the last decade, BM research has greatly benefited from parallel advancements in nanotechnology and bioelectronics, resulting in multifaceted systems for a variety of sensing and drug discovery applications. As such, BMs coated on electroactive surfaces are a particularly promising label-free platform to investigate interfacial phenomena, as well as interactions with drugs at the first point of contact: the bacterial membrane. Another common approach suggests the use of lipid-coated nanoparticles as a drug carrier system for therapies for infectious diseases and cancer. Herein, we discuss emerging platforms that make use of BMs for biosensing, bioimaging, drug delivery/discovery, and immunotherapy, focusing on bacterial infections and cancer. Further, we detail the synthesis and characteristics of BMs, followed by various models for utilizing them in biomedical applications. The key research areas required to augment the characteristics of bacterial membranes to facilitate wider applicability are also touched upon. Overall, this review provides an interdisciplinary approach to exploit the potential of BMs and current emerging technologies to generate novel solutions to unmet clinical needs.

Solid and Liquid Surface-Supported Bacterial Membrane Mimetics as a Platform for the Functional and Structural Studies of Antimicrobials

Increasing antibiotic resistance has provoked the urgent need to investigate the interactions of antimicrobials with bacterial membranes. The reasons for emerging antibiotic resistance and innovations in novel therapeutic approaches are highly relevant to the mechanistic interactions between antibiotics and membranes. Due to the dynamic nature, complex compositions, and small sizes of native bacterial membranes, bacterial membrane mimetics have been developed to allow for the in vitro examination of structures, properties, dynamics, and interactions. In this review, three types of model membranes are discussed: monolayers, supported lipid bilayers, and supported asymmetric bilayers; this review highlights their advantages and constraints. From monolayers to asymmetric bilayers, biomimetic bacterial membranes replicate various properties of real bacterial membranes. The typical synthetic methods for fabricating each model membrane are introduced. Depending on the properties of lipids and their biological relevance, various lipid compositions have been used to mimic bacterial membranes. For example, mixtures of phosphatidylethanolamines (PE), phosphatidylglycerols (PG), and cardiolipins (CL) at various molar ratios have been used, approaching actual lipid compositions of Gram-positive bacterial membranes and inner membranes of Gram-negative bacteria. Asymmetric lipid bilayers can be fabricated on solid supports to emulate Gram-negative bacterial outer membranes. To probe the properties of the model bacterial membranes and interactions with antimicrobials, three common characterization techniques, including quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), and neutron reflectometry (NR) are detailed in this review article. Finally, we provide examples showing that the combination of bacterial membrane models and characterization techniques is capable of providing crucial information in the design of new antimicrobials that combat bacterial resistance.

Antimicrobial Polymers of Linear and Bottlebrush Architecture: Probing the Membrane Interaction and Physicochemical Properties

Polymeric antimicrobial peptide mimics are a promising alternative for the future management of the daunting problems associated with antimicrobial resistance. However, the development of successful antimicrobial polymers (APs) requires careful control of factors such as amphiphilic balance, molecular weight, dispersity, sequence, and architecture. While most of the earlier developed APs focus on random linear copolymers, the development of APs with advanced architectures proves to be more potent. It is recently developed multivalent bottlebrush APs with improved antibacterial and hemocompatibility profiles, outperforming their linear counterparts. Understanding the rationale behind the outstanding biological activity of these newly developed antimicrobials is vital to further improving their performance. This work investigates the physicochemical properties governing the differences in activity between linear and bottlebrush architectures using various spectroscopic and microscopic techniques. Linear copolymers are more solvated, thermo-responsive, and possess facial amphiphilicity resulting in random aggregations when interacting with liposomes mimicking Escheria coli membranes. The bottlebrush copolymers adopt a more stable secondary conformation in aqueous solution in comparison to linear copolymers, conferring rapid and more specific binding mechanism to membranes. The advantageous physicochemical properties of the bottlebrush topology seem to be a determinant factor in the activity of these promising APs.

Structural Organization of Cardiolipin-Containing Vesicles as Models of the Bacterial Cytoplasmic Membrane

The bacterial cytoplasmic membrane is the innermost bacterial membrane and is mainly composed of three different phospholipid species, i.e., phosphoethanolamine (PE), phosphoglycerol (PG), and cardiolipin (CL). In particular, PG and CL are responsible for the negative charge of the membrane and are often the targets of cationic antimicrobial agents. The growing resistance of bacteria toward the available antibiotics requires the development of new and more efficient antibacterial drugs. In this context, studying the physicochemical properties of the bacterial cytoplasmic membrane is pivotal for understanding drug-membrane interactions at the molecular level as well as for designing drug-testing platforms. Here, we discuss the preparation and characterization of PE/PG/CL vesicle suspensions, which contain all of the main lipid components of the bacterial cytoplasmic membrane. The vesicle suspensions were characterized by means of small-angle neutron scattering, dynamic light scattering, and electron paramagnetic spectroscopy. By combining solution scattering and spectroscopy techniques, we propose a detailed description of the impact of different CL concentrations on the structure and dynamics of the PE/PG bilayer. CL induces the formation of thicker bilayers, which exhibit higher curvature and are overall more fluid. The experimental results contribute to shed light on the structure and dynamics of relevant model systems of the bacterial cytoplasmic membrane.

A Study of the Interaction of a New Benzimidazole Schiff Base with Synthetic and Simulated Membrane Models of Bacterial and Mammalian Membranes

Biological membranes are complex dynamic systems composed of a great variety of carbohydrates, lipids, and proteins, which together play a pivotal role in the protection of organisms and through which the interchange of different substances is regulated in the cell. Given the complexity of membranes, models mimicking them provide a convenient way to study and better understand their mechanisms of action and their interactions with biologically active compounds. Thus, in the present study, a new Schiff base (Bz-Im) derivative from 2-(m-aminophenyl)benzimidazole and 2,4-dihydroxybenzaldehyde was synthesized and characterized by spectroscopic and spectrometric techniques. Interaction studies of (Bz-Im) with two synthetic membrane models prepared with 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and DMPC/1,2-dimyristoyl-sn-glycero-3-phosphoglycerol (DMPG) 3:1 mixture, imitating eukaryotic and prokaryotic membranes, respectively, were performed by applying differential scanning calorimetry (DSC). Molecular dynamics simulations were also developed to better understand their interactions. In vitro and in silico assays provided approaches to understand the effect of Bz-Im on these lipid systems. The DSC results showed that, at low compound concentrations, the effects were similar in both membrane models. By increasing the concentration of Bz-Im, the DMPC/DMPG membrane exhibited greater fluidity as a result of the interaction with Bz-Im. On the other hand, molecular dynamics studies carried out on the erythrocyte membrane model using the phospholipids POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine), SM (N-(15Z-tetracosenoyl)-sphing-4-enine-1-phosphocholine), and POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) revealed that after 30 ns of interaction, both hydrophobic interactions and hydrogen bonds were responsible for the affinity of Bz-Im for PE and SM. The interactions of the imine with POPG (1-Palmitoyl-2-Oleoyl-sn-Glycero-3-Phosphoglycerol) in the E. coli membrane model were mainly based on hydrophobic interactions.

Interaction of cationic surfactants with DPPC membranes: effect of a novel Nα-benzoylated arginine-based compound

Cationic amino acid-based surfactants are known to interact with the lipid bilayer of microorganism resulting in cell death through a disruption of the membrane topology. To elucidate the interaction of a cationic surfactant synthesized in our lab, investigations involving N^α-benzoyl-arginine decyl amide (Bz-Arg-NHC₁₀), and model membranes composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) were done. Bz-Arg-NHC₁₀was able to penetrate into DPPC monolayers up to a critical pressure of 59.6 mN m^-1. Differential scanning calorimetry revealed that as the concentration of Bz-Arg-NHC₁₀ increased, the main transition temperature of DPPC slightly decreased. Atomic force microscopy (AFM) in situ experiments performed on supported DPPC bilayers on mica allowed monitoring the changes induced by Bz-Arg-NHC₁₀. DPPC bilayer patches were partially removed, mainly in borders and bilayer defects for 50 µM Bz-Arg-NHC₁₀ solution. Increasing the concentration to 100 µM resulted in a complete depletion of the supported bilayers. Surface plasmon resonance (SPR) experiments, carried out with fully DPPC bilayers covered chips, showed a net increase of the SPR signal, which can be explained by Bz-Arg-NHC₁₀ adsorption. When patchy DPPC bilayers were formed on the substrate, a SPR signal net decrease was obtained, which is consistent with the phospholipids' removal observed in the AFM images. The results obtained suggest that the presence of the benzoyl group attached to the polar head of our compound would be the responsible of the increased antimicrobial activity against gram-negative bacteria when compared with other arginine-based surfactants.

Composition effects on photooxidative membrane destabilization by TiO2 nanoparticles

Membrane interactions and photooxidative membrane destabilization of titanium dioxide (TiO₂) nanoparticles were investigated, focusing on the effects of membrane composition, notably phospholipid headgroup charge and presence of cholesterol. For this, we employed a battery of state-of-the-art methods for studies of bilayers formed by zwitterionic palmitoyloleoylphosphatidylcholine (POPC) containing also polyunsaturated palmitoylarachidonoylphosphocholine (PAPC), as well as its mixtures with anionic palmitoyloleoylphosphatidylglycerol (POPG) and cholesterol. It was found that the TiO₂ nanoparticles display close to zero charge at pH 7.4, resulting in aggregation. At pH 3.4, in contrast, the 6 nm TiO₂ nanoparticles are well dispersed due to a strongly positive ζ-potential. Mirroring this pH dependence, TiO₂ nanoparticles were observed to bind to negatively charged lipid bilayers at pH 3.4, but much less so at pH 7.4. While nanoparticle binding has some destabilizing effect alone, illumination with ultraviolet (UV) light accentuates membrane destabilization, a result of oxidative stress caused by generated reactive oxygen species (ROS). Neutron reflectivity (NR), quartz crystal microbalance (QCM), and small-angle X-ray scattering (SAXS) results all demonstrate that membrane composition strongly influences membrane interactions and photooxidative destabilization of lipid bilayers. In particular, the presence of anionic POPG makes the bilayers more sensitive to oxidative destabilization, whereas a stabilizing effect was observed in the presence of cholesterol. Also, structural aspects of peroxidation were found to depend strongly on membrane composition, notably the presence of anionic phospholipids. The results show that membrane interactions and UV-induced ROS generation act in concert and need to be considered together to understand effects of lipid membrane composition on UV-triggered oxidative destabilization by TiO₂ nanoparticles, e.g., in the context of oxidative damage of bacteria and cells.

Impact of antimicrobial peptides on E. coli-mimicking lipid model membranes: correlating structural and dynamic effects using scattering methods

Using X-rays and neutrons we address the effect of AMPs on structure and dynamics of lipids in bacterial model membranes.

Understanding the Structure and Dynamics of Complex Biomembrane Interactions by Neutron Scattering Techniques

The membrane is one of the key structural materials of biology at the cellular level. Composed predominantly of a bilayer of lipids with embedded and bound proteins, it defines the boundaries of the cell and many organelles essential to life and therefore is involved in almost all biological processes. Membrane-specific interactions, such as drug binding to a membrane receptor or the interactions of an antimicrobial compound with the lipid matrix of a pathogen membrane, are of interest across the scientific disciplines. Herein we present a review, aimed at nonexperts, of the major neutron scattering techniques used in membrane studies: small-angle neutron scattering, neutron membrane diffraction, neutron reflectometry, quasielastic neutron scattering, and neutron spin echo. Neutron scattering techniques are well suited to studying biological membranes. The nondestructive nature of cold neutrons means that samples can be measured for long periods without fear of beam damage from ultraviolet, electron, or X-ray radiation, and neutron beams are highly penetrating, thus offering flexibility in samples and sample environments. Most important is the strong difference in neutron scattering lengths between the two most abundant forms of hydrogen, protium and deuterium. Changing the relative amounts of protium/deuterium in a sample allows the production of a series of neutron scattering data sets, enabling the observation of differing components within complex membrane architectures. This approach can be as simple as using the naturally occurring neutron contrast between different biomolecules to study components in a complex by changing the solution H₂O/D₂O ratio or as complex as selectively labeling individual components with hydrogen isotopes. This review presents an overview of each experimental technique with the neutron instrument configuration, related sample preparation and sample environment, and data analysis, highlighted by a special emphasis on using prominent neutron contrast to understand structure and dynamics. This review gives researchers a practical introduction to the often enigmatic suite of neutron beamlines, thereby lowering the barrier to taking advantage of these large-facility techniques to achieve new understandings of membranes and their interactions with other molecules.

Peptide discs as precursors of biologically relevant supported lipid bilayers

Supported lipid bilayers (SLBs) are commonly used to investigate the structure and dynamics of biological membranes. Vesicle fusion is a widely exploited method to produce SLBs. However, this process becomes less favoured when the vesicles contain complex lipid mixtures, e.g. natural lipid extracts. In these cases, it is often necessary to change experimental parameters, such as temperature, to unphysiological values to trigger the SLB formation. This may induce lipid degradation and is also not compatible with including membrane proteins or other biomolecules into the bilayers. Here, we show that the peptide discs, ~10 nm discoidal lipid bilayers stabilized in solution by a self-assembled 18A peptide belt, can be used as precursors for SLBs. The characterizations by means of neutron reflectometry and attenuated total reflectance-FTIR spectroscopy show that SLBs were successfully formed both from synthetic lipid mixtures (surface coverage 90-95%) and from natural lipid mixtures (surface coverage ~85%). Traces of 18A peptide (below 0.02 M ratio) left at the support surface after the bilayer formation do not affect the SLB structure. Altogether, we demonstrate that peptide disc formation of SLBs is much faster than the SLB formation by vesicle fusion and without the need of altering any experimental variable from physiologically relevant values.

Structure, Formation, and Biological Interactions of Supported Lipid Bilayers (SLB) Incorporating Lipopolysaccharide

Biomimetic membrane systems play a crucial role in the field of biosensor engineering. Over the years, significant progress has been achieved creating artificial membranes by various strategies from vesicle fusion to Langmuir transfer approaches to meet an ever-growing demand for supported lipid bilayers on various substrates such as glass, mica, gold, polymer cushions, and many more. This paper reviews the diversity seen in the preparation of biologically relevant model lipid membranes which includes monolayers and bilayers of phospholipid and other crucial components such as proteins, characterization techniques, changes in the physical properties of the membranes during molecular interactions and the dynamics of the lipid membrane with biologically active molecules with special emphasis on lipopolysaccharides (LPS).

Role of lipopolysaccharides and lipoteichoic acids on C-Chrysophsin-1 interactions with model Gram-positive and Gram-negative bacterial membranes

Antimicrobial peptides (AMPs) are attractive as biomaterial coatings because they have broad spectrum activity against different microbes, with a low likelihood of incurring antimicrobial resistance. Direct action against the bacterial membrane is the most common mechanism of action (MOA) of AMPs, with specific MOAs dependent on membrane composition, peptide concentration, and environmental factors that include temperature. Chrysophsin-1 (CHY1) is a broad spectrum salt-tolerant AMP that is derived from a marine fish. A cysteine modification was made to the peptide to facilitate attachment to a surface, such as a biomedical device. The authors used quartz crystal microbalance with dissipation monitoring to study how temperature (23 and 37 °C) and lipid composition influence the MOA of cysteine-modified peptide (C-CHY1) with model membranes comprised of supported lipid bilayers (SLBs). These two temperatures were used so that the authors could better understand the differences in behavior between typical lab temperatures and physiologic conditions. The authors created model membranes that mimicked properties of Gram-negative and Gram-positive bacteria in order to understand how the mechanisms might differ for different types of bacterial systems. SLB models of Gram-positive bacterial membranes were formed using combinations of phosphatidylcholine, phosphatidylglycerol (PG), and S. aureus-derived lipoteichoic acid (LTA). SLB models of Gram-negative bacterial membranes were formed using combinations of phosphatidylethanolamine (PE), PG, and E. coli-derived lipopolysaccharides (LPS). The molecules that distinguish Gram-positive and Gram-negative membranes (LTA and LPS) have the potential to alter the MOA of C-CHY1 with the SLBs. The authors' results showed that the MOA for the Gram-positive SLBs was not sensitive to temperature, but the LTA addition did have an effect. Specifically, similar trends in frequency and dissipation changes across all overtones were observed, and the same mechanistic trends were observed in the polar plots at 23 and 37 °C. However, when LTA was added, polar plots showed an association between C-CHY1 and LTA, leading to SLB saturation. This was demonstrated by significant changes in dissipation, while the frequency (mass) was not increasing after the saturation point. For the Gram-negative SLBs, the composition did not have a significant effect on MOA, but the authors saw more differences between the two temperatures studied. The authors believe this is due to the fact that the gel-liquid crystal transition temperature of PE is 25 °C, which means that the bilayer is more rigid at 23 °C, compared to temperatures above the transition point. At 23 °C, a significant energetic shift would be required to allow for additional AMP insertion. This could be seen in the polar plots, where there was a steep slope but there was very little mass addition. At 37 °C, the membrane is more fluid and there is less of an energetic requirement for insertion. Therefore, the authors observed greater mass addition and fewer changes in dissipation. A better understanding of C-CHY1 MOA using different SLB models will allow for the more rational design of future therapeutic solutions that make use of antimicrobial peptides, including those involving biomaterial coatings.

Design and use of model membranes to study biomolecular interactions using complementary surface-sensitive techniques

Cellular membranes are complex structures and simplified analogues in the form of model membranes or biomembranes are used as platforms to understand fundamental properties of the membrane itself as well as interactions with various biomolecules such as drugs, peptides and proteins. Model membranes at the air-liquid and solid-liquid interfaces can be studied using a range of complementary surface-sensitive techniques to give a detailed picture of both the structure and physicochemical properties of the membrane and its resulting interactions. In this review, we will present the main planar model membranes used in the field to date with a focus on monolayers at the air-liquid interface, supported lipid bilayers at the solid-liquid interface and advanced membrane models such as tethered and floating membranes. We will then briefly present the principles as well as the main type of information on molecular interactions at model membranes accessible using a Langmuir trough, quartz crystal microbalance with dissipation monitoring, ellipsometry, atomic force microscopy, Brewster angle microscopy, Infrared spectroscopy, and neutron and X-ray reflectometry. A consistent example for following biomolecular interactions at model membranes is used across many of the techniques in terms of the well-studied antimicrobial peptide Melittin. The overall objective is to establish an understanding of the information accessible from each technique, their respective advantages and limitations, and their complementarity.

Supported Lipid Bilayer Formation: Beyond Vesicle Fusion

Supported lipid bilayers (SLBs) are cell-membrane-mimicking platforms that can be formed on solid surfaces and integrated with a wide range of surface-sensitive measurement techniques. SLBs are useful for unravelling details of fundamental membrane biology and biophysics as well as for various medical, biotechnology, and environmental science applications. Thus, there is high interest in developing simple and robust methods to fabricate SLBs. Currently, vesicle fusion is a popular method to form SLBs and involves the adsorption and spontaneous rupture of lipid vesicles on a solid surface. However, successful vesicle fusion depends on high-quality vesicle preparation, and it typically works with a narrow range of material supports and lipid compositions. In this Feature Article, we summarize current progress in developing two new SLB fabrication techniques termed the solvent-assisted lipid bilayer (SALB) and bicelle methods, which have compelling advantages such as simple sample preparation and compatibility with a wide range of material supports and lipid compositions. The molecular self-assembly principles underpinning the two strategies and important experimental parameters are critically discussed, and recent application examples are presented. Looking forward, we envision that these emerging SLB fabrication strategies can be widely adopted by specialists and nonspecialists alike, paving the way to enriching our understanding of lipid membrane properties and realizing new application possibilities.