Building biomimetic membrane at a gold electrode surface

Uncoated gold nanoparticles create fewer and less localized defects in model prokaryotic than in model eukaryotic lipid membranes

The rise of the populations of antibiotic resistant bacteria represents an increasing threat to human health. In addition to the synthesis of new antibiotics, which is an extremely expensive and time-consuming process, one of the ways to combat bacterial infections is the use of gold nanoparticles (Au NPs) as the vehicles for targeted delivery of therapeutic drugs. Since such a strategy requires the investigation of the effect of Au NPs (with and without drugs) on both bacterial and human cells, we investigated how the presence of coating-free Au NPs affects the physicochemical properties of lipid membranes that model prokaryotic (PRO) and eukaryotic (EU) cells. PRO/EU systems prepared as multilamellar liposomes (MLVs) and hybrid structures (HSs) from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylglycerol (DPPG)/1,2-dipalmitoyl-sn-glycero-3-phosphoserine (DPPS) in the absence (MLVs)/presence (HSs) of differently distributed Au NPs (sizes ∼20 nm) reported stabilization of the gel phase of PRO systems in comparison with EU one (DSC data of PRO/EU were Tm(MLVs) ≈ 41.8 °C/42.0 °C, Tm¯ (HSs) ≈ 43.1 °C/42.4 °C, whereas UV-Vis response Tm(MLVs) ≈ 41.5 °C/42.0 °C, Tm¯ (HSs) ≈ 42.9 °C/41.1 °C). Vibrational spectroscopic data unraveled a substantial impact of Au NPs on the non-polar part of lipid bilayers, emphasizing the increase of kink and gauche conformers of the hydrocarbon chain. By interpreting the latter as Au NPs-induced defects, which exert the greatest effect when Au NPs are found exclusively outside the lipid membrane, these findings suggested that Au NPs reduced the compactness of EU-based lipid bilayers much more than in analogous PRO systems. Since the uncoated Au NPs manifested adverse effects when applied as antimicrobials, the results obtained in this work contribute towards recognizing AuNP functionalization as a strategy in tuning and reversing this effect.

Encapsulation of (-)-epigallocatechin gallate (EGCG) within phospholipid-based nanovesicles using W/O emulsion-transfer methods: Masking bitterness and delaying release of EGCG

A novel phospholipid-based nanovesicle (PBN) was developed to encapsulate (-)-epigallocatechin gallate (EGCG), a major polyphenol in green tea, to mask its bitter taste and expand its application in food products. The PBN was formed using W/O emulsion-transfer methods and showed a multilayer membrane nanovesicle structure (around 200 nm) observed with TEM. The PBN possessed a high encapsulation efficiency (92.1%) for EGCG. The bitterness of EGCG was significantly reduced to 1/12 after encapsulation. Fourier transform infrared spectroscopy (FTIR) indicated the EGCG mainly interacted with the upper chain/glycerol/head group region of the lipid bilayerin PBN. Quartz crystal microbalance with dissipation (QCM-D) showed the addition of γ-cyclodextrin in PBN enhanced EGCG's adsorption with phospholipids and allowed for its good sustained release. Encapsulating EGCG in PBN inhibited its complexation with mucin, reducing bitterness and astringency. This provides a new method to improve EGCG's flavor, potentially expanding its application in the food industry.

Effect of Anionic Lipids on Mammalian Plasma Cell Membrane Properties

The effect of lipid composition on models of the inner leaflet of mammalian cell membranes has been investigated. Grazing incidence X-ray diffraction and X-ray and neutron reflectivity have been used to characterize lipid packing and solvation, while electrochemical and infrared spectroscopic methods have been employed to probe phase behavior in an applied electric field. Introducing a small quantity of the anionic lipid dimyristoylphosphatidylserine (DMPS) into bilayers of zwitterionic dimyristoylphosphatidylethanolamine (DMPE) results in a significant change in the bilayer response to an applied field: the tilt of the hydrocarbon chains increases before returning to the original tilt angle on detachment of the bilayer. Equimolar mixtures, with slightly closer chain packing, exhibit a similar but weaker response. The latter also tend to incorporate more solvent during this electrochemical phase transition, at levels similar to those of pure DMPS. Reflectivity measurements reveal greater solvation of lipid layers for DMPS > 30 mol %, matching the greater propensity for DMPS-rich bilayers to incorporate water. Taken together, the data indicate that the range of 10-35 mol % DMPS provides optimum bilayer properties (in flexibility and function as a barrier), which may explain why the DMPS content of cell membranes tends to be found within this range.

Influence of the Lipid Backbone on Electrochemical Phase Behavior

Sphingolipids are an important class of lipids found in mammalian cell membranes with important structural and signaling roles. They differ from another major group of lipids, the glycerophospholipids, in the connection of their hydrocarbon chains to their headgroups. In this study, a combination of electrochemical and structural methods has been used to elucidate the effect of this difference on sphingolipid behavior in an applied electric field. N-Palmitoyl sphingomyelin forms bilayers of similar coverage and thickness to its close analogue di-palmitoyl phosphatidylcholine. Grazing incidence diffraction data show slightly closer packing and a smaller chain tilt angle from the surface normal. Electrochemical IR results at low charge density show that the difference in tilt angle is retained on deposition to form bilayers. The bilayers respond differently to increasing electric field strength: chain tilt angles increase for both molecules, but sphingomyelin chains remain tilted as field strength is further increased. This behavior is correlated with disruption of the hydrogen-bonding network of small groups of sphingomyelin molecules, which may have significance for the behavior of molecules in lipid rafts in the presence of strong fields induced by ion gradients or asymmetric distribution of charged lipids.

Cell-membrane-inspired polymers for constructing biointerfaces with efficient molecular recognition

Fabrication of devices that accurately recognize, detect, and separate target molecules from mixtures is a crucial aspect of biotechnology for applications in medical, pharmaceutical, and food sciences. This technology has also been recently applied in solving environmental and energy-related problems. In molecular recognition, biomolecules are typically complexed with a substrate, and specific molecules from a mixture are recognized, captured, and reacted. To increase sensitivity and efficiency, the activity of the biomolecules used for capture should be maintained, and non-specific reactions on the surface should be prevented. This review summarizes polymeric materials that are used for constructing biointerfaces. Precise molecular recognition occurring at the surface of cell membranes is fundamental to sustaining life; therefore, materials that mimic the structure and properties of this particular surface are emphasized in this article. The requirements for biointerfaces to eliminate nonspecific interactions of biomolecules are described. In particular, the major issue of protein adsorption on biointerfaces is discussed by focusing on the structure of water near the interface from a thermodynamic viewpoint; moreover, the structure of polymer molecules that control the water structure is considered. Methodologies enabling stable formation of these interfaces on material surfaces are also presented.

Effect of Molecular Structure on Electrochemical Phase Behavior of Phospholipid Bilayers on Au(111)

Lipid bilayers form the basis of biological cell membranes, selective and responsive barriers vital to the function of the cell. The structure and function of the bilayer are controlled by interactions between the constituent molecules and so vary with the composition of the membrane. These interactions also influence how a membrane behaves in the presence of electric fields they frequently experience in nature. In this study, we characterize the electrochemical phase behavior of dipalmitoylphosphatidylcholine (DPPC), a glycerophospholipid prevalent in nature and often used in model systems and healthcare applications. DPPC bilayers were formed on Au(111) electrodes using Langmuir-Blodgett and Langmuir-Schaefer deposition and studied with electrochemical methods, atomic force microscopy (AFM) and in situ polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). The coverage of the substrate determined with AFM is in accord with that estimated from differential capacitance measurements, and the bilayer thickness is slightly higher than for bilayers of the similar but shorter-chained lipid, dimyristoylphosphatidylcholine (DMPC). DPPC bilayers exhibit similar electrochemical response to DMPC bilayers, but the organization of molecules differs, particularly at negative charge densities. Infrared spectra show that DPPC chains tilt as the charge density on the metal is increased in the negative direction, but, unlike in DMPC, the chains then return to their original tilt angle at the most negative potentials. The onset of the increase in the chain tilt angle coincides with a decrease in solvation around the ester carbonyl groups, and the conformation around the acyl chain linkage differs from that in DMPC. We interpret the differences in behavior between bilayers formed from these structurally similar lipids in terms of stronger dispersion forces between DPPC chains and conclude that relatively subtle changes in molecular structure may have a significant impact on a membrane's response to its environment.

Proton Removal Kinetics That Govern the Hydrogen Peroxide Oxidation Activity of Heterogeneous Bioinorganic Platforms

Precise regulation of proton-coupled electron-transfer (PCET) rates holds the key to simultaneously optimizing the turnover frequency and product selectivity of redox reactions that are central to the realization of renewable energy schemes in a sustainable future. In this work, a self-assembled monolayer (SAM) of a Ru complex electrografted onto a glassy carbon (GC) electrode was prepared as a heterogeneous electrocatalytic interface to facilitate the hydrogen peroxide (H₂O₂) oxidation half-cell reaction of a direct hydrogen peroxide/hydrogen peroxide fuel cell. A functional lipid membrane embedded with catalytic amounts of proton carriers was appended on top of the Ru SAM to construct a hybrid bilayer membrane (HBM) platform that can modulate the thermodynamics and kinetics of proton- and electron-transfer steps independently. The performances of the as-prepared Ru SAMs and HBMs toward H₂O₂ oxidation were investigated using electrochemical means, kinetic isotope effect (KIE) studies, and Tafel analyses. Proton carriers featuring borate, phosphate, and nitrile headgroups were found to dictate the transmembrane proton removal rate, thereby controlling the H₂O₂ oxidation activity. The first significance of this work was the expansion of HBM platforms to GC substrates to overcome the limited redox potential window on gold thiol systems, thereby enabling electrochemical investigations of anodic reactions at the SAM-lipid interface. The second highlight of this work was demonstrating for the first time that deprotonation kinetics can be taken advantage of to enhance the electrocatalytic oxidation performance of a metal complex anchored at the SAM-lipid interface of a HBM platform. When the knowledge gaps regarding how PCET steps govern redox pathways are closed, the advances achieved using our unique bioinorganic platform are envisioned to accelerate the understanding and optimization of electrocatalytic processes involving proton- and electron- transfer steps that are fundamental to the development of high-performance energy devices.

Tuning stable noble metal nanoparticles dispersions to moderate their interaction with model membranes

HYPOTHESIS

The properties of stable gold (Au) nanoparticle dispersions can be tuned to alter their activity towards biomembrane models.

EXPERIMENTS

Au nanoparticle coating techniques together with rapid electrochemical screens of a phospholipid layer on fabricated mercury (Hg) on platinum (Pt) electrode have been used to moderate the phospholipid layer activity of Au nanoparticle dispersions. Screening results for Au nanoparticle dispersions were intercalibrated with phospholipid large unilamellar vesicle (LUV) interactions using a carboxyfluorescein (CF) leakage assay. All nanoparticle dispersions were characterised for size, by dynamic light scattering (DLS) and transmission electron microscopy (TEM).

FINDINGS

Commercial and high quality home synthesised Au nanoparticle dispersions are phospholipid monolayer active whereas Ag nanoparticle dispersions are not. If Au nanoparticles are coated with a thin layer of Ag then the particle/lipid interaction is suppressed. The electrochemical assays of the lipid layer activity of Au nanoparticle dispersions align with LUV leakage assays of the same. Au nanoparticles of decreasing size and increasing dispersion concentration showed a stronger phospholipid monolayer/bilayer interaction. Treating Au nanoparticles with cell culture medium and incubation of Au nanoparticle dispersions in phosphate buffered saline (PBS) solutions removes their phospholipid layer interaction.

Protective Role of Sphingomyelin in Eye Lens Cell Membrane Model against Oxidative Stress

In the eye lens cell membrane, the lipid composition changes during the aging process: the proportion of sphingomyelins (SM) increases, that of phosphatidylcholines decreases. To investigate the protective role of the SMs in the lens cell membrane against oxidative damage, analytical techniques such as electrochemistry, high-resolution mass spectrometry (HR-MS), and atomic force microscopy (AFM) were applied. Supported lipid bilayers (SLB) were prepared to mimic the lens cell membrane with different fractions of PLPC/SM (PLPC: 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine). The SLBs were treated with cold physical plasma. A protective effect of 30% and 44% in the presence of 25%, and 75% SM in the bilayer was observed, respectively. PLPC and SM oxidation products were determined via HR-MS for SLBs after plasma treatment. The yield of fragments gradually decreased as the SM ratio increased. Topographic images obtained by AFM of PLPC-bilayers showed SLB degradation and pore formation after plasma treatment, no degradation was observed in PLPC/SM bilayers. The results of all techniques confirm the protective role of SM in the membrane against oxidative damage and support the idea that the SM content in lens cell membrane is increased during aging in the absence of effective antioxidant systems to protect the eye from oxidative damage and to prolong lens transparency.

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.

A combination of electrochemistry and mass spectrometry to monitor the interaction of reactive species with supported lipid bilayers

Reactive oxygen and nitrogen species (RONS), e.g. generated by cold physical plasma (CPP) or photodynamic therapy, interfere with redox signaling pathways of mammalian cells, inducing downstream consequences spanning from migratory impairment to apoptotic cell death. However, the more austere impact of RONS on cancer cells remains yet to be clarified. In the present study, a combination of electrochemistry and high-resolution mass spectrometry was developed to investigate the resilience of solid-supported lipid bilayers towards plasma-derived reactive species in dependence of their composition. A 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayer was undisturbed by 200 µM H2O2 (control) but showed full permeability after CPP treatment and space-occupying oxidation products such as PoxnoPC, PAzePC, and POPC hydroperoxide were found. Electron paramagnetic resonance spectroscopy demonstrated the presence of hydroxyl radicals and superoxide anion/hydroperoxyl radicals during the treatment. In contrast, small amounts of the intramembrane antioxidant coenzyme Q10 protected the bilayer to 50% and LysoPC was the only POPC derivative found, confirming the membrane protective effect of Q10. Such, the lipid membrane composition including the presence of antioxidants determines the impact of pro-oxidant signals. Given the differences in membrane composition of cancer and healthy cells, this supports the application of cold physical plasma for cancer treatment. In addition, the developed model using the combination of electrochemistry and mass spectrometry could be a promising method to study the effect of reactive species or mixes thereof generated by chemical or physical sources.

Altering model cell membranes by means of localized magnetic heating

Isolated iron oxide magnetic nanoparticles (MNPs), 12 nm in diameter, coated with oleic acid molecules as capping agents have been deposited by the Langmuir-Blodgett (LB) method onto a model cell membrane incorporating 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and Cholesterol (Chol) in the 1:1 ratio, which was also fabricated by the LB technique. Atomic Force Microscopy (AFM) experiments showed that the application of an alternating magnetic field results in the embedding of the MNPs through the phospholipidic layer. These experimental results reveal that the heating of individual MNPs may induce a local increase in the fluidity of the film with a large control of the spatial and temporal specificity.

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.

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.

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.

Effect of Interfacial Water on the Nanomechanical Properties of Negatively Charged Floating Bilayers Supported on Gold Electrodes

Floating lipid bilayers composed of phosphatidylglycerols and cardiolipin were deposited on gold electrodes premodified with 1-thio-β-d-glucose monolayer by spreading of small unilamellar vesicles. The resulting lipid membrane was homogeneous, and its thickness was ∼5.0 nm. Electrochemical characterization combined with surface-enhanced infrared absorption spectroscopy revealed that negative polarization of the electrode leads to accumulation of water molecules in the interfacial region between lipid membrane and the thioglucose film. Moreover, the buildup of water layer was demonstrated to affect the nanomechanical properties of the membrane. The latter was manifested by well-pronounced decrease of Young's modulus of the lipid bilayer correlating with increasing hydration. This effect was ascribed to the decoupling of the membrane from supporting thioglucose film due to the accumulation of interfacial water. As a result, the effective stiffness of the supporting layer is lower and it alters the nanomechanical behavior of lipid membrane. Our results provide strong experimental proof for the correlation between elastic properties of floating lipid membrane and the amount of water accumulated in the submembrane region.

Impact of Electric Fields on the Nanoscale Behavior of Lipid Monolayers at the Surface of Graphite in Solution

The nanoscale organization and dynamics of lipid molecules in self-assembled membranes is central to the biological function of cells and in the technological development of synthetic lipid structures as well as in devices such as biosensors. Here, we explore the nanoscale molecular arrangement and dynamics of lipids assembled in monolayers at the surface of highly ordered pyrolytic graphite (HOPG), in different ionic solutions, and under electrical potentials. Using a combination of atomic force microscopy and fluorescence recovery after photobleaching, we show that HOPG is able to support fully formed and fluid lipid membranes, but mesoscale order and corrugations can be observed depending on the type of the lipid considered (1,2-dioleoyl- sn-glycero-3-phosphocholine, 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS), and 1,2-dioleoyl-3-trimethylammoniumpropane) and the ion present (Na⁺, Ca²⁺, Cl^-). Interfacial solvation forces and ion-specific effects dominate over the electrostatic changes induced by moderate electric fields (±1.0 V vs Ag/AgCl reference electrode) with particularly marked effects in the presence of calcium, and for DOPS. Our results provide insights into the interplay between the molecular, ionic, and electrostatic interactions and the formation of dynamical ordered structures in fluid lipid 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.

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) study of the interaction between protein aggregates and biomimetic membranes

EC-SERS is used for the first time to characterize protein aggregate–biomembrane interactions.

In Situ STM and Vibrational Study of Nanometer-Scale Reorganization of a Phospholipid Monolayer Accompanied by Potential-Driven Headgroup Digestion

In situ dynamic observation of model biological cell membranes, formed on a water/gold substrate interface, has been performed by the combination of electrochemical scanning tunneling microscopy and reflection infrared absorption vibrational spectroscopy. Monolayers of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) were formed on alkanethiol-modified gold surfaces in a buffer solution, and the microscopic phase transitions driven by electrochemical potential control were observed more in detail than our previous study on the same system [Electrochem. Commun. 2007, 9, 645-650]. This time the transitions were associated with the chemistry of DHPC by the aid of vibrational spectroscopy and the utilization of deuterium-labeled DHPC molecules. A negative potential shift solidifies the fluidic lipid layers into static striped or grainy features without notable chemical reactions. The first positive potential shift over the virginal DHPC monolayer breaks DHPC into choline and the corresponding phosphatidic acid (DHPA). This is the first case of a phospholipid electrochemical reaction microscopically detected at the solid surface.

Reversible Monolayer-Bilayer Transition in Supported Phospholipid LB Films under the Presence of Water: Morphological and Nanomechanical Behavior

Mixed monolayer Langmuir-Blodgett (LB) films of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol (Chol) in the 1:1 ratio have been prepared onto solid mica substrates. Upon immersion in water or in an aqueous HEPES solution (pH 7.4) the monolayer LB films were spontaneously converted into well-organized bilayers leaving free mica areas. The process has been demonstrated to be reversible upon removal of the aqueous solution, resulting in remarkably free of defects monolayers that are homogeneously distributed onto the mica. In addition, the nanomechanical properties exhibited by the as-formed bilayers have been determined by means of AFM breakthrough force studies. The bilayers formed by immersion of the monolayer in an aqueous media exhibit nanomechanical properties and stability under compression analogous to those of DPPC:Chol supported bilayers obtained by other methods previously described in the literature. Consequently, the hydration of a monolayer LB film has been revealed as an easy method to produce well-ordered bilayers that mimic the cell membrane and that could be used as model cell membranes.

Formation of Ordered Phospholipid Monolayer on a Hydrophilically Modified Au(111) Substrate

The molecular arrangement of phospholipid molecules was investigated on a hydrophilically modified gold surface within an aqueous solution by scanning tunneling microscopy. By suspending phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC) nanoparticles in the aqueous electrolyte surrounding a hydrophilically modified gold (111) substrate with 3-mercaptopropionic acid (SH-C2H4-COOH, 3-MPA), well-ordered adlattices of POPC were observed. Traces of particle fusion were visualized before formation of the adlattice. Addition of cholesterol to the suspension seems to facilitate accommodation of POPC on this surface. The observed unit cells of POPC adlattices had dimensions of 0.5 nm × 1.9-2.5 nm. By high-resolution imaging, each unit cell was discerned to be occupied by one upright POPC molecule. The POPC + cholesterol suspension also leads to formation of a flat integrated POPC layer, which may be a lipid bilayer covering the surface.

Physicochemical Studies on Orientation and Conformation of a New Bacteriocin BacSp222 in a Planar Phospholipid Bilayer

The behavior, secondary structure, and orientation of a recently discovered bacteriocin-like peptide BacSp222 in a lipid model system supported at a gold electrode was investigated by chronocoulometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and attenuated total reflectance infrared (ATR-IR) spectroscopy. The IR spectra show that the secondary structure of BacSp222 is predominantly α-helical. Analysis of the spectra in the amide I region shows that the α-helical fragment of the peptide is inserted into bilayer at the potential range at which the bilayer is stable and attached to the Au(111) surface, i.e., from -0.5 to 0.3 V vs Ag/AgCl. Insertion of BacSp222 to the membrane significantly changes the conformation of the acyl chains of lipid molecules, from all-trans to partially melted; however, the chains become less tilted. Based on these results, we propose that BacSp222 interacts with the DMPC bilayer through the barrel-stave pore formation. In this model, α-helix of BacSp222 inserts into the membrane with an angle between the α-helix axis and membrane normal equal to ∼18°. The changes in orientation of the α-helical fragment of the peptide indicate that the orientation of BacSp222 with respect to the bilayer surface is potential-dependent. The peptide is inserted into the membrane driven by the electrostatic field generated by negative charge at the metal surface. It is not inserted at negative potentials where the membrane is detached from the metal and no longer exposed to the electrostatic field of the metal.

Surface-Enhanced Infrared Spectroscopy and Neutron Reflectivity Studies of Ubiquinone in Hybrid Bilayer Membranes under Potential Control

Surface-enhanced infrared adsorption spectroscopy (SEIRAS) and neutron reflectometry (NR) were employed to characterize ubiquinone (UQ) containing hybrid bilayer membranes. The biomimetic membrane was prepared by fusing phospholipid vesicles on a hydrophobic octadecanethiol monolayer self-assembled on a thin gold film. Using SEIRAS, the assembly of the membrane is monitored in situ. The presence of ubiquinone is verified by the characteristic carbonyl peaks from the quinone ester. A well-ordered distal lipid leaflet results from fusion of vesicles with and without the addition of ubiquinone. With applied potential, the hybrid bilayer membrane in the absence of UQ behaves in the same way as previously reported solid supported phospholipid membranes. When ubiquinone is incorporated in the membrane, electric field induced changes in the distal leaflet are suppressed. Changes in the infrared vibrations of the ubiquinone due to applied potential indicate the head groups are located in both polar and nonpolar environments. The spectroscopic data reveal that the isoprenoid unit of the ubiquinone is likely lying in the midplane of the lipid bilayer while the head has some freedom to move within the hydrophobic core. The SEIRAS experiments show redox behavior of UQ incorporated in a model lipid membrane that are otherwise inaccessible with traditional electrochemistry techniques.

Effect of Deuteration on Phase Behavior of Supported Phospholipid Bilayers: A Spectroelectrochemical Study

Differences in molecular organization of two sides of a chemically symmetric, planar bilayer supported on a Au(111) substrate have been monitored with charge density measurements and in situ polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Isotopic substitution of the hydrogen atoms in the hydrocarbon chains with deuterium atoms in one monolayer was employed to allow the monitoring of C-H vibrations from that monolayer alone. Charge density measurements of bilayers formed from dimyristoylphosphatidylethanolamine (DMPE) showed that the effect of placing the deuterated layer next to the substrate or electrolyte had little impact on the electrical barrier properties. In situ PM-IRRAS studies revealed that the structure of the two monolayers was the same at negative potentials, where the bilayer is separated from the Au substrate, but different at more positive potentials or small charge densities, where the bilayer is expected to be directly adsorbed on the Au surface. Thus, the differences observed for the related molecule dimyristoylphosphatidylcholine (DMPC) persist in planar structures, although to a lesser extent. A small but observable variation in the tilt angle was also apparent in the spectra of both isotopically asymmetric DMPE bilayers during the electrochemical phase transition. The fact that this effect was not previously observed for hydrogenous bilayers means that the dynamic behavior of deuterated DMPE and/or of bilayers composed of different monolayers is different from that of hydrogenous DMPE bilayers. These results have implications for future studies in which isotopic substitution is used to extract selectively information from one layer or component of lipid bilayers in spectroscopic or neutron measurements.

Characterization of a Self-Assembled Monolayer of 1-Thio-β-D-Glucose with Electrochemical Surface Enhanced Raman Spectroscopy Using a Nanoparticle Modified Gold Electrode

Preparation of a nanoparticle modified gold substrate designed for characterization of hydrophilic self-assembled monolayers (SAMs) of 1-thio-β-D-glucose (TG) with electrochemical surface-enhanced Raman spectroscopy (EC-SERS) is presented. Citrate stabilized gold nanoparticles were deposited on a polycrystalline gold electrode and subjected to an electrochemical desorption procedure to completely remove all traces of adsorbed citrate. Complete desorption of citrate was confirmed by recording cyclic voltammetry curves and SERS spectra. The citrate-free nanoparticle modified gold electrode was then incubated in a 1 mg mL(-1) aqueous solution of TG for 16 h prior to being characterized by EC-SERS. The SERS spectra confirmed that at potentials more negative than -0.10 V vs SCE thioglucose forms a monolayer in which the majority of the molecules preserve their lactol ring structure and only a small fraction of molecules appear to be oxidized. At potentials more positive than -0.10 V, the oxidation of TG molecules becomes prominent, and at potentials more positive than 0.20 V vs SCE, the monolayer of TG consists chiefly of oxidized product. The SERS spectra collected in the double layer region suggest the SAM of TG is well hydrated and hence can be used for hydrophilic modifications of a gold surface.

Mobile lipid bilayers on gold surfaces through structure-induced lipid vesicle rupture

Forming fluid supported lipid bilayers (SLBs) on a gold surface can enable various lipid-membrane-associated biomolecular interactions to be investigated by several surface sensing techniques, such as surface plasmon resonance and scanning tunneling microscopy. However, forming fluid SLBs on a gold surface through lipid vesicle deposition continues to pose a challenge. In this study, we constructed nanograting structures on a gold surface to induce lipid vesicle rupture for forming a mobile layer of SLBs. Observations based on fluorescence recovery after photobleaching showed that SLBs on the prepared grating supports had some fluidity, while SLBs on the planar support had no fluidity. The anisotropic fluorescence intensity recovery shape changes observed in the SLBs on the grating support suggested that a second layer of SLBs partially formed on top of the first layer in contact with the gold surface and extended along the grating structure. Comparisons of the relative amounts of second bilayer and the fluorescence recovery fractions on supports with various grating edge densities suggested that the second layer formed at the edge regions and that the coverage ratio was directly proportional to the grating edge density. All of these results showed that the grating edges could serve as vesicle-rupture-inducing sites for the formation of a mobile second SLB on a gold surface. The formation of the second layer of SLBs at the edge regions but not in the flat regions enabled us to determine the second layer locations and provided us with an opportunity to pattern mobile lipid bilayers on gold surfaces by controlling the edge locations.

A reusable device for electrochemical applications of hydrogel supported black lipid membranes

Black lipid membranes (BLMs) are significant in studies of membrane transport, incorporated proteins/ion transporters, and hence in construction of biosensor devices. Although BLMs provide an accepted mimic of cellular membranes, they are inherently fragile. Techniques are developed to stabilize them, such as hydrogel supports. In this paper, we present a reusable device for studies on hydrogel supported (hs) BLMs. These are formed across an ethylene tetrafluoroethylene (ETFE) aperture array supported by the hydrogel, which is during in situ polymerization covalently "sandwiched" between the ETFE substrate and a gold electrode microchip, thus allowing direct electrochemical studies with the integrated working electrodes. Using electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy and contact angle measurements, we demonstrate the optimized chemical modifications of the gold electrode microchips and plasma modification of the ETFE aperture arrays facilitating covalent "sandwiching" of the hydrogel. Both fluorescence microscopy and EIS were used to demonstrate the induced spontaneous thinning of a deposited lipid solution, leading to formation of stabilized hsBLMs on average in 10 min. The determined specific membrane capacitance and resistance were shown to vary in the range 0.31-0.49 μF/cm(2) and 45-65 kΩ cm(2), respectively, corresponding to partially solvent containing BLMs with an average life time of 60-80 min. The characterized hsBLM formation and devised equivalent circuit models lead to a schematic model to illustrate lipid molecule distribution in hydrogel-supported apertures. The functionality of stabilized hsBLMs and detection sensitivity of the platform were verified by monitoring the effect of the ion transporter valinomycin.