Polar Bilayer Research Articles

Engineering interfacial polarization switching in van der Waals multilayers.

In conventional ferroelectric materials, polarization is an intrinsic property limited by bulk crystallographic structure and symmetry. Recently, it has been demonstrated that polar order can also be accessed using inherently non-polar van der Waals materials through layer-by-layer assembly into heterostructures, wherein interfacial interactions can generate spontaneous, switchable polarization. Here we show that deliberate interlayer rotations in multilayer van der Waals heterostructures modulate both the spatial ordering and switching dynamics of polar domains. The engendered tunability is unparalleled in conventional bulk ferroelectrics or polar bilayers. By means of operando transmission electron microscopy we show how alterations of the relative rotations of three WSe2 layers produce structural polytypes with distinct arrangements of polar domains with either a global or localized switching response. Furthermore, the presence of uniaxial strain generates structural anisotropy that yields a range of switching behaviours, coercivities and even tunable biased responses. We also provide evidence of mechanical coupling between the two interfaces of the trilayer, a key consideration for the control of switching dynamics in polar multilayer structures more broadly.

The N-terminal helices of amphiphysin and endophilin have different capabilities of membrane remodeling

Amphiphysin and endophilin are two members of the N-BAR protein family. We have reported membrane interactions of the helix 0 of endophilin (H0-Endo). Here we investigate membrane modulations caused by the helix 0 of amphiphysin (H0-Amph). Electron paramagnetic resonance (EPR) spectroscopy was used to explore membrane properties. H0-Amph was found to reduce lipid mobility, make the membrane interior more polar, and decrease lipid chain orientational order. The EPR data also showed that for anionic membranes, H0-Endo acted as a more potent modulator. For instance, at peptide-to-lipid (P/L) ratio of 1/20, the peak-to-peak splitting was increased by 0.27 G and 1.89 G by H0-Amph and H0-Endo, respectively. Similarly, H0-Endo caused a larger change in the bilayer polarity than H0-Amph (30% versus 12% at P/L = 1/20). At P/L = 1/50, the chain orientational order was decreased by 26% and 66% by H0-Amph and H0-Endo, respectively. The different capabilities were explained by considering hydrophobicity score distributions. We employed atomic force microscopy to investigate membrane structural changes. Both peptides caused the formation of micron-sized holes. Interestingly, only H0-Amph induced membrane fusion as evidenced by the formation of high-rise regions. Lastly, experiments of giant unilamellar vesicles showed that H0-Amph and H0-Endo generated thin tubules and miniscule vesicles, respectively. Together, our studies showed that both helices are effective in altering membrane properties; the observed changes might be important for membrane curvature induction. Importantly, comparisons between the two peptides revealed that the degree of membrane remodeling is dependent on the sequence of the N-terminal helix of the N-BAR protein family.

The helix 0 of endophilin modifies membrane material properties and induces local curvature

The amphipathic helix 0 of endophilin (i.e., H0-Endo) is important to membrane binding, but its function of curvature generation remains controversial. We used electron paramagnetic resonance (EPR) spectroscopy to study effects of H0-Endo on membrane material properties. We found that H0-Endo reduced lipid chain mobility and increased bilayer polarity, i.e., making the bilayer interior more polar. Lipid-dependent examination revealed that anionic lipids augmented the effect of H0-Endo, while cholesterol had a minimal impact. Our EPR spectroscopy of magnetically aligned bicelles showed that as the peptide-to-lipid ratio increased, the lipid chain orientational order decreased gradually, followed by a sudden loss. We discuss an interfacial-bound model of the amphipathic H0-Endo to account for all EPR data. We used atomic force microscopy and fluorescence microscopy to explore membrane morphological changes. We found that H0-Endo caused the formation of micron-sized holes in mica-supported planar bilayers. Hole formation is likely caused by two competing forces – the adhesion force exerted by the substrate represses bilayer budging, whereas the line tension originating from peptide clustering has a tendency of destabilizing bilayer organization. In the absence of substrate influences, membrane curvature induction was manifested by generating small vesicles surrounding giant unilamellar vesicles. Our results of membrane perforation and vesiculation suggest that the functionality of H0-Endo is more than just coordinating membrane binding of endophilin.

A Multiparametric Fluorescence Probe to Understand the Physicochemical Properties of Small Unilamellar Lipid Vesicles in Poly(ethylene glycol)-Water Medium.

FDAPT (2-formyl-5-(4'-N,N-dimethylaminophenyl)thiophene) efficiently senses the minimum alteration of lipid bilayer microenvironment with all six different fluorescence parameters namely emission wavelength, fluorescence intensity, steady-state anisotropy, and their corresponding time-dependent parameters (Sahu et al., J. Phys. Chem. B 2018, 122, 7308-7318). In the present work, the effect of poly(ethylene glycol) on the small unilamellar vesicle is demonstrated with the emission behavior of the FDAPT probe. A medium and a high molecular weight PEG were chosen to perturb the lipid vesicles. The alteration of the bilayer polarity, water content inside bilayer, lipid packing density in the perturbed vesicles reflect significant changes in different fluorescence parameters of FDAPT probe. The effect of PEG on the unilamellar vesicle was rationalized with the alteration of the emission behavior, fluorescence lifetime, steady-state anisotropy and anisotropy decay of the probe. The simple and convenient fluorescence measurements provide new insights into the effect of PEG on the packing density, water volume, micro polarity, and microviscosity of the small unilamellar vesicle. The physiological understanding was extended to rationalize the cryoprotecting behavior of PEG.

Built-In Electric Field Hindering Photogenerated Carrier Recombination in Polar Bilayer SnO/BiOX (X = Cl, Br, I) for Water Splitting

Alloying in 2D materials plays a more and more important role due to wide range bandgap tunability and integrating the advantages of HER and OER. Here, the novel bilayers of SnO/BiOX (X= Cl, Br and I) bilayer are constructed to integrate the advantages of narrow bandgap and separating photo-generated carriers. The bandgap of the bilayers can be tuned from 1.09 to 1.84 eV, remarkably improving the utilization of solar energy. The large difference in effective masses and built-in electric field effectively hamper the fast recombination of photo-generated carries, which highly enhances the photocatalytic efficiency. Besides that, the type-II band alignment guarantee the two half reactions could occur at different surfaces. Moreover, the optical absorption (the strong transition between band edges and high joint density of states) and band-edge level further confirm the SnO/BiOX (X= Cl and Br) bilayer is a promising candidate for overall water-splitting.

Ergosterol-Induced Ordered Phase in Ternary Lipid Mixture Systems of Unsaturated and Saturated Phospholipid Membranes.

The sterol ergosterol (Erg) is ubiquitous in the membranes of lower eukaryotes such as fungi. To investigate the interactions between Erg and phosphocholine (PC) molecules, we studied ternary lipid mixture systems composed of unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), saturated 1,2-dipalmitoyl-sn-glycero-3-phophocholine (DPPC), and Erg. Bilayer membrane fluidity and polarity were systematically analyzed using fluorescent probes. The presence of ≥30 mol % of Erg exhibited a significant ordering effect and stabilized membrane properties when temperatures increased, suggesting that Erg has a similar function to cholesterol (Chol) in comparable lipid systems that form a liquid-ordered phase. Erg was also observed to have a significant condensing effect at the monolayer level in saturated PC-enriched systems. The phase behavior of Erg in bilayer systems was compared with that of Chol, with the data suggesting that Erg behaves in a similar manner to Chol in membranes enriched with saturated lipids.

Polar Bilayer Cathode for Advanced Lithium–Sulfur Battery: Synergy Between Polysulfide Conversion and Confinement

Despite possessing 5-fold higher specific energy density compared to commercial lithium-ion batteries, the insulating nature of sulfur and its reductive derivatives along with the uncontrollable m...

Spacer nature and composition as key factors for structural tailoring of anionic/cationic mixed gemini micelles: Interaction and solubilization studies

Strategies (composition, morphology and spacer nature variations) are reported for aqueous solubility enhancement of hydrophobic material. Critical micelle concentration (cmc) of mixed oppositely charged gemini surfactants has been studied conductometrically. Phosphoric acid, P, P′-1,4-butanedieyl, P, P′ didodecylester, disodium salt is used as an anionic component while cationic geminis, with different spacers (polymethylene, ethylenediamine, isosorbide, and ethylene glycol), are used as another component. Based on micellar interaction parameter (βm), both synergistic and antagonistic interactions are observed. Zeta-potential shows variation in charge (or charge reversal) of the mixed aggregate. Small angle neutron scattering, dynamic light scattering and transmission electron microscopy show spacer nature dependent formation of micelles/vesicles. Molar solubilization ratio, of polycyclic aromatic hydrocarbon (PAH), suggests higher solubilization in vesicles. Solubilization depends on the vesicle electrostatics and bi-layer thickness. Potential key factors of the process seem bi-layer polarity and interaction with individual PAHs. Approach might be employed to enhance aqueous solubility/bioavailability of other hydrophobic materials such as drugs, dyes or pesticides.

Lipid Extraction by α-Synuclein Generates Semi-Transmembrane Defects and Lipoprotein Nanoparticles.

Modulations of synaptic membranes play an essential role in the physiological and pathological functions of the presynaptic protein α-synuclein (αSyn). Here we used solution atomic force microscopy (AFM) and electron paramagnetic resonance (EPR) spectroscopy to investigate membrane modulations caused by αSyn. We used several lipid bilayers to explore how different lipid species may regulate αSyn–membrane interactions. We found that at a protein-to-lipid ratio of ∼1/9, αSyn perturbed lipid bilayers by generating semi-transmembrane defects that only span one leaflet. In addition, αSyn coaggregates with lipid molecules to produce ∼10 nm-sized lipoprotein nanoparticles. The obtained AFM data are consistent with the apolipoprotein characteristic of αSyn. The role of anionic lipids was elucidated by comparing results from zwitterionic and anionic lipid bilayers. Specifically, our AFM measurements showed that anionic bilayers had a larger tendency of forming bilayer defects; similarly, our EPR measurements revealed that anionic bilayers exhibited more substantial changes in lipid chain mobility and bilayer polarity. We also studied the effect of cholesterol. We found that cholesterol increased the capability of αSyn in inducing bilayer defects and altering lipid chain mobility and bilayer polarity. These data can be explained by an increase in the lipid headgroup–headgroup spacing and/or specific cholesterol−αSyn interactions. Interestingly, we found an inhibitory effect of the cone-shaped phosphatidylethanolamine lipids on αSyn-induced bilayer remodeling. We explained our data by considering interlipid hydrogen-bonding that can stabilize bilayer organization and suppress lipid extraction. Our results of lipid-dependent membrane modulations are likely relevant to αSyn functioning.

Effects of the Interlayer Interaction and Electric Field on the Band Gap of Polar Bilayers: A Case Study of Sc2CO2

The two-dimensional Sc2CO2 monolayer has net dipole moments as compared with other functional transition metal carbides. This unique feature may provide potential routes to tune its electronic properties. Herein, the band gap engineering of Sc2CO2 bilayers is investigated by the density functional calculation. We found that the band gap of Sc2CO2 bilayers is not only dependent on the relative stacking position and distance but also on the magnitude and direction of external electric field. The competition between the interlayer van der Waals and electrostatic interactions has been revealed as the dominant mechanism for the band gap modulation of polar bilayers. This works demonstrate that polar bilayers exhibit richer electronic properties than the nonpolar one. The interlayer electric field may be utilized to fabricate novel electronic and optoelectronic devices.

Active membrane cholesterol as a physiological effector

Sterols associate preferentially with plasma membrane sphingolipids and saturated phospholipids to form stoichiometric complexes. Cholesterol in molar excess of the capacity of these polar bilayer lipids has a high accessibility and fugacity; we call this fraction active cholesterol. This review first considers how active cholesterol serves as an upstream regulator of cellular sterol homeostasis. The mechanism appears to utilize the redistribution of active cholesterol down its diffusional gradient to the endoplasmic reticulum and mitochondria, where it binds multiple effectors and directs their feedback activity. We have also reviewed a broad literature in search of a role for active cholesterol (as opposed to bulk cholesterol or lipid domains such as rafts) in the activity of diverse membrane proteins. Several systems provide such evidence, implicating, in particular, caveolin-1, various kinds of ABC-type cholesterol transporters, solute transporters, receptors and ion channels. We suggest that this larger role for active cholesterol warrants close attention and can be tested easily.

Dendronization-induced phase-transfer, stabilization and self-assembly of large colloidal Au nanoparticles.

The phase-transfer of CTAB-coated aqueous, spherical gold nanoparticles, with metallic core diameters ranging from ca. 27 to 54 nm, into organic solvents by exchanging the primitive polar bilayer with lipophilic, disulfide dendritic ligands is reported. The presence of such a thick nonpolar organic shell around these large nanoparticles enhances their stabilization against aggregation, in addition to enabling their transfer into a variety of solvents such as chloroform, toluene or tetrahydrofuran. Upon the slow evaporation of a chloroform suspension deposited on a solid support, the dendronized hybrids were found to self-assemble into ring structures of various diameters. Moreover, their self-assembly at the liquid-air interface affords the formation of fairly long-range ordered monolayers, over large areas, that can then be entirely transferred onto solid substrates.

Graphene-silicene bilayer: A nanocapacitor with permanent dipole and piezoelectricity effect

Using density functional theory, we study the electronic properties of a graphene-silicene bilayer (GSB). A single layer of silicene binds to the graphene layer with adhesion energy of about 25 meV/atom. This adhesion energy between the two layers follows accurately the well-known $\ensuremath{-}1/{z}^{2}$ dispersion energy as found between two infinite parallel plates. In small flakes of GSB with hydrogenated edges, negative charge is transferred from the graphene layer to the silicene layer, producing a permanent and a switchable polar bilayer, while in an infinite GSB, the negative charge is transferred from the silicene layer to the graphene layer. The graphene-silicene bilayer is a good candidate for a nanocapacitor with piezoelectric capabilities. We found that the permanent dipole of the bilayer can be tuned by an external perpendicular electric field.

Magnetic excitation spectrum ofLuFe2O4measured with inelastic neutron scattering

We report neutron inelastic scattering measurements and analysis of the spectrum of magnons propagating within the Fe2O4 bilayers of LuFe2O4. The observed spectrum is consistent with six magnetic modes and a single prominent gap, which is compatible with a single bilayer magnetic unit cell containing six spins. We model the magnon dispersion by linear spin-wave theory and find very good agreement with the domain-averaged spectrum of a spin-charge bilayer superstructure comprising one Fe3+ -rich monolayer and one Fe2+ -rich monolayer. These findings indicate the existence of polar bilayers in LuFe2O4, contrary to recent studies that advocate a charge-segregated non-polar bilayer model. Weak scattering observed below the magnon gap suggests that a fraction of the bilayers contain other combinations of charged monolayers not included in the model. Refined values for the dominant exchange interactions are reported.

Relationships between membrane water molecules and Patman equilibration kinetics at temperatures far above the phosphatidylcholine melting point

The naphthalene-based fluorescent probes Patman and Laurdan detect bilayer polarity at the level of the phospholipid glycerol backbone. This polarity increases with temperature in the liquid–crystalline phase of phosphatidylcholines and was observed even 90°C above the melting temperature. This study explores mechanisms associated with this phenomenon. Measurements of probe anisotropy and experiments conducted at 1M NaCl or KCl (to reduce water permittivity) revealed that this effect represents interactions of water molecules with the probes without proportional increases in probe mobility. Furthermore, comparison of emission spectra to Monte Carlo simulations indicated that the increased polarity represents elevation in probe access to water molecules rather than increased mobility of relevant bilayer waters. Equilibration of these probes with the membrane involves at least two steps which were distinguished by the membrane microenvironment reported by the probe. The difference in those microenvironments also changed with temperature in the liquid–crystalline phase in that the equilibrium state was less polar than the initial environment detected by Patman at temperatures near the melting point, more polar at higher temperatures, and again less polar as temperature was raised further. Laurdan also displayed this level of complexity during equilibration, although the relationship to temperature differed quantitatively from that experienced by Patman. This kinetic approach provides a novel way to study in molecular detail basic principles of what happens to the membrane environment around an individual amphipathic molecule as it penetrates the bilayer. Moreover, it provides evidence of unexpected and interesting membrane behaviors far from the phase transition.

Behavior of pyrene as a polarity probe in palmitoylsphingomyelin and palmitoylsphingomyelin/cholesterol bilayers: A molecular dynamics simulation study

Pyrene is a polycyclic aromatic hydrocarbon noted for its remarkable optical spectroscopic properties. Among its uses as a fluorescent probe, measurement of lipid bilayer's equivalent polarity through the pyrene Ham effect stands out. To this effect, the ratio of the intensities of the first and third vibronic bands (I1/I3) in its emission spectrum of pyrene is measured. However, issues concerning the precise location of bilayer-inserted pyrene and the possibility of probe-induced perturbation of host bilayer properties are potential sources of concern in this regard. Atomistic molecular dynamics simulations constitute a useful method for the characterization of lipid membrane systems, and, in particular, to understand the behavior of fluorescence probes upon incorporation in lipid bilayers. In this report, we present a detailed characterization of the behavior of pyrene in fluid N-palmitoylsphingomyelin (PSM) and PSM/cholesterol membranes, with emphasis on the degree of proximity between the probe and water molecules inside bilayers, related to the use of pyrene to measure equivalent lipid bilayer polarity. It is concluded that pyrene exerts minor effects on bilayer properties, with slight local disordering being apparent for high cholesterol content. Whereas rotation and lateral diffusion of pyrene are greatly slowed in cholesterol-rich systems, its relative transverse location is not significantly affected. While hydration of PSM bilayers, as sensed by pyrene, is already low compared to that of fluid phosphatidylcholine, it becomes even smaller for high cholesterol mole fraction at the studied temperature.

The New Fluorescent Membrane Probe Ahba: A Comparative Study with the Largely Used Laurdan

Lipid bilayers have been largely used as model systems for biological membranes. Hence, their structures, and alterations caused on them by biological active molecules, have been the subject of many studies. Accordingly, fluorescent probes incorporated into lipid bilayers have been extensively used for characterizing lipid bilayer fluidity and/or polarity. However, for the proper analysis of the alterations undergone by a membrane, a comprehensive knowledge of the fluorescent properties of the probe is fundamental. Therefore, the present work compares fluorescent properties of a relative new fluorescent membrane probe, 2-amino-N-hexadecyl-benzamide (Ahba), with the largely used probe 6-dodecanoyl-N,N-dimethyl-2-naphthylamine (Laurdan), using both static and time resolved fluorescence. Both Ahba and Laurdan have the fluorescent moiety close to the bilayer surface; Ahba has a rather small fluorescent moiety, which was shown to be very sensitive to the bilayer surface pH. The main goal was to point out the fluorescent properties of each probe that are most sensitive to structural alterations on a lipid bilayer. The two probes were incorporated into bilayers of the well-studied zwitterionic lipid dimyristoyl phosphatidylcholine (DMPC), which exhibits a gel-fluid transition around 23 °C. The system was monitored between 5 and 50 °C, hence allowing the study of the two different lipid structures, the gel and fluid bilayer phases, and the transition between them. As it is known, the fluorescent emission spectrum of Laurdan is highly sensitive to the bilayer gel-fluid transition, whereas the Ahba fluorescence spectrum was found to be insensitive to changes in bilayer structure and polarity, which are known to happen at the gel-fluid transition. However, both probes monitor the bilayer gel-fluid transition through fluorescence anisotropy measurements. With time-resolved fluorescence, it was possible to show that bilayer structural variations can be monitored by Laurdan excited state lifetimes changes, whereas Ahba lifetimes were found to be insensitive to bilayer structural modifications. Through anisotropy time decay measurements, both probes could monitor structural bilayer changes, but the limiting anisotropy was found to be a better parameter than the rotational correlation time. It is interesting to have in mind that the relatively small fluorophore of Ahba (o-Abz) could possibly be bound to a phospholipid hydrocarbon chain, not disturbing much the bilayer packing and being a sensitive probe for the bilayer core.

Charge Order inLuFe2O4: An Unlikely Route to Ferroelectricity

We present the refinement of the crystal structure of charge-ordered LuFe2O4, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast with the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The nonpolar bilayers are inconsistent with charge order based ferroelectricity.

The ascorbyl palmitate–polyethyleneglycol 400–water system phase behavior

Ascorbyl palmitate (Asc16) in polyethyleneglycol 400 (PEG 400)–water mixtures at weight fractions (w/w) between 0.05 and 1.0 were studied by differential scanning calorimetry (DSC) and polarizing microscopy (PM) at different temperatures. The employed PEG 400–water proportions were: 0–25–50% and 75% of polymer. A complete phase diagram was determined for each PEG 400–water mixture.A cubic mesophase and two (probably three) lamellar mesophases were detected in different regions of the phase diagrams. The addition of PEG 400 to the Asc16–water system shifts the limits of the liquid crystalline domains to lower temperature and surfactant concentration. At weight fraction of PEG 400≥50%, the limits of the domain of existence of cubic mesophase shift to low surfactant concentration compared with water-rich systems. The hydrated crystals are Asc16.2·6H2O. If the proportion of water is lower than that value, a mixture of hydrated and anhydrous crystals appears. Heating these crystals produce waxy crystals having melted hydrocarbon bilayers retaining their crystalline structure because the polar bilayers are still rigid.

The ascorbyl palmitate-water system: Phase diagram and state of water

This work addresses the ascorbyl palmitate (Asc16) aqueous systems. It has been studied through differential scanning calorimetry (DSC) and polarizing microscopy (PM) at different temperatures over a wide range of concentrations (weight fractions (w/w) between 0.05 and 1.0), allowing the determination of a complete phase diagram. When possible, the findings were corroborated with literature data. Systems below C ≈ 0.48 wt fraction show hydrated crystals in an isotropic liquid, which lead to a lamellar liquid crystal when heated to about 60 °C. Above this concentration, the phase transition occurs at about 80 °C, giving a cubic liquid crystal which in turn becomes lamellar liquid crystal at about 90 °C. The texture of this lamellar mesophase is different to that produced at lower concentrations. It has been determined, through DSC analysis of the melting peaks of water, computer simulation of the hydration of Asc16 molecules and a model of the hydrated aggregates (hydrated crystals and lamellar mesophase), that low concentration liquid crystals exist provided bulk water is present in the system. Two other kinds of water but bulk were detected: a first hydration shell, with water molecules strongly attached by hydrogen bonds to the oxygen and hydrogen atoms of the polar headgroup, which does not melt and is then undetectable by DSC (11.47 ± 0.95 water molecules per surfactant molecule) and a second hydration layer containing 59 ± 17 water molecules per surfactant one (apart from those in the first hydration layer). The number of water molecules in the second hydration layer decreases as the surfactant concentration increases, and vanishes at C = 0.62 ± 0.01 wt fraction (computer simulation and structure model data) or at C = 0.667 ± 0.007 (DSC). At very high concentrations the formation of mesophases is preceded by the melting of the chains, giving a waxy texture when viewed at the polarizing microscope, maintaining the structure of crystals because the anhydrous polar bilayer remains “solid”.