Thickness and refractive index of DPPC and DPPE monolayers by multiple-beam interferometry

Spray freeze dried cannabidiol with dipalmitoylphosphatidylcholine (DPPC) for inhalation and solubility enhancement

Pulmonary delivery is an efficient route of administration to deliver cannabidiol (CBD) due to the high bioavailability and fast onset of action. The major formulation challenge is the poor aqueous solubility of CBD. This study aimed to produce inhalable CBD powders with enhanced solubility and characterise their solid-state properties. CBD was spray freeze dried with mannitol or trehalose dihydrate with and without dipalmitoylphosphatidylcholine (DPPC). All four powders had acceptable yields at > 70 % with porous and spherical particles. The two crystalline mannitol powders contained less residual solvent than both amorphous trehalose ones. The addition of DPPC did not affect the crystallinity and residual solvent level of the powders. Instead, DPPC made the particles more porous, decreased the particle size from 19-23 µm to 11-13 µm, and increased CBD solubility from 0.36 µg/mL to over 2 µg/mL. The two DPPC powders were dispersed from a low resistance RS01 inhaler, showing acceptable aerosol performance with emitted fractions at 91-93 % and fine particle fractions < 5 µm at 34-43 %. These formulations can be used as a platform to deliver CBD and other cannabinoids by inhalation.

Cooperative Tridentate Hydrogen-Bonding Interactions Enable Strong Underwater Adhesion

Multidentate hydrogen-bonding interactions are a promising strategy to improve underwater adhesion. Molecular and macroscale experiments have revealed an increase in underwater adhesion by incorporating multidentate H-bonding groups, but quantitatively relating the macroscale adhesive strength to cooperative hydrogen-bonding interactions remains challenging. Here, we investigate whether tridentate alcohol moieties incorporated in a model epoxy act cooperatively to enhance adhesion. We first demonstrate that incorporation of tridentate alcohol moieties leads to comparable adhesive strength with mica and aluminum in air and in water. We then show that the presence of tridentate groups leads to energy release rates that increase with an increase in crack velocity in air and in water, while materials lacking these groups do not display rate-dependent adhesion. We model the rate-dependent adhesion to estimate the activation energy of the interfacial bonds. Based on our data, we estimate the lifetime of these bonds to be between 2 ms and 6 s, corresponding to an equilibrium activation energy between 23k_BT and 31k_BT. These values are consistent with tridentate hydrogen bonding, suggesting that the three alcohol groups in the Tris moiety bond cooperatively form a robust adhesive interaction underwater.

A compendium of single extracellular vesicle flow cytometry

Flow cytometry (FCM) offers a multiparametric technology capable of characterizing single extracellular vesicles (EVs). However, most flow cytometers are designed to detect cells, which are larger than EVs. Whereas cells exceed the background noise, signals originating from EVs partly overlap with the background noise, thereby making EVs more difficult to detect than cells. This technical mismatch together with complexity of EV-containing fluids causes limitations and challenges with conducting, interpreting and reproducing EV FCM experiments. To address and overcome these challenges, researchers from the International Society for Extracellular Vesicles (ISEV), International Society for Advancement of Cytometry (ISAC), and the International Society on Thrombosis and Haemostasis (ISTH) joined forces and initiated the EV FCM working group. To improve the interpretation, reporting, and reproducibility of future EV FCM data, the EV FCM working group published an ISEV position manuscript outlining a framework of minimum information that should be reported about an FCM experiment on single EVs (MIFlowCyt-EV). However, the framework contains limited background information. Therefore, the goal of this compendium is to provide the background information necessary to design and conduct reproducible EV FCM experiments. This compendium contains background information on EVs, the interaction between light and EVs, FCM hardware, experimental design and preanalytical procedures, sample preparation, assay controls, instrument data acquisition and calibration, EV characterization, and data reporting. Although this compendium focuses on EVs, many concepts and explanations could also be applied to FCM detection of other particles within the EV size range, such as bacteria, lipoprotein particles, milk fat globules, and viruses.

Plasmonic lipogels: driving co-assembly of composites with peptide-based gels for controlled drug release

Supramolecular short peptide-based gels are promising materials for the controlled release of drugs (e.g. chemotherapeutic drugs) owing to the biocompatibility and similarity to cell matrix. However, the drug encapsulation and control over its release, mainly the hydrophilic drugs, can be a cumbersome task. This can be overcome through encapsulation/compartmentalization of drugs in liposomes, which can also enable spatiotemporal control and enhanced drug release through a trigger, such as photothermia. Having this in mind, we explored the assembly of silica-coated gold nanoparticles and liposomes (storage units) with dehydropeptide-based hydrogels as a proof-of-concept to afford peptide-based NIR light-responsive lipogels. Several liposomes compositions were assessed that displayed influence on the final assembly properties by combining with silica-coated gold nanorods (∼106 nm). Gold nanospheres (∼11 nm) were used to study the preparation method, which revealed the importance of initially combine liposomes with nanoparticles and then the gelator solution to achieve a closer proximity of the nanoparticles to the liposomes. The control over a hydrophilic model drug, 5(6)-carboxyfluorescein, was only achieved by its encapsulation in liposomes, in which the presence of silica-coated nanorods further enabled the use of photothermia to induce the liposomes phase transition and stimulate the drug release. Further, both composites, the liposomes and silica-coated gold nanorods, induced a lower elastic modulus, but also provided an enhanced gelation kinetics. Hereby, this work advances fabrication strategies for the development of short peptide-based hydrogels towards on-demand, sustained and controlled release of hydrophilic drugs through photothermia under NIR light irradiation.

Characterization of lipid bilayers adsorbed to functionalized air/water interfaces

Lipid bilayers immobilized in planar geometries, such as solid-supported or "floating" bilayers, have enabled detailed studies of biological membranes with numerous experimental techniques, notably X-ray and neutron reflectometry. However, the presence of a solid support also has disadvantages as it complicates the use of spectroscopic techniques as well as surface rheological measurements that would require surface deformations. Here, in order to overcome these limitations, we investigate lipid bilayers adsorbed to inherently soft and experimentally well accessible air/water interfaces that are functionalized with Langmuir monolayers of amphiphiles. The bilayers are characterized with ellipsometry, X-ray scattering, and X-ray fluorescence. Grazing-incidence X-ray diffraction reveals that lipid bilayers in a chain-ordered state can have significantly different structural features than regular Langmuir monolayers of the same composition. Our results suggest that bilayers at air/water interfaces may be well suited for fundamental studies in the field of membrane biophysics.

Computation of Förster Resonance Energy Transfer in Lipid Bilayer Membranes

Calculations of Förster Resonance Energy Transfer (FRET) often neglect the influence of different chromophore orientations or changes in the spectral overlap. In this work, we present two computational approaches to estimate the energy transfer rate between chromophores embedded in lipid bilayer membranes. In the first approach, we assess the transition dipole moments and the spectral overlap by means of quantum chemical calculations in implicit solvation, and we investigate the alignment and distance between the chromophores in classical molecular dynamics simulations. In the second, all properties are evaluated integrally with hybrid quantum mechanical/molecular mechanics (QM/MM) calculations. Both approaches come with advantages and drawbacks, and despite the fact that they do not agree quantitatively, they provide complementary insights on the different factors that influence the FRET rate. We hope that these models can be used as a basis to optimize energy transfers in nonisotropic media.

Interferometric Probing of Physical and Chemical Properties of Solutions: Noncontact Investigation of Liquids

Interferometry is a highly versatile tool for probing physical and chemical phenomena. In addition to the benefit of noncontact investigations, even spatially resolved information can be obtained by choosing a suitable setup. This review presents the evolution of the various setups that have evolved since the first interferometers were developed in the mid-nineteenth century and highlights the benefits, limitations, and typical areas of application. This review focuses on interferometry based on electromagnetic waves in the near-infrared and visible range applied to liquid samples, categorizes the chemical/physical properties (e.g., pressure, temperature, composition) and phenomena (e.g., evaporation, crystal growth, diffusion, thermophoresis) that can be assessed, and presents a comprehensive literature review of specific existing applications. Finally, it discusses some fundamental open questions with respect to geometric considerations and overlapping effects. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

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.

Template-Stripped Ultra-Smooth Aluminum Films (0.2 nm RMS) for the Surface Forces Apparatus

We present a method for the fabrication of ultra-smooth (0.2 nm RMS), aluminum substrates through template stripping (TS). The method relies on the use of mica as a template in combination with thermal evaporation of Al at high (>10 nm/s) rates under vacuum (≤1 × 10^-7 Torr). As a reactive metal, Al is usually not considered a viable option for TS off oxide templates. However, under these conditions, the adhesion between the Al film and mica is poor, enabling the removal of the template under water without any mica residue. We verify the absence of mica using atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measurements. We establish the suitability of these films for surface forces measurements. Multiple-bean interferometry in transmission yields high quality fringes allowing for the measurement of force-distance curves. The adhesion the films to mica is significantly higher than the adhesion of thermally evaporated Al (0.9 nm RMS). Preliminary results suggest that the TS-Al surface displays a higher corrosion resistance. The fabrication method will enable important experiments on this widely used material.

Unmodified Clay Nanosheets at the Air-Water Interface

Quasi-two-dimensional (2D) nanolayers, such as graphene oxide or clay layers, adhere to gas-liquid or liquid-liquid interfaces. Particularly, clays are of wide general interest in this context because of their extensive and crucial use as Pickering emulsion stabilizers, as well as for their ability to provide colloidosome capsules. So far, clays could only be localized at oil-water or air-saline-water interfaces in aggregated states, while our results now show that clay nanosheets without any modification can be located at air-deionized-water interfaces. The clay mineral used in the present work is synthetic fluorohectorite with a very high aspect ratio and superior quality in homogeneity and charge distribution compared to other clay minerals. This clay mineral is more suitable for achieving unmodified clay anchoring to fluid interfaces compared to other clay minerals used in previous works. In this context, we studied clay nanosheet organization at the air-water interface by combining different experimental methods: Langmuir-Blodgett trough studies, scanning electron microscopy (SEM) studies of film deposits, grazing-incidence X-ray off-specular scattering (GIXOS), and Brewster angle microscopy (BAM). Clay films formed at the air-water interface could be transferred to solid substrates by the Langmuir-Schaefer method. The BAM results indicate a dynamic equilibrium between clay sheets on the interface and in the subphase. Because of this dynamic equilibrium, the Langmuir monolayer surface pressure does not change significantly when pure clay sheets are spread on the liquid surface. However, also, GIXOS results confirm that there are clay nanosheets at the air-water interface. In addition, we find that clay sheets modified by a branched polymer are much more likely to be confined to the interface.

Low-flux scanning electron diffraction reveals substructures inside the ordered membrane domain

Ordered/disordered phase separation occurring in bio-membranes has piqued researchers’ interest because these ordered domains, called lipid rafts, regulate important biological functions. The structure of the ordered domain has been examined with artificial membranes, which undergo macroscopic ordered/disordered phase separation. However, owing to technical difficulties, the local structure inside ordered domains remains unknown. In this study, we employed electron diffraction to examine the packing structure of the lipid carbon chains in the ordered domain. First, we prepared dehydrated monolayer samples using a rapid-freezing and sublimation protocol, which attenuates the shrinkage of the chain-packing lattice in the dehydration process. Then, we optimised the electron flux to minimise beam damage to the monolayer sample. Finally, we developed low-flux scanning electron diffraction and assessed the chain packing structure inside the ordered domain formed in a distearoylphosphatidylcholine/dioleoylphosphatidylcholine binary monolayer. Consequently, we discovered that the ordered domain contains multiple subdomains with different crystallographic axes. Moreover, the size of the subdomain is larger in the domain centre than that near the phase boundary. To our knowledge, this is the first study to reveal the chain packing structures inside an ordered domain.

Yeast Sphingolipid-Enriched Domains and Membrane Compartments in the Absence of Mannosyldiinositolphosphorylceramide

The relevance of mannosyldiinositolphosphorylceramide [M(IP)₂C] synthesis, the terminal complex sphingolipid class in the yeast Saccharomyces cerevisiae, for the lateral organization of the plasma membrane, and in particular for sphingolipid-enriched gel domains, was investigated by fluorescence spectroscopy and microscopy. We also addressed how changing the complex sphingolipid profile in the plasma membrane could influence the membrane compartments (MC) containing either the arginine/ H⁺ symporter Can1p (MCC) or the proton ATPase Pma1p (MCP). To achieve these goals, wild-type (wt) and ipt1Δ cells, which are unable to synthesize M(IP)₂C accumulating mannosylinositolphosphorylceramide (MIPC), were compared. Living cells, isolated plasma membrane and giant unilamellar vesicles reconstituted from plasma membrane lipids were labelled with various fluorescent membrane probes that report the presence and organization of distinct lipid domains, global order, and dielectric properties. Can1p and Pma1p were tagged with GFP and mRFP, respectively, in both yeast strains, to evaluate their lateral organization using confocal fluorescence intensity and fluorescence lifetime imaging. The results show that IPT1 deletion strongly affects the rigidity of gel domains but not their relative abundance, whereas no significant alterations could be perceived in ergosterol-enriched domains. Moreover, in these cells lacking M(IP)₂C, a clear alteration in Pma1p membrane distribution, but no significant changes in Can1p distribution, were observed. Thus, this work reinforces the notion that sphingolipid-enriched domains distinct from ergosterol-enriched regions are present in the S. cerevisiae plasma membrane and suggests that M(IP)₂C is important for a proper hydrophobic chain packing of sphingolipids in the gel domains of wt cells. Furthermore, our results strongly support the involvement of sphingolipid domains in the formation and stability of the MCP, possibly being enriched in this compartment.

Structure of DPPC Monolayers at the Air/Buffer Interface: A Neutron Reflectometry and Ellipsometry Study

Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine, known as DPPC, at the air/water interface are extensively used as model systems of biomembranes and pulmonary surfactant. The properties of these monolayers have been mainly investigated by surface pressure–area isotherms coupled with different complementary techniques such as Brewster angle microscopy, for example. Several attempts using neutron reflectometry (NR) or ellipsometry have also appeared in the literature. Here, we report structural information obtained by using NR and ellipsometry on DPPC monolayers in the liquid condensed phase. On one side, NR can resolve the thickness of the aliphatic tails and the degree of hydration of the polar headgroups. On the other side, ellipsometry gives information on the refractive index and, therefore, on the physical state of the monolayer. The thickness and surface excess obtained by multiple-angle-of-incidence ellipsometry (MAIE) is compared with the results from NR measurements yielding a good agreement. Besides, a novel approach is reported to calculate the optical anisotropy of the DPPC monolayer that depends on the orientation of the aliphatic chains. The results from both NR and ellipsometry are also discussed in the context of the existing results for DPPC monolayers at the air/water interface. The differences observed are rationalized by the presence of buffer molecules interacting with phospholipids.

Whole-Brain Profiling of Cells and Circuits in Mammals by Tissue Clearing and Light-Sheet Microscopy

Tissue clearing and light-sheet microscopy have a 100-year-plus history, yet these fields have been combined only recently to facilitate novel experiments and measurements in neuroscience. Since tissue-clearing methods were first combined with modernized light-sheet microscopy a decade ago, the performance of both technologies has rapidly improved, broadening their applications. Here, we review the state of the art of tissue-clearing methods and light-sheet microscopy and discuss applications of these techniques in profiling cells and circuits in mice. We examine outstanding challenges and future opportunities for expanding these techniques to achieve brain-wide profiling of cells and circuits in primates and humans. Such integration will help provide a systems-level understanding of the physiology and pathology of our central nervous system.

How Well Can You Tailor the Charge of Lipid Vesicles?

Knowledge and control of surface charge or potential is important for tailoring colloidal interactions. In this work, we compare widely used zeta potential (ζ) measurements of charged lipid vesicle surface potential to direct measurements using the surface force apparatus (SFA). Our measurements show good agreement between the two techniques. On varying the fraction of anionic lipids dimyristoylphosphatidylserine (DMPS) or dimyristoylphosphatidylglycerol (DMPG) mixed with zwitterionic dimyristoylphosphatidylcholine (DMPC) from 0 to 100 mol % we observed a near-linear increase in membrane surface charge or potential up to 20-30 mol % charged lipids beyond which charge saturation occurred in physiological (high) salt conditions. Similarly, in low salt concentrations, a linear increase in charge/potential was found but only up to ∼5-10 mol % charged lipids beyond which the surface charge or potential leveled off. While a lower degree of ionization is expected due to the lower dielectric constant (ε ∼ 4) of the lipid acyl chain environment, increasing intramembrane electrostatic repulsion between neighboring charged lipid head groups at higher charge loading contributes to charge suppression. Measured potentials in physiological salt solutions were consistent with predictions using the Gouy-Chapman-Stern-Grahame (GCSG) model of the electrical double layer with Langmuir binding of counterions, but in low salt conditions, the model significantly overestimated the surface charge/potential. The much lower ionization in low salt (maximum ∼1-2% of total lipids ionized) instead was consistent with counterion condensation at the bilayer surface which limited the charge that could be obtained. The strong interplay between membrane composition, lipid headgroup ionization, electrolyte concentration, and solution pH complicates exact prediction and tuning of membrane surface charge for applications. However, the theoretical frameworks used here can provide guidelines to understand this interplay and establish a range of achievable potentials for a system and predict the response to triggers like pH and salt concentration changes.

Lipid and Protein Transfer between Nanolipoprotein Particles and Supported Lipid Bilayers

A nanolipoprotein particle (NLP) is a lipid bilayer disc stabilized by two amphipathic "scaffold" apolipoproteins. It has been most notably utilized as a tool for solubilizing a variety of membrane proteins while preserving structural and functional properties. Transfer of functional proteins from NLPs into model membrane systems such as supported lipid bilayers (SLBs) would enable new opportunities, for example, two-dimensional protein crystallization and studies on protein-protein interactions. This work used fluorescence microscopy and atomic force microscopy to investigate the interaction between NLPs and SLBs. When incubated with SLBs, NLPs were found to spontaneously deliver lipid and protein cargo. The impact of membrane composition on lipid exchange was explored, revealing a positive correlation between the magnitude of lipid transfer and concentration of defects in the target SLB. Incorporation of lipids capable of binding specifically to polyhistidine tags encoded into the apolipoproteins also boosted transfer of NLP cargo. Optimal conditions for lipid and protein delivery from NLPs to SLBs are proposed based on interaction mechanisms.

Optimizing multiple beam interferometry in the surface forces apparatus: Novel optics, reflection mode modeling, metal layer thicknesses, birefringence, and rotation of anisotropic layers

Multiple beam interferometry (MBI) evolved as a powerful tool for the simultaneous evaluation of thin film thicknesses and refractive indices in Surface Forces Apparatus (SFA) measurements. However, analysis has relied on simplifications for providing fast or simplified analysis of recorded interference spectra. Here, we describe the implementation of new optics and a generalized fitting approach to 4 × 4 transfer matrix method simulations for the SFA. Layers are described by dispersive complex refractive indices, thicknesses, and Euler angles that can be fitted, providing modeling for birefringent or colored layers. Normalization of data by incident light intensities is essential for the implementation of a fitting approach. Therefore, a modular optical system is described that can be retrofit to any existing SFA setup. Real-time normalization of spectra by white light is realized, alignment procedures are considerably simplified, and direct switching between transmission and reflection modes is possible. A numerical approach is introduced for constructing transfer matrices for birefringent materials. Full fitting of data to the simulation is implemented for arbitrary multilayered stacks used in SFA. This enables self-consistent fitting of mirror thicknesses, birefringence, and relative rotation of anisotropic layers (e.g., mica), evaluation of reflection and transmission mode spectra, and simultaneous fitting of thicknesses and refractive indices of media confined between two surfaces. In addition, a fast full spectral fitting method is implemented for providing a possible real-time analysis with up to 30 fps. We measure and analyze refractive indices of confined cyclohexane, the thickness of lipid bilayers, the thickness of metal layers, the relative rotation of birefringent materials, contact widths, as well as simultaneous fitting of both reflection and transmission mode spectra of typical interferometers. Our analyses suggest a number of best practices for conducting SFA and open MBI in an SFA for increasingly complex systems, including metamaterials, multilayered anisotropic layers, and chiral layers.

Ellipsometry of human tears

PURPOSE

The outer surface layer of tears is presumably composed of lipid. The thickness of this layer is considered critical to retard evaporation. Prior thickness measurements differ widely. Advances in ellipsometry have availed more precise and accurate measurements for thin films. The range in thickness of the surface layer of tears was studied by ellipsometry to uncover the source of prior discrepancies.

METHODS

Tear surface layers of normal and dry eye subjects were measured by in-vitro ellipsometry. Lateral and Z resolutions of ∼1 μm and 0.1 nm, were achieved respectively. Thicknesses were derived from matrices and a Levenberg-Marquardt multivariate regression algorithm to Fresnel equations for multi-layered films.

RESULTS

Ellipsometric measurements of pooled and individual human tears in-vitro revealed a larger overall range (0-500 nm) of surface film thicknesses than previously reported by any one study. Each sample showed thin areas (0-2.6 nm) with interspersed thicker regions (∼200-500 nm). Repeat measurements of a single donor collected at weekly intervals showed a broad range of surface thicknesses within and between samples. Thickness measurements from a dry eye subject overlapped that of normal subjects.

CONCLUSION

The data show that published disparity in surface film thickness may be attributable to limitations of prior methodologies. The range and overlap of surface film thicknesses challenge less rigorous methodologies that claim to segregate normal and dry eye.

Preparation and Characterization of Solid-Supported Lipid Bilayers Formed by Langmuir-Blodgett Deposition: A Tutorial

The structure, phase behavior, and properties of cellular membranes are derived from their composition, which includes phospholipids, sphingolipids, sterols, and proteins with various levels of glycosylation. Because of the intricate nature of cellular membranes, a plethora of in vitro studies have been carried out with model membrane systems that capture particular properties such as fluidity, permeability, and protein binding but vastly simplify the membrane composition in order to focus in detail on a specialized property or function. Supported lipid bilayers (SLB) are widely used as archetypes for cellular membranes, and this instructional review primarily focuses on the preparation and characterization of SLB systems formed by Langmuir deposition methods. Typical characterization methods, which take advantage of the planar orientation of SLBs, are illustrated, and references that go into more depth are included. This invited instructional review is written so that nonexperts can quickly gain in-depth knowledge regarding the preparation and characterization of SLBs. In addition, this work goes beyond traditional instructional reviews to provide expert readers with new results that cover a wider range of SLB systems than those previously reported in the literature. The quality of an SLB is frequently not well described, and details such as topological defects can influence the results and conclusions of an individual study. This article quantifies and compares the quality of SLBs fabricated from a variety of gel and fluid compositions, in correlation with preparation techniques and parameters, to generate general rules of thumb to guide the construction of designed SLB systems.

A Comparative Study of Phosphatidylcholine versus Phosphatidylserine-Based Solid Supported Membranes for the Preparation of Liposome-Rich Interfaces

Solid supported membranes (SSMs) are usually formed by an hybrid octadecanethiol/phosphatidylcholine (PC) bilayer supported by a gold electrode. Recently, it was shown that phosphatidylserine (PS) in place of PC can promote a more effective accumulation of lipid vesicles on the SSM surface when Ca²⁺ and Mg²⁺ ions are present in the external environment. Here we performed a detailed comparative study of the vesicle adsorption process onto PC- and PS-SSMs by employing surface plasmon resonance (SPR), electrochemical impedance spectroscopy (EIS), and atomic force microscopy (AFM). SPR analysis has demonstrated a higher affinity of the PS-SSM surface for the phospholipid vesicles. Both SPR and EIS measurements suggest that adsorption of lipid vesicles on the PC-SSM tends to a saturating value, whereas a continuous and progressive vesicle adsorption occurs on the PS-SSM surface following subsequent liposome additions. AFM analysis pointed out a systematic flattening of the adsorbed vesicles on the PS-SSM surface. We interpreted our results as due to the strong coordinating action of the high amount of divalent cations accumulated at the negatively charged PS-SSM surface, whereas a lower amount of cations is present on the dipolar PC-SSM surface, which can therefore adsorb only a limited number of vesicles.

Hollow organosilica beads as reference particles for optical detection of extracellular vesicles

Essentials Standardization of extracellular vesicle (EV) measurements by flow cytometry needs improvement. Hollow organosilica beads were prepared, characterized, and tested as reference particles. Light scattering properties of hollow beads resemble that of platelet-derived EVs. Hollow beads are ideal reference particles to standardize scatter flow cytometry research on EVs.

SUMMARY

Background The concentration of extracellular vesicles (EVs) in body fluids is a promising biomarker for disease, and flow cytometry remains the clinically most applicable method to identify the cellular origin of single EVs in suspension. To compare concentration measurements of EVs between flow cytometers, solid polystyrene reference beads and EVs were distributed in the first ISTH-organized interlaboratory comparison studies. The beads were used to set size gates based on light scatter, and the concentration of EVs was measured within the size gates. However, polystyrene beads lead to false size determination of EVs, owing to the mismatch in refractive index between beads and EVs. Moreover, polystyrene beads gate different EV sizes on different flow cytometers. Objective To prepare, characterize and test hollow organosilica beads (HOBs) as reference beads to set EV size gates in flow cytometry investigations. Methods HOBs were prepared with a hard template sol-gel method, and extensively characterized for morphology, size, and colloidal stability. The applicability of HOBs as reference particles was investigated by flow cytometry with HOBs and platelet-derived EVs. Results HOBs proved to be monodisperse with a homogeneous shell thickness. Two-angle light-scattering measurements by flow cytometry confirmed that HOBs have light-scattering properties similar to those of platelet-derived EVs. Conclusions Because the structure and light-scattering properties HOBs resemble those of EVs, HOBs with a given size will gate EVs of the same size. Therefore, HOBs are ideal reference beads with which to standardize optical measurements of the EV concentration within a predefined size range.

Properties of Artificial Phospholipid Membranes Containing Lauryl Gallate or Cholesterol

Lauryl gallate (LG) is an antioxidant agent. However, it exhibits poor solubility in water. Its interactions with the membrane result in structure evolution thus affecting the membrane functionality. In this paper the Brewster angle microscope coupled with the Langmuir trough was applied to determine the morphology, phase behaviour, thickness and miscibility of ternary Langmuir monolayers with equal mole fractions of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC); 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and an increasing mole fraction of LG. The results were discussed as regards analogous systems where cholesterol (Chol) was the third component. Moreover, the phosphatidylcholine–lauryl gallate (PC–LG) interactions were monitored by the attenuated total reflectance Fourier transform infrared spectroscopy and time-of-flight secondary ion mass spectrometry. Besides lipid composition, the addition of LG was found to be a significant factor to modulate the model membrane properties. The LG molecules adjust themselves to the PC monolayer structure. The hydrophobic fragment is dipped into the membrane interior while the hydroxyl groups of phenolic gallate moiety associate with the polar groups of PC mainly through hydrogen bonding inducing the compacting effect. LG is found to be deeply submerged within DOPC, closer to the double bonds, and its insertion practically does not affect the DPPC/DOPC membrane fluidity. This is crucial for getting more profound insight into the role of LG in stabilizing the non-raft domains, mostly exposed to oxidation in which LG can co-localize and serve its antioxidant function.

Microwave measurement of giant unilamellar vesicles in aqueous solution

A microwave technique is demonstrated to measure floating giant unilamellar vesicle (GUV) membranes in a 25 μm wide and 18.8 μm high microfluidic channel. The measurement is conducted at 2.7 and 7.9 GHz, at which a split-ring resonator (SRR) operates at odd modes. A 500 nm wide and 100 μm long SRR split gap is used to scan GUVs that are slightly larger than 25 μm in diameter. The smaller fluidic channel induces flattened GUV membrane sections, which make close contact with the SRR gap surface. The used GUVs are synthesized with POPC (16:0–18:1 PC 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), SM (16:0 Egg Sphingomyelin) and cholesterol at different molecular compositions. It is shown that SM and POPC bilayers have different dielectric permittivity values, which also change with measurement frequencies. The obtained membrane permittivity values, e.g. 73.64-j6.13 for POPC at 2.7 GHz, are more than 10 times larger than previously reported results. The discrepancy is likely due to the measurement of dielectric polarization parallel with, other than perpendicular to, the membrane surface. POPC and SM-rich GUV surface sections are also clearly identified. Further work is needed to verify the obtained large permittivity values and enable accurate analysis of membrane composition.

Contact-free calibration of an asymmetric multi-layer interferometer for the surface force balance

The Surface Force Balance (SFB, also known as Surface Force Apparatus, SFA) has provided important insights into many phenomena within the field of colloid and interface science. The technique relies on using white light interferometry to measure the distance between surfaces with sub-nanometer resolution. Up until now, the determination of the distance between the surfaces required a so-called "contact calibration," an invasive procedure during which the surfaces are brought into mechanical contact. This requirement for a contact calibration limits the range of experimental systems that can be investigated with SFB, for example, it precludes experiments with substrates that would be irreversibly modified or damaged by mechanical contact. Here we present a non-invasive method to measure absolute distances without performing a contact calibration. The method can be used for both "symmetric" and "asymmetric" systems. We foresee many applications for this general approach including, most immediately, experiments using single layer graphene electrodes in the SFB which may be damaged when brought into mechanical contact.

Refractive Index Imaging of Cells with Variable-Angle Near-Total Internal Reflection (TIR) Microscopy

The refractive index in the interior of single cells affects the evanescent field depth in quantitative studies using total internal reflection (TIR) fluorescence, but often that index is not well known. We here present method to measure and spatially map the absolute index of refraction in a microscopic sample, by imaging a collimated light beam reflected from the substrate/buffer/cell interference at variable angles of incidence. Above the TIR critical angle (which is a strong function of refractive index), the reflection is 100%, but in the immediate sub-critical angle zone, the reflection intensity is a very strong ascending function of incidence angle. By analyzing the angular position of that edge at each location in the field of view, the local refractive index can be estimated. In addition, by analyzing the steepness of the edge, the distance-to-substrate can be determined. We apply the technique to liquid calibration samples, silica beads, cultured Chinese hamster ovary cells, and primary culture chromaffin cells. The optical technique suffers from decremented lateral resolution, scattering, and interference artifacts. However, it still provides reasonable results for both refractive index (~1.38) and for distance-to-substrate (~150 nm) for the cells, as well as a lateral resolution to about 1 µm.

Ballistic Penetration of Highly Charged Nanoaerosol Particles through a Lipid Monolayer

To be used as a drug, inhaled nanoaerosol particles (NAPs) must first penetrate the lipid layer on top of the lung fluid before they will be able to reach the lung epithelium. We investigated how the penetration of NAPs through a model lipid monolayer (LM) depends upon their charging level and size. It was shown that deposition of NAPs 20-200 nm in diameter and charged to the Rayleigh limit gradually increased the surface tension of a dipalmitoylphosphatidylcholine monolayer (DPPC), indicating a loss of lipid molecules from the monolayer. This phenomenon was reproduced with a variety of NAPs produced from glucose, proteins, and polymers. Transfer of the lipid material into the subphase was documented by direct visualization of lipid nanoparticles in the subphase with atomic force microscopy after deposition of glucose NAPs on a DPPC monolayer, followed by collection of the lipid nanoparticles on a mica surface. Partial restoration of tension upon storage indicates that some of the lipid may return to the monolayer. Experiments with the deposition of highly charged calibrated polystyrene nanoparticles showed that the amount of lipid removed from the surface was roughly proportional to the overall surface area of the deposited NAPs. When the number of charges on the NAPs was reduced from their Rayleigh level of 10³-10⁴ units to 1-10 units, no notable changes in monolayer surface tension were observed even with prolonged deposition of such NAPs. It was therefore concluded that only highly charged NAPs of a certain size acquire sufficient speed from their attraction by mirror charges to enable ballistic penetration through a lipid monolayer.

The insertion of Polybia-MP1 peptide into phospholipid monolayers is regulated by its anionic nature and phase state

Polybia-MP1 or simply MP1 (IDWKKLLDAAKQIL-NH₂) is a peptide with broad-spectrum bactericidal activity and a strong inhibitory effect against cancer cells. The aim of this work was to evaluate the effect of biophysical properties such as membrane texture and film thickness on MP1 interaction with neutral and anionic lipid membranes. For this purpose, we first explored the peptide's surface behavior. MP1 showed high surface activity, adsorbing onto bare air/aqueous interfaces up to higher surface pressures than the collapse pressure of MP1 Langmuir films. The MP1-lipid membrane interaction was studied using Langmuir phosphatidylcholine and phosphatidylserine (PS) monolayers as model membrane systems. PS was chosen since this negatively charged lipid was found predominantly on the outer leaflet of tumor cells, and it enhances MP1 activity for PS-containing membranes to a greater extent than for other negatively charged lipids. MP1 incorporated into anionic PS monolayers, which show a liquid-expanded (LE) phase or LE-liquid-condensed (LC) phase coexistence, up to lipid-packing densities higher than those of cell membranes. The mixed lipid/MP1 films were explored by Brewster angle microscopy and atomic force microscopy. MP1 partitioned preferentially into the LE phase state of PS films, and were thus excluded from the coexisting LC phase. This interaction had strong electrostatic bases: in pure water, the lipid-peptide interaction was strong enough to induce formation of reversible lipid-peptide 3D structures associated with the interface. MP1 incorporation into the LE phase was accompanied by a shift of the phase transition pressure to higher values and a thinning of the lipid film. These results showed a clear correlation between peptide penetration capacity and the presence or induction of the thin LE phase. This capacity to regulate membrane physical properties may be of relevance in the binding, incorporation and membrane selectivity of this promising antitumor peptide.

Interaction Forces between Lipid Rafts

Cellular membranes containing sphingolipids and cholesterol have been shown to self-organize into lipid rafts-specialized domains that host integral membrane proteins and modulate the bioactivity of cells. In this work, force-distance profiles between raft membranes in the liquid-ordered phase consisting of singly unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a complex mixture of brain sphingomyelin (BSM), and cholesterol were measured using the surface force apparatus (SFA). Two distinct force profiles were detected corresponding to uniform raft membranes and raft membranes with a higher level of topological membrane defects (heterogeneous) as corroborated by atomic force microscopy (AFM) scans. In all cases a weak, long-range electrostatic repulsion was observed with some variation in the surface charge density. The variation in electrostatic repulsion was attributed to charged lipid species primarily from the constituent lipids in the BSM mixture. The adhesion between the uniform raft membranes was comparable to our previous work with pure component, liquid-ordered POPC-DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine)-cholesterol membranes. Raft membranes with more topological defects adhered more strongly owing to hydrophobic attraction between exposed acyl chains. Even though the rafts were in the liquid-ordered phase and membrane defects were present in the contact region, the raft membranes were stable, and no structural rearrangement was observed throughout the measurements. Our findings demonstrate that liquid-ordered membranes are stable to mechanical loading and not particularly sensitive to compositional variation.

Density and Phase State of a Confined Nonpolar Fluid

Measurements of the mean refractive index of a spherelike nonpolar fluid, octamethytetracylclosiloxane (OMCTS), confined between mica sheets, demonstrate direct and conclusive experimental evidence of the absence of a first-order liquid-to-solid phase transition in the fluid when confined, which has been suggested to occur from previous experimental and simulation results. The results also show that the density remains constant throughout confinement, and that the fluid is incompressible. This, along with the observation of very large increases (many orders of magnitude) in viscosity during confinement from the literature, demonstrate that the molecular motion is limited by the confining wall and not the molecular packing. In addition, the recently developed refractive index profile correction method, which enables the structural perturbation inherent at a solid-liquid interface and that of a liquid in confinement to be determined independently, was used to show that there was no measurable excess or depleted mass of OMCTS near the mica surface in bulk films or confined films of only two molecular layers.

Simulation Method Linking Dense Microalgal Culture Spectral Properties in the 400-750 nm Range to the Physiology of the Cells

This work describes a method to model the optical properties over the (400-750 nm) spectral range of a dense microalgal culture using the chemical and physical properties of the algal cells. The method was based on a specific program called AlgaSim coupled with the adding-doubling method: at the individual cell scale, AlgaSim simulates the spectral properties of one model, three-layer spherical algal cell from its size and chemical composition. As a second step, the adding-doubling method makes it possible to retrieve the total transmittance of the algal medium from the optical properties of the individual algal cells. The method was tested by comparing the simulated total transmittance spectra for dense marine microalgal cultures of Isochrysis galbana (small flagellates) and Phaeodactylum tricornutum (diatoms) to spectra measured using an experimental spectrophotometric setup. Our study revealed that the total transmittance spectra simulated for the quasi-spherical cells of Isochrysis galbana were in good agreement with the measured spectra over the whole spectral range. For Phaeodactylum tricornutum, large differences between simulated and measured spectra were observed over the blue part of the transmittance spectra, probably due to non-spherical shape of the algal cells. Prediction of the algal cell density, mean size and pigment composition from the total transmittance spectra measured on algal samples was also investigated using the reversal of the method. Mean cell size was successfully predicted for both species. The cell density was also successfully predicted for spherical Isochrysis galbana, with a relative error below 7%, but not for elongated Phaeodactylum tricornutum with a relative error up to 26%. The pigments total quantity and composition, the carotenoids:chlorophyll ratio in particular, were also successfully predicted for Isochrysis galbana with a relative error below 8%. However, the pigment predictions and measurements for Phaeodactylum tricornutum showed large discrepancies, with a relative error up to 88%. These results give strong support for the development of a promising tool providing rapid and accurate estimations of biomass and physiological status of a dense microalgal culture based on only light transmittance properties.

Characterization of solid-supported dipalmitoylphosphatidylcholine membranes containing cholesterol

The incorporation of cholesterol into dipalmitoylphosphatidylcholine (DPPC) membranes, even in small amounts, has been shown to significantly alter the properties of the membrane. In this work, force-distance interaction profiles of DPPC membranes containing 8 mol % cholesterol obtained using the surface force apparatus are analyzed in the context of high-resolution structural characterization by atomic force microscopy and neutron reflectometry. The adhesion between the mixed membranes was greater than that for pure DPPC and was variable-depending on the number of defects in the outer membrane leaflets. These defects were only detectable by atomic force microscopy and had an average size of 230 ± 30 nm and 1-5% surface density in the outer leaflet. The adhesion between the membranes monotonically increased as the thickness of the membrane decreased-in direct correlation with the number of defects present (exposed hydrophobic groups) in the membrane contact region. Because of the low diffusion rate of gel-phase membranes, the interaction force profiles were stable and no membrane restructuring was observed.

Analyzing refractive index profiles of confined fluids by interferometry

This work describes an interferometry data analysis method for determining the optical thickness of thin films or any variation in the refractive index of a fluid or film near a surface. In particular, the method described is applied to the analysis of interferometry data taken with a surface force apparatus (SFA). The technique does not require contacting or confining the fluid or film. By analyzing interferometry data taken at many intersurface separation distances out to at least 300 nm, the properties of a film can be quantitatively determined. The film can consist of material deposited on the surface, like a polymer brush, or variation in a fluid's refractive index near a surface resulting from, for example, a concentration gradient, depletion in density, or surface roughness. The method is demonstrated with aqueous polyethylenimine (PEI) adsorbed onto mica substrates, which has a large concentration and therefore refractive index gradient near the mica surface. The PEI layer thickness determined by the proposed method is consistent with the thickness measured by conventional SFA methods. Additionally, a thorough investigation of the effects of random and systematic error in SFA data analysis and modeling via simulations of interferometry is described in detail.