Effect of pH on the interfacial tension of lipid bilayer membrane

Ascorbic acid reduction pretreatment enhancing metal regulation to improve methane production from anaerobic digestion of waste activated sludge

Conversion of waste activated sludge (WAS) to methane by anaerobic digestion (AD) is often limited by the slow rate of hydrolysis, and the presence of metal ions in sludge is regarded as a critical factor hindering sludge hydrolysis. This study developed a novel strategy to remove Fe from WAS by using ascorbic acid (VC) as a reducing agent under acidic conditions. The feasibility of reduction pretreatment in improving methane production of AD and its intrinsic mechanism were investigated. Results indicate that, under VC doses of 100 mmol/L and pH of 3.50, pretreatment removed 47.60 % of Fe, 59.88 % of Ca, and 51.86 % of Mg contained in the sludge. The removal of metal ions facilitated the disruption of sludge flocculation structure and extracellular polymeric substance (EPS) layers, leading to a 14.78 % increase in cell lysis and a decrease in fractal dimension values to 2.08. Batch AD experiments showed that VC pretreatment improved methane production, with an optimized net methane yield of 190.22 mL/g·VS, an increase of 134.75 % compared to raw WAS. The pretreatment affected the interfacial interaction energy of the sludge, leading to a transformation in the sludge surfaces from hydrophilic to hydrophobic, reducing the interaction between sludge molecules and increasing the number of binding sites available for enzymatic reactions. According to a study of microbial communities, it was found that VC pretreatment caused an increase in the presence of essential functional microbes responsible for hydrolysis, acidification, and methanation. This increase in acetoclastic and hydrogenotrophic methanogens resulted in a substantial enhancement in methane production. These results can be used to develop better pretreatment methods to enhance AD performance.

Physical Chemistry of Chloroquine Permeation through the Cell Membrane with Atomistic Detail

We provide a molecular-level description of the thermodynamics and mechanistic aspects of drug permeation through the cell membrane. As a case study, we considered the antimalaria FDA approved drug chloroquine. Molecular dynamics simulations of the molecule (in its neutral and protonated form) were performed in the presence of different lipid bilayers, with the aim of uncovering key aspects of the permeation process, a fundamental step for the drug's action. Free energy values obtained by well-tempered metadynamics simulations suggest that the neutral form is the only permeating protomer, consistent with experimental data. H-bond interactions of the drug with water molecules and membrane headgroups play a crucial role for permeation. The presence of the transmembrane potential, investigated here for the first time in a drug permeation study, does not qualitatively affect these conclusions.

Electrostatic Potentials Caused by the Release of Protons from Photoactivated Compound Sodium 2-Methoxy-5-nitrophenyl Sulfate at the Surface of Bilayer Lipid Membrane

Lateral transport and release of protons at the water-membrane interface play crucial roles in cell bioenergetics. Therefore, versatile techniques need to be developed for investigating as well as clarifying the main features of these processes at the molecular level. Here, we experimentally measured the kinetics of binding of protons released from the photoactivated compound sodium 2-methoxy-5-nitrophenyl sulfate (MNPS) at the surface of a bilayer lipid membrane (BLM). We developed a theoretical model of this process describing the damage of MNPS coupled with the release of the protons at the membrane surface, as well as the exchange of MNPS molecules and protons between the membrane and solution. We found that the total change in the boundary potential difference across the membrane, ∆ϕb, is the sum of opposing effects of adsorption of MNPS anions and release of protons at the membrane-water interface. Steady-state change in the ∆ϕb due to protons decreased with the concentration of the buffer and increased with the pH of the solution. The change in the concentration of protons evaluated from measurements of ∆ϕb was close to that in the unstirred water layer near the BLM. This result, as well as rate constants of the proton exchange between the membrane and the bulk solution, indicated that the rate-limiting step of the proton surface to bulk release is the change in the concentration of protons in the unstirred layer. This means that the protons released from MNPS remain in equilibrium between the BLM surface and an adjacent water layer.

Macrophage Cell Membrane Coating on Piperine-Loaded MIL-100(Fe) Nanoparticles for Breast Cancer Treatment

Piperine (PIP), a compound found in Piper longum, has shown promise as a potential chemotherapeutic agent for breast cancer. However, its inherent toxicity has limited its application. To overcome this challenge, researchers have developed PIP@MIL-100(Fe), an organic metal-organic framework (MOF) that encapsulates PIP for breast cancer treatment. Nanotechnology offers further treatment options, including the modification of nanostructures with macrophage membranes (MM) to enhance the evasion of the immune system. In this study, the researchers aimed to evaluate the potential of MM-coated MOFs encapsulated with PIP for breast cancer treatment. They successfully synthesized MM@PIP@MIL-100(Fe) through impregnation synthesis. The presence of MM coating on the MOF surface was confirmed through SDS-PAGE analysis, which revealed distinct protein bands. Transmission electron microscopy (TEM) images demonstrated the existence of a PIP@MIL-100(Fe) core with a diameter of around 50 nm, surrounded by an outer lipid bilayer layer measuring approximately 10 nm in thickness. Furthermore, the researchers evaluated the cytotoxicity indices of the nanoparticles against various breast cancer cell lines, including MCF-7, BT-549, SKBR-3, and MDA. The results demonstrated that the MOFs exhibited between 4 and 17 times higher cytotoxicity (IC₅₀) in all four cell lines compared to free PIP (IC₅₀ = 193.67 ± 0.30 µM). These findings suggest that MM@PIP@MIL-100(Fe) holds potential as an effective treatment for breast cancer. The study's outcomes highlight the potential of utilizing MM-coated MOFs encapsulated with PIP as an innovative approach for breast cancer therapy, offering improved cytotoxicity compared to free PIP alone. Further research and development are warranted to explore the clinical translation and optimize the efficacy and safety of this treatment strategy.

pH-Modulated Nanoarchitectonics for Enhancement of Multivalency-Induced Vesicle Shape Deformation at Receptor-Presenting Lipid Membrane Interfaces

Multivalent ligand-receptor interactions between receptor-presenting lipid membranes and ligand-modified biological and biomimetic nanoparticles influence cellular entry and fusion processes. Environmental pH changes can drive these membrane-related interactions by affecting membrane nanomechanical properties. Quantitatively, however, the corresponding effects on high-curvature, sub-100 nm lipid vesicles are scarcely understood, especially in the multivalent binding context. Herein, we employed the label-free localized surface plasmon resonance (LSPR) sensing technique to track the multivalent attachment kinetics, shape deformation, and surface coverage of biotin ligand-functionalized, zwitterionic lipid vesicles with different ligand densities on a streptavidin receptor-coated supported lipid bilayer under varying pH conditions (4.5, 6, 7.5). Our results demonstrate that more extensive multivalent interactions caused greater vesicle shape deformation across the tested pH conditions, which affected vesicle surface packing as well. Notably, there were also pH-specific differences, i.e., a higher degree of vesicle shape deformation was triggered at a lower multivalent binding energy in pH 4.5 than in pH 6 and 7.5 conditions. These findings support that the nanomechanical properties of high-curvature lipid membranes, especially the membrane bending energy and the corresponding responsiveness to multivalent binding interactions, are sensitive to solution pH, and indicate that multivalency-induced vesicle shape deformation occurs slightly more readily in acidic pH conditions relevant to biological environments.

Molecular dynamics insights on temperature and pressure effects on electroporation

Electroporation is a cell-level phenomenon caused by an ionic imbalance in the membrane, being of great relevance in various fields of knowledge. A dependence of the pore formation kinetics on the environmental conditions (temperature and pressure) of the cell membrane has already been reported, but further clarification regarding how these variables affect the pore formation/resealing dynamics and the transport of molecules through the membrane is still lacking. The objective of the present study was to investigate the temperature (288-348 K) and pressure (1-5000 atm) effects on the electroporation kinetics using coarse-grained molecular dynamics simulations. Results shown that the time for pore formation and resealing increased with pressure and decreased with temperature, whereas the maximum pore radius increased with temperature and decreased with pressure. This behavior influenced the ion migration through the bilayer, and the higher ionic mobility was obtained in the 288 K/1000 atm simulations, i.e., a combination of low temperature and (not excessively) high pressure. These results were used to discuss some experimental observations regarding the extraction of intracellular compounds applying this technique. This study contributes to a better understanding of electroporation under different thermodynamic conditions and to an optimal selection of processing parameters in practical applications which exploit this phenomenon.

A Physicochemical Consideration of Prebiotic Microenvironments for Self-Assembly and Prebiotic Chemistry

The origin of life on Earth required myriads of chemical and physical processes. These include the formation of the planet and its geological structures, the formation of the first primitive chemicals, reaction, and assembly of these primitive chemicals to form more complex or functional products and assemblies, and finally the formation of the first cells (or protocells) on early Earth, which eventually evolved into modern cells. Each of these processes presumably occurred within specific prebiotic reaction environments, which could have been diverse in physical and chemical properties. While there are resources that describe prebiotically plausible environments or nutrient availability, here, we attempt to aggregate the literature for the various physicochemical properties of different prebiotic reaction microenvironments on early Earth. We introduce a handful of properties that can be quantified through physical or chemical techniques. The values for these physicochemical properties, if they are known, are then presented for each reaction environment, giving the reader a sense of the environmental variability of such properties. Such a resource may be useful for prebiotic chemists to understand the range of conditions in each reaction environment, or to select the medium most applicable for their targeted reaction of interest for exploratory studies.

Heterosynaptic plasticity in biomembrane memristors controlled by pH

Abstract

In biology, heterosynaptic plasticity maintains homeostasis in synaptic inputs during associative learning and memory, and initiates long-term changes in synaptic strengths that nonspecifically modulate different synapse types. In bioinspired neuromorphic circuits, heterosynaptic plasticity may be used to extend the functionality of two-terminal, biomimetic memristors. In this article, we explore how changes in the pH of droplet interface bilayer aqueous solutions modulate the memristive responses of a lipid bilayer membrane in the pH range 4.97-7.40. Surprisingly, we did not find conclusive evidence for pH-dependent shifts in the voltage thresholds (V*) needed for alamethicin ion channel formation in the membrane. However, we did observe a clear modulation in the dynamics of pore formation with pH in time-dependent, pulsed voltage experiments. Moreover, at the same voltage, lowering the pH resulted in higher steady-state currents because of increased numbers of conductive peptide ion channels in the membrane. This was due to increased partitioning of alamethicin monomers into the membrane at pH 4.97, which is below the pKa (~5.3-5.7) of carboxylate groups on the glutamate residues of the peptide, making the monomers more hydrophobic. Neutralization of the negative charges on these residues, under acidic conditions, increased the concentration of peptide monomers in the membrane, shifting the equilibrium concentrations of peptide aggregate assemblies in the membrane to favor greater numbers of larger, increasingly more conductive pores. It also increased the relaxation time constants for pore formation and decay, and enhanced short-term facilitation and depression of the switching characteristics of the device. Modulating these thresholds globally and independently of alamethicin concentration and applied voltage will enable the assembly of neuromorphic computational circuitry with enhanced functionality.

Impact statement

We describe how to use pH as a modulatory "interneuron" that changes the voltage-dependent memristance of alamethicin ion channels in lipid bilayers by changing the structure and dynamical properties of the bilayer. Having the ability to independently control the threshold levels for pore conduction from voltage or ion channel concentration enables additional levels of programmability in a neuromorphic system. In this article, we note that barriers to conduction from membrane-bound ion channels can be lowered by reducing solution pH, resulting in higher currents, and enhanced short-term learning behavior in the form of paired-pulse facilitation. Tuning threshold values with environmental variables, such as pH, provide additional training and learning algorithms that can be used to elicit complex functionality within spiking neural networks.

Graphical abstract

Supplementary information

The online version contains supplementary material available at 10.1557/s43577-022-00344-z.

Micrometer-size double-helical structures from phospholipid-modified carbon nanotubes

Biomolecular self-assembly plays a key role in the life system. Herein, double-helical phospholipid-modified carbon nanotube structures were constructed via the self-assembly of phospholipids on carbon nanotubes. These micrometer size spring structures may find potential applications in biocompatible microrobots.

Reversible photonic hydrogel sensors via holographic interference lithography

Continuous monitoring of physiological conditions and biomarkers via optical holographic sensors is an area of growing interest to facilitate the expansion of personalised medicine. Here, a facile laser-induced dual polymerization method is developed to fabricate holographic hydrogel sensors for the continuous and reversible colorimetric determination of pH variations over a physiological range in serum (pH 7-9). Readout parameters simulated through a Finite-difference time-domain Yee's algorithm retrieve the spectral response through expansion. Laser lithography of holographic hydrogel sensor fabrication is achieved via a single 355 nm laser pulse to initiate polymerization of ultrafine hydrogel fringes. Eliminating the requirement for complex processing of toxic components and streamlining the synthetic procedure provides a simpler route to mass production. Optimised pH-responsive hydrogels contain amine bearing functional co-monomers demonstrating reversible Bragg wavelength shifts of 172 nm across the entire visible wavelength range with pH variation from 7.0 to 9.0 upon illumination with broadband light. Photolithographic recording of information shows the ability to convey detailed information to users for qualitative identification of pH. Holographic sensor reversibility over 20 cycles showed minimal variation in replay wavelength supporting reliable and consistent readout, with optimised sensors showing rapid response times of <5 min. The developed sensors demonstrate the application to continuous monitoring in biological fluids, withstanding interference from electrolytes, saccharides, and proteins colorimetrically identifying bovine serum pH over a physiological range. The holographic sensors benefit point-of-care pH analysis of biological analytes which could be applied to the identification of blood gas disorders and wound regeneration monitoring through colorimetric readouts.

Systematic analysis of specific and nonspecific auxin effects on endocytosis and trafficking

The phytohormone auxin and its directional transport through tissues are intensively studied. However, a mechanistic understanding of auxin-mediated feedback on endocytosis and polar distribution of PIN auxin transporters remains limited due to contradictory observations and interpretations. Here, we used state-of-the-art methods to reexamine the auxin effects on PIN endocytic trafficking. We used high auxin concentrations or longer treatments versus lower concentrations and shorter treatments of natural indole-3-acetic acid (IAA) and synthetic naphthalene acetic acid (NAA) auxins to distinguish between specific and nonspecific effects. Longer treatments of both auxins interfere with Brefeldin A-mediated intracellular PIN2 accumulation and also with general aggregation of endomembrane compartments. NAA treatment decreased the internalization of the endocytic tracer dye, FM4-64; however, NAA treatment also affected the number, distribution, and compartment identity of the early endosome/trans-Golgi network, rendering the FM4-64 endocytic assays at high NAA concentrations unreliable. To circumvent these nonspecific effects of NAA and IAA affecting the endomembrane system, we opted for alternative approaches visualizing the endocytic events directly at the plasma membrane (PM). Using total internal reflection fluorescence microscopy, we saw no significant effects of IAA or NAA treatments on the incidence and dynamics of clathrin foci, implying that these treatments do not affect the overall endocytosis rate. However, both NAA and IAA at low concentrations rapidly and specifically promoted endocytosis of photo-converted PIN2 from the PM. These analyses identify a specific effect of NAA and IAA on PIN2 endocytosis, thus, contributing to its polarity maintenance and furthermore illustrate that high auxin levels have nonspecific effects on trafficking and endomembrane compartments.

Oral genistein-loaded phytosomes with enhanced hepatic uptake, residence and improved therapeutic efficacy against hepatocellular carcinoma

Genistein (Gen) is one of the most potent soy isoflavones used for hepatocellular carcinoma (HCC) treatment. Low aqueous solubility and first-pass metabolism are the main obstacles resulting in low Gen oral bioavailability. The current study aims to introduce phytosomes as an approach to improve Gen solubility, protect it from metabolism by complexation with phospholipids (PL), and get used to PL in Gen lymphatic delivery. Different forms of PL namely: Lipiod® S100, Phosal® 53 MCT, and Phosal®75 SA were used in phytosomes preparation GP, GPM, and GPL respectively. The effect of formulation components on Gen absorption, metabolism, and liver accumulation was evaluated following oral administration to rats. Cytotoxicity and cellular uptake studies were applied on HepG2 cells and in-vivo anti-tumor studies were applied to the DEN-mice model. Results revealed that GP and GPL remarkably accumulated Gen aglycone in hepatic cells and minimized the metabolic effect on Gen. They significantly increased the intracellular accumulation of Gen in its complex form in HepG2 cells. Their cytotoxicity is time-dependent according to the complex stability. The enhanced in-vivo anti-tumor effect was observed for GP and GPL compared to Gen suspension on DEN-induced HCC in mice. In conclusion, Gen-phytosomes can represent a promising approach for liver cancer treatment.

Membrane tension may define the deadliest virus infection

This manuscript describes the potentially significant role of interfacial tension in viral infection. Our hypothesis is based on evidence from drop coalescence hydrodynamics. A change in membrane tension can trigger fusion between the vesicle and cell such that genetic material, like viral RNA, can subsequently be transported to the cell interior. In other cases, RNA may reside near the cell membrane inside the cell, which could make their removal energetically unfavorable because of hydrodynamic interactions between membrane and RNA. Interfacial tension of the virus membrane can be modulated by temperature, among many other factors, of the mucosa layer. We discuss our hypothesis within the scope of recent SARS-CoV-2 studies where temperature-dependent membrane surface tension could be impacted through different atmospheric conditions, air conditioning systems, and the use of masks.

Role of pH level on the morphology and growth rate of myelin figures

The myelin figure (MF) is one of the basic structures of lipids, and the study of their formation and the effect of various parameters on their growth is useful in understanding several biological processes. In this paper, we address the influence of the pH degree of the surrounding medium on MF dynamics. We introduce a tunable shearing digital holographic microscopy arrangement to obtain quantitative and volumetric information about the complex growth of MFs. Our results show that (1) the time evolution of relative length and volume changes of MFs follows a power-law, (2) the acidity facilitates the growth rate, and (3) the acidic environment causes the formation of thicker MFs.

The Equilibria in Lipid-Lipoic Acid Systems: Monolayers, Microelectrophoretic and Interfacial Tension Studies

In this examination, we investigated the effect of lipoic acid (LA) on the properties of biological membrane models (monolayers, bilayers, and liposomes) formed from phosphatidylcholine (PC) or phosphatidylserine (PS) using the Langmuir, microelectrophoresis, and interfacial tension methods. The Langmuir technique allowed us to calculate the π–A isotherms and determine the molecular surface areas of pure and mixed monolayers. Using mathematical equations, we established that LA and the lipids formed complexes at a 1:1 ratio. The interfacial tension method was based on Young and Laplace’s equation. We assumed the formation of a 1:1 complex in the PC–LA system. Using the mathematical relationships, we derived the parameters characterizing the resulting complex, i.e., the surface occupied by the complex and the interfacial tension and stability constant of the formed complex. The microelectrophoretic method was used to determine the dependence of the zeta potential of the lipid membranes as a function of the pH (pH 2 to 10) of the electrolyte solution. The results indicate that modification of PC or PS membranes with LA affects changes in the zeta potential and the isoelectric point values.

Optimization of the Inverted Emulsion Method for High-Yield Production of Biomimetic Giant Unilamellar Vesicles

In the field of bottom-up synthetic biology, lipid vesicles provide an important role in the construction of artificial cells. Giant unilamellar vesicles (GUVs), due to their membrane's similarity to natural biomembranes, have been widely used as cellular mimics. So far, several methods exist for the production of GUVs with the possibility to encapsulate biological macromolecules. The inverted emulsion-based method is one such technique, which has great potential for rapid production of GUVs with high encapsulation efficiencies for large biomolecules. However, the lack of understanding of various parameters that affect production yields has resulted in sparse adaptation within the membrane and bottom-up synthetic biology research communities. Here, we optimize various parameters of the inverted emulsion-based method to maximize the production of GUVs. We demonstrate that the density difference between the emulsion droplets, oil phase, and the outer aqueous phase plays a crucial role in vesicle formation. We also investigated the impact that centrifugation speed/time, lipid concentration, pH, temperature, and emulsion droplet volume has on vesicle yield and size. Compared to conventional electroformation, our preparation method was not found to significantly alter the membrane mechanical properties. Finally, we optimize the parameters to minimize the time from workbench to microscope and in this way open up the possibility of time-sensitive experiments. In conclusion, our findings will promote the usage of the inverted emulsion method for basic membrane biophysics studies as well as the development of GUVs for use as future artificial cells.

The Amphoteric and Hydrophilic Properties of Cartilage Surface in Mammalian Joints: Interfacial Tension and Molecular Dynamics Simulation Studies

In this paper, we explain the amphoteric character of the cartilage surface by studying a lipid bilayer model built from phospholipids. We examined the interfacial tension values and molecular dynamics simulation in solutions of varying pH. The effects of negative and positive charge density (or fixed charges) on the (cartilage/cartilage) friction coefficient were investigated. In physiological (or synovial) fluid, after the isoelectric point (pI), the curve of interfacial tension decreases rapidly as it reaches pH 7.4 and then approaches a constant value at higher pH. It was shown that the curve of the interfacial tension curve exhibits a maximum value at the isoelectric point with a Gaussian shape feature. The phospholipid bilayers facilitate an almost frictionless contact in the joint. Moreover, the slippage of the bilayer and the short-range repulsion between the surfaces of the negatively charged cartilage surfaces are the main determinants of the low frictional properties of the joint.

Active Accumulation of Spherical Analytes on Plasmonic Hot Spots of Double-Bent Au Strip Arrays by Multiple Dip-Coating

To achieve sensitive plasmonic biosensors, it is essential to develop an efficient method for concentrating analytes in hot spots, as well as to develop plasmonic nanostructures for concentrating light. In this study, target analytes were delivered to the surface of double-bent Au strip arrays by a multiple dip-coating method; they were self-aligned in the valleys between neighboring Au strips by capillary forces. As the valleys not only accommodate target analytes but also host strong electromagnetic fields due to the interaction between adjacent strips, sensitive measurement of target analytes was possible by monitoring changes in the wavelength of a localized surface plasmon resonance. Using the proposed plasmonic sensor and target delivery method, the adsorption and saturation of polystyrene beads 100 nm in size on the sensor surface were monitored by the shift of the resonance wavelength. In addition, the pH-dependent stability of exosomes accumulated on the sensor surface was successfully monitored by changing the pH from 7.4 to 4.0.

Understanding the Unique Role of Phospholipids in the Lubrication of Natural Joints: An Interfacial Tension Study

Some solid lubricants are characterized by a layered structure with weak (van der Waals) inter-interlayer forces which allow for easy, low-strength shearing. Solid lubricants in natural lubrication are characterized by phospholipid bilayers in the articular joints and phospholipid lamellar phases in synovial fluid. The influence of the acid–base properties of the phospholipid bilayer on the wettability and properties of the surface have been explained by studying the interfacial tension of spherical lipid bilayers based on a model membrane. In this paper, we show that the phospholipid multi-bilayer can act as an effective solid lubricant in every aspect, ranging from a ‘corrosion inhibitor’ in the stomach to a load-bearing lubricant in bovine joints. We present evidence of the outstanding performance of phospholipids and argue that this is due to their chemical inertness and hydrophilic–hydrophobic structure, which makes them amphoteric and provides them with the ability to form lamellar structures that can facilitate functional sliding. Moreover, the friction coefficient can significantly change for a given phospholipid bilayer so it leads to a lamellar-repulsive mechanism under highly charged conditions. After this, it is quickly transformed to result in stable low-friction conditions.

Articular cartilage. Strong adsorption and cohesion of phospholipids with the quaternary ammonium cations providing satisfactory lubrication of natural joints

Much evidence supports the hypothesis that surface-active phospholipid (SAPL) molecules on articular cartilage (AC) adsorbed to negatively-charged proteoglycan matrix form phospholipid (membrane), are negatively charged surface (-PO₄^-) and hydrophilic. In Hills cartilage model (1984) phospholipids adsorbed to cartilage surface act as boundary lubricants making the surface extremely hydrophobic. Hydrophobic surface of AC has gained no support in all experimental facts presented in this paper and the current literature showing that AC is amphoteric and hydrophilic with the negatively charged surface (-PO₄^-). The interfacial energy of the model membrane of spherical lipid bilayers evident from phosphatidylcholine "bell-shaped curve" has amphoteric character and lowest energy in lubrication at a pH 7.4 ± 1 of the natural joint.

Exchange dynamics between amphiphilic block copolymers and lipidic membranes through hydrophobic pyrene probe transfer

Vectorization has experienced significant development over the last few years and has been used to control the distribution of active ingredients to a target by their association with a vector. However, controlled drug delivery suffers from "burst release" as the drugs are released before the targeted site. Very few studies have examined the collective mechanisms of fission-fusion on micelles in the transport and expulsion of active ingredients. Endocytosis and exocytosis of cells are examples of fusion and fission in biological matter. Understanding these dynamics becomes crucial for the design and the control of new materials and new processes effective in controlled drug delivery. In this work, a study of the exchange dynamics between amphiphilic block copolymers and lipid membranes for vectorization of hydrophobic molecules using a fluorescence technique is presented. A highly hydrophobic alkylated pyrene, PyC₁₈, is used as a fluorescent probe that can be exchanged between amphiphilic block copolymer micelles and liposomes via different mechanisms. It is demonstrated that the exchange dynamics evaluated for different liposome concentrations is a collective mechanism characterized by having two rate constants.

Mathematical principles and methods of biological surface lubrication with phospholipids bilayers

This paper presents a mini-review of investigations performed by the authors in the field of hydrodynamic theory of lubrication of biological systems and synthetic processing of results to indicate the influence of biologically live material properties on biological liquid viscosity variations. The goal of the presented study was to demonstrate a new principle of a general mathematical theory applied to the mechanism of hydrodynamic lubrication of human joint cartilage surfaces with phospholipids bilayer and to indicate analytical solutions of hydrodynamic pressure, temperature and bio-fluid velocity components. Moreover, 3D variations of dynamic synovial fluid viscosity are assessed, particularly its variations across the entire film thickness. A new 3D analytical and numerical model has been elaborated on the basis of tribology methods, based on the assumptions of an ultra-thin boundary layer of non-Newtonian fluid. The analysed elements also included phospholipid concentrations, power hydrogen ion and collagen fiber concentrations in synovial, biological fluids, as well as electrostatic field, cartilage or biological surface wettability. The obtained results of our analysis demonstrate relationships which occur among hydrodynamic pressure, human joint load carrying capacity and phospholipid bilayer in the cartilage superficial layer. According to the best knowledge of the Authors, the obtained results may find applications in a broad scope of spatiotemporal models in biology and health science.

Enhancement of pharmacokinetic and pharmacological behavior of ocular dorzolamide after factorial optimization of self-assembled nanostructures

Dorzolamide hydrochloride is frequently administered for the control of the intra-ocular pressure associated with glaucoma. The aim of this study is to develop and optimize self-assembled nanostructures of dorzolamide hydrochloride and L-α-Phosphatidylcholine to improve the pharmacokinetic parameters and extend the drug pharmacological action. Self-assembled nanostructures were prepared using a modified thin-film hydration technique. The formulae compositions were designed based on response surface statistical design. The prepared self-assembled nanostructures were characterized by testing their drug content, particle size, polydispersity index, zeta potential, partition coefficient, release half-life and extent. The optimized formulae having the highest drug content, zeta potential, partition coefficient, release half-life and extent with the lowest particle size and polydispersity index were subjected to further investigations including investigation of their physicochemical, morphological characteristics, in vivo pharmacokinetic and pharmacodynamic profiles. The optimized formulae were prepared at pH 8.7 (F5 and F6) and composed of L-α-Phosphatidylcholine and drug mixed in a ratio of 1:1 and 2:1 w/w, respectively. They showed significantly higher Cmax, [Formula: see text] and [Formula: see text] at the aqueous humor with extended control over the intra-ocular pressure, when compared to the marketed product; Trusopt®. The study introduced novel and promising self-assembled formulae able to permeate higher drug amount through the cornea and achieve sustained pharmacological effect at the site of action.

The effect of hydronium ions on the structure of phospholipid membranes

This work studies the mechanisms by which hydronium ions modulate the structure of phospholipid bilayers.

Pore formation in lipid membrane II: Energy landscape under external stress

Lipid membranes are extremely stable envelopes allowing cells to survive in various environments and to maintain desired internal composition. Membrane permeation through formation of transversal pores requires substantial external stress. Practically, pores are usually formed by application of lateral tension or transmembrane voltage. Using the same approach as was used for obtaining continuous trajectory of pore formation in the stress-less membrane in the previous article, we now consider the process of pore formation under the external stress. The waiting time to pore formation proved a non-monotonous function of the lateral tension, dropping from infinity at zero tension to a minimum at the tension of several millinewtons per meter. Transmembrane voltage, on the contrary, caused the waiting time to decrease monotonously. Analysis of pore formation trajectories for several lipid species with different spontaneous curvatures and elastic moduli under various external conditions provided instrumental insights into the mechanisms underlying some experimentally observed phenomena.

Development and characterization of polymer-coated liposomes for vaginal delivery of sildenafil citrate

Vaginal administration of sildenafil citrate has shown recently to develop efficiently the uterine lining with subsequent successful embryo implantation following in vitro fertilization. The aim of the present study was to develop sildenafil-loaded liposomes coated with bioadhesive polymers for enhanced vaginal retention and improved drug permeation. Three liposomal formulae were prepared by thin-film method using different phospholipid:cholesterol ratios. The optimal liposomal formulation was coated with bioadhesive polymers (chitosan and HPMC). A marked increase in liposomal size and zeta potential was observed for all coated liposomal formulations. HPMC-coated liposomes showed the greater bioadhesion and higher entrapment efficiency than chitosan-coated formulae. The in vitro release studies showed prolonged release of sildenafil from coated liposomes as compared to uncoated liposomes and sildenafil solution. Ex vivo permeation study revealed the enhanced permeation of coated relative to uncoated liposomes. Chitosan-coated formula demonstrated highest drug permeation and was thus selected for further investigations. Transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) confirmed the successful coating of the liposomes by chitosan. Histopathological in vivo testing proved the efficacy of chitosan-coated liposomes to improve blood flow to the vaginal endometrium and to increase endometrial thickness. Chitosan-coated liposomes can be considered as potential novel drug delivery system intended for the vaginal administration of sildenafil, which would prolong system's retention at the vaginal site and enhance the permeation of sildenafil to uterine blood circulation.

Interfacial pH and polarity detection of amphiphilic self-assemblies using a single Schiff-base molecule

The interfacial pH and polarity for different amphiphilic self-assemblies are estimated at a similar interfacial depth utilizing a unique Schiff-base molecule containing two identical phenol-conjugated-imine moieties.

Somatostatin receptor targeted liposomes with Diacerein inhibit IL-6 for breast cancer therapy

Selective targeting to the tumor niche remains a major challenge in successful cancer therapy. Somatostatin receptor 2 (SSTR2) is overexpressed in breast cancer cells thus making this receptor an attractive target for selective guidance of ligand-conjugated drug liposomes to the tumor site. In this study, a synthetic somatostatin analogue (SST) was used as SSTR2 targeting agent and Diacerein was employed as therapeutic molecule. Diacerein loaded liposomes (DNL) were prepared and they were further decorated with the synthetic and stable analogue of somatostatin (SST-DNL). Fabricated liposomes were nano-size in range and biocompatible. SST-DNL displayed significantly better anti-tumor efficacy as compared to free Diacerein (DN) and DNL in breast cancer models. Enhanced apoptosis in breast cancer cells was detected in SST-DNL treated groups as monitored by cell cycle analysis and changes in expression level of apoptotic/anti-apoptotic proteins Bcl-2, Bax, cleaved Caspase 3 and PARP. SST-DNL more effectively inhibited the oncogenic IL-6/IL-6R/STAT3/MAPK/Akt signalling pathways as compared to DN or DNL in cancer cells. In addition, SST-DNL effectively suppressed angiogenesis and cancer cell invasion. In vivo tumor growth in a MDA-MB-231 mouse xenograft model was significantly suppressed following SST-DNL treatment. In xenograft model, immunohistochemistry of Ki-67 and CD-31 indicated that SST-DNL improved the anti-proliferative and anti-angiogenic impacts of Diacerein. In vivo pharmacokinetic studies in rats showed enhanced circulation time in the DNL or SST-DNL treated groups as compared to free DN. Considering all of these findings, we conclude that SST-DNL provides a novel strategy with better efficacy for breast cancer therapy.

Association of alkali metal cations with phosphatidylcholine liposomal membrane surface

Interactions of alkali metal cations (Li⁺, Na⁺, K⁺, Cs⁺) with phosphatidylcholine (PC) liposomal membranes were investigated through experimental studies and theoretical considerations. Using a microelectrophoresis technique, charge densities of experimental membrane surfaces were measured as a function of the pH of electrolyte solutions. Equilibria between the PC liposomal membranes and monovalent ions were mathematically analyzed and described quantitatively through a previously proposed theoretical model. Association constants between functional groups of PC and the studied ions were determined and used to define theoretical curves of membrane surface charge density versus pH. Theoretical and experimental data were compared to verify the model. The PC membrane was found to have the highest affinity for lithium ions, among the ions tested.

Migration of phospholipid vesicles in response to OH(-) stimuli

We demonstrate migration of phospholipid vesicles in response to a pH gradient. Upon simple micro-injection of a NaOH solution, the vesicles linearly moved to the tip of the micro-pipette and the migration velocity was proportional to the gradient of OH(-) concentration. Vesicle migration was characteristic of OH(-) ions and no migration was observed for monovalent salts or nonionic sucrose solutions. The migration of vesicles is quantitatively described by the surface tension gradient model where the hydrolysis of the phospholipids by NaOH solution decreases the surface tension of the vesicle. The vesicles move toward a direction where the surface energy decreases. Thus the chemical modification of lipids produces a mechanical force to drive vesicles.

Tribological efficacy and stability of phospholipid-based membrane lubricants in varying pH chemical conditions

In this study, the authors examine the influence of joint chemical environment by measuring changes in the tribological properties (friction coefficient and charge density) of contacting surfaces of normal and degenerated cartilage samples in bath solutions of varying pH (2.0-9.0). Bovine articular cartilage samples (n = 54) were subjected to several surface measurements, including interfacial energy, contact angle, and friction coefficient, at varying pH. The samples were delipidized and then subjected to the same measurement protocols. Our results reveal that the interfacial energy and charge density, which have been shown to be related to friction coefficient, decrease with pH in the acidic range and approach constant values at physiological (or synovial fluid) pH of 7.4 and beyond it, i.e., toward basic pH domain. The authors conclude that this rather complex response explains the long-term efficacy with respect to ageing and associated pH changes, of the phospholipid layers that facilitate the almost frictionless, hydration-lubrication involving contact in the mammalian musculoskeletal system.

Sustained Epigenetic Drug Delivery Depletes Cholesterol-Sphingomyelin Rafts from Resistant Breast Cancer Cells, Influencing Biophysical Characteristics of Membrane Lipids

Cell-membrane lipid composition can greatly influence biophysical properties of cell membranes, affecting various cellular functions. We previously showed that lipid synthesis becomes altered in the membranes of resistant breast cancer cells (MCF-7/ADR); they form a more rigid, hydrophobic lipid monolayer than do sensitive cell membranes (MCF-7). These changes in membrane lipids of resistant cells, attributed to epigenetic aberration, significantly affected drug transport and endocytic function, thus impacting the efficacy of anticancer drugs. The present study's objective was to determine the effects of the epigenetic drug, 5-aza-2'-deoxycytidine (DAC), delivered in sustained-release nanogels (DAC-NGs), on the composition and biophysical properties of membrane lipids of resistant cells. Resistant and sensitive cells were treated with DAC in solution (DAC-sol) or DAC-NGs, and cell-membrane lipids were isolated and analyzed for lipid composition and biophysical properties. In resistant cells, we found increased formation of cholesterol-sphingomyelin (CHOL-SM) rafts with culturing time, whereas DAC treatment reduced their formation. In general, the effect of DAC-NGs was greater in changing the lipid composition than with DAC-sol. DAC treatment also caused a rise in levels of certain phospholipids and neutral lipids known to increase membrane fluidity, while reducing the levels of certain lipids known to increase membrane rigidity. Isotherm data showed increased lipid membrane fluidity following DAC treatment, attributed to decrease levels of CHOL-SM rafts (lamellar beta [Lβ] structures or ordered gel) and a corresponding increase in lipids that form lamellar alpha-structures (Lα, liquid crystalline phase). Sensitive cells showed marginal or insignificant changes in lipid profile following DAC-treatment, suggesting that epigenetic changes affecting lipid biosynthesis are more specific to resistant cells. Since membrane fluidity plays a major role in drug transport and endocytic function, treatment of resistant cells with epigenetic drugs with altered lipid profile could facilitate anticancer drug transport to overcome acquired drug resistance in a combination therapy.

β2-Microglobulin amyloid fibril-induced membrane disruption is enhanced by endosomal lipids and acidic pH

Although the molecular mechanisms underlying the pathology of amyloidoses are not well understood, the interaction between amyloid proteins and cell membranes is thought to play a role in several amyloid diseases. Amyloid fibrils of β2-microglobulin (β2m), associated with dialysis-related amyloidosis (DRA), have been shown to cause disruption of anionic lipid bilayers in vitro. However, the effect of lipid composition and the chemical environment in which β2m-lipid interactions occur have not been investigated previously. Here we examine membrane damage resulting from the interaction of β2m monomers and fibrils with lipid bilayers. Using dye release, tryptophan fluorescence quenching and fluorescence confocal microscopy assays we investigate the effect of anionic lipid composition and pH on the susceptibility of liposomes to fibril-induced membrane damage. We show that β2m fibril-induced membrane disruption is modulated by anionic lipid composition and is enhanced by acidic pH. Most strikingly, the greatest degree of membrane disruption is observed for liposomes containing bis(monoacylglycero)phosphate (BMP) at acidic pH, conditions likely to reflect those encountered in the endocytic pathway. The results suggest that the interaction between β2m fibrils and membranes of endosomal origin may play a role in the molecular mechanism of β2m amyloid-associated osteoarticular tissue destruction in DRA.

The effect of pH on the electrical capacitance of phosphatidylcholine-phosphatidylserine system in bilayer lipid membrane

This paper reports measurements on the pH dependence of the electrical capacitance of lipid membranes formed by 1:1 phosphatidylcholine-phosphatidylserine mixtures. A theoretical model was developed to describe this dependence, in which the contributions of functional groups (as the active centers of adsorption of the hydrogen and hydroxide ions) to the overall membrane capacitance were assumed to be additive. The proposed model was verified experimentally using electrochemical impedance spectroscopy. The theoretical predictions agreed with the experimental results over the measured pH range. A minimum corresponding to the isoelectric point appeared in both the theoretical equation and the experimental data.

Biophysics of cell membrane lipids in cancer drug resistance: Implications for drug transport and drug delivery with nanoparticles

In this review, we focus on the biophysics of cell membrane lipids, particularly when cancers develop acquired drug resistance, and how biophysical changes in resistant cell membrane influence drug transport and nanoparticle-mediated drug delivery. Recent advances in membrane lipid research show the varied roles of lipids in regulating membrane P-glycoprotein function, membrane trafficking, apoptotic pathways, drug transport, and endocytic functions, particularly endocytosis, the primary mechanism of cellular uptake of nanoparticle-based drug delivery systems. Since acquired drug resistance alters lipid biosynthesis, understanding the role of lipids in cell membrane biophysics and its effect on drug transport is critical for developing effective therapeutic and drug delivery approaches to overcome drug resistance. Here we discuss novel strategies for (a) modulating the biophysical properties of membrane lipids of resistant cells to facilitate drug transport and regain endocytic function and (b) developing effective nanoparticles based on their biophysical interactions with membrane lipids to enhance drug delivery and overcome drug resistance.

Electrochemical impedance spectroscopy as a method for electrical characterization of the bilayers formed from lipid-amino acid systems

Bilayer lipid membranes composed of phosphatidylcholine and isoleucine or phosphatidylcholine and tyrosine were investigated using electrochemical impedance spectroscopy. Interaction between membrane components causes significant deviations from the additivity rule which can be explained by formation of the domain structures. The surface area of domains was calculated based on derived equations. We suggest that the stoichiometry of phosphatidylcholine-isoleucine domain is equal 3:1. In the case of tyrosine-modified phosphatidylcholine membranes, domain with stoichiometry 7:1 should be taken into consideration.

Relationship between wettability and lubrication characteristics of the surfaces of contacting phospholipid-based membranes

The wettability of the articular surface of cartilage depends on the condition of its surface active phospholipid overlay, which is structured as multi-bilayer. Based on a hypothesis that the surface of cartilage facilitates the almost frictionless lubrication of the joint, we examined the characteristics of this membrane surface entity in both its normal and degenerated conditions using a combination of atomic force microscopy, contact angle measurement, and friction test methods. The observations have led to the conclusions that (1) the acid-base equilibrium condition influences the lubrication effectiveness of the surface of cartilage and (2) the friction coefficient is significantly dependent on the hydrophobicity of the surface of the tissue, thereby confirming the hypothesis tested in this paper. Both wettability angle and interfacial energy were obtained for varying conditions of the cartilage surface both in its wet, dry and lipid-depleted conditions. The interfacial energy also increased with mole fraction of the lipid species reaching an asymptotic value after 0.6. Also, the friction coefficient was found to decrease to an asymptotic level as the wettability angle increased. The result reveal that the interfacial energy increased with pH till pH = 4.0, and then decreased from pH = 4.0 to reach equilibrium at pH = 7.0.

Proton-induced endocytosis is dependent on cell membrane fluidity, lipid-phase order and the membrane resting potential

Recently it has been shown that decreasing the extracellular pH of cells stimulates the formation of inward membrane invaginations and vesicles, accompanied by an enhanced uptake of macromolecules. This type of endocytosis was coined as proton-induced uptake (PIU). Though the initial induction of inward membrane curvature was rationalized in terms of proton-based increase of charge asymmetry across the membrane, the dependence of the phenomenon on plasma membrane characteristics is still unknown. The present study shows that depolarization of the membrane resting potential elevates PIU by 25%, while hyperpolarization attenuates it by 25%. Comparison of uptake in suspended and adherent cells implicates that the resting-potential affects PIU through remodeling the actin-cytoskeleton. The pH at the external interface of the cell membrane rather than the pH gradient across it determines the extent of PIU. PIU increases linearly upon temperature increase in the range of 4-36°C, in correlation with the membrane fluidity. The plasma membrane fluidity and the lipid phase order are modulated by enriching the cell's membrane with cholesterol, tergitol, dimethylsulfoxide, 6-ketocholestanol and phloretin and by cholesterol depletion. These treatments are shown to alter the extent of PIU and are better correlated with membrane fluidity than with the lipid phase order. We suggest that the lipid phase order and fluidity influence PIU by regulating the lipid order gradient across the perimeter of the lipid-condensed microdomains (rafts) and alter the characteristic tension line that separates the higher ordered lipid-domains from the lesser ordered ones.

The interfacial tension of the lipid membrane formed from lipid-amino acid systems

The interfacial tension of lipid membranes composed of phosphatidylcholine (lecithin, PC)-valine (Val), phosphatidylcholine-isoleucine (Ile), phosphatidylcholine-tyrosine (Tyr), and phosphatidylcholine-phenylalanine (Phe) has been studied. The membrane components formed 1:1 complexes. The interfacial tension measurements were used to determine the membrane surface concentration A (3)(-1), the membrane interfacial tension γ(3), and the stability constant K.