Bilayer lipid membrane permeation and rupture due to hole formation

Fine-tuning of membrane permeability by reversible photoisomerization of aryl-azo derivatives of thymol embedded in lipid nanoparticles

Responsiveness of liposomes to external stimuli, such as light, should allow a precise spatial and temporal control of release of therapeutic agents or ion transmembrane transport. Here, some aryl-azo derivatives of thymol are synthesized and embedded into liposomes from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine to obtain light-sensitive membranes whose photo-responsiveness, release behaviour, and permeability towards Cl- ions are investigated. The hybrid systems are in-depth characterized by dynamic light scattering, atomic force microscopy and Raman spectroscopy. In liposomal bilayer the selected guests undergo reversible photoinduced isomerization upon irradiation with UV and visible light, alternately. Non-irradiated hybrid liposomes retain entrapped 5(6)-carboxyfluorescein (CF), slowing its spontaneous leakage, whereas UV-irradiation promotes CF release, due to guest trans-to-cis isomerization. Photoisomerization also influences membrane permeability towards Cl- ions. Data processing, according to first-order kinetics, demonstrates that Cl- transmembrane transport is enhanced by switching the guest from trans to cis but restored by back-switching the guest from cis to trans upon illumination with blue light. Finally, the passage of Cl- ions across the bilayer can be fine-tuned by irradiation with light of longer λ and different light-exposure times. Fine-tuning the photo-induced structural response of the liposomal membrane upon isomerization is a promising step towards effective photo-dynamic therapy.

Development, Characterization and Pharmacological Evaluation of Cannabidiol-Loaded Long Circulating Niosomes

Cannabidiol (CBD) is a promising drug candidate with pleiotropic pharmacological activity, whose low aqueous solubility and unfavorable pharmacokinetics have presented obstacles to its full clinical implementation. The rational design of nanocarriers, including niosomes for CBD encapsulation, can provide a plausible approach to overcoming these limitations. The present study is focused on exploring the feasibility of copolymer-modified niosomes as platforms for systemic delivery of CBD. To confer steric stabilization, the niosomal membranes were grafted with newly synthesized amphiphilic linear or star-shaped 3- and 4-arm star-shaped copolymers based on polyglycidol (PG) and poly(ε-caprolactone) (PCL) blocks. The niosomes were prepared by film hydration method and were characterized by DLS, cryo-TEM, encapsulation efficacy, and in vitro release. Free and formulated cannabidiol were further investigated for cytotoxicity and pro-apoptotic and anti-inflammatory activities in vitro in three human tumor cell lines. The optimal formulation, based on Tween 60:Span60:Chol (3.5:3.5:3 molar ration) modified with 2.5 mol% star-shaped 3-arm copolymer, is characterized by a size of 235 nm, high encapsulation of CBD (94%), and controlled release properties. Niosomal cannabidiol retained the antineoplastic activity of the free agent, but noteworthy superior apoptogenic and inflammatory biomarker-modulating effects were established at equieffective exposure vs. the free drug. Specific alterations in key signaling molecules, implicated in programmed cell death, cancer cell biology, and inflammation, were recorded with the niosomal formulations.

Histological Response to 5 kHz Irreversible Electroporation in a Porcine Liver Model

Unlike necrosis by thermal ablation, irreversible electroporation (IRE) is known to induce apoptosis by disrupting plasma membrane integrity with electric pulses while preserving the structure of blood vessels and bile ducts in liver tissue without a heat sink effect. This study aimed to investigate thermal damage and histopathological effects in the porcine liver by high-frequency electric pulses (5 kHz) which is much higher than the widely used 1 Hz. The electric field and thermal distributions of 5 kHz electric pulses were compared with those of 1 Hz in numerical simulations. 5 kHz-IRE was applied on pigs under ultrasound imaging to guide the electrode placement. The animals underwent computed tomography (CT) examination immediately and 1 day after IRE. After CT, IRE-treated tissues were taken and analyzed histologically. CT revealed that hepatic veins were intact for 1-day post-IRE. Histopathologically, the structure of the portal vein was intact, but endothelial cells were partially removed. In addition, the hepatic artery structure from which endothelial cells were removed were not damaged, while the bile duct structure and cholangiocytes were intact. The thermal injury was observed only in the vicinity of the electrodes as simulated in silico. 5 kHz-IRE generated high heat due to its short pulse interval, but the thermal damage was limited to the tissue around the electrodes. The histopathological damage caused by 5 kHz-IRE was close to that caused by 1 Hz-IRE. If a short-time treatment is required for reasons such as anesthesia, high-frequency IRE treatment is worth considering. Our observations will contribute to a better understanding of the IRE phenomena and search for advanced therapeutic conditions.

Calixarene-based artificial ionophores for chloride transport across natural liposomal bilayer: Synthesis, structure-function relationships, and computational study

An amphiphilic calix[6]arene, alone or complexed with an axle to form a pseudo-rotaxane, has been embedded into liposomes prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and the permeability of the membrane-doped liposomes towards Cl^- ions has been evaluated by using lucigenin as the fluorescent probe. The pseudo-rotaxane promotes transmembrane transport of Cl^- ions more than calix[6]arene does. Surprisingly, the quenching of lucigenin was very fast for liposomes doped with the positively charged axle alone. Molecular dynamics (MD) simulations and quantum-chemical calculations were also carried out for providing a semi-quantitative support to the experimental results.

Embedding calix[4]resorcinarenes in liposomes: Experimental and computational investigation of the effect of resorcinarene inclusion on liposome properties and stability

Two calix[4]resorcinarenes, which differ in the length of the alkyl chain on the methylene bridge between the aromatic rings, have been embedded in unilamellar liposomes prepared from 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine in three host/guest ratios, following two different procedures. The effect of the insertion of the guests has been evaluated through the measurements of the viscosity and the kinetic stability of the liposomal systems by means of the fluorescent probes pyrene and 5(6)-carboxyfluorescein. The presence of the guests reduces the viscosity of the liposomes, suggesting a modification of the bilayer structure. However, this does not affect liposome stability. A calix[4]resorcinarene cavitand with a more rigid conformation compared to the parent resorcinarene, has been also synthetized and embedded in liposomes. The free energy of the insertion of the substrates in the lipid bilayer has been evaluated through Molecular Dynamics simulations.

Effect of the Incorporation of Functionalized Cyclodextrins in the Liposomal Bilayer

Liposomes loaded with drug–cyclodextrin complexes are widely used as drug delivery systems, especially for species with low aqueous solubility and stability. Investigation of the intimate interactions of macrocycles with liposomes are essential for formulation of efficient and stable drug-in-cyclodextrin-in-liposome carriers. In this work, we reported the preparation of unilamellar vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) embedded with native β-cyclodextrin and two synthetic derivatives: heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMCD) and heptakis(2,3-di-O-acetyl)-β-cyclodextrin (DACD). We then studied the effect of these macrocycles on the liposomal size, membrane viscosity, and liposomal stability at different temperatures and concentrations. We observed that TMCD and DACD affected vesicle size and the change of size was related to CD concentration. Irrespective of its nature, the macrocycle established interactions with the phospholipidic head groups, preventing cyclodextrins to diffuse into the lipid bilayer, as confirmed by molecular dynamics simulations. Such supramolecular structuring improves liposome stability making these colloid systems promising carriers for biologically active compounds.

Design of Prototype Formulations for In Vitro Dermal Delivery of the Natural Antioxidant Ferulic Acid Based on Ethosomal Colloidal Systems

Ferulic acid (FA), a naturally occurring antioxidant, is currently used to prevent skin damage. However, FA is very unstable upon exposure to UV radiation and other factors, which decrease its shelf-life and effectiveness. Therefore, in this work, different prototypes of ethosomal FA vesicular systems were designed and developed to provide protection against different environmental factors. A two-level fractional factorial design was employed using particle size, zeta potential (ZP), incorporation efficiency (EE), polydispersity index (PDI), and the existing relationship between length and width of vesicles or aspect ratio (AR) as response variables. The optimal formulation was characterized using differential scanning calorimetry (DSC), infrared analysis, UV-Vis absorption, in-vitro permeability, and thermal degradation studies. Depending on the processing conditions, the EE and particle size varied between 3 and 87% and 470 and 1208 nm, respectively. Membrane studies indicated that the free product released ~4.8% of the compound, whereas the encapsulated material released ~7.1%. Because of their enhanced permeability, ethosomes could be a promising alternative for the topical administration of antioxidants to reduce the oxidative damage caused by solar radiation.

Effect of ubiquinone-10 on the stability of biomimetic membranes of relevance for the inner mitochondrial membrane

Ubiquinone-10 (Q10) plays a pivotal role as electron-carrier in the mitochondrial respiratory chain, and is also well known for its powerful antioxidant properties. Recent findings suggest moreover that Q10 could have an important membrane stabilizing function. In line with this, we showed in a previous study that Q10 decreases the permeability to carboxyfluorescein (CF) and increases the mechanical strength of 1-palmitoyl-2-oleyl-sn-glycero-phosphocholine (POPC) membranes. In the current study we report on the effects exerted by Q10 in membranes having a more complex lipid composition designed to mimic that of the inner mitochondrial membrane (IMM). Results from DPH fluorescence anisotropy and permeability measurements, as well as investigations probing the interaction of liposomes with silica surfaces, corroborate a membrane stabilizing effect of Q10 also in the IMM-mimicking membranes. Comparative investigations examining the effect of Q10 and the polyisoprenoid alcohol solanesol on the IMM model and on membranes composed of individual IMM components suggest, moreover, that Q10 improves the membrane barrier properties via different mechanisms depending on the lipid composition of the membrane. Thus, whereas Q10's inhibitory effect on CF release from pure POPC membranes appears to be directly and solely related to Q10's lipid ordering and condensing effect, a mechanism linked to Q10's ability to amplify intrinsic curvature elastic stress dominates in case of membranes containing high proportions of palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE).

Lipid-Based Drug Delivery Systems in Cancer Therapy: What Is Available and What Is Yet to Come

Cancer is a leading cause of death in many countries around the world. However, the efficacy of current standard treatments for a variety of cancers is suboptimal. First, most cancer treatments lack specificity, meaning that these treatments affect both cancer cells and their normal counterparts. Second, many anticancer agents are highly toxic, and thus, limit their use in treatment. Third, a number of cytotoxic chemotherapeutics are highly hydrophobic, which limits their utility in cancer therapy. Finally, many chemotherapeutic agents exhibit short half-lives that curtail their efficacy. As a result of these deficiencies, many current treatments lead to side effects, noncompliance, and patient inconvenience due to difficulties in administration. However, the application of nanotechnology has led to the development of effective nanosized drug delivery systems known commonly as nanoparticles. Among these delivery systems, lipid-based nanoparticles, particularly liposomes, have shown to be quite effective at exhibiting the ability to: 1) improve the selectivity of cancer chemotherapeutic agents; 2) lower the cytotoxicity of anticancer drugs to normal tissues, and thus, reduce their toxic side effects; 3) increase the solubility of hydrophobic drugs; and 4) offer a prolonged and controlled release of agents. This review will discuss the current state of lipid-based nanoparticle research, including the development of liposomes for cancer therapy, different strategies for tumor targeting, liposomal formulation of various anticancer drugs that are commercially available, recent progress in liposome technology for the treatment of cancer, and the next generation of lipid-based nanoparticles.

Long-term adsorption kinetics of asphaltenes at the oil-water interface: a random sequential adsorption perspective

Previous studies indicated that asphaltenes adsorbed as monomers on oil-water interfaces and the early stage kinetics of the process was controlled by diffusion and hence dependent on oil viscosity. By measuring interfacial tension (IFT) as a function of surface coverage during droplet expansions in pendant drop experiments, it was also concluded that the IFT data could be interpreted with a Langmuir equation of state (EoS), which was independent of oil viscosity, time of adsorption, and bulk asphaltenes concentration. The surface excess coverage was calculated to be ∼0.3 nm(2)/molecule, which suggested adsorption in face-on configuration of asphaltenes monomers at the interface and average PAH core per molecule of about 6 for the asphaltenes investigated, consistent with the Yen-Mullins model. The current study focuses on the kinetics of asphaltenes adsorption at longer times and higher interfacial coverage. Long-term IFT data have been measured by the pendant drop method for different asphaltenes concentrations and for different bulk viscosities of the oil phase (0.5-28 cP). The data indicate that when coverage reaches 35-40%, the adsorption rates slow down considerably compared to the diffusion-controlled rates at the very early stages. The surface pressure increase rate (or IFT decrease rate) at these higher coverages is now independent of oil viscosity but dependent upon both surface pressure itself and asphaltene monomer concentration. The long-term asymptotic behavior of surface coverage is found to be consistent with the predictions from surface diffusion-mediated random sequential adsorption (RSA) theory which indicates a linear dependency of surface coverage on 1/√t and an asymptotic limit very close to 2D random close packing of polydispersed disks (85%). From these observations RSA theory parameters were extracted that enabled description of adsorption kinetics for the range of conditions above surface coverage of 35%.

Effect of gas type on foam film permeability and its implications for foam flow in porous media

The aim of this paper is to provide a perspective on the effect of gas type on the permeability of foam films stabilized by different types of surfactant and to present a critical overview of the tracer gas experiments, which is the common approach to determine the trapped fraction of foam in porous media. In these experiments some part of the gas is replaced by a "tracer gas" during the steady-state stage of the experiments and trapped fraction of foam is determined by fitting the effluent data to a capacitance mass-transfer model. We present the experimental results on the measurement of the gas permeability of foam films stabilized with five surfactants (non-ionic, anionic and cationic) and different salt concentrations. The salt concentrations assure formation of either common black (CBF) or Newton black films (NBF). The experiments are performed with different single gasses. The permeability of the CBF is in general higher than that of the NBF. This behavior is explained by the higher density of the surfactant molecules in the NBF compared to that of CBF. It is also observed that the permeability coefficient, K(cm/s), of CBF and NBF for non-ionic and cationic surfactants are similar and K is insensitive to film thickness. Compared to anionic surfactants, the films made by the non-ionic surfactant have much lower permeability while the films made by the cationic surfactant have larger permeability. This conclusion is valid for all gasses. For all types of surfactant the gas permeability of foam film is largely dependent on the dissolution of gas in the surfactant solution and increases with increasing gas solubility in the bulk liquid. The measured values of K are consistent with rapid diffusion of tracer gasses through trapped gas adjacent to flowing gas in porous media, and difficulties in interpreting the results of tracer-foam experiments with conventional capacitance models. The implications of the results for foam flow in porous media and factors leading to difficulties in the modeling of trapped fraction of foam are discussed in detail. To avoid complications in the interpretation of the results, the best tracer would be one with a permeability close to the permeability of the gas in the foam. This puts a lower limit on the effective diffusion coefficient for tracer in an experiment.

Fine-tuning of POPC liposomal leakage by the use of beta-cyclodextrin and several hydrophobic guests

The effect of entrapped beta-cyclodextrin (beta-CD) on the stability of multilamellar vesicles (MLVs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), prepared by the dehydration-rehydration method, was studied by monitoring the release of 5(6)-carboxyfluorescein encapsulated into the liposomes. Different hydrophobic guests, such as Fullerene C(60), have been incorporated into the POPC bilayer in order to modify the membrane composition. The kinetic results as well as ESI-MS measurements evidenced that the destabilizing activity of beta-CD is due to the formation of beta-CD inclusion complexes and the consequent removal of selected bilayer constituents from the liposomal membrane. Hence, when beta-CD was added to the liposomes in the form of a strong, water-soluble 2:1 beta-CD/C(60) inclusion complex, such a destabilizing effect was not observed. However, the same beta-CD/C(60) inclusion complex does not form as a result of C(60) extraction from the bilayer. This may be attributed either to the overwhelming concentration of POPC with respect to C(60) and/or to the fact that C(60) is largely aggregated in the bilayer. Turbidimetric and fluorimetric determinations of lamellarity and entrapped volume of the studied MLVs provided further evidence of the alteration of the liposomal bilayer as a consequence of the addition of beta-CD and/or the presence of the studied guests.

Liposome destabilization by a 2,7-diazapyrenium derivative through formation of transient pores in the lipid bilayer

The effect of the luminescent heteroaromatic electron acceptor N,N'-dimethyl-2,7-diazapyrenium dichloride (DM-DAP(2+)) on the stability of 1-palmitoyl-2-oleoylphosphatydilcholine (POPC) liposomes is determined on the basis of the rate of release of different fluorescent probes entrapped within the liposome. The experiments show that DM-DAP(2+) exerts a substantial destabilizing action on the liposomal bilayer, particularly at low concentrations. Molecular dynamics simulations suggest that the activity of DM-DAP(2+) is related to its tendency to surround itself with water molecules, conceivably favoring the formation of transient pores across the bilayer.

Gas permeability of foam films stabilized by an alpha-olefin sulfonate surfactant

The gas permeability of equilibrium foam films stabilized with an alpha-olefin sulfonate surfactant was measured. The permeability coefficient, K (cm/s), was obtained as a function of the electrolyte (NaCl) concentration, surfactant concentration, and temperature. The addition of salt to the film-forming solution leads to a decrease of the film thickness, which was complemented by an increase of K up to a certain value. Above that critical salt concentration, the gas permeability decreases even though the film thickness also decreases. We explain this effect as a result of interplay of the film thickness and the adsorption monolayer permeability for the permeability of the whole film, i.e., the thermodynamic state of the film. The classical theories that explain the process were applied. The gas permeability of the film showed an unexpected increase at surfactant concentrations well above the critical micelle concentration. The origin of this effect remains unclear and requires further studies to be clarified. The experiments at different temperatures allowed the energy barrier of the permeability process to be estimated.

Foam film permeability: theory and experiment

The mass transfer of gas through foam films is a prototype of various industrial and biological processes. The aim of this paper is to give a perspective and critical overview of studies carried out to date on the mass transfer of gas through foam films. Contemporary experimental data are summarized, and a comprehensive overview of the theoretical models used to explain the observed effects is given. A detailed description of the processes that occur when a gas molecule passes through each layer that forms a foam film is shown. The permeability of the film-building surfactant monolayers plays an important role for the whole permeability process. It can be successfully described by the models used to explain the permeability of surfactant monolayers on aqueous sub-phase. For this reason, the present paper briefly discusses the surfactant-induced resistance to mass transfer of gases through gas-liquid interface. One part of the paper discusses the experimental and theoretical aspects of the foam film permeability in a train of foam films in a matrix or a cylinder. This special case is important to explain the gas transfer in porous media or in foams. Finally, this paper will highlight the gaps and challenges and sketch possible directions for future research.

Foam films stabilized with dodecyl maltoside. 2. Film stability and gas permeability

The gas permeability and stability of foam films stabilized by n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) were determined. The permeability coefficient (K, cm/s) and the mean film lifetime were measured as a function of the surfactant concentration. The films are less permeable than those stabilized by other surfactants at comparable conditions. The permeability coefficient decreases with increasing surfactant concentration. It does not show a remarkable dependence on the salt concentration. Stable Newton black foam films (NBFs) are formed above a surfactant concentration of 3.9 x 10(-)(6) M beta-C(12)G(2) in the presence of 0.2 M NaCl. The theory of nucleation hole formation in NBFs was applied to describe the observed dependencies of the permeability and film stability on the surfactant concentration. The theory gave satisfactory relation to the experiment.

Effects of fullerene guests on the stability of 1-palmitoyl-2-oleoylphosphatidylcholine liposomes

Membrane stability of extruded large unilamellar vesicles () formed by 1-palmitoyl-2-oleoylphosphatidylcholine () containing fullerene C or an amphiphilic fullerene derivative, 2-[2-(2-fulleropyrrolidin-1-ylethoxy)-ethoxy]-ethanol (), has been investigated by spectrofluorimetrically monitoring the spontaneous release of entrapped carboxyfluorescein (). Under controlled conditions of temperature, osmolarity and pH, these guests increase the stability of the liposomal membrane as shown by the decrease in the rate of outflux of . The stability conferred to the liposomes by C and has been compared with that conferred by the well-known stabilizing guest 1,2-dipalmitoyl--glycero-3-phosphoethanolamine--[methoxy-(polyethylene glycol)-2000] ammonium salt (). The addition of amphiphilic molecules, such as non-ionic surfactants, which intercalate into the membrane bilayer, and of sucrose or NaCl, which induce a hyposmotic stress, has been extensively investigated in order to get information on how to modulate the release of entrapped . This information could hopefully be useful in the formulation of new drug delivery systems as well as for getting a deeper understanding of the mechanisms of the formation/enclosure of channels through the membrane. The viscosity and the micropolarity of the membrane have been measured fluorimetrically by using pyrene as the probe. An interesting increase of the gel-liquid crystal phase transition temperature has been observed for POPC liposomes hosting C.

Local anesthetics facilitate ion transport across lipid planar bilayer membranes under an electric field: Dependence on type of lipid bilayer

In order to elucidate the role of structural change of lipid membrane bilayer in the mode of action of local anesthetic, we studied the effects of local anesthetics, charged tetracaine and uncharged benzocaine, on ion permeability across various lipid planar bilayers (PC, mixed PC/PS (4/1, mol/mol); mixed PC/PE (1/1, mol/mol); mixed PC/SM (4/1, mol/mol)) under a constant applied voltage. The membrane conductances increased in the order of PC<<PC/PS<or=PC/SM<<PC/PE. When the constant voltage of -100 or -70 mV was applied through the lipid bilayer membranes in the presence of positively charged tetracaine, the fluctuating current pulses with the large amplitude generated, but not appeared in the absence of tetracaine. The addition of uncharged benzocaine generated the fluctuating currents with the small amplitude. Both charged tetracaine and uncharged benzocaine facilitated electrophoretically the transport of small ions such as KCl in the buffer solution through the fluctuating pores in the lipid bilayer membranes formed by interaction with the local anesthetic under the negative applied membrane potential. The current pulses also contained actual transport of charged tetracaine together with the transport of the small ions. The amplitude and the duration time of the electrical current generated by adding the local anesthetics were dependent on the type of the lipid, the applied voltage and its voltage polarity.

Cardanol as a replacement for cholesterol into the lipid bilayer of POPC liposomes

Large unilamellar liposomes were prepared by hydration of 1-palmitoyl-2-oleylphosphatydilcholine (POPC) films and subsequent extrusion of the obtained liposomal suspension. Inclusion of cholesterol and cardanol brings about a stabilization of the membranes of the liposomes, as determined by their rates of release of entrapped 5(6)-carboxyfluorescein. The liposome breakdown was promoted by a non-ionic surfactant (Triton X-100) and the kinetic measurements were carried out by fluorimetry in water at 25 degrees C. Morphological analyses of giant POPC liposomes in the presence and in the absence of both guests were also performed. The results obtained suggest the use of cardanol (an easy available natural product) as a replacement for cholesterol as a new possibility for stabilizing liposomes in drug targetting.

Linkage identity is a major factor in determining the effect of PEG-ylated surfactants on permeability of phosphatidylcholine liposomes

The permeability effects induced by single-chained and double-chained poly(ethylene glycol)-surfactants were investigated by measuring the leakage of the fluorescent dye 5(6)-carboxy fluorescein from EPC liposomes. The standard incorporated amount of the surfactants was 5 mol%. Depending on the size of the poly(ethylene glycol) chain and especially on the type of linkage between the polymer and the hydrophobic moiety different leakage profiles were obtained. The presence of a long PEG-polymer resulted in a slower leakage compared with a short analogue. More importantly, the linkage identity was decisive for whether an overall reduction or increase in permeability was obtained. When the hydrocarbon chains were attached to the PEG chain via an ether or an ester the leakage increased compared to pure EPC liposomes. In contrast, if the link was an amide, the leakage was significantly reduced. This effect is assumed to originate from headgroup-headgroup interactions, and most probably hydrogen bonding, between amide and phosphate groups of the PEG-surfactant and the EPC, respectively.

Chain-melting phase transition and short-range molecular interactions in phospholipid foam bilayers

Occurrence of two-dimensional chain melting phase transition in foam bilayers was established for the first time. Microscopic horizontal foam bilayers [Newton black films (NBF)] were investigated by the microinterferometric method of Scheludko-Exerowa. The foam bilayers were formed from water-ethanol solutions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) and egg phosphatidylcholine (Egg PC) and samples of amniotic fluid (AF) at different temperatures. The influence of temperature on the foam bilayer thickness h(w) and on the critical concentration Cc for formation of foam bilayer was studied. It was shown that in the range of the main phase transition the temperature dependence of h(w) and C(c) changed specifically in the case of DMPC and DPPC foam bilayers. The thickness of the foam bilayers increased with decreasing temperature in the range of the main phase transition due to the melting of hydrocarbon tails of phospholipid molecules. These changes took place at the temperatures of the bulk chain-melting phase transitions, as determined by differential scanning calorimetry (DSC) for both aqueous, and water/ethanol DMPC, DPPC, and DPPC dispersions. An effect of the 'disperse medium' on h(w) was found for foam bilayers from DPPC. The results that foam bilayers could have different thickness at different temperatures disproved the current concept that NBF acquired constant thickness at concentrations higher than C(el,cr). The data for Cc were analysed on the basis of the hole-nucleation theory of bilayer stability of Kashchiev and Exerowa. This theory considered the amphiphile bilayer as a two-dimensional ordered system with short-range molecular interactions between the first neighbour molecules (as in a crystal). The short-range molecular interactions were presented by the parameter binding energy Q of an amphiphile molecule in the bilayer. The binding energy Q of two neighbouring phospholipids was calculated for the gel (30-60 kT) and liquid crystalline state (16-18 kT) of the bilayers from DMPC, DPPC, Egg PC, AF. Concentration/temperature phase diagram of DPPC foam bilayers that defined regions of gaseous (ruptured), gel and liquid crystalline foam bilayers were drawn. The values of Q obtained for various samples were very close and vary from 5.3 x 10(-20) to 9.4 x 10(-20) (approx. 13-22 kT) which indicated that in all cases the foam bilayers were in liquid-crystalline state. This is an important result since the parameter studied-threshold concentration (threshold dilution) is crucial for a very successful assessment of the risk for respiratory distress syndrome (RDS) in newborns and could be employed in medicine for assessment of other respiratory disturbances. It is to be expected that foam bilayers from phospholipids could be used as a model for investigation of short-range forces in biological structures, of interaction between membranes, etc.

Interface-mediated oscillatory phenomena

Oscillatory transport processes which occur in the far from equilibrium region have assumed great significance from the viewpoint of science of complexity. Oscillatory phenomena in the chemical reaction systems have been subjected to intense investigations both from theoretical and experimental angles. In the present review an effort has been made to bring transport processes other than conventional chemical reactions into focus: transport processes mediated by solid-liquid and liquid-liquid interfaces have been discussed. Transport through membranes including liquid membranes, liquid-liquid interfaces and the recently reported hydrodynamic oscillator have been covered. Applications of these systems in areas such as fabrication of sensors, phase transfer catalysis and, of course, the obvious biological action, e.g. excitation of biomembranes and tissues, have been reviewed. Theoretical frameworks proposed to rationalize the phenomena have also been critically reviewed.

Effect of adsorption of bovine serum albumin on liposomal membrane characteristics

The effect of adsorption of bovine serum albumin (BSA) on the membrane characteristics of liposomes at pH 7.4 was examined in terms of zeta potential, micropolarity, microfluidity and permeability of liposomal bilayer membranes, where negatively charged L-alpha-dipalmitoylphosphatidylglycerol (DPPG)/L-alpha-dipalmitoylphosphatidylcholine (DPPC), negatively charged dicetylphosphate (DCP)/DPPC and positively charged stearylamine (SA)/DPPC mixed liposomes were used. BSA with negative charges adsorbed on negatively charged DPPG/DPPC mixed liposomes but did not adsorb on negatively charged DCP/DPPC and positively charged SA/DPPC mixed liposomes. Furthermore, the adsorption amount of BSA on the mixed DPPG/DPPC liposomes increased with increasing the mole fraction of DPPG in spite of a possible electrostatic repulsion between BSA and DPPG. Thus, the adsorption of BSA on liposomes was likely to be related to the hydrophobic interaction between BSA and liposomes. The microfluidity of liposomal bilayer membranes near the bilayer center decreased by the adsorption of BSA, while the permeability of liposomal bilayer membranes increased by the adsorption of BSA on liposomes. These results are considered to be due to that the adsorption of BSA brings about a phase separation in liposomes and that a temporary gap is consequently formed in the liposomal bilayer membranes, thereby the permeability of liposomal bilayer membranes increases by the adsorption of BSA.

Effects of lysozyme and bovine serum albumin on membrane characteristics of dipalmitoylphosphatidylglycerol liposomes

The effects of adsorption of two kinds of proteins on the membrane characteristics of liposomes were examined at pH 7.4 in terms of adsorption amounts of proteins on liposomes, penetrations of proteins into liposomal bilayer membranes, phase transition temperature, microviscosity and permeability of liposomal bilayer membranes, using positively charged lysozyme (LSZ) and negatively charged bovine serum albumin (BSA) as proteins and negatively charged L-alpha-dipalmitoylphosphatidylglycerol (DPPG) liposomes. The saturated adsorption amount of LSZ was 720 g per mol of liposomal DPPG, while that of BSA was 44 g per mol of liposomal DPPG. The penetration of LSZ into DPPG lipid membranes was greater than that of BSA. The microviscosity in the hydrophobic region of liposomal bilayer membranes increased due to adsorption (penetration) of LSZ or BSA, while the permeability of liposomal bilayer membranes increased. The gel-liquid crystalline phase transition temperature of liposomal bilayer membranes was not affected by adsorption of LSZ or BSA, while the DSC peak area (heat of phase transition) decreased with increasing adsorption amount of LSZ or BSA. It is suggested that boundary DPPG makes no contribution to the phase transition and that boundary DPPG and bulk DPPG are in the phase-separated state, thereby increasing the permeability of liposomal bilayer membranes through adsorption of LSZ or BSA. A possible schematic model for the adsorption of LSZ or BSA on DPPG liposomes was proposed.

Effect of grafted polyethylene glycol (PEG) on the size, encapsulation efficiency and permeability of vesicles

Liposomes have been prepared by the vesicle extrusion method (VETs) from mixtures of dipalmitoylphosphatidylcholine (DPPC), phosphatidylinositol (PI) and dipalmitoylphosphatidylethanolamine with covalently linked poly(ethylene glycol) molecular mass 5000 and 2000 (DPPE-PEG 5000 and DPPE-PEG 2000) covering a range of 0-7.5 mole%. The encapsulation of D-glucose has been studied and found to be markedly dependent on the mole% DPPE-PEG. The permeability of the liposomes to D-glucose has been measured both as a function of temperature and liposome composition. The permeability coefficients for D-glucose increase with mole% DPPE-PEG 5000 and with temperature over the range 25-50 degrees C. The activation energies for glucose permeability range from 90 to 23 kJ mol(-1). The decrease in activation energy with increasing temperature is attributed to an increasing number of bilayer defects as the liposome content of PEG-grafted lipid is increased. The dependence of D-glucose encapsulation as a function of PEG-grafted lipid content is discussed in terms of the conformation of the PEG molecules on the inner surface of the bilayer. For liposomes containing DPPE-PEG 5000 the relative percentage encapsulation of glucose, assuming that the PEG surface layer excludes glucose, is comparable to that predicted from the mushroom and brush conformational models.

Effects of PEG-lipids on permeability of phosphatidylcholine/cholesterol liposomes in buffer and in human serum

The permeability of liposomal membranes was studied as a function of the amount of incorporated PEG-lipid. The fluorescent dyes ethidium, propidium and 5(6)-carboxy fluorescein were used as markers for measurements of spontaneous leakage. The results show that addition of up to 8 mol% of PEG(2000)-DSPE into liposomal membranes of DSPC/Cho and EPC/Cho reduces the permeability of carboxyfluorescein in buffer solution. In contrast, the leakage of the more amphiphilic dye ethidium was not to any measurable extent affected by PEG-lipid inclusion. Another important difference was that ethidum leakage showed a clear dependence on temperature whereas leakage of carboxyfluorescein from pegylated liposomes did not. We conclude that the mechanisms by which the two dyes permeate the liposomal bilayer are qualitatively different. Both ethidium and carboxyfluorescein did interact with human serum components in a way that made measurements in serum unreliable. The more hydrophilic ethidium analogue propidium was shown not to interact with human serum components to any detectable extent. This made propidium suitable for permeability determinations in human serum. It was found that liposomes composed of pure EPC or EPC with 5 mol% DSPE-PEG, displayed a dramatic increase in permeability when subjected to a medium composed of 20% human serum in buffer. Addition of 40 mol% cholesterol to the EPC bilayers reduced the observed release rate in human serum substantially, whereas no stabilizing effect was observed upon PEG-lipid inclusion.

Phospholipid free thin liquid films with grafted poly(ethylene glycol)-2000: formation, interaction forces and phase states

Free thin liquid films (foam films) formed from aqueous dispersions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylethanolamine with covalently bound poly-(ethylene glycol) of molecular weight 2000 (DPPE-PEG-2000) were studied by the thin liquid film microinterferometric technique of Scheludko and Exerowa in the temperature range 14-36 degrees C. The surface tension kinetics of the dispersions were studied in order to ensure equilibration of the foam films. These measurements showed that the rate of surface coverage depends slightly on the temperature and does not reach equilibrium values within reasonable time intervals for the dispersions containing only one amphiphile (DPPE-PEG-2000). The destruction of the vesicles at the air/(aqueous dispersion) interface was much faster for the dispersions containing DMPC/DPPE-PEG-2000 mixtures above 23 degrees C, the temperature of the chain-melting phase transition of the main lipid component (DMPC). The dependence of the equilibrium thickness of the foam films on the electrolyte concentration was measured for 1 and 9 mol% DPPE-PEG-2000 at 28 degrees C in the range 10-3 to 0.5 M NaCl. These results indicate that at the low electrolyte concentrations the electrostatic and van der Waals interactions are dominant similar to the foam films stabilized with DMPC alone. At the high electrolyte concentrations the steric repulsion of the PEG layers becomes dominant. The temperature-composition dependence of the bilayer thickness was measured for the foam bilayers at 0.14 M NaCl. The data for the foam bilayer thickness and the comparison with the phase diagrams of PC/PE-PEG dispersions, show that the DMPC/DPPE-PEG-2000 foam bilayers are able to exist in two phase states characterised by different conformations (mushroom and brush) of the grafted polymer.

Thermally induced proliferation of pores in a model fluid membrane

The growth of thermally induced pores in a two-dimensional model fluid membrane is investigated by Monte Carlo simulation. Holes appear in the membrane via an activated process, and their subsequent growth is controlled by an edge energy per unit length or line tension. The barrier height and line tension, together with a lateral tension, are the independent parameters of the model. In the resulting phase diagram, a rupture transition separates an intact membrane from a disintegrated state. The approach to the ruptured state shows distinct regimes. Reducing the barrier height at large line tension produces multiple, quasi-independent, small holes whose behavior is dominated by their edge energy, whereas at lower line tensions shape fluctuations of the holes facilitate their coalescence into a single large hole. At a small value of line tension and large barrier height, a single hole spontaneously permeabilizes the membrane in an entropically driven phase transition. Entropy dominates pore growth for line tensions not far below those measured for artificial vesicles. Permeabilization of lipid bilayers by certain peptides involves perturbing lipid-lipid cohesive energies, and our simulations show that at small line tensions the entropy of hole shape fluctuations destroys the model membrane's stability.

Effect of grafted PEG-2000 on the size and permeability of vesicles

The permeability of PEG-derivatized vesicles produced by the extrusion technique (VETs) to encapsulated D-glucose has been studied. Vesicles were prepared from mixtures of dipalmitoylphosphatidylcholine (DPPC), phosphatidylinositol (PI) and dipalmitoylphosphatidylethanolamine with covalently attached poly(ethylene glycol) of molecular weight 2000 (DPPE-PEG2000). The occurrence of an irreversible temperature-induced size transition of the PEG-derivatized VETs was detected close to the temperature of the main phase transition of the predominant phospholipid component by photon correlation spectroscopy. The permeability of the vesicles towards radioactive D-glucose was studied by a method similar to the one proposed by Johnson and Bangham at 25, 37 and 50 degrees C for VETs with DPPE-PEG content from 0 to 9 mol%. A complex effect of the PEG-coverage was found with a maximum leakage in the range of the transition of the polymer from the mushroom to the brush conformation.