Effects of electrically-induced constant tension on giant unilamellar vesicles using irreversible electroporation

Probability and kinetics of rupture and electrofusion in giant unilamellar vesicles under various frequencies of direct current pulses

Irreversible electroporation induces permanent permeabilization of lipid membranes of vesicles, resulting in vesicle rupture upon the application of a pulsed electric field. Electrofusion is a phenomenon wherein neighboring vesicles can be induced to fuse by exposing them to a pulsed electric field. We focus how the frequency of direct current (DC) pulses of electric field impacts rupture and electrofusion in cell-sized giant unilamellar vesicles (GUVs) prepared in a physiological buffer. The average time, probability, and kinetics of rupture and electrofusion in GUVs have been explored at frequency 500, 800, 1050, and 1250 Hz. The average time of rupture of many 'single GUVs' decreases with the increase in frequency, whereas electrofusion shows the opposite trend. At 500 Hz, the rupture probability stands at 0.45 ± 0.02, while the electrofusion probability is 0.71 ± 0.01. However, at 1250 Hz, the rupture probability increases to 0.69 ± 0.03, whereas the electrofusion probability decreases to 0.46 ± 0.03. Furthermore, when considering kinetics, at 500 Hz, the rate constant of rupture is (0.8 ± 0.1)×10-2 s-1, and the rate constant of fusion is (2.4 ± 0.1)×10-2 s-1. In contrast, at 1250 Hz, the rate constant of rupture is (2.3 ± 0.8)×10-2 s-1, and the rate constant of electrofusion is (1.0 ± 0.1)×10-2 s-1. These results are discussed by considering the electrical model of the lipid bilayer and the energy barrier of a prepore.

Determination of pore edge tension from the kinetics of rupture of giant unilamellar vesicles using the Arrhenius equation: effects of sugar concentration, surface charge and cholesterol

The pore edge tension (Γ) of a membrane closely intertwines with membrane stability and plays a vital role in the mechanisms that facilitate membrane resealing following pore formation caused by electrical and mechanical tensions. We have explored a straightforward procedure to determine Γ by fitting the inverse of the tension-dependent logarithm of the rate constant of rupture of giant unilamellar vesicles (GUVs) using the Arrhenius equation. The GUVs were prepared using a combination of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) and 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) in a physiological environment. The effects of sugar concentration, membrane surface charge density, and membrane cholesterol concentration on Γ have been investigated. The values of Γ increase with sugar concentration in the physiological buffer, measuring 9.6 ± 0.3, 10.4 ± 0.1, and 16.2 ± 0.1 pN for 40, 100, and 300 mM, respectively. A higher concentration of anionic lipids (70 mol% of DOPG) significantly reduces Γ. An increasing trend of Γ with cholesterol content was observed; specifically, the values of Γ were 11.9 ± 0.9, 13.9 ± 0.7, and 16.2 ± 0.4 pN for 15, 29, and 40 mol% cholesterol, respectively. Thus, the presence of higher anionic lipids in the bilayer led to a decrease in membrane stability. In contrast, the presence of higher sugar concentrations in the buffer and increased cholesterol concentration in the membranes enhanced membrane stability.

A review on the measurement of the bending rigidity of lipid membranes

This review provides an overview of the latest developments in both experimental and simulation techniques used to assess the bending rigidity of lipid membranes. It places special emphasis on experimental methods that utilize model vesicles to manipulate lipid compositions and other experimental parameters to determine the bending rigidity of the membrane. It also describes two commonly used simulation methods for estimating bending rigidity. The impact of various factors on membrane bending rigidity is summarized, including cholesterol, lipids, salt concentration, surface charge, membrane phase state, peptides, proteins, and polyethylene glycol. These factors are shown to influence the bending rigidity, contributing to a better understanding of the biophysical properties of membranes and their role in biological processes. Furthermore, the review discusses future directions and potential advancements in this research field, highlighting areas where further investigation is required.

Effects of membrane potentials on the electroporation of giant unilamellar vesicles

Living organisms maintain a resting membrane potential, which plays an important role in various biophysical and biological processes. In the context of medical applications, irreversible electroporation (IRE) is a non-thermal and minimally invasive technique that utilizes precisely controlled electric field pulses of micro- to millisecond durations to effectively ablate cancer and tumor cells. Previous studies on IRE-induced rupture of cell-mimetic giant unilamellar vesicles (GUVs) have primarily been conducted in the absence of membrane potentials. In this study, we investigated the electroporation of GUVs, including parameters such as the rate constant of rupture and the probability of rupture, in the presence of various negative membrane potentials. The membranes of GUVs were prepared using lipids and channel forming proteins. As the membrane potential increased from 0 to -90 mV, the rate constant of rupture showed a significant increase from (7.5 ± 1.6)×10-3 to (35.6 ± 5.5)×10-3 s-1. The corresponding probability of rupture also exhibited a notable increase from 0.40 ± 0.05 to 0.68 ± 0.05. To estimate the pore edge tension, the electric tension-dependent logarithm of the rate constant was fitted with the Arrhenius equation for different membrane potentials. The presence of membrane potential did not lead to any significant changes in the pore edge tension. The increase in electroporation is reasonably explained by the decrease in the prepore free energy barrier. The choice of buffer used in GUVs can significantly influence the kinetics of electroporation. This study provides valuable insights that can contribute to the application of electroporation techniques in the biomedical field.

Methods to mechanically perturb and characterize GUV-based minimal cell models

Cells shield organelles and the cytosol via an active boundary predominantly made of phospholipids and membrane proteins, yet allowing communication between the intracellular and extracellular environment. Micron-sized liposome compartments commonly known as giant unilamellar vesicles (GUVs) are used to model the cell membrane and encapsulate biological materials and processes in a cell-like confinement. In the field of bottom-up synthetic biology, many have utilized GUVs as substrates to study various biological processes such as protein-lipid interactions, cytoskeletal assembly, and dynamics of protein synthesis. Like cells, it is ideal that GUVs are also mechanically durable and able to stay intact when the inner and outer environment changes. As a result, studies have demonstrated approaches to tune the mechanical properties of GUVs by modulating membrane composition and lumenal material property. In this context, there have been many different methods developed to test the mechanical properties of GUVs. In this review, we will survey various perturbation techniques employed to mechanically characterize GUVs.

Updates on Responsive Drug Delivery Based on Liposome Vehicles for Cancer Treatment

Liposomes are well-known nanoparticles with a non-toxic nature and the ability to incorporate both hydrophilic and hydrophobic drugs simultaneously. As modern drug delivery formulations are produced by emerging technologies, numerous advantages of liposomal drug delivery systems over conventional liposomes or free drug treatment of cancer have been reported. Recently, liposome nanocarriers have exhibited high drug loading capacity, drug protection, improved bioavailability, enhanced intercellular delivery, and better therapeutic effect because of resounding success in targeting delivery. The site targeting of smart responsive liposomes, achieved through changes in their physicochemical and morphological properties, allows for the controlled release of active compounds under certain endogenous or exogenous stimuli. In that way, the multifunctional and stimuli-responsive nanocarriers for the drug delivery of cancer therapeutics enhance the efficacy of treatment prevention and fighting over metastases, while limiting the systemic side effects on healthy tissues and organs. Since liposomes constitute promising nanocarriers for site-targeted and controlled anticancer drug release, this review focuses on the recent progress of smart liposome achievements for anticancer drug delivery applications.

Effects of sugar concentration on the electroporation, size distribution and average size of charged giant unilamellar vesicles

We investigated the effects of sugar concentration on the electroporation, size distribution and average size of giant unilamellar vesicles (GUVs). GUVs were prepared from 40 mol% of 1,2-dioleoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DOPG) and 60 mol% of 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) lipids. Pulsed electric field was applied to the 40%DOPG/60%DOPC-GUVs and it induced lateral electric tension (σ_c) in the membranes of vesicles. The σ_c-induced probability of rupture (P_pore) and the rate constant of rupture (k_p) of GUVs under the sugar concentration, c = 40, 100 and 300 mM, were determined. Both the P_pore and k_p increased with the increase of σ_c, but higher tension was required to generate the same values of P_pore and k_p with increasing c. We also investigated average sizes of GUVs from the size distribution of vesicles under various sugar concentrations. With the increase of c, the peak of the size distribution histograms shifted to the region of smaller vesicles. The average size decreased 1.6-fold when c increased from 10 to 300 mM. These investigations help to understand various biomedical, biophysical, and biochemical processes in vesicles and cells. Electroporation, size distribution and average size of charged GUVs were investigated under various sugar concentrations. The sugar concentration influences the electroporation of vesicles and the average size of GUVs.

Actin protein inside DMPC GUVs and its mechanical response to AC electric fields

Cells are dynamic systems with complex mechanical properties, regulated by the presence of different species of proteins capable to assemble (and disassemble) into filamentous forms as required by different cells functions. Giant unilamellar vesicles (GUVs) of DMPC (1,2-dimyristoyl-sn-glycero-3-phosphocholine) are systems frequently used as a simplified model of cells because they offer the possibility of assaying separately different stimuli, which is no possible in living cells. Here we present a study of the effect of acting protein on mechanical properties of GUVs, when the protein is inside the vesicles in either monomeric G-actin or filamentous F-actin. For this, rabbit skeletal muscle G-actin is introduced inside GUVs by the electroformation method. Protein polymerization inside the GUVs is promoted by adding to the solution MgCl₂ and the ion carrier A23187 to allow the transport of Mg⁺² ions into the GUVs. To determine how the presence of actin changes the mechanical properties of GUVs, the vesicles are deformed by the application of an AC electric field in both cases with G-actin and with polymerized F-actin. The changes in shape of the vesicles are characterized by optical microscopy and from them the bending stiffness of the membrane are determined. It is found that G-actin has no appreciable effect on the bending stiffness of DMPC GUVs, but the polymerized actin makes the vesicles more rigid and therefore more resistant to deformations. This result is supported by evidence that actin filaments tend to accumulate near the membrane.

Quantification of pulsed electric field for the rupture of giant vesicles with various surface charges, cholesterols and osmotic pressures

Electropermeabilization is a promising phenomenon that occurs when pulsed electric field with high frequency is applied to cells/vesicles. We quantify the required values of pulsed electric fields for the rupture of cell-sized giant unilamellar vesicles (GUVs) which are prepared under various surface charges, cholesterol contents and osmotic pressures. The probability of rupture and the average time of rupture are evaluated under these conditions. The electric field changes from 500 to 410 Vcm^-1 by varying the anionic lipid mole fraction from 0 to 0.60 for getting the maximum probability of rupture (i.e., 1.0). In contrast, the same probability of rupture is obtained for changing the electric field from 410 to 630 Vcm^-1 by varying the cholesterol mole fraction in the membranes from 0 to 0.40. These results suggest that the required electric field for the rupture decreases with the increase of surface charge density but increases with the increase of cholesterol. We also quantify the electric field for the rupture of GUVs containing anionic mole fraction of 0.40 under various osmotic pressures. In the absence of osmotic pressure, the electric field for the rupture is obtained 430 Vcm^-1, whereas the field is 300 Vcm^-1 in the presence of 17 mOsmL^-1, indicating the instability of GUVs at higher osmotic pressures. These investigations open an avenue of possibilities for finding the electric field dependent rupture of cell-like vesicles along with the insight of biophysical and biochemical processes.

Effects of cholesterol on the size distribution and bending modulus of lipid vesicles

The influence of cholesterol fraction in the membranes of giant unilamellar vesicles (GUVs) on their size distributions and bending moduli has been investigated. The membranes of GUVs were synthesized by a mixture of two elements: electrically neutral lipid 1, 2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol and also a mixture of three elements: electrically charged lipid 1,2-dioleoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DOPG), DOPC and cholesterol. The size distributions of GUVs have been presented by a set of histograms. The classical lognormal distribution is well fitted to the histograms, from where the average size of vesicle is obtained. The increase of cholesterol content in the membranes of GUVs increases the average size of vesicles in the population. Using the framework of Helmholtz free energy of the system, the theory developed by us is extended to explain the experimental results. The theory determines the influence of cholesterol on the bending modulus of membranes from the fitting of the proper histograms. The increase of cholesterol in GUVs increases both the average size of vesicles in population and the bending modulus of membranes.

PoET: automated approach for measuring pore edge tension in giant unilamellar vesicles

Motivation

A reliable characterization of the membrane pore edge tension of single giant unilamellar vesicles (GUVs) requires the measurement of micrometer sized pores in hundreds to thousands of images. When manually performed, this procedure has shown to be extremely time-consuming and to generate inconsistent results among different users and imaging systems. A user-friendly software for such analysis allowing quick processing and generation of reproducible data had not yet been reported.

Results

We have developed a software (PoET) for automatic pore edge tension measurements on GUVs. The required image processing steps and the characterization of the pore dynamics are performed automatically within the software and its use allowed for a 30-fold reduction in the analysis time. We demonstrate the applicability of the software by comparing the pore edge tension of GUVs of different membrane compositions and surface charges. The approach was applied to electroporated GUVs but is applicable to other means of pore formation.

Availability and implementation

The complete software is implemented in Python and available for Windows at https://dx.doi.org/10.17617/3.7h.

Supplementary information

Supplementary data are available at Bioinformatics Advances online.

Recent developments in the kinetics of ruptures of giant vesicles under constant tension

External tension in membranes plays a vital role in numerous physiological and physicochemical phenomena. In this review, recent developments in the constant electric- and mechanical-tension-induced rupture of giant unilamellar vesicles (GUVs) are considered. We summarize the results relating to the kinetics of GUV rupture as a function of membrane surface charge, ions in the bathing solution, lipid composition, cholesterol content in the membrane, and osmotic pressure. The mechanical stability and line tension of the membrane under these conditions are discussed. The membrane tension due to osmotic pressure and the critical tension of rupture for various membrane compositions are also discussed. The results and their analysis provide a biophysical description of the kinetics of rupture, along with insight into biological processes. Future directions and possible developments in this research area are included.

A new purification technique to obtain specific size distribution of giant lipid vesicles using dual filtration

A new purification technique is developed for obtaining distribution of giant unilamellar vesicles (GUVs) within a specific range of sizes using dual filtration. The GUVs were prepared using well known natural swelling method. For filtration, different combinations of polycarbonate membranes were implemented in filter holders. In our experiment, the combinations of membranes were selected with corresponding pore sizes-(i) 12 and 10 μm, (ii) 12 and 8 μm, and (iii) 10 and 8 μm. By these filtration arrangements, obtained GUVs size distribution were in the ranges of 6-26 μm, 5-38 μm and 5-30 μm, respectively. In comparison, the size distribution range was much higher for single filtration technique, for example, 6-59 μm GUVs found for a membrane with 12 μm pores. Using this technique, the water-soluble fluorescent probe, calcein, can be removed from the suspension of GUVs successfully. The size distributions were analyzed with lognormal distribution. The skewness became smaller (narrow size distribution) when a dual filtration was used instead of single filtration. The mode of the size distribution obtained in dual filtration was also smaller to that of single filtration. By continuing this process of purification for a second time, the GUVs size distribution became even narrower. After using an extra filtration with dual filtration, two different size distributions of GUVs were obtained at a time. This experimental observation suggests that different size specific distributions of GUVs can be obtained easily, even if GUVs are prepared by different other methods.

Effects of osmotic pressure on the irreversible electroporation in giant lipid vesicles

Irreversible electroporation (IRE) is a nonthermal tumor/cell ablation technique in which a series of high-voltage short pulses are used. As a new approach, we aimed to investigate the rupture of giant unilamellar vesicles (GUVs) using the IRE technique under different osmotic pressures (Π), and estimated the membrane tension due to Π. Two categories of GUVs were used in this study. One was prepared with a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol (chol) for obtaining more biological relevance while other with a mixture of DOPG and DOPC, with specific molar ratios. We determined the rate constant (kp) of rupture of DOPG/DOPC/chol (46/39/15)-GUVs and DOPG/DOPC (40/60)-GUVs induced by constant electric tension (σc) under different Π. The σc dependent kp values were fitted with a theoretical equation, and the corresponding membrane tension (σoseq) at swelling equilibrium under Π was estimated. The estimated membrane tension agreed well with the theoretical calculation within the experimental error. Interestingly, the values of σoseq were almost same for both types of synthesized GUVs under same osmotic pressure. We also examined the sucrose leakage, due to large osmotic pressure-induced pore formation, from the inside of DOPG/DOPC/chol(46/39/15)-GUVs. The estimated membrane tension due to large Π at which sucrose leaked out was very similar to the electric tension at which GUVs were ruptured without Π. We explained the σc and Π induced pore formation in the lipid membranes of GUVs.

Analysis of purification of charged giant vesicles in a buffer using their size distribution

We have analyzed the purification of charged giant unilamellar vesicles (GUVs) prepared in a buffer containing various concentrations of salt using their size distribution. The membranes of GUVs were synthesized by a mixture of dioleoylphosphocholine (DOPC) and dioleoylphosphatidylglycerol (DOPG) lipids. The DOPG mole fractions (X) in the membranes of GUVs were 0.10, 0.25, 0.40, 0.55, 0.70, 0.90 in a physiological buffer containing 162 mM salt. In addition, for a fixed value of X the concentrations of salt (C) in the buffer were 12, 62, 112, 162, 212, 312, 362 mM. The size distribution histograms of experimentally investigated unpurified and purified GUVs were fitted with the lognormal distribution and obtained the multiplication factor [Formula: see text] for mean ([Formula: see text]) and [Formula: see text] for standard deviation ([Formula: see text]) of the lognormal distribution. The key parameters [Formula: see text] and [Formula: see text] were responsible for changing the average size and size distribution of unpurified GUVs to purified ones. The theoretically fitting equation of experimentally obtained X- and C-dependent values of [Formula: see text] and [Formula: see text] provided the calibration equation for estimating the average size of purified GUVs theoretically for any values of X and C. The estimated size of purified GUVs increased with the increase in electrostatic effect (i.e., increase in vesicle surface charge density or decrease in salt concentration in buffer). The estimated size of purified GUVs varied with X and C, which supported the previous report qualitatively. These investigations might be helpful in the field of cell/chemical biology for understanding the process of purification of vesicles/cells investigated by any other techniques.

Pulsed Electric Fields Promote Liposome Buddings

Background: Liposomes have been a useful tool to analyze membrane behavior. Various studies have attempted to induce biological activities, for example, buddings, divisions, and endocytosis, on liposomes, focusing on lipid rafts that move along electric fields. Materials and Methods: Liposomes consisting of soybean lecithin, phosphatidylcholine, and cholesterol were prepared, with inner and outer liquid conductivities of 0.595 and 1.564 S/m, respectively. Results: We tried to induce buddings by pulsed electric fields (PEFs) on liposomes. Results demonstrated that 1.248 kV/cm, 400 μs PEF promoted postpulse liposome buddings, which were preceded by a membrane relaxation. Although a transient thick area (a lipid raft-like area) on the membrane just after PEF application preceded buddings, it was not the sufficient factor for buddings. Conclusion: We established a brief model as follows: 1.248 kV/cm, 400 μs PEF induced the lipid membrane relaxation without electroporation to trigger buddings. The current results could be a new frontier in bioelectrics.

An investigation into the critical tension of electroporation in anionic lipid vesicles

Irreversible electroporation (IRE) is a technique for the disruption of localized cells or vesicles by a series of short and high-frequency electric pulses which has been used for tissue ablation and treatment in certain diseases. It is well reported that IRE induces lateral tension in the membranes of giant unilamellar vesicles (GUVs). The GUVs are prepared by a mixture of anionic lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. Here the influence of DOPG mole fraction, XDOPG, on the critical tension of electroporation in GUVs has been investigated in sodium chloride-containing PIPES buffer. The critical tension decreases from 9.0 ± 0.3 to 6.0 ± 0.2 mN/m with the increase of XDOPG from 0.0 to 0.60 in the membranes of GUVs. Hence an increase in XDOPG greatly decreases the mechanical stability of membranes. We develop a theoretical equation that fits the XDOPG dependent normalized critical tension, and obtain a binding constant for the lipid-ion interaction of 0.75 M-1. The decrease in the energy barrier for formation of the nano-size nascent or prepore state, due to the increase in XDOPG, is the main factor explaining the decrease in critical tension of electroporation in vesicles.

Proof-of-Concept of Electrical Activation of Liposome Nanocarriers: From Dry to Wet Experiments

The increasing interest toward biocompatible nanotechnologies in medicine, combined with electric fields stimulation, is leading to the development of electro-sensitive smart systems for drug delivery applications. To this regard, recently the use of pulsed electric fields to trigger release across phospholipid membranes of liposomes has been numerically studied, for a deeper understanding of the phenomena at the molecular scale. Aim of this work is to give an experimental validation of the feasibility to control the release from liposome vesicles, using nanosecond pulsed electric fields characterized by a 10 ns duration and intensity in the order of MV/m. The results are supported by multiphysics simulations which consider the coupling of three physics (electromagnetics, thermal and pore kinetics) in order to explain the occurring physical interactions at the microscopic level and provide useful information on the characteristics of the train of pulses needed to obtain quantitative results in terms of liposome electropermeabilization. Finally, a complete characterization of the exposure system is also provided to support the reliability and validity of the study.

Electrostatic effects on the electrical tension-induced irreversible pore formation in giant unilamellar vesicles

Irreversible electroporation (IRE) is a new technique in which a series of short pulses with high frequency electrical energy is applied on the targeted regions of cells or vesicles for their destruction or rupture formation. IRE induces lateral tension in the membranes of vesicles. We have investigated the electrostatic interaction effects on the constant electrical tension-induced rate constant of irreversible pore formation in the membranes of giant unilamellar vesicles (GUVs). The electrostatic interaction has been varied by changing the salt concentration in buffer and the surface charge density of membranes. The membranes of GUVs are synthesized by a mixture of negatively charged lipid dioleoylphosphatidylglycerol (DOPG) and neutral lipid dioleoylphosphatidylcholine (DOPC) using the natural swelling method. The rate constant of pore formation increases with the decrease of salt concentration in buffer along with the increase of surface charge density of membranes. The tension dependent probability of pore formation and the rate constant of pore formation are fitted to the theoretical equation, and obtained the line tension of membranes. The decrease in energy barrier of a prepore due to electrostatic interaction is the key factor causing an increase of rate constant of pore formation.

Influence of cholesterol on electroporation in lipid membranes of giant vesicles

Irreversible electroporation (IRE) is primarily a nonthermal ablative technology that uses a series of high-voltage and ultra-short pulses with high-frequency electrical energy to induce cell death. This paper presents the influence of cholesterol on the IRE-induced probability of pore formation and the rate constant of pore formation in giant unilamellar vesicles (GUVs). The GUVs are prepared by a mixture of dioleoylphosphatidylglycerol (DOPG), dioleoylphosphatidylcholine (DOPC) and cholesterol using the natural swelling method. An IRE signal of frequency 1.1 kHz is applied to the membranes of GUVs. The probability of pore formation and the rate constant of pore formation events are obtained using statistical analysis from several single GUVs. The time-dependent fraction of intact GUVs among all those examined is fitted to a single exponential decay function from where the rate constant of pore formation is calculated. The probability of pore formation and the rate constant of pore formation decreases with an increase in cholesterol content in the membranes of GUVs. Theoretical equations are fitted to the tension-dependent rate constant of pore formation and to the probability of pore formation, which allows us to obtain the line tension of membranes. The obtained line tension increases with an increase in cholesterol in the membranes. The increase in the energy barrier of the prepore state, due to the increase of cholesterol in membranes, is the main factor explaining the decrease in the rate constant of pore formation.

Kinetics of irreversible pore formation under constant electrical tension in giant unilamellar vesicles

Stretching in the plasma membranes of cells and lipid membranes of vesicles plays important roles in various physiological and physicochemical phenomena. Irreversible electroporation (IRE) is a minimally invasive non-thermal tumor ablation technique where a series of short electrical energy pulses with high frequency is applied to destabilize the cell membranes. IRE also induces lateral tension due to stretching in the membranes of giant unilamellar vesicles (GUVs). Here, the kinetics of irreversible pore formation under constant electrical tension in GUVs has been investigated. The GUVs are prepared by a mixture of dioleoylphosphatidylglycerol and dioleoylphosphatidylcholine using the natural swelling method. An IRE signal of frequency 1.1 kHz is applied to the GUVs through a gold-coated electrode system. Stochastic pore formation is observed for several 'single GUVs' at a particular constant tension. The time course of the fraction of intact GUVs among all the examined GUVs is fitted with a single-exponential decay function from which the rate constant of pore formation in the vesicle, k_p, is calculated. The value of k_p increases with an increase of membrane tension. An increase in the proportion of negatively charged lipids in a membrane gives a higher k_p. Theoretical equations are fitted to the tension-dependent k_p and to the probability of pore formation, which allows us to obtain the line tension of the membranes. The decrease in the energy barrier for formation of the nano-size nascent or prepore state, due to the increase in electrical tension, is the main factor explaining the increase of k_p.

Location of Peptide-Induced Submicron Discontinuities in the Membranes of Vesicles Using ImageJ

Cell penetrating peptide transportan 10 and antimicrobial peptide melittin formed submicron pores in the lipid membranes of vesicles which are explained by the leakage of water-soluble fluorescent probes from the inside of vesicles to the outside. It is hypothesized that these submicron pores induce submicron discontinuities in the membranes. Considering this hypothesis, a technique has developed to locate the submicron discontinuities in the membranes of giant unilamellar vesicles (GUVs) using ImageJ. In this technique, at first the edges of membrane of a 'single GUV' are detected and then these edges are used to locate the submicron discontinuities. Two continuous rings are observed after applying the ImageJ in GUVs which indicated the edges of membrane. In contrast, the submicron discontinuations are detected at the edges of transportan 10 and melittin induced pore formed membranes. This investigation might be helpful for the elucidation of mechanism of the peptide-induced pore formation in the membranes of vesicles.

Deformation and poration of giant unilamellar vesicles induced by anionic nanoparticles

The interaction of anionic magnetite nanoparticles (MNPs) of size 18 nm with negatively charged giant unilamellar vesicles (GUVs) formed from a mixture of neutral dioleoylphosphatidylcholine (DOPC) and negatively charged dioleoylphosphatidylglycerol (DOPG) lipids has been investigated. It has been obtained that NPs induces the deformation of spherical GUVs. The reaction of other GUVs on NPs consists in the appearance of pores in their membranes. We focused the effect of electrostatics on the interaction of charged membranes with MNPs. To study the influence of the surface charge of GUVs on the processes under consideration, we varied the fraction of DOPG in the vesicles from 0 to 100%. We examined the influence of salt concentration in the range of 50-300 mM NaCl concentration. To describe the degree of deformation, a special parameter compactness was introduced. The pore formation in the membranes of GUVs was investigated by the leakage of sucrose. The compactness increases with time and also NPs concentration. The fraction of deformed GUVs increases with the increase of surface charge density of membranes as well as the decrease of salt concentration in buffer. The value of compactness for neutral membrane is 1.25 times higher than that of charged ones. The fraction of deformed GUVs become constant after 20 min, however it increases with NPs concentration. The time taken for stochastic pore formation is less for charged membrane than neutral one. The physical mechanism explaining the experimental results obtained in these investigations.

Electrostatic interaction effects on the size distribution of self-assembled giant unilamellar vesicles

The influence of electrostatic conditions (salt concentration of the solution and vesicle surface charge density) on the size distribution of self-assembled giant unilamellar vesicles (GUVs) is considered. The membranes of GUVs are synthesized by a mixture of dioleoylphosphatidylglycerol and dioleoylphosphatidylcholine in a physiological buffer using the natural swelling method. The experimental results are presented in the form of a set of histograms. The log-normal distribution is used for statistical treatment of results. It is obtained that the decrease of salt concentration and the increase of vesicle surface charge density of the membranes increase the average size of the GUV population. To explain the experimental results, a theory using the Helmholtz free energy of the system describing the GUV vesiculation is developed. The size distribution histograms and average size of GUVs under various conditions are fitted with the proposed theory. It is shown that the variation of the bending modulus due to changing of electrostatic parameters of the system is the main factor causing a change in the average size of GUVs.