Effect of a PEG lipid (DSPE-PEG2000) and freeze-thawing process on phospholipid vesicle size and lamellarity

Liposomes containing distearoylphosphatidylethanolamine with covalently linked polyethylene glycol of molecular weight 2,000 (DSPE-PEG2000) covering a range of 0–30 mol% were prepared by a mechanical dispersion or detergent-removal method. The effects of DSPE-PEG2000 on particle sizes and lamellarity of liposomes were investigated. The average diameters of vesicles prepared from both methods decreased when the concentration of DSPE-PEG2000 was increased. The decrease in vesicle size with increase in DSPE-PEG2000 was ascribed to the steric hindrance of strongly hydrated PEG. The significant decrease in the sizes of DSPE-PEG2000-containing EggPC vesicles prepared by the detergent-removal method could be explained by the postvesiculation size growth in the process of micelle–vesicle transition. For DMPC vesicles prepared by the detergent-removal method, electron micrographs showed that inclusion of DSPE-PEG2000 promoted vesicle formation. Based on the results of investigation of calcein entrapment efficiency, we concluded that the lamellarity of liposomes is reduced as PEG lipid concentration is increased. Fragmentation of multilamellar vesicles into smaller unilamellar vesicles occurred more readily when the liposome suspension was subjected to repetitive freeze-thawing. After five cycles of freezing and thawing, vesicles containing more than 0.5 mol% DSPE-PEG2000 were fragmented into unilamellar vesicles with diameters smaller than 300 nm.     

A facile approach for assembling lipid bilayer membranes on template-stripped gold

Lipid vesicles are designed with functional chemical groups to promote vesicle fusion on template-stripped gold (TS Au) surfaces that does not spontaneously occur on unfunctionalized Au surfaces. Three types of vesicles were exposed to TS Au surfaces: (1) vesicles composed of only 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids; (2) vesicles composed of lipid mixtures of 2.5 mol % of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-pyridyldithio)propionate] (DSPE-PEG-PDP) and 97.5 mol % of POPC; and (3) vesicles composed of 2.5 mol % of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(poly(ethylene glycol))-2000] (DSPE-PEG) and 97.5 mol % POPC. Atomic force microscopy (AFM) topography and force spectroscopy measurements acquired in a fluid environment confirmed tethered lipid bilayer membrane (tLBM) formation only for vesicles composed of 2.5 mol % DSPE-PEG-PDP/97.5 mol % POPC, thus indicating that the sulfur-containing PDP group is necessary to achieve tLBM formation on TS Au via Au-thiolate bonds. Analysis of force-distance curves for 2.5 mol % DSPE-PEG-PDP/97.5 mol % POPC tLBMs on TS Au yielded a breakthrough distance of 4.8 ± 0.4 nm, which is about 1.7 nm thicker than that of POPC lipid bilayer membrane formed on mica. Thus, the PEG group serves as a spacer layer between the tLBM and the TS Au surface. Fluorescence microscopy results indicate that these tLBMs also have greater mechanical stability than solid-supported lipid bilayer membranes made from the same vesicles on mica. The described process for assembling stable tLBMs on Au surfaces is compatible with microdispensing used in array fabrication.     

Aggregation and fusion of phospholipid vesicles

Membrane fusion and aggregation of phospholipid vesicles are reviewed and discussed. The fusion process is viewed as consisting of several stages: aggregation and close apposition of the particles, destabilization, and finally, merging of the bilayers. A procedure is presented which yields both the rate constant of the dimerization (C₁₁) and the rate constant for fusion of the dimers (f₁₁), which is a direct measure of the probability that two apposed vesicles will fuse. Experimental methods used in the study of membrane fusion are reviewed, primarily with respect to their capacity to monitor the kinetics of vesicle fusion. A few kinetic studies on the mixing of aqueous contents, leakage and increase in size of fusing vesicles are presented in detail.The range of C₁₁ values for Ca²⁺-induced aggregation and fusion of small unilamellar vesicles (SUV, ～ 125 Å radius) composed of phosphatidylserine (PS) is 10⁶ to 5 × 10⁷ M^-1 in the presence of Ca²⁺ concentrations from 1.15 to 2 mM, respectively. For larger PS vesicles (LUV, ～ 500 Å radius) C₁₁ = 6.5 × 10⁷ M^-1s^-1 in the presence of 5 mM Ca²⁺. These values are in good agreement with theoretical calculations based on van der Waals and electrostatic interactions, in which binding of cations is explicitly taken into account. The rate constants of fusion, f₁₁, are 5 s^-1 for PS SUV and 0.08 s^-1 for LUV in the presence of 2 mM and 5 mM Ca²⁺, respectively. The significance of these fusion rate constants to the duration of the fusion event is discussed.Factors affecting fusion such as cations, temperature, membrane composition vesicle concentration and size are reviewed and analyzed. Di- or tri-valent cations induce fusion of acidic phospholipid vesicles (except for phosphatidylinositol) in either pure or mixed form. Among the neutral phospholipids, phosphatidylcholine (PC) inhibits but phosphatidylethanolamine (PE) sustains or enhances the fusion capacity of acidic phospholipid vesicles. Monovalent cations induce reversible aggregation of negatively charged vesicles, but they inhibit the fusion induced by divalent cations such as Ca²⁺ or Mg²⁺. Fusion of neutral phospholipid vesicles, and it occurs the cation-induced fusion of acidic phospholipid vesicles, and it occurs only at temperatures below the gel to liquid crystalline phase transition temperature Tc. This is in contrast to the acidic phospholipid vesicle fusion which is greatly enhanced when the temperature is above the Tc of the phospholipid.     

Study on size growth in the vesicle formation upon detergent removal from phospholipid/ detergent mixed micelles

When 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS) was removed from the mixed CHAPS/EggPC micelles, large vesicles were prepared by dialysis or by slow step-by-step dilution, but small vesicles were prepared by fast one-step dilution. When sodium cholate was removed from the sodium cholate/EggPC micelles, small vesicles formed either by dialysis or by dilution; however, in the presence of 5 mM Ca²⁺ large vesicles were produced by dialysis, while small vesicles were prepared by dilution. The size growth was related to a detergent-induced fusion of the vesicles containing a large amount of detergent. Using spectrophotometry, quasielastic light scattering and freeze–fracture electron microscopy the fusion events were investigated both through the process of vesicle solubilization by adding detergent and through the process of vesicle formation by diluting a mixed micelle. The results suggest that a rapid CHAPS-induced fusion of the vesicles led to the large resultant vesicles and that no fusion of vesicles containing sodium cholate is responsible for the formation of small vesicles. Furthermore, the ultimate vesicle size related to rapid or slow detergent removal is dependent on the kinetic aspects of the fusion. Received: 19 August 1999 Accepted: 18 February 2000     

Physicochemical properties of PEG-grafted liposomes

Egg phosphatidylcholine (EggPC) or dimyristoylphosphatidylcholine (DMPC) liposomes containing polyethylene glycol (PEG)-lipids covering a range of 0-30 mol% have been prepared by Extrusion method. The physicochemical properties including size evolution and calcein permeation were evaluated to investigate the effect of PEG-lipids on bilayer structure. The results from quasielasetic light scattering (QELS), freeze-fracture microscopy, and gel exclusion chromatography revealed that presence of low concentration of PEG-lipid results in decreasing of vesicle size and further increase in the PEG-lipid concentrations lead to a transition from the lamellar membranes to micelles. The permeability for calcein increased with increase in concentration of distearoylphosphatidylethanolamine (DSPE)-PEG. On the other hand, the permeability decreased with low amount of cholesterol-PEG (blow 20% cholesterol-PEG) and increased with high amount of it. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the composition of the bilayer. The concentration of DSPE-PEG2000 incorporated into vesicles without damaging vesicle structures were <20 mol% for EggPC and <10% for DMPC.     

Effects of poly(ethylene glycol) (PEG) concentration on the permeability of PEG-grafted liposomes

Poly(ethylene glycol)-grafted liposomes (PEG-liposomes) were prepared from dipalmitoylphosphatidylcholine (DPPC) with various amounts of distearoyl-N-monomethoxy poly(ethylene glycol)-succinyl-phosphatidylethanolamines (DSPE-PEG) with PEG molecular weights of 1000, 2000, 3000 and 5000. The effects of DSPE-PEG concentration on the permeability of PEG-liposomes were investigated using carboxyfluorescein (CF). In the gel state, the CF leakage from PEG-liposomes was decreased with increasing mole fractions of DSPE-PEG for all PEG molecular weights. In the liquid-crystalline state, the CF leakage from PEG-liposomes containing DSPE-PEG1000 gradually increased with increasing mole fractions of DSPE-PEG, while that of PEG-liposomes whose molecular weight in PEG units was above 2000 rapidly decreased by the addition of DSPE-PEG. Furthermore, no effect of PEG molecular weight on CF leakage was observed. The relationship between the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) (or 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH)) and the mole fraction of DSPE-PEG for PEG-liposomes was also investigated. No significant changes in fluorescence polarization of DPH for liposomal bilayer membranes was observed in the gel and liquid-crystalline states due to the addition of DSPE-PEG, while that of TMA-DPH was decreased compared with that of liposomes without DSPE-PEG in both states.     

Effect of freeze-thawing on lipid bilayer-protected gold nanoparticles

In this study, varieties of lipid bilayer-protected gold nanoparticles (AuNPs) were synthesized through a simple wet chemical method, and then the effect of freeze-thawing on the as-prepared AuNPs was investigated. The freeze-thawing process induced fusion or fission of lipid bilayers tethered on the AuNPs. The UV-vis spectra and transmission electron microscopy experiments revealed that the disruption of lipid bilayer structures on the nanoparticles led to the fusion or aggregation of AuNPs. The role of freeze-thawing in the evolution of lipid bilayer-protected AuNPs was studied. The addition of adequate sucrose, a well-known cryoprotectant, effectively prevented the fusion or aggregation of lipid bilayer-protected AuNPs undergoing the freeze-thawing process. The possible mechanism of sucrose preserving the integrity of the lipid bilayer-protected AuNPs was also discussed.     

Mixing behavior of a poly(ethylene glycol)-grafted phospholipid in monolayers at the air/water interface

Mixed phospholipid monolayers hosting a poly(ethylene glycol) (PEG)-grafted distearoylphosphatidylethanolamine with a PEG molecular weight of 5000 (DSPE-PEG5000) spread at the air/water interface were used as model systems to study the effect of PEG-phospholipids on the lateral structure of PEG-grafted membrane-mimetic surfaces. DSPE-PEG5000 has been found to mix readily with distearoylphosphoethanolamine-succinyl (DSPE-succynil), a phospholipid whose structure resembles closely that of the phospholipid part of the DSPE-PEG5000 molecule. However, properties of mixed monolayers such as morphology and stability varied significantly with DSPE-PEG5000 content. In particular, our surface pressure, epifluorescence microscopy (EFM), and Brewster angle microscopy (BAM) studies have shown that mixtures containing 1-9 mol % of DSPE-PEG5000 form stable condensed monolayers with no sign of microscopic phase separation at surface pressures above approximately 25 mN/m. Yet, at 1 mol % of DSPE-PEG5000 in mixed monolayers, the two components have been found to behave nearly immiscibly at surface pressures below approximately 25 mN/m. For monolayers containing 18-75 mol % of DSPE-PEG5000, a high-pressure transition has been observed in the low-compressibility region of their isotherms, which has been identified on the basis of continuous BAM imaging of monolayer morphology, as reminiscent of the collapse nucleation in a pure DSPE-PEG5000 monolayer. Thus, the comparative analysis of our surface pressure, EFM, and BAM data has revealed that there exists a rather narrow range of mixture compositions with DSPE-PEG5000 content between 3 and 9 mol %, where somewhat homogeneous distribution of DSPE-PEG5000 molecules and high pressure stability can be achieved. This finding can be useful to "navigating" through possible mixture compositions while developing guidelines to the rational design of membrane-mimetic surfaces with highly controlled bio-nonfouling properties.     

Formation of tubules and giant vesicles from large multilamellar vesicles

This study reports an observation of submicrometer multilamellar vesicles (MLVs) prepared by simply freeze-thawing a phospholipid dispersion at full hydration that transformed into giant vesicles (GVs) and tubules (TUs) when confined between microscope glass slides. Cover slide cleaning and surface treatment did not hamper the formation of GVs or TUs. However, when small unilamellar vesicles (SUV) were prepared or when MLVs were not confined but rather freely moved between the glass slides or when the phospholipid was in its gel phase, neither GVs nor TUs were observed. Altogether, our results suggested that MLVs would play a role as a lipid reservoir and that the liquid flow between the glass slides induces the peeling of the external bilayers, yielding the formation of tubules and giant unilamellar vesicles.     

Shear-induced permeation and fusion of lipid vesicles

This paper introduces a novel approach to controlling membrane permeability in free unilamellar vesicles using shearing in the presence of a detergent with a large head-group to tune pore formation. Such shear-induced permeation could offer a simple means of postencapsulating bioactive molecules to prepare vesicle vectors for drug delivery. Using UV absorption, fluorescence emission, dynamic light scattering, and electron microscopy, we investigated the membrane permeability and the morphology of unilamellar lipid vesicles (diameter in the range 50-400 nm) subjected to a shear stress in the presence of a small amount of nonionic surfactant (Brij 76). Shear-induced leakage and fusion events were observed. We analyzed the significance of the vesicle size, the shear rate, and the surfactant-to-lipid ratio for the observed phenomena. The present approach is evaluated for postloading of preformed vesicles.     

Spatiotemporal Propagation of a Minimal Catalytic RNA Network in GUV Protocells by Temperature Cycling and Phase Separation

Compartmentalization is key to many cellular processes and a critical bottleneck of any minimal life approach. In cells, a complex chemistry is responsible for bringing together or separating biomolecules at the right place at the right time. Lipids, nucleic acids and proteins self-organize, thereby creating boundaries, interfaces and specialized microenvironments. Exploiting reversible RNA-based liquid-liquid phase separation (LLPS) inside giant unilamellar vesicles (GUVs), we present an efficient system capable of propagating an RNA-based enzymatic reaction across a population of GUVs upon freezing-thawing (FT) temperature cycles. We report that compartmentalization in the condensed RNA-rich phase can accelerate such an enzymatic reaction. In the decondensed state, RNA substrates become homogeneously dispersed, enabling content exchange between vesicles during freeze-thawing. This work explores how a minimal reversible phase separation system in lipid vesicles could help to implement spatiotemporal control in cyclic processes, as required for minimal cells.     

Phospholipase D-mediated aggregation, fusion, and precipitation of phospholipid vesicles

Large unilamellar vesicles with a diameter of 100 nm were prepared from the zwitterionic phospholipid POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) at pH 8.0. After addition to these vesicles of the enzyme phospholipase D (PLD) from Streptomyces sp. AA586 at 40 degrees C, the terminal phosphate ester bond of POPC was hydrolyzed, yielding the negatively charged POPA (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid) and the positively charged choline. While the reaction yield in the presence of 1 mM Ca2+ reached 100%, the yield was only approximately 68% in the absence of Ca2+. Furthermore, in the absence of Ca2+, the size of the vesicles did not change significantly with time upon PLD addition, as judged from turbidity, dynamic light scattering, and electron microscopy measurements. In the presence of 1 mM Ca2+, however, PLD addition resulted in vesicle aggregation, fusion, and precipitation, originating from the interaction of Ca2+ ions with the negatively charged phospholipids formed in the membranes. Vesicle fusion was monitored by using a novel fusion assay system involving vesicles containing entrapped trypsin and vesicles containing entrapped chymotrypsinogen A. After vesicle fusion, chymotrypsinogen A transformed into a-chymotrypsin, catalyzed by trypsin inside the fused vesicles. The alpha-chymotrypsin formed could be detected with benzoyl-L-Tyr-p-nitroanilide as a membrane permeable chymotrypsin substrate. The observed vesicle precipitation occurring after vesicle fusion in the presence of 1 mM Ca2+ was correlated with an increase of the main phase transition temperature, Tm, of POPA to values above 40 degrees C.     

Spontaneous formation of vesicles from mixed amphiphiles with dispersed molecular weight: Monte Carlo simulation

The spontaneous formation of vesicles from amphiphiles with dispersed molecular weight (MW) as well as with mono-MW has been studied by a lattice Monte Carlo simulation. Both pure and mixed amphiphiles were self-assembled into vesicles under appropriate conditions. When mixed amphiphiles were examined, the amphiphiles with longer hydrophilic blocks preferred to segregate into the outer monolayer of the resultant vesicles, which is consistent with the experimental observations in recent literature. The kinetic study reveals that the increase of vesicle size is mainly caused by the mechanism of vesicle fusion at the early stage, and the evaporation-condensation mechanism cannot be neglected at the late stage. The fusion of vesicles is accompanied by translocation of chains from the outer monolayer to the inner monolayer. For mixed amphiphiles, the degree of segregation exhibits a size dependence of the vesicle. Compared to the chains with shorter hydrophilic blocks, those with longer hydrophilic blocks exhibit stronger trends to translocate from the outer monolayer to the inner one in vesicle self-adjustment, which leads to the quasi-equilibrium asymmetric distribution of the hydrophilic blocks in the post-fused vesicles.     

Poly(ethylene glycol)-conjugated phospholipids in aqueous micellar solutions: hydration, static structure, and interparticle interactions

By means of dielectric relaxation spectroscopy (DRS) and small-angle X-ray scattering (SAXS), we have investigated hydration behavior, solvent dynamics, and static structures of aqueous solutions of poly(ethylene glycol)-conjugated distearoyl phosphatidylethanolamine (DSPE-PEG) (molecular weight of PEG: M(PEG)= 2000, 5000, and 12,000 Da). A quantitative analysis of the bulk-water relaxation amplitude revealed the effective hydration number of a DSPE-PEG molecule per ethylene oxide monomer unit to be approximately 5.0-5.5, virtually independent of M(PEG). The overall hydration number of a DSPE-PEG molecule is ca. 20% higher than that of the corresponding normal PEG (without DSPE). This is attributed to both hydration of a charged head group of phosphoric acid in DSPE and a packing effect of PEG chains into micellar structures. The pair-distance distribution functions, p(r), extracted from the GIFT analysis of SAXS intensities show that the DSPE-PEGs form spherical-like micelles in water having the maximum diameter of approximately 16, 22, and 31 nm, respectively, for M(PEG) = 2000, 5000, and 12,000 Da and nearly identical aggregation numbers of 72 (+/-10%). The DSPE-PEG micelles behave as charged colloids whose interparticle interaction potential can be approximated by the screened Coulomb potential model. The extracted pair correlation function g(r) demonstrates that both electrostatic repulsion induced by the charged head group and excluded volume effects of the fully hydrated PEG layer contribute to repulsive interactions among the PEG-lipid micelles. This should be a key factor for the function of PEG lipids as a stabilizer of liposomes.     

Control of mechanically activated polymersome fusion: Factors affecting fusion

Previously, it was found that extruded (200 nm) polymer vesicles are capable of fusion into giant polymersomes using agitation in the presence of salt. In this study, several factors contributing to this phenomenon, including the effects of (i) polymer vesicle concentration, (ii) agitation speed and duration, and (iii) variation of the salt and its concentration are investigated. To accomplish these goals dynamic light scattering is used in conjunction with fluorescence microscopy, which provides insight into vesicles above the practical limit for DLS characterization. Increasing the concentration of the polymer dramatically increases the production of giant vesicles through the increased collisions of polymersomes. Likewise, increasing the frequency of agitation increases the efficiency of fusion, although ultimately the size of vesicle that could be produced is limited due to the high shear involved. Finally, salt‐mediation of the fusion process was not limited to NaCl, but is instead a general effect facilitated by the presence of solvated ionic compounds, albeit with different salts initiating fusion at different concentrations. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 297–303     

Effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the permeability of liposomal bilayer membranes

The effects of poly(ethylene glycol) (PEG) chain length of PEG-lipid on the membrane characteristics of liposomes were investigated by differential scanning calorimetry (DSC), freeze-fracture electron microscopy (FFEM), fluorescence polarization measurement and permeability measurement using carboxyfluorescein (CF). PEG-liposomes were prepared from mixtures of dipalmitoyl phosphatidylcholine (DPPC) and distearoyl phosphatidylethanolamines with covalently attached PEG molecular weights of 1000, 2000, 3000 and 5000 (DSPE-PEG). DSC and FFEM results showed that the addition of DSPE-PEG to DPPC in the preparation of liposomes caused the lateral phase separation both in the gel and liquid-crystalline states. The fluidity in the hydrocarbon region of liposomal bilayer membranes was not significantly changed by the addition of DSPE-PEG, while that in the interfacial region was markedly increased. From these results, it was anticipated that the CF leakage from PEG-liposomes is accelerated compared with DPPC liposomes. However, CF leakage from liposomes containing DSPE-PEG with a 0.060 mol fraction was depressed compared with regular liposomes, and the leakage decreased with increasing PEG chain length. Furthermore, the CF leakage from liposomes containing DSPE-PEG with a 0.145 mol fraction was slightly increased compared with that of liposomes containing DSPE-PEG with a 0.060 mol fraction. It is suggested that the solute permeability from the PEG-liposomes was affected by not only properties of the liposomal bilayer membranes such as phase transition temperature, phase separation and membrane fluidity, but also the PEG chain of the liposomal surface.     

The Role of Lateral Tension in Calcium-Induced DPPS Vesicle Rupture

We assess the role of lateral tension in rupturing anionic dipalmitoylphosphatidyserine (DPPS), neutral dipalmitoylphosphatidylcholine (DPPC), and mixed DPPS-DPPC vesicles. Binding of Ca(2+) is known to have a significant impact on the effective size of DPPS lipids and little effect on the size of DPPC lipids in bilayer structures. In the present work we utilized laser transmission spectroscopy (LTS) to assess the effect of Ca(2+)-induced stress on the stability of the DPPS and DPPC vesicles. The high sensitivity and resolution of LTS has permitted the determination of the size and shape of liposomes in solution. The results indicate a critical size after which DPPS single shell vesicles are no longer stable. Our measurements indicate Ca(2+) promotes bilayer fusion up to a maximum diameter of ca. 320 nm. These observations are consistent with a straightforward free-energy-based model of vesicle rupture involving lateral tension between lipids regulated by the binding of Ca(2+). Our results support a critical role of lateral interactions within lipid bilayers for controlling such processes as the formation of supported bilayer membranes and pore formation in vesicle fusion. Using this free energy model we are able to infer a lower bound for the area dilation modulus for DPPS (252 pN/nm) and demonstrate a substantial free energy increase associated with vesicle rupture.     

Stability and Size Control of Reverse Vesicles

The stability and size control of reverse vesicles have been investigated for a sucrose monoalkanoate/hexaethylene glycol hexadecyl ether/decane/water system. The stability is highly dependent on the surfactant mixing ratio, amount of added water, and vesicle size. The size distribution of reverse vesicles produced by simple mixing is very large, but larger vesicles can be removed by means of the extrusion method and reasonably homogeneously size-distributed reverse vesicles can be obtained. If a probe-type ultrasonicator is used, the reverse vesicles obtained are homogeneous and of very small size (50-70 nm in diameter) and they are considered to be of the unilamellar type.     

Photoinduced increase in vesicle size and role of photoresponsive malachite green leuconitrile derivative in vesicle fusion

The influence of photoirradiation on vesicles containing a Malachite Green leuconitrile derivative carrying a long alkyl chain, affording photogenerated amphiphilicity, was investigated. The photoresponsive Malachite Green leuconitrile derivative was embedded in the vesicle bilayer of two single-tailed amphiphiles with oppositely charged head groups consisting of cetyltrimethylammonium chloride (CTAC) and sodium octyl sulfate (SOS). Transmission electron microscopy, which was used for observing photoinduced structural change in the vesicles, demonstrated that photoirradiation of the vesicles containing the Malachite Green leuconitrile derivative increased the average size of the vesicle diameter from 116 to 243 nm in the [CTAC]/[SOS] = 0.48 system. The mechanism for vesicle enlargement was studied with fluorescent probe molecules. The photoinduced change in the vesicle size can be explained by the destabilization of the vesicle bilayer, which is perturbed by photogenerated amphiphilicity. In addition, it was shown that the fusion process arising from the destabilized bilayer contributed to the increase in vesicle size.     

Effect of polyethylene glycol-2000 on amino acid surfactant-based vesicles

Polyethylene glycol (PEG)-modified amino acid surfactant-based vesicles were prepared to improve the stability and cellular delivery of drugs. The vesicles comprised PEG-2000, sodium N-lauroylsarcosinate hydrate (SNLS), 1-decanol, and deionized water. The complex showed vesicular structures that were almost the same as the original vesicles, and their size distribution was (100–150 nm). Transmission electron microscopy (TEM) results revealed that no fusion occurred at 1.25 wt.% PEG concentration. The steric hindrance present among the vesicles prevented aggregation of the particles. No visual phase separation was observed for 6 months at room temperature 28?ºC. At higher molar concentration of PEG, fusion and wrinkling occurred owing to the association of PEG chains. The decreasing bending curvature led to the formation of fused vesicles with multilayer structure, as revealed by TEM and differential scanning calorimetry (DSC).