Anomalous solubilization behavior of dimyristoylphosphatidylcholine liposomes induced by sodium dodecyl sulfate micelles

The solubilization dynamics of dimyristoylphosphatidylcholine (DMPC) liposomes, as induced by sodium dodecyl sulfate (SDS), were investigated; this investigation was motivated by several types of atypical behavior that were observed in the solubilization in this system. The liposomes and surfactants were mixed in a microchip, and the solubilization reaction of each liposome was observed using a microscope. We found that solubilization occurred not only via a uniform dissolution of the liposome membrane, but also via a dissolution involving the rapid motion of the liposome, or via active emission of protrusions from the liposome surface. We statistically analyzed the distribution of these patterns and considered hypotheses accounting for the solubilization mechanism based on the results. When the SDS concentration was lower than the critical micelle concentration (CMC), the SDS monomers entered the liposome membrane, and mixed micelles were emitted. When the SDS concentration was higher than the CMC, the SDS micelles directly attacked the liposome membrane, and many SDS molecules were taken up; this caused instability, and atypical solubilization patterns were triggered. The size dependence of the solubilization patterns was also investigated. When the particle size was smaller, the SDS molecules were found to be homogeneously dispersed throughout the whole membrane, which dissolved uniformly. In contrast, when the particle size was larger, the density of SDS molecules increased locally, instability was induced, and atypical dissolution patterns were often observed.     

Solubilizing interactions of octylphenol surfactants with liposomes modeling the stratum corneum lipid composition

 The interaction of a series of polyethoxylated octylphenols (ethylene oxide units average 8.5–20.0) with liposomes modeling the stratum corneum (SC) lipid composition (40% ceramides, 25% cholesterol, 25% palmitic acid and 10% of cholesteryl sulfate) was investigated. The surfactant/lipid molar ratios (Re) and the bilayer/aqueous-phase partition coefficients (K) were determined by monitoring the changes in the static light scattering of the system during solubilization. The fact that free concentration for each surfactant tested was always similar to its critical micelle concentration (CMC) indi-cates that the liposome solubilization was mainly ruled by the formation of mixed micelles. The Re and K para-meters for liposome saturation fell as the surfactant HLB increased. Thus, at this interaction step the higher the surfactant HLB, the higher the ability of these surfactants to saturate SC liposomes and the lower their degree of partitioning into liposomes. However, the maximum solubilizing ability was achieved at intermediate HLB values. Thus, the octylphenols with 20 and 12.5 ethylene oxide units showed, respectively, the highest power of saturation and solubilization of SC structures in terms of the total surfactant amounts needed to produce these effects. Different trends in the interaction of these surfactants with SC liposomes were observed when comparing the Re and K parameters with those reported for PC ones. Thus, whereas the SC liposomes were more resistant to the surfactant action, the affinity of these surfactants with these bilayer structures was higher in all cases. Received: 3 March 1997 Accepted: 22 May 1997     

Dodecyl maltoside as a solubilizing agent of stratum corneum lipid liposomes

The lytic interactions of the nonionic surfactant dodecyl maltoside (DM) with liposomes formed by a mixture of lipids modeling the stratum corneum (SC) lipid composition were investigated. To this end, the surfactant to lipid molar ratios (Re) and the normalized bilayer/aqueous phase partition coefficients (K) were determined by monitoring the changes in the static light-scattering (SLS) of the system during solubilization. The fact that the free surfactant concentration was always similar to its critical micelle concentration indicates that the liposome solubilization was mainly ruled by formation of mixed micelles. In addition, the linear dependence established between the level of SLS and Re indicates a progressive incorporation of DM in the liposomes as well as the progressive formation of mixed micelles. DM showed in all cases lower bilayer activity (higher Re values) and greater affinity with vesicles (higher K values) than those reported for its interaction with phosphatidylcholine (PC) liposomes. Thus, whereas the SC lipid liposomes were more resistant to the action of this surfactant, its degree of partitioning into SC bilayers was higher throughout the solubilization process than that exhibited in PC vesicles. Comparison of the present Re values with those reported for the lytic interaction of dodecyl glucoside (DG) with SC liposomes reveals that in the case of DM the bilayer activity was more than three times higher than that for DG in spite of the identical alkyl chain length. Received: 19 July 2001 Accepted: 10 October 2001     

Influence of the hydrophobic tail of alkyl glucosides on their ability to solubilize stratum corneum lipid liposomes

The lytic interactions of a series of alkyl glucosides (alkyl chain lengths ranging from C₈ to C₁₂) with liposomes formed by a mixture of lipids modeling the stratum corneum (SC) lipid composition were investigated. The surfactant-to-lipid molar ratios (Re) and the normalized bilayer/aqueous phase partition coefficients (K) were determined by monitoring the changes in the static light-scattering (SLS) of the system during solubilization. The fact that the free surfactant concentrations were always similar to their critical micelle concentrations indicates that the liposome solubilization was mainly ruled by the formation of mixed micelles. At the two interaction levels studied (100 and 0% SLS) the nonyl glucoside showed the highest ability to saturate and to solubilize liposomes (lowest Re values), whereas the dodecyl glucoside showed the highest degree of partitioning into liposomes or affinity with these structures (highest K values). Comparison of the data for octyl glucoside with that reported for the interaction of this surfactant with phosphatidylcholine (PC) liposomes shows that whereas the SC lipid liposomes were more resistant to the action of this surfactant (higher Re values), its degree of partitioning into SC bilayers was both in the saturation and solubilization of liposomes similar to that exhibited in PC vesicles (similar K values). Received: 27 November 2000/Accepted: 19 February 2001     

Stable polymeric nanoballoons: lyophilization and rehydration of cross-linked liposomes

The cross-linking of supramolecular assemblies of hydrated lipids is an effective method to stabilize these assemblies to disruption by surfactants or aqueous alcohol. The heterobifunctional lipids, Acryl/DenPC(16,18) and Sorb/DenPC(18,21), are examples of a new class of polymerizable lipid designed for the creation of cross-linked lipid structures. The robust nature of cross-linked liposomes was demonstrated by lyophilization of the liposomes followed by their essentially complete redispersion in water. The resulting liposomes were compared to the original sample by quasi-elastic light scattering and transmission electron microscopy. There was no major change in the size or structure of the cross-linked liposomes after rehydration of the freeze-dried powder of liposomes. Moreover, the rehydrated cross-linked liposomes continued to be resistant to surfactant solubilization. Neutral cross-linked liposomes were predominantly redispersed after freeze-drying with the aid of bath sonication. The small amount of residual liposome aggregation observed with neutral liposomes could be prevented by incorporating a surface charge into the liposome or attaching hydrophilic polymers, for example, poly(ethylene glycol), onto the liposome.     

Influence of the level of cholesteryl sulfate in the solubilization of stratum corneum lipid liposomes by sodium dodecyl sulfate

 The role played by cholesteryl sulfate (Chol-sulf) in the solubilization of liposomes modeling the stratum corneum (SC) lipids by sodium dodecyl sulfate (SDS) was studied. We determined the surfactant-to-lipid molar ratios and the bilayer/aqueous phase surfactant partition coefficients of this interaction by varying the proportion of Chol-sulf, the relative proportions of the others lipids remaining constant. These parameters were determined by monitoring the changes in the static light scattering of the system during solubilization. The fact that the free surfactant concentration was always similar to its critical micelle concentration indicates that the liposome solubilization was mainly ruled by the formation of mixed micelles. The SDS ability to saturate and solubilize SC liposomes decreased as the proportion of Chol-sulf in the bilayers increased until a minimum was reached for a Chol-sulf proportion of about 15%. Inversely, the SDS partitioning into liposomes (or affinity with these bilayers) increased as the proportion of Chol-sulf increased until a maximum was reached at similar Chol-sulf proportions (10–15%). Hence, in these Chol-sulf proportions (similar to that existing in the intercellular lipids, which was 10%) the ability of SDS molecules to interact with liposomes exhibits a minimum despite their enhanced partitioning into liposomes. These effects may be related to the reported dependencies of the level of Chol-sulf on the abnormalities in the skin barrier function and on the SC intercellular cohesion. Received: 12 October 1999 Accepted: 20 January 2000     

Effect of the electrostatic charge on the mechanism inducing liposome solubilization: a kinetic study by synchrotron radiation SAXS

The anionic surfactant sodium dodecyl sulfate (SDS) was used to induce the initial steps of the solubilization of liposomes. The structural transformations as well as the kinetics associated with this initial period were studied by means of time-resolved small-angle X-ray scattering (SAXS) using a synchrotron radiation source. Neutral and electrically charged (anionic and cationic) liposomes were used to investigate the effect of the electrostatic charges on the kinetics of these initial steps. The mechanism that induces the solubilization process consisted of adsorption of surfactant on the bilayers and desorption of mixed micelles from the liposomes surface to the aqueous medium. In all cases the time needed for desorption of the first mixed micelles was shorter than that for complete adsorption of the surfactant on the liposomes surface. The present work demonstrates that adsorption of the SDS molecules on negatively charged liposomes was slower and release of mixed micelles from the surface of these liposomes was faster than for neutral liposomes. In contrast, in the case of positively charged liposomes, the adsorption and release processes were, respectively, faster and slower than those for neutral vesicles.     

Solubilization of unilamellar phospholipid bilayers by nonionic surfactants

The mechanisms governing the solubilization of neutral or electrically charged unilamellar liposomes by a series of octylphenol polyethoxylated surfactants (average of ethylene oxide units between 8.5 and 20.0) were investigated. Solubilization was detected as a decrease in light-scattering of liposome suspensions. To this end, in accordance with the nomenclature adopted by Lichtenberg, three parameters were considered as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant (a) saturated the liposomesRe _sat; (b) resulted in a 50% solubilization of vesiclesRe _50% and (c) led to a total solubilization of liposomesRe _sol. These parameters, corresponded to theRe at which light scattering starts to decrease, reaches 50% of the original value and shows no further decrease.It is noteworthy that theRe _sat parameter decreases as the EO contents or the surfactant critical micellar concentration (CMC) increases. However, theRe _50% and theRe _sol parameters show the lowest values for the surfactant with 12.5 EO units in its molecular structure regardless of the electrical charge of the lipid bilayers. As a consequence, these last parameters are not linearly correlated with the CMC of these surfactants. The CMC values of the surfactant/lipid systems at 0.5 mM lipid concentration corresponded in all cases to the surfactant concentration at which liposomes were saturated by surfactants (S _sat).     

水溶性离子液体溶解磷脂脂质体的初步研究

以磷脂脂质体为模拟生物膜,研究了新型绿色溶剂离子液体(ILs)对生物膜的影响. 对ILs-脂质体混悬液进行浊度、 差示扫描量热(DSC)和³¹P核磁共振(³¹P NMR)表征. 结果表明,1-正烷基-3-甲基-咪唑([C_nmim])阳离子的水溶性ILs与脂质体相互作用,能破坏脂质体的片层结构并溶解磷脂组分. [C_nmim]阳离子对脂质体的溶解起关键作用,阴离子类型和取代烷基链长对ILs的溶解性质也有显著影响. 初步探讨了ILs溶解脂质体的可行性机理.     

Monitoring of membrane collapse and enzymatic reaction with single giant liposomes embedded in agarose gel

Giant liposomes are often used as models for studies on cell membranes. We embedded giant liposomes in agarose gel to fix them for assays. Giant liposomes of dioleoylphosphatidylcholine were embedded in 1% (w/v) agarose gel with a low melting temperature: While only 20–25% of giant liposomes survived embedment, their size distribution was unaffected. Using a confocal laser scanning microscope, we monitored dynamic changes in individual agarose gel-embedded giant liposomes induced by the addition of a surfactant (Triton X-100). The permeation and collapse could be clearly discriminated from each other. Invaginated buds on liposome membranes could also be captured as intermediate structures. Additionally, an enzymatic (β-glucosidase) reaction encapsulated within the target liposome was triggered by the external addition of a non-fluorescent substrate and successfully monitored. These results suggest that embedment in agarose gel is useful for the simple fixation of giant liposomes for biochemical and biophysical assays.     

Interactions of mixed surfactant solution with liposome membrane

Interactions of the mixed surfactant solution of dodecylamido propyl dimethyl aminoacetate and sodium dodecyl sulfate with the liposomal membrane were studied. Lytic activities of the surfactants were measured as a function of the concentrations of surfactant and phospholipid and the composition of mixed surfactants. The solubilization limits of phospholipid by surfactants were determined from the change of their aggregation behavior in suspensions at equilibrium by means of quasi-elastic light scattering. The mixed surfactant solutions showed lower lytic activity than single component surfactant solution in spite of the strong adsorption onto the liposome surface. This was attributed to low solubilization power of binary mixture for phospholipid.     

Physicochemical aspects of the liposome-wool interaction in wool dyeing

Despite the promising application of liposomes in wool dyeing, little is known about the mechanism of liposome interactions with the wool fiber and dyestuffs. The kinetics of wool dyeing by two dyes, Acid Green 27 (hydrophobic) and Acid Green 25 (hydrophilic), were compared in three experimental protocols: (1) without liposomes, (2) in the presence of phosphatidylcholine (PC) liposomes, and (3) with wool previously treated with PC liposomes. Physicochemical interactions of liposomes with wool fibers were studied under experimental dyeing conditions with particular interest in the liposome affinity to the fiber surface and changes in the lipid composition of the wool fibers. The results obtained indicate that the presence of liposomes favors the retention of these two dyes in the dyeing bath, this effect being more pronounced in case of the hydrophobic dye. Furthermore, the liposome treatment is accompanied by substantial absorption of PC by wool fibers with simultaneous partial solubilization of their polar lipids (more evident at higher temperatures). This may result in structural modification of the cell membrane complex of wool fibers, which could account for a high level of the dye exhaustion observed at the end of the liposome dyeing process.     

微电极阵列芯片的脂质体电融合研究

利用旋转蒸发法制作基于大豆卵磷脂的一种大型脂质体,在微电极阵列芯片上进行脂质体电融合实验研究.在电融合过程中,利用介电电泳力实现脂质体在微流控芯片中的排队,再利用高场强的电脉冲使脂质体膜发生可逆性电穿孔,在持续的介电电泳力作用下,使穿孔的脂质体实现融合.芯片上脂质体的融合率可以达到20％左右.而且,玻璃基底材料和低深宽比的通道结构更有利于脂质体融合过程的观察与控制.     

Solubilization of phospholipid bilayers by C14-alkyl betaine/anionic mixed surfactant systems

The mechanisms governing the solubilizing interactions between zwitterionic/anionic mixed surfactant systems at different molar fractions of the zwitterionic surfactant (X _zwitter) and unilamellar liposomes were investigated. Solubilization was detected as a decrease in static lightscattering of liposome suspensions. Three parameters were regarded as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant system a) saturated the liposomesRe _sat; b) resulted in 50% solubilization of liposomesRe _50%, and c) led to a complete solubilization of these structuresRe _sol. These parameters corresponded to theRe at which light scattering stars to decrease, reaches 50% of the original value and shows no further decrease. From these parameters the surfactant partition coefficients for these three steps (K _sat,K _50% andK _sol) were also determined. The mixed systems were formed byN-tetradecyl-N, N-dimethylbetaine (C₁₄-Bet) and sodium dodecyl sulphate (SDS) in PIPES buffer at pH 7.20. Liposomes were formed by egg phosphatidylcholine and phosphatidic acid (9:1 molar ratio). When the range ofX _zwitter was about 0.4–0.6Re andK parameters showed a maximum, whereas the critical micelle concentration (CMC) of these systems exhibited a minimum. Given that the ability of the surfactant systems to solubilize liposomes is inversely related toRe _sol, this capacity appeared to be directly correlated with the CMC of the systems. The progressive rise ofK during the process indicates that an increasing surfactant partition equilibrium governs the interaction process from the saturation until the solubilization of vesicles, the free surfactant concentration remaining almost constant with similar values to the CMC for each mixed system studied.     

Flow manipulation for sweeping with a cationic surfactant in microchip capillary electrophoresis

Flow manipulation in sweeping microchip capillary electrophoresis (CE) is complicated by the free liquid communication between channels at the intersection, especially when the electroosmotic flows are mismatched in the main channel. Sweeping in traditional CE with cationic micelles is an effective way to concentrate anionic analytes. However, it is a challenge to transfer this method onto microchip CE because the dynamic coating process on capillary walls by cationic surfactants is interrupted when the sample solution free of surfactants is introduced into the microchip channels. This situation presents a difficulty in the sample loading, injection and dispensing processes. By adding surfactant at a concentration around the critical micelle concentration and by properly designing the voltage configuration, the flows in a microchip were effectively manipulated and this sweeping method was successfully moved to microchip CE using tetradecyltrimethylammonium bromide (TTAB). The sweeping effect of cationic surfactant in the sample solution was discussed theoretically and studied experimentally in traditional CE. The flows in a microchip were monitored with fluorescence imaging, and the injection and sweeping processes were studied by locating the detection point along the separation channel. A detection enhancement of up to 500-fold was achieved for 5-carboxyfluorescein.     

Electroformation and collection of giant liposomes on an integrated microchip

In this work, a microchip was developed for the electroformation and collection of giant liposomes. On the chip, a reactor chamber array was expressly designed so that the influences of multiple parameters could be explored rapidly and efficiently to screen for a suitable condition. Furthermore, in order to overcome the difficulty in the collection of giant liposomes, a tailor-made collection chamber was also integrated on this chip.     

Silicone-stabilized liposomes

The present work is focused on stabilization of liposomes by covering their surface with a thin silicone layer. The appropriate silicone monomer was obtained by the hydrosilylation of vinylmethyldimethoxysilane with 1,3,5,7-tetramethylcyclotetrasiloxane. The surface potential of egg yolk phosphatidylcholine vesicles was modified by incorporation of a cationic double-tailed surfactant, dimethyldioctadecylammonium bromide (DODAB), yielding cationic liposomes. The silicone material was deposited on the cationic liposomes in base-catalyzed polycondensation/polymerization processes of the monomer at the liposomal surface. In order to initialize the processes pH of the liposomal dispersion was adjusted to the values of 8.5 or 10.2. The formed structures were characterized using dynamic light scattering (DLS) and zeta potential measurements. The DLS measurements show that the size of covered liposomes decrease during the reaction and the zeta potential turned to negative value, as can be expected. The morphology of the structures was evaluated using transmission cryo-electron microscopy (cryo-TEM). The cryo-TEM micrographs revealed the presence of the covered liposomes of sizes lower than the initial liposomes, which is in line with DLS measurements. However, some disintegration of the liposomes occurred during the covering procedure, especially at high pH value. Using the surfactant lysis and calcein-release study it was shown that silicone-covered liposomes are stable.     

Interactions between nonionic Triton X surfactants and cholesterol-containing phosphatidylcholine liposomes

Solubilization kinetics experiments were developed to study the effects of the polyethylene glycol chain length of Triton X surfactants on their interactions with the cholesterol-containing phosphatidylcholine vesicles. An empirical liposome stability ratio was used to describe the vesicle solubilization process. The effectiveness of Triton X surfactants in solubilizing vesicles decreases with increasing polyethylene glycol chain length of surfactants. It was also shown that vesicles containing the intercalated surfactant molecules with the largest number of ethylene glycol units per molecule exhibited the exceedingly retarded solubilization behavior. Independent experiments based on a thermodynamic approach provide supporting evidence for the conclusions obtained from solubilization kinetics experiments.     

Investigation of Factors Affecting Controlled Release from Photosensitive DMPC and DSPC Liposomes

An investigation of liposomes comprised of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) lipids with cholesterol and zinc phthalocyanine (ZnPC) revealed that several fundamental liposome properties are influenced by composition and by lipid-specific features. DMPC and DSPC liposomes were synthesized, and their compositional changes, encapsulation capacities, morphologies, and release properties were evaluated. In this research, liposome degradation, lysis, and content release were initiated by photolysis, i.e., rupture induced by exposure to light. A controlled release mechanism was created through the introduction of photosensitizers (i.e., ZnPC) embedded within the cholesterol-stabilized liposome membrane. The light wavelength and light exposure time accelerated photodegradation properties of DMPC liposomes compared to DSPC liposomes, which exhibited a slower release rate. Morphological changes in the liposomes were strongly influenced by light wavelength and light exposure time. For both the DMPC and DSPC liposomes, visible light with wavelengths in the red end of the spectrum and broad spectrum ambient lighting (400?C700?nm) were more effective for lysis than UV-A light (365?nm). Heating liposomes to 100?°C decreased the stability of liposomes compared to liposomes kept at room temperatures. In addition, the optimal lipid-to-cholesterol-to-photoactivator ratio that produced the most stable liposomes was determined.     

Solubilization kinetics of phospholipid vesicles by sodium taurocholate

The solubilization kinetics of phospholipid vesicles, about 100 nm in diameter and composed of egg phosphatidylcholine (EPC) and EPC/cholesterol in molar ratio 7/3, by sodium taurocholate (TC) used as a model bile salt were investigated by monitoring the turbidity at 500 nm and by quasielastic light scattering (QELS). The solubilization process was found to be dependent on the rate of TC addition. Although the solubilization profiles were identical whatever the rate of TC addition, an increase in the amount of TC needed to solubilize phosphatidylcholine liposomes was observed at higher rates. These results suggest that at low TC concentrations the permeability of the membrane to taurocholate is the rate-limiting step of the solubilization. In the case of cholesterol-containing vesicles, the effect of the rate of addition of TC was observed only at the solubilization characteristic points, called B and C, corresponding to a sharp decrease in the turbidity. This suggests that cholesterol greatly reduces the permeability of the membrane. In addition, the kinetic process was found to be independent of the micellar concentration of the detergent added to the aqueous medium, indicating that the solubilization of liposomes by TC was independent of the initial state of aggregation of the detergent. The calculated values of lipid/TC aggregates and of the partition coefficient show that the kinetic effect observed at high TC concentrations prior to complete solubilization might also be due to the diffusion of the detergent into the membranes. This gives rise to the differences in composition of the aggregates as a consequence of the variation in the rate of TC addition. In addition, QELS scattered intensity variations confirm the presence of a kinetic process for the solubilization of liposomes by TC. In conclusion, our results suggest that solubilization of lipid vesicles by TC is governed by kinetic parameters that might be controlled by liposome membrane permeability at low TC concentrations and by the lateral diffusion of the detergent into aggregates at higher TC concentrations.