Liposome solubilization induced by surfactant molecules in a microchip

The dynamics of liposome solubilization was monitored by dynamic light scattering and optical microscopy. A newly designed Y-shape microchannel connected to a room was incorporated into a microchip and the reaction processes of the liposome suspension and surfactant solution were observed in the room after mixing the two fluids and stopping the flow. By using this microchip, we succeeded in real-time monitoring of liposome solubilization and the following dynamic processes of solubilization were proposed: 1) Deformed liposomes become spherical. 2) The liposome size increases until the surfactant/liposome ratio in the liposome membrane reaches a threshold value. 3) Mixed micelles of surfactants and phospholipids are released and the liposomes collapse.     

可控组装方法制备脂质体

利用可控组装方法制备了单仓脂质体。这种制备方法分为两个步骤: (a)利用超声振荡形成油包水(W/O)的微乳液, (b)微乳中小水滴在离心作用下通过油水界面的单分子膜并被其包裹, 在水相中形成脂质体。脂质体直径在50-200nm。脂质体的双分子膜间基本没有残留有机溶剂存在, 并且在制备过程中包封的试剂没有泄漏。可控组装方法制备脂质体具有操作简单、生成脂质体一般不需进一步分离、包封效率较高(57%)等特点。     

Characterization of solvation properties of lipid bilayer membranes in liposome electrokinetic chromatography

The nature of solute interactions with biomembrane-like liposomes, made of naturally occurring phospholipids and cholesterol, was characterized using electrokinetic chromatography (EKC). Liposomes were used as a pseudo-stationary phase in EKC that provided sites of interactions for uncharged solutes. The retention factors of uncharged solutes in liposome EKC are directly proportional to their liposome-water partition coefficients. Linear solvation energy relationship (LSER) models were developed to unravel the contributions from various types of interactions for solute partitioning into liposomes. Size and hydrogen bond acceptor strength of solutes are the main factors that determine partitioning into lipid bilayers. This falls within the general behavior of solute partitioning from an aqueous into organic phases such as octanol and micelles. However, there exist subtle differences in the solvation properties of liposomes as compared to those of octanol and various micellar pseudo-phases such as aggregates of sodium dodecyl sulfate (SDS), sodium cholate (SC), and tetradecylammonium bromide (TTAB). Among these phases, the SDS micelles are the least similar to the liposomes, while octanol, SC, and TTAB micelles exhibit closer solvation properties. Subsequently, higher correlations are observed between partitioning into liposomes and the latter three phases than that into SDS.     

Liposome formation of selectively-polymerized diene-containing phospholipids and their postpolymerization

Small and large unilamellar liposomes composed of 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) are prepared by sonication and extrusion, respectively. They are polymerized with water-insoluble radical initiator, azobis(isobutyronitrile) (AIBN) which can selectively polymerize diene groups in 1-acyl chains of the lipids. Polymerized liposomes are freeze-dried to obtain the polymerized liposome powder. There are two methods to redisperse lyophilized liposomes into water. The extrusion is an effective method to disperse them because the energy at extrusion is necessary only for redispersion, whereas the excess energy at sonication gives damage on liposome structure. There is no difference in stability between polymerized liposomes before and after redispersion with extrusion. DODPC polymers, obtained from free radical-initiated polymerization with AIBN, are linear and have polymerizable diene groups in 2-acyl chains. The liposome powder is therefore soluble in organic solvents. Reconstruction of polymerized liposomes is performed with lipid polymers having low or high molecular weight. The lipid polymers having high molecular weight provide stable large unilamellar liposomes by ethanol injection, but unstable small unilamellar liposomes are formed by sonication. The liposomes reconstructed from lipid polymers having low molecular weight are unstable regardless of their size. After reconstruction of liposomes selectively polymerized by AIBN, diene groups in 2-acyl chains are polymerized by water-soluble radical initiator or UV-irradiation to yield highly crosslinked structure. Their stability is improved remarkably by this postpolymerization.     

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.     

电容法研究卵磷脂/氨基酸/H2O胶束和囊泡体系

运用电容法研究卵磷脂/氨基酸/H2O胶束和囊泡体系结构与性质. 卵磷脂的临界胶束浓度和囊泡生成浓度可由体系电容-卵磷脂浓度关系曲线求得.随着卵磷脂浓度增加, 体系电容增加, 卵磷脂由胶束形成囊泡. 随着氨基酸浓度增加, 胶束、囊泡半径增大, 体系电容减小. 氨基酸能促进卵磷脂形成胶束和囊泡, 使得卵磷脂临界胶束浓度和囊泡生成浓度减小, 其影响的强弱顺序为组氨酸>色氨酸>>甘氨酸.     

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.     

电容法研究卵磷脂/氨基酸/H_2O胶束和囊泡体系

运用电容法研究卵磷脂/氨基酸/H2O胶束和囊泡体系结构与性质.卵磷脂的临界胶束浓度和囊泡生成浓度可由体系电容-卵磷脂浓度关系曲线求得.随着卵磷脂浓度增加,体系电容增加,卵磷脂由胶束形成囊泡.随着氨基酸浓度增加,胶束、囊泡半径增大,体系电容减小.氨基酸能促进卵磷脂形成胶束和囊泡,使得卵磷脂临界胶束浓度和囊泡生成浓度减小,其影响的强弱顺序为组氨酸色氨酸垌甘氨酸.     

Characterisation of electroosmotic flow in capillary electrochromatography columns

Immobilized liposome chromatography (ILC) has been proven to be a useful method for the study or rapid screening of drug-membrane interactions. To obtain an adequate liposomal membrane phase for ILC, unilamellar liposomes were immobilized in gel beads by avidin-biotin binding. The retardation of 15 basic drugs on the liposome column could be converted to membrane partitioning coefficients, K(LM). The effects of small or large unilamellar liposomes and multilamellar liposomes on the drug-membrane partitioning were compared. The K(LM) values for both small and large liposomes were similar, but higher than those for the multilamellar liposomes. The basic drugs showed stronger partitioning into negatively charged liposomes than into either neutral liposomes or positively charged liposomes. The membrane fluidity of the immobilized liposomes was modulated by incorporating cholesterol into the liposomal membranes, by changing the acyl chain length and degree of unsaturation of the phospholipids, and by changing the temperature for ILC runs. Our data show that K(LM) obtained using ILC correlated well with those reported by batch studies using free liposomes. It is concluded that negatively charged or cholesterol-containing large unilamellar liposomes are suitable models for the ILC analysis of drug-membrane interactions.     

Complexation of polycations to anionic liposomes: composition and structure of the interfacial complexes

Poly(N-ethyl-4-vinylpyridinium bromide) (a polycation with a degree of polymerization of 1100) was adsorbed onto liposomes composed of egg lecithin with a 0.05-0.20 molar fraction (nu) of anionic headgroups provided by cardiolipin (a doubly anionic lipid). According to electrophoretic mobility data, this led to total charge neutralization of the liposomes, whereupon the liposomes adopted a positive charge as additional polymer continued to adsorb. Although the liposomes aggregated at the charge-neutralization point, they disassembled into individual liposomes after becoming positively charged. The degree of polymer adsorption was shown to reach a limit. Thus, by measuring the free polymer content in a liposome suspension, it was possible to determine the polymer concentration at which the liposome surface became saturated with polymer. Beyond this point, an electrostatic/steric barrier at the surface suppressed further adsorption. Dynamic light scattering studies of liposomes with and without adsorbed polymer allowed calculation of the polymer film thickness which ranged from 22 to 35 nm as the molar fraction of cardiolipin (nu) increased from 0.05 to 0.20. The greater the content on the anionic lipid in the bilayer, the thicker the polymer film. The maximum number of polymer molecules adsorbed onto the liposomes was estimated: 1-2 molecules for nu = 0.05; 3 molecules for nu = 0.1; 4- molecules for nu = 0.15; and 6 molecules for nu = 0.2. The polymer appears to lie on the liposome surface, rather than embedding into the bilayer, because addition of NaCl easily dislodges the polymer from the liposome into the bulk water.     

Immunoliposomes assemblied layer by layer from reverse micelles

A method is described for the coupling of the HBsAb antibody on to a liposome by reverse micelles passing through the modified organic/water interface. Incorporation of the fatty acid modified HBsAb into liposomal membranes was studied as a function of the extent of fatty acid coupling. Results show that the incorporation of HBsAb is proportional to the lipid coupling degree. The HBsAb incorporated into the liposomal membrane by this method remains an antigen binding activity shown by the liposome dialysis assay.     

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     

A study on the characterization and stability of rhenium(III) chloride-incorporated liposomes

Nanoliposomes are important carriers capable of packaging drugs for various delivery applications through passive targeting tumor sites by enhancing permeability and retention effect. Radiolabeled liposomes have potential applications in radiotherapy and diagnostic imaging. However, the physico-chemical instability of liposomes during manufacturing and storage limits their extensive application. Therefore, considerable numbers of studies have been made on the stability of liposomes over the last few years in order to overcome this problem. In this study, we attempted to prepare polymer-coated liposomes using water-soluble chitosan in order to enhance the stability of rhenium(III) chloride-incorporated liposomes. They were characterized by an electrophoretic light-scattering spectrophotometer, Fourier transform infrared spectroscopy (FT-IR), UV–Vis spectrometer, and phase-contrast microscopy. The chitosan-coated liposomes are spherical and the particle size is about 800–850 nm. Incorporation of chitosan into the liposome bilayer decreased rhenium(III) chloride release from the liposome due to an increased rigidity of the liposome membrane structure. Chitosan-coated liposomes showed a higher stability compared with the stability of non-coated liposomes. The release characteristics of rhenium(III) chloride encapsulated in the liposome were taken as a measure of stability of the liposome membrane.     

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     

Block liposomes from curvature-stabilizing lipids: connected nanotubes, -rods, or -spheres

We report on the discovery of block liposomes, a new class of chain-melted (liquid) vesicles, with membranes comprised of mixtures of the membrane-curvature-stabilizing multivalent lipid MVLBG2 of colossal charge +16 e and neutral 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC). In a narrow MVLBG2 composition range (8-10 mol%), cryo-TEM revealed block liposomes consisting of distinctly shaped, yet connected, nanoscale spheres, pears, tubes, or rods. Unlike typical liposome systems, where spherical vesicles, tubular vesicles, and cylindrical micelles are separated on the macroscopic scale, within a block liposome, shapes are separated on the nanometer scale. Diblock (pear-tube) and triblock (pear-tube-pear) liposomes contain nanotubes with inner lumen diameter of 10-50 nm. Diblock (sphere-rod) liposomes were found to contain micellar nanorods approximately 4 nm in diameter and several micrometers in length, analogous to cytoskeletal filaments of eukaryotic cells. Block liposomes may find a range of applications in chemical and nucleic acid delivery and as building blocks in the design of templates for hierarchical structures.     

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     

Attenuated total reflectance infrared studies of liposome adsorption at the solid-liquid interface

Interfacial interactions between liposomes and the solid–liquid interface (i.e. a ZnSe internal reflection element, modified to mimic a biological surface) were studied by Fourier transform infrared (FTIR) spectroscopy in attenuated total reflectance (ATR) mode. Both conventional liposomes, containing lecithin and cholesterol and Stealth^® liposomes containing poly(ethylene)glycol (PEG)5000- or PEG2000-lipids were investigated. IR bands due to the liposome components were observed to increase with time and enabled the liposome adsorption kinetics and thermodynamics to be quantified. The liposome solution conditions, surface properties and compositions have all been shown to influence liposome adsorption. Free energies of adsorption were determined to be in the range from −10.0 to −11.0 kJ mol⁻¹ and slightly reduced by PEG incorporation. The adsorption rate constant is decreased with increased solution pH and decreased ionic strength; this reflects the importance of electrostatics in controlling liposome adsorption. Increasing the level and molecular weight of PEG incorporation in the liposomes significantly reduced both the rate and extent of liposome adsorption; steric hindrance is considered to play a key role. Findings from this research will improve the understanding of liposome interaction during drug delivery, give insight into the actions of liposomes in the body and may form the basis for improved liposome formulations.     

Azide Quenching of Singlet Oxygen in Suspensions of Microenvironments of Neutral and Surface Charged Liposomes and Micelles

The azide anion is often used as a physical quencher of singlet oxygen, the important active intermediate in photosensitized oxidation. An observed effect of azide on the rate of a reaction is considered an indication to the involvement of singlet oxygen. In most biological photosensitizations, the light‐absorbing sensitizer is located in a membrane or in an intracellular organelle, whereas azide is water soluble. The quenching it causes relies on a physical encounter with singlet oxygen during the latter's short lifetime. This can happen either if azide penetrates into the membrane's lipid phase or if singlet oxygen is intercepted when diffusing in the aqueous phase. We demonstrate in this article the difference, in liposomes’ suspension, between the effect of azide when using a water‐soluble and membrane‐bound chemical targets of singlet oxygen, whereas this difference does not exist when micelles are used. We explain the difference on the population of sensitizer and target in the liposome vs micelle. We also show the effect that exists on azide quenching of singlet oxygen by electrically charged lipids in liposomes. This is a result of the accumulation or dilution of azide in the debye layer near the membranes’ surface, due to the surface Gouy–Chapman potential.     

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     

Dynamic and structural aspects of PEGylated liposomes monitored by NMR

Proton-detected NMR diffusion and (31)P NMR chemical shifts/bandwidths measurements were used to investigate a series of liposomal formulations where size and PEGylation extent need to be controlled for ultrasound mediated drug release. The width of the (31)P line is sensitive to aggregate size and shape and self-diffusion (1)H NMR conveys information about diffusional motion, size, and PEGylation extent. Measurements were performed on the formulations at their original pH, osmolality, and lipid concentration. These contained variable amounts of PEGylated phospholipid (herein referred to as PEG-lipid) and cholesterol. At high levels of PEG-lipid (11.5 and 15 mol%) the self-diffusion (1)H NMR revealed the coexistence of two entities with distinct diffusion coefficients: micelles (1.3 to 3x10(-11) m(2)/s) and liposomes (approximately 5x10(-12) m(2)/s). The (31)P spectra showed a broad liposome signal and two distinct narrow lines that were unaffected by temperature. The narrow lines arise from mixed micelles comprising both PEG-lipids and phospholipids. The echo decay in the diffusion experiments could be described as a sum of exponentials revealing that the exchange of PEG-lipid between liposomes and micellar aggregates is slower than the experimental observation time. For low amounts of PEG-lipid (1 and 4.5 mol%) the (31)P spectra consisted of a broad signal typically obtained for liposomes and the diffusion data were best described by a single exponential decay attributed solely to liposomes. For intermediate amounts of PEG-lipid (8 mol%), micellization started to occur and the diffusion data could no longer be fitted to a single or bi-exponential decay. Instead, the data were best described by a log-normal distribution of diffusion coefficients. The most efficient PEG-lipid incorporation in liposomes (about 8 mol%) was achieved for lower molecular weight PEG (2000 Da vs 5000 Da) and when the PEG-lipid acyl chain length matched the acyl chain length of the liposomal core phospholipid. Simultaneously to the PEGylation extent, self-diffusion (1)H NMR provides information about the size of micelles and liposomes. The size of the micellar aggregates decreased as the PEG-lipid content was increased while the liposome size remained invariant.