Temperature-responsive phase transition of polymer vesicles: real-time morphology observation and molecular mechanism

Novel thermosensitive polymer vesicles with controlled temperature-responsive phase transition at the lower critical solution temperature (LCST) varying from 8 to 81 degrees C were prepared via self-assembly of amphiphilic hyperbranched star copolymers having a hydrophobic hyperbranched poly[3-ethyl-3-(hydroxymethyl)oxetane] (HBPO) core and many hydrophilic polyethylene oxide (PEO) arms. Real-time optical microscopic observation revealed that the polymer vesicles have undergone sequential morphology changes including enrichment, aggregation, fusion, and vesicle-to-membrane transformation near the LCST. Molecular-level investigation indicates that the LCST transition results from the decreasing water solubility of the polymer vesicles with increasing temperature based on the partial dehydration of the PEO vesicle corona. On the basis of these results, a LCST transition mechanism, in view of the molecular configuration, balance of hydrophilic and hydrophobic moieties, and the vesicle morphology transformations, was proposed. As far as we know, the work presented here is the first demonstration of thermosensitive vesicles based on PEO, and the finding may be useful to design the thermosensitive core-shell structures by introducing the PEO segments.     

Novel polymeric vesicles with pH-induced deformation character for advanced drug delivery

pH-responsive amphiphilic graft macromolecules consisting of a polyphosphazene backbone, hydrophilic PEG branches and pH-sensitive DPA were successfully synthesized and characterized. The copolymer can self-assemble into vesicles in an aqueous solution with unique inner structure and homogeneously encapsulate both lipophilic and hydrophilic molecules. The pH-dependent structure change of vesicles was also observed by DLS and TEM. Dox-loaded vesicles exhibit a sharp pH-responsive drug release profile and dramatically enhance the cytotoxicity of Dox against Dox-resistant MCF-7/adr cells. These results suggest such vesicles based on pH-responsive polyphosphazene hold great potential for specific drug therapy.     

三维宏观网络状囊泡薄膜的分级自组装研究

分别合成以疏水性超支化聚醚(HBPO)为核,以亲水性聚环氧乙烷(EO)和聚甲基丙烯酸N,N-二甲氨基乙酯(DMAEMA)为臂的两亲性超支化多臂共聚物HBPO-star-PEO和HBPO-star-PDMAEMA.通过两者在水溶液中的复合自组装制备得到具有pH响应性的巨型聚合物囊泡(1~10μm),并用zeta电位仪,激光共聚焦显微镜及光学显微镜对囊泡的自组装行为进行了研究.结果表明,在等电点以前,复合囊泡始终以单个囊泡形式存在;随着溶液pH的升高,囊泡逐步线型缔合成串珠结构;在更高的pH下,囊泡进一步二次聚集形成具有宏观尺度的三维蜘蛛网状超分子结构,这是一类新的自组装体.     

Optically Triggered Dissociation of Kinetically Stabilized Block Copolymer Vesicles in Aqueous Solution

We demonstrate a strategy for using an optical stimulus to trigger the dissociation of block copolymer (BCP) vesicles in aqueous solution. The BCP, comprising hydrophilic poly(ethylene oxide) (PEO) and a block of poly(methacrylic acid) bearing a number of spiropyran methacrylate comonomer units (P(MAA‐co‐SPMA)), was allowed to firstly self‐assemble into large vesicles in aqueous solution at pH = 3 with protonated carboxylic acid groups, and then become kinetically stable at pH = 8 due to the glassy vesicle membrane of P(MAA‐co‐SPMA). Fast dissociation of the vesicles was achieved through a cascade of events triggered by UV‐induced isomerization from neutral spiropyran to charged merocyanine in the membrane.

     

Influence of hydrocarbon surfactant on the aggregation behavior of silicone surfactant: observation of intermediate structures in the vesicle-micelle transition

Intermediate structures of the aggregates in the aqueous solution of an ABA-type silicone surfactant and in the process of an SDS-induced vesicle-micelle transition are reported. In single ABA silicone surfactant aqueous solutions, large multilamellar vesicles (MLV), small single lamellar vesicles (SLV), threadlike micelles (TLM), and spheroidal micelles were observed. Interestingly, a large amount of TLMs were found entrapped into the large MLVs, but not in SLVs. Disintegration of the small vesicles inside the MLVs indicates that the entrapped TLM are from the disintegrated membrane of the entrapped small vesicles. Addition of SDS induced a transition from vesicles or threadlike micelles to spheroidal micelles. The intermediate structures, such as the appearance of small holes in the vesicle membrane, the budding of threadlike micelles from the membrane fracture, and the clusters of spheroidal micelles, were observed with increase of the SDS concentration. The electrical conductivity measurements indicated that complex micelles of SDS and silicone surfactant were formed in the solution due to the interaction between the SDS and PEO part of the silicone surfactant.     

光交联固化的聚合物囊泡用于降温触发的药物释放

The stability of nanocarriers in physiological environments is of importance for biomedical applications. Among the existing crosslinking approaches for enhancing the structural integrity and stability, photocrosslinking has been considered to be an ideal crosslinking chemistry, as it is non-toxic and cost-effective, and does not require an additional crosslinker or generate by-products. Meanwhile, most current temperature-responsive nanocarriers are designed and synthesized for drug release by increasing temperature. However, heating may induce cell damage during triggered drug release. Therefore, lowering temperature-triggered nanocarriers need to be developed for drug delivery and safe drug release during therapeutic hypothermia. In this study, we prepared an amphiphilic block copolymer, poly(ethylene oxide)-block-poly[N-isopropyl acrylamide-stat-7-(2-methacryloyloxyethoxy)-4-methylcoumarin]-block-poly(acrylic acid) [PEO₄₃-b-P(NIPAM₇₁-stat-CMA₈)-b-PAA₁₃], by reversible addition fragmentation chain transfer (RAFT) polymerization. Successful synthesis of the polymer was verified by proton nuclear magnetic resonance (¹H NMR) and size exclusion chromatography (SEC). The copolymers self-assembled into vesicles in aqueous solution, with the P(NIPAM-stat-CMA) block forming an inhomogeneous membrane and the PEO chains and PAA chains forming mixed coronas. The cavity of this vesicle could be utilized to load hydrophilic drugs. The CMA groups could undergo photocrosslinking and enhance the stability of vesicles in biological applications, and the PNIPAM moiety endowed the vesicle with temperature-responsive properties. Upon decreasing the temperature, the vesicles swelled and released the loaded drugs. The size distribution and morphology of the vesicles were characterized by dynamic light scattering (DLS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) experiments. After staining with phosphotungstic acid, the hollow morphology of the vesicles with a phase-separated inhomogeneous membrane was observed by TEM and SEM. The DLS results showed that the hydrodynamic diameter of the vesicles was 208 nm and the polydispersity was 0.075. The size of the vesicles observed by TEM was between 180 and 200 nm, which was in accordance with that measured by DLS. To verify the drug loading capacity and controlled release ability of the vesicle, a water-soluble antibiotic was encapsulated in the vesicles. The experimental results showed that the drug loading content was 10.4% relative to the vesicles and the drug loading efficiency was approximately 32.7%. For vesicles containing the same amount of antibiotics, the release rate at 25 ℃ was 35% higher than that at 37 ℃ after 12 h in aqueous solution. Overall, this photocrosslinked vesicle with temperature-responsive properties facilitates lowering temperature-triggered drug release during therapeutic hypothermia.     

Effect of cholesterol or phospholipids incorporation on vesicle formation of muramyldipeptide derivative B30-MDP

Summary The muramyldipeptide derivative B30-MDP has immunoadjuvant activity and vesicleforming ability in aqueous solutions. To assist in the clinical application of B30-MDP to liposomal vaccine, we investigated the physicochemical properties including membrane fluidity, surface charge and particle size of B30-MDP vesicles containing cholesterol, dipalmitoylphosphatidyl-choline (DPPC) or dipalmitoylphosphatidylglycerol (DPPG).The membrane fluidity of B30-MDP/cholesterol vesicles was slightly influenced by cholesterol concentration and temperature. The membrane fluidity of B30-MDP/phospholipid vesicle was dependent on temperature. ESR spectra clearly showed the good miscibility of cholesterol with B30-MDP and the occurrence of phase separation between B30-MDP and phospholipid.The surface charge and particle size of B30-MDP/cholesterol vesicles were hardly influenced by cholesterol concentration in the membrane because the membrane surface was covered with the hydrophilic region of B30-MDP. The effect of this hydrophilic region of B30-MDP on the surface charge and particle size of B30-MDP/phospholipid vesicle was greater than that of phospholipid.This study showed that the membrane structure of B30-MDP/cholesterol vesicle differed from that of B30-MDP/phospholipid vesicle. Further, the hydrophilic region of B30-MDP is considered to play an important role in the physicochemical properties and formation of the vesicle.     

Polymer encapsulation within giant lipid vesicles

We report encapsulation of polymers and small molecules within individual giant lipid vesicles (GVs; 3-80 microm), as determined by confocal fluorescence microscopy. Polymer-bound or free dyes were encapsulated within GVs by including these molecules in the aqueous solution during vesicle formation via gentle hydration. Encapsulation efficiencies of individual GVs (EE(ind)) were determined from the fluorescence intensity ratio inside vs outside the vesicle. EE(ind) varied considerably from vesicle to vesicle, with interior solute concentrations for GVs within the same batch ranging from much less than to slightly more than the initial concentration. The majority of GVs had high internal concentrations of polymer or small-molecule encapsulants equal to or slightly greater than the external concentration. EE(ind) decreased for high molecular weight polymers (e.g., dextran 500 000), but was relatively insensitive to the GV diameter, membrane composition, or incubation temperature in our experiments. Knowledge of EE(ind) is important for quantitative evaluation of reactions occurring within GVs (e.g., enzymatic processes) and for optimizing encapsulation conditions.     

Synthesis,Characterization and Aqueous Solution Behavior of a pH-responsive Double Hydrophilic Block Copolymer

The pH-responsive double hydrophilic block copolymer poly(ethylene glycol)-b-poly(methacylic acid-co-4-vinyl benzylamine hydrochloride salt) (PEG-b-PMAA/PVBAHS) was synthesized. A series of PEG-b-PMAA/PVBAHS with different molecule weights and compositions were characterized by IR, ¹H-NMR, elemental analysis and TGA. With different MAA/VBAHS ratio, the PEG-b-PMAA/PVBAHS copolymers had the different isoelectric point (IEP). Supermolecular structures of the block copolymers could be formed by the interionic interactions at different solution pH. Experiment results showed that the structures of the pH-responsive copolymers in aqueous solution could be changed at different pH environments. The aggregation of this double hydrophilic block copolymer in aqueous solution was determined by both of solution pH and copolymer composition.     

pH-responsive aqueous foams stabilized by hairy latex particles

Polystyrene (PS) latex particles carrying pH-responsive poly[2-(diethylamino)ethyl methacrylate] (PDEA) hair (PDEA-PS particles) were synthesized by dispersion polymerization and characterized in terms of diameter, diameter distribution, morphology, chemical composition, surface chemistry, and pH-response using scanning electron microscopy (SEM), elemental microanalysis, (1)H nuclear magnetic resonance spectroscopy, the laser diffraction method, and zeta potential measurements. The hairy particles can act as pH-responsive stabilizers of aqueous foams by adsorption at the air-water surface. Above pH 8.0, where particles have nonprotonated PDEA hair, which is relatively hydrophobic, particle-stabilized foams are stable for at least 1 month. Optical microscopy and SEM confirmed that flocculated PDEA-PS latex particles were adsorbed at the air-water interface and stabilized the aqueous foams. At pH 6.1 and 7.1, relatively stable foams can be prepared that remain stable for at least 24 h. SEM studies indicated that the PDEA-PS particles were adsorbed at the air-water interface as a monolayer at pH 6.1. At pH 5.1 and 3.1, where the particles have cationic water-soluble PDEA hairs with hydrophilic character, no foam was formed. Rapid defoamation can be induced by lowering the solution pH; the addition of acid caused the in situ protonation of 2-(diethylamino)ethyl methacrylate residues, which impart water-soluble hydrophilic character to the PDEA hair, and the PDEA-PS particles desorbed from the air-water interface. The foaming and defoaming cycles could be repeated at least five times.     

Energy transfer of ionic dyes in mixed surfactant vesicle

Stable vesicles composed of cationic and anionic single-tailed-surfactant were prepared, and their image obtained by electron microscopy with negative staining technique. Significant fluorescence enhancement for acridine orange in vesicle with regards to water has been observed. In heterogeneous vesicle solution composed of mixed cationic and anionic surfactants for the energy transfer between acridine orange (D) and pyronine (A), the Förster dipole-dipole model was valid, and it is interesting to note that the energy transfer rate constant (k_ET) was smaller than that in homogeneous aqueous solution. On the inside and outside of the stable vesicle, immiscible water solution of acridine orange and pyronine could be obtained, and the distance calculated from the energy transfer between D and A separated by the bilayer membrane implied that the location of ionic dye molecules was in the Gouy-Chapman layers of the vesicles. Furthermore, due to the electrostatic absorption of the dye molecules to charged headgroups of surfactants, acridine orange and pyronine accumulated and aggregated to the vesicle bilayer membrane.     

Preparation of primary amine-based block copolymer vesicles by direct dissolution in water and subsequent stabilization by sol-gel chemistry

A new amphiphilic biocompatible diblock copolymer, poly(epsilon-caprolactone)-block-poly(2-aminoethyl methacrylate), PCL-b-PAMA, was synthesized in three steps by (i) ring-opening polymerization of epsilon-caprolactone, (ii) end-group modification by esterification, and (iii) atom transfer radical polymerization (ATRP) of 2-aminoethyl methacrylate hydrochloride (AMA) in its hydrochloride salt form. This copolymer forms block copolymer vesicles with the hydrophobic PCL block forming the vesicle membrane. Unusually, these vesicles are easily prepared by direct dissolution in water without using organic co-solvents, pH adjustment, or even stirring. These vesicles can be stabilized by aqueous sol-gel chemistry using tetramethyl orthosilicate (TMOS) as the silica precursor. It is well-known that cationic polymers can catalyze silica formation, but in this particular case, it seems that the TMOS precursor is solubilized within the hydrophobic PCL membrane. Thus, the neutral membrane actually directs silica formation, rather than the cationic PAMA chains. The final vesicle morphology and the silica content depend on the silicification conditions. Provided that the TMOS/AMA molar ratio does not exceed 10:1, silicification is solely confined within the PCL membrane. At higher ratios, silica nanoparticles (5-12 nm) are also observed on the outer surface of the silicified vesicles. However, these nanoparticles appear to be only weakly adsorbed, since they can be easily removed by dialysis. The mean hydrodynamic diameter of the silicified vesicles varies from 175 to 205 nm with solution pH due to (de)protonation of the externally expressed PAMA chains. Calcination of the silicified vesicles at 800 degrees C leads to the formation of hollow silica particles. 1H NMR, transmission electron microscopy (TEM), dynamic light scattering (DLS), aqueous electrophoresis, and thermogravimetric analysis (TGA) were employed to characterize the vesicles, both before and after silicification.     

Effect of chain lengths of PEO-PPO-PEO on small unilamellar liposome morphology and stability: an AFM investigation

The morphology and stability of small unilamellar egg yolk phosphatidylcholine (EggPC) liposomes modified with the Pluronic copolymer (poly (oxyethylene)-poly (oxypropylene)-poly (oxyethylene) (PEO-PPO-PEO)) with different compositions on mica surface have been investigated using atomic force microscopy. Morphology studies reveal significant morphological changes of liposomes upon incorporating the Pluronic copolymer. Bilayers are observed for Pluronic with small hydrophilic (PEO) chain lengths such as L81 [(PEO)2(PPO)40(PEO)2] and L121 [(PEO)4(PPO)60(PEO)4]; bilayer and vesicle coexistence is observed for P85 [(PEO)26(PPO)39.5(PEO)26] and F87 [(PEO)61.1(PPO)39.7(PEO)61.1]; and stable vesicles are observed for F88 [(PEO)103.5(PPO)39.2(PEO)103.5], F127 [(PEO)100(PPO)65(PEO)100], and F108 [(PEO)132.6(PPO)50.3(PEO)132.6]. The micromechanical properties of Pluronic-modified EggPC vesicles were studied by analyzing AFM approaching force curve. The bending modulus (k(c)) of the Pluronic-modified EggPC vesicles increased several-fold compared with that of the pure EggPC vesicles. The significant difference is due to the enhanced rigidity of the EggPC vesicles as a result of the incorporation of PPO molecules and PEO chains. Based on the analysis of onset point by AFM and diameters of vesicles by light scattering, it was concluded that the favorable model to describe the polymer-bilayer interaction is the membrane-spanning model.     

Sugar-derived tricatenar catanionic surfactant: synthesis, self-assembly properties, and hydrophilic probe encapsulation by vesicles

A new sugar-derived tricatenar catanionic surfactant (TriCat) was developed to obtain stable vesicles that could be exploited for drug encapsulation. The presence of the sugar moiety led to the formation of highly hydrophilic stoichiometric catanionic surfactant systems. The three hydrophobic chains permitted vesicles to form spontaneously. The self-assembly properties (morphology, size, and stability) of TriCat were examined in water and in buffer solution. Encapsulation studies of a hydrophilic probe, arbutin, commonly used in cosmetics for its whitening properties, were performed to check the impermeability of the vesicle bilayer. The enhancement of hydrophobic forces by the three chains of TriCat prevented surfactant equilibrium between the bilayer and the solution and enabled the probe to be retained in the aqueous cavity of the vesicles for at least 30 h. Thus, the present study suggests that this tricatenar catanionic surfactant could be a promising delivery system for hydrophilic drugs.     

Templated nanostructured PS‐b‐PEO nanotubes

With anodic aluminum oxide (AAO) membranes as wetting templates, nanotubes of the cylinder‐forming polystyrene‐block‐poly(ethylene oxide) (PS‐b‐PEO) copolymer were generated. The PS‐b‐PEO solution was introduced into the cylindrical nanopores of an AAO membrane by capillary force and polymeric nanotubes formed after solvent evaporation. Because of the water solubility of the cylindrical PEO microdomains and the orientation of the cylindrical PEO microdomains with respect to the nanotube walls, the nanotubes were permeable to aqueous media. PS‐b‐PEO nanotubes were also prepared on the interior walls of amorphous carbon nanotubes (a‐CNTs). Because of the unique water permeability of the PEO microdomains, an avenue for functionalizing the interior of the a‐CNTs is enabled. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2912–2917, 2007     

Thermosensitive sol–gel transition behaviors of poly(ethylene oxide)/aliphatic polyester/poly(ethylene oxide) aqueous solutions

The thermoreversible gelation of Pluronic [poly(ethylene oxide) (PEO)–polypropylene oxide (PPO)–PEO] aqueous solutions originates from micelle formation and micelle volume changes due to PEO–water and PPO–water lower critical solution temperature behavior. The micelle volume fraction is known to dominate the sol–gel transition behavior of Pluronic aqueous solutions. Triblock copolymers of PEO and aliphatic polyesters, instead of PPO, were prepared by hexamethylene diisocyanate coupling and dicyclohexyl carbodiimide coupling. Through changes in the molecular weight and hydrophobicity of the polyester middle block, the hydrophobic–hydrophilic balance of each block was systematically controlled. The following aliphatic polyesters were used: poly(hexamethylene adipate) (PHA), poly(ethylene adipate) (PEA), and poly(ethylene succinate) (PESc). With the hydrophobicity and molecular weight of the middle block increasing, the critical micelle concentration at the same critical micelle temperature decreased, and the absolute value of the micellization free energy increased. The micelle size was rather insensitive to temperature but slightly decreased with increasing temperature. PEO–PHA–PEO and PEO–PEA–PEO triblock copolymers needed high polymer concentrations to form gels. This was ascribed to the tight aggregation of PHA and PEA chains in the micelle core due to strong hydrophobic interactions, which induced the contraction of the micelle core. However, because of the relatively hydrophilic core, a PEO–PESc–PEO aqueous solution showed gelation at a low polymer concentration. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 772–784, 2004     

Polymersomes with ionic liquid interiors dispersed in water

We describe polymersomes with ionic liquid interiors dispersed in water. The vesicles are prepared via a simple and spontaneous migration of poly(butadiene-b-ethylene oxide) (PB-PEO) block copolymer vesicles from a hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), to water at room temperature. As PB is insoluble in both water and [EMIM][TFSI] and PEO is well solvated in both media, the vesicles feature a PB membrane with PEO brushes forming both interior and exterior coronas. The robust and stable PB-PEO vesicles migrate across the liquid-liquid interface with their ionic liquid interiors intact and form a stabilized aqueous dispersion of vesicles enclosing microscopic ionic liquid pools. The nanostructure of the vesicles with ionic liquid interiors dispersed in water is characterized by direct visualization using cryogenic transmission electron microscopy. Upon heating, the vesicles can be quantitatively transferred back to [EMIM][TFSI], thus enabling facile recovery. The reversible transport capability of the shuttle system is demonstrated by the use of distinct hydrophobic dyes, which are selectively and simultaneously loaded in the vesicle membrane and interior. Furthermore, the fluorescence of the loaded dyes in the vesicles enables probing of the microenvironment of the vesicular ionic liquid interior through solvatochromism and direct imaging of the vesicles using laser scanning confocal microscopy. This vesicle system is of particular interest as a nanocarrier or nanoreactor for reactions, catalysis, and separations using ionic liquids.     

Cytomimetic large-scale vesicle aggregation and fusion based on host-guest interaction

Herein, we have shown a large-scale cell-mimetic (cytomimetic) aggregation process by using cell-sized polymer vesicles as the building blocks and intervesicular host-guest molecular recognition interactions as the driving force. We first prepared the hyperbranched polymer vesicles named branched polymersomes (BPs) around 5-10 μm through the aqueous self-assembly of a hyperbranched multiarm copolymer of HBPO-star-PEO [HBPO = hyperbranched poly(3-ethyl-3-oxetanemethanol); PEO = poly(ethylene oxide)]. Subsequently, adamantane-functionalized BPs (Ada-BPs) or β-cyclodextrin-functionalized BPs (CD-BPs) were prepared through the coassembly of HBPO-star-PEO and Ada-modified HBPO-star-PEO (HBPO-star-PEO-Ada), or of HBPO-star-PEO and CD-modified HBPO-star-PEO (HBPO-star-PEO-CD), respectively. Macroscopic vesicle aggregates were obtained by mixing CD-BPs and Ada-BPs. The intervesicular host-guest recognition interactions between β-CD units in CD-BPs and Ada units in Ada-BPs, which were proved by (1)H nuclear Overhauser effect spectroscopy (NOESY) spectrum and the fluorescence probe method, are responsible for the vesicle aggregation. Additionally, the vesicle fusion events happened frequently in the process of vesicle aggregation, which were certified by double-labeling fluorescent assay, real-time observation, content mixing assay, and component mixing assay.     

Enantioselectivity of vesicle-forming chiral surfactants in capillary electrophoresis. Role of the surfactant headgroup structure

Two vesicle-forming single-tailed amino acid derivatized surfactants sodium N-[4-n-dodecyloxybenzoyl]-L-leucinate (SDLL) and sodium N-[4-n-dodecyloxybenzoyl]-L-isoleucinate (SDLIL) have been synthesized and used as pseudo-stationary phase in micellar electrokinetic chromatography to evaluate the role of steric factor of amino acid headgroup and hydrophobic/hydrophilic interactions for enantiomeric separations. The aggregation behavior of the surfactants has been studied in aqueous buffered solution using surface tension and fluorescence probe techniques. Results of these studies have suggested formation of vesicles in aqueous solutions. Microenvironment of the vesicle, which determines the depth of penetration of the analytes into vesicle was determined by fluorescence probe technique using pyrene, N-phenyl-1-naphthylamine (NPN), and 1,6-diphenyl-1,3,5-hexatriene (DPH) as probe molecules. Atropisomeric compounds (+/-)-1,1'-bi-2-naphthol (BOH), (+/-)-1,1'-binaphthyl-2,2'-diamine (BDA), (+/-)-1,1'-binaphthyl-2,2'-diylhydrogen phosphate (BNP) and Tr?ger's base (TB) and chiral compound benzoin (BZN) has been enantioseparated. The separations were optimized with respect to surfactant concentration, pH, and borate buffer concentration. SDLL was found to provide better resolution for BOH, BNP, and BZN. On the other hand, SDLIL offers better resolution for BDA. The chromatographic results have been discussed in the light of the aggregation behavior of the surfactants and the interaction of the solutes with the vesicles.     

DIRECT OBSERVATION OF ELECTRON TRANSFER ACROSS A LIPID BILAYER: PULSED LASER PHOTOLYSIS OF AN ASYMMETRIC VESICLE SYSTEM CONTAINING CHLOROPHYLL,METHYL VIOLOGEN AND EDTA*

Abstract— Suspensions of vesicles composed of chlorophyll a (Chi) and phospholipid that were asymmetric with respect to aqueous solutions of methyl viologen (MV₂₊), an electron acceptor, and EDTA, an electron donor, were investigated using both flash and steady-state photolysis techniques. It was shown that Chl-photosensitized electron transfer occurred across the walls of the vesicles from EDTA to MV₂₊. Flash photolysis indicated that MV₂₊ dissolved in the interior aqueous compartments of the vesicles oxidized only those triplet excited state Chi molecules that were dissolved in the inner monolayers of the vesicle walls. The resultant radical products, Chi₊ and MV₊, recombined with a halftime of the order of 10_-4s. EDTA, added externally to the vesicles, competed effectively with MV₊ as a reducing agent for Chl₊. This places a lower limit of 10₄ s_-1 on the rate constant for transmembrane electron transfer. Compartmentalization by the vesicle wall of the competing pathways for the reduction of Chi₊ resulted in a nonlinear dependence of the rate constant of Chl₊ decay on EDTA concentration. The magnitude of the rate constant of electron transfer through the membrane and the way that the kinetics of Chl₊ decay depended on the concentration of Chi in the membrane strongly suggest that the electron transfer occurred by electron exchange between Chi and Chl₊.