Membrane-forming properties of pseudoglyceryl backbone based gemini lipids possessing oxyethylene spacers

Five pseudoglyceryl backbone based gemini lipids possessing varying lengths of oxyethylene [(-CH2-O-CH2-)n] spacers between cationic ammonium head groups have been synthesized, where n varies from 1 to 5. The membrane-forming properties of these gemini cationic lipids have been investigated. All the gemini lipids formed stable suspensions in water. The presence of membranous aggregates in such lipid suspensions was evidenced by transmission electron microscopy. The membrane-forming characteristics of these gemini lipids were compared with those of the corresponding monomeric lipid with one head group to understand the effect of lipid dimerization. The lipid suspensions were further characterized by dynamic light scattering and zeta potential measurements. Except for the gemini lipid with -CH2-CH2-O-CH2-CH2- spacer (2a), zeta potential of aggregates of all other gemini lipids were significantly greater than that of monomeric lipid suspensions. X-ray diffraction studies with lipid cast films revealed the increase in membrane bilayer width with increase in the length of the spacer (-CH2-O-CH2-)n. Clear thermotropic phase transitions typical of membranous assemblies were observed for all the lipid suspensions by high sensitivity differential scanning calorimetry. Aggregates of gemini lipid 2a bearing one oxyethylene [-(CH2-CH2-O-CH2-CH2)-] unit between headgroups manifested the highest phase transition temperature as compared to other gemini analogues as well as that of monomeric lipid 1. The phase transitions were reversible and exhibited large hysteresis, indicating that the observed phase transitions were of first order. To probe the surface hydration of these membranous aggregates, Paldan fluorescence studies were performed. These studies indicated the high polarity of the vesicular surface of gemini lipid 2a both in the gel and fluid melted phase as compared to vesicles of other gemini lipids.     

Surface tension and aggregation properties of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer

A series of novel cationic gemini surfactants with diethylammonium headgroups and a diamido spacer were synthesized, and their surface and bulk properties were investigated by surface tension, electrical conductivity, fluorescence, viscosity, dynamic light scattering (DLS), and transmission electron microscopy (TEM) measurements. An interesting phenomenon, that is, the obvious decline in surface tension upon increasing concentration above the critical micelle concentration (cmc), was found in these gemini surfactant solutions, and two explanations were proposed. This surface tension behavior could be explained by the rapid increase in the counterion activity in the bulk phase or the continued filling of the interface with increasing surfactant concentration above the cmc. More interestingly, not only vesicles but also the surfactant-concentration-induced vesicle to larger aggregate (spongelike aggregate) transition and the salt-induced vesicle and spongelike aggregate to micelle transition were found in the aqueous solutions of these gemini surfactants. The spongelike aggregate that is first reported in the cationic gemini surfactant-water binary system is probably caused by the adhesion and fusion of vesicles at high surfactant concentration.     

Molecular aggregates of partially fluorinated quaternary ammonium salt gemini surfactants

The size and shape of novel partially fluorinated gemini surfactant 1,2-bis[dimethyl-(3-perfluoroalkyl-2-hydroxypropyl)ammonium]ethane bromide (CnFC3-2-C3CnF, where n=4, 6, and 8) were investigated in aqueous solution by means of light scattering and transmission electron microscopy (TEM). The sizes of these molecular aggregates changed with increasing carbon number of the alkyl chain and concentration. For example, the apparent hydrodynamic radius by dynamic light scattering was 18 nm at a concentration of cmcx5 for n=4, 115 nm at the cmcx15 for n=6, and 62 nm at the cmcx30 for n=8, at 298.2 K. The shapes of CnFC3-2-C3CnF aggregates drastically changed with the alkyl chain length; the aggregates were mainly in the form of large or irregular small aggregates (n=4), string-like aggregates (n=6), and vesicles (n=8). The bromide-ion activity was measured using a bromide-ion-selective electrode to determine the degree of counterion binding to the aggregates. The degree of counterion binding to aggregate was very small compared with that in the typical hydrogenated gemini surfactants. These results indicated that the small curvature of large aggregates was not influenced by an electrostatic repulsion between the cationic head groups in the case of the bulky molecular volume of fluorinated gemini surfactants.     

Properties of mixed DOTAP-DPPC bilayer membranes as reported by differential scanning calorimetry and dynamic light scattering measurements

We have investigated the effect of a cationic lipid [DOTAP] on both the thermotropic phase behavior and the structural organization of aqueous dispersions of dipalmitoyl-phosphatidylcholine [DPPC] by means of high-sensitivity differential scanning calorimetry and dynamic light scattering measurements. We find that the incorporation of increasing quantities of DOTAP progressively reduces the temperature and the enthalpy of the gel-to-liquid crystalline transition. We are further showing that, in mixed DOTAP-DPPC systems, the reduction of the phase transition temperature is accompanied by a reduction of the average size of the structures present in the aqueous mixtures, whatever the DOTAP concentration is. These results, which extend a previous investigation by Campbell et al. (Campbell, R. B.; Balasubramanian, S. V.; Straubinger, R. M.; Biochim. Biosphys. Acta 2001, 27, 1512.) limited to a DOTAP concentration below 20 mol %, confirm that the insertion of cationic head groups in zwitterionic phosphatidylcholine bilayers facilitates the formation of stable, relatively small, unilamellar vesicles. This self-assembling restructuring from an aqueous multilamellar structure toward a liposomal phase is favored by decreasing the phospholipid phase transition temperature and by increasing the temperature of the system. This reduction of the average size and the appearance of a stable liposomal phase is also promoted by a heating and cooling thermal treatment.     

Membrane-forming properties of gemini lipids possessing aromatic backbone between the hydrocarbon chains and the cationic headgroup

Membrane-forming properties of five new gemini cationic lipids possessing an aromatic backbone between the headgroup and hydrocarbon chains have been presented. These gemini lipids differ by the number of polymethylene units [-(CH(2))(n)-] between the cationic ammonium -[N(+)(CH(3))(2)]- headgroups. The membrane-forming properties of these gemini lipids have been studied in detail by transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray diffraction (XRD), high-sensitivity differential scanning calorimetry (DSC), Paldan fluorescence studies, and UV-vis absorption spectroscopy. The electron micrographs and dynamic light scattering of their aqueous suspensions confirmed the formation of vesicular-type aggregates. The vesicle sizes and morphologies were found to depend strongly on the n-value of the spacer. Information on the thermotropic and hydration properties of the resulting vesicles was obtained from differential scanning calorimetry and temperature-dependent Paldan fluorescence studies, respectively. Examination of the thermotropic phase-transition properties of the lipid aggregates revealed interesting features of these lipids, which were found to depend on the length of the spacer chain. Paldan fluorescence studies indicate that the membranes of the gemini lipids are less hydrated as compared to that of the monomeric counterpart in their solid-gel state. In contrast in their fluid, liquid-crystalline phase, the hydration of gemini lipid aggregates was found to depend strongly on the length of the spacer. UV-vis absorption studies suggest an apparent H-type aggregate formation in the gemini lipid membranes in the gel states. In fluid state of the lipid membranes, H-aggregate formation was found to be enhanced depending on the length of the spacer. Such an understanding of the properties upon membrane formation from this new class of gemini lipids will be useful for further development of related gene delivery systems.     

Supramolecular vesicles of cationic gemini surfactants modulated by p-sulfonatocalix[4]arene

Supramolecular binary vesicles were constructed by host-guest complex formation between p-sulfonatocalix[4]arene and three cationic gemini surfactants, which were identified by UV-vis, dynamic laser scattering, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, and surface tension experiments. The critical aggregation concentration of gemini surfactants decreased pronouncedly by a factor of ca. 1000 owing to the complexation of p-sulfonato-calix[4]arene.     

Interaction of antimicrobial arginine-based cationic surfactants with liposomes and lipid monolayers

The present work examines the relationship between the antimicrobial activity of novel arginine-based cationic surfactants and the physicochemical process involved in the perturbation of the cell membrane. To this end, the interaction of these surfactants with two biomembrane models, namely, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) multilamellar lipid vesicles (MLVs) and monolayers of DPPC, 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] sodium salt (DPPG), and Escherichia coli total lipid extract, was investigated. For the sake of comparison, this study included two commercial antimicrobial agents, hexadecyltrimethylammonium bromide and chlorhexidine dihydrochloride. Changes in the thermotropic phase transition parameters of DPPC MLVs in the presence of the compounds were studied by differential scanning calorimetry analysis. The results show that variations in both the transition temperature (Tm) and the transition width at half-height of the heat absorption peak (deltaT1/2) were consistent with the antimicrobial activity of the compounds. Penetration kinetics and compression isotherm studies performed with DPPC, DPPG, and E. coli total lipid extract monolayers indicated that both steric hindrance effects and electrostatic forces explained the antimicrobial agent-lipid interaction. Overall, in DPPC monolayers single-chain surfactants had the highest penetration capacity, whereas gemini surfactants were the most active in DPPG systems. The compression isotherms showed an expansion of the monolayers compared with that of pure lipids, indicating an insertion of the compounds into the lipid molecules. Owing to their cationic character, they are incorporated better into the negatively charged DPPG than into zwitterionic DPPC lipid monolayers.     

Lateral organization of a membrane protein in a supported binary lipid domain: direct observation of the organization of bacterial light-harvesting complex 2 by total internal reflection fluorescence microscopy

A unique method is described for directly observing the lateral organization of a membrane protein (bacterial light-harvesting complex LH2) in a supported lipid bilayer using total internal reflection fluorescence (TIRF) microscopy. The supported lipid bilayer consisted of anionic 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1'-glycerol)] (DOPG) and 1,2-distearoly-sn-3-[phospho-rac-(1'-glycerol)] (DSPG) and was formed through the rupture of a giant vesicle on a positively charged coverslip. TIRF microscopy revealed that the bilayer was composed of phase-separated domains. When a suspension of cationic phospholipid (1,2-dioleoyl-sn-glycero-3-ethylphosphocholine: EDOPC) vesicles (approximately 400 nm in diameter), containing LH2 complexes (EDOPC/LH2 = 1000/1), was put into contact with the supported lipid bilayer, the cationic vesicles immediately began to fuse and did so specifically with the fluid phase (DOPG-rich domain) of the supported bilayer. Fluorescence from the incorporated LH2 complexes gradually (over approximately 20 min) spread from the domain boundary into the gel domain (DSPG-rich domain). Similar diffusion into the domain-structured supported lipid membrane was observed when the fluorescent lipid (1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-lissamine-rhodamine B sulfonyl: N-Rh-DOPE) was incorporated into the vesicles instead of LH2. These results indicate that vesicles containing LH2 and lipids preferentially fuse with the fluid domain, after which they laterally diffuse into the gel domain. This report describes for first time the lateral organization of a membrane protein, LH2, via vesicle fusion and subsequent lateral diffusion of the LH2 from the fluid to the gel domains in the supported lipid bilayer. The biological implications and applications of the present study are briefly discussed.     

Effect of hydrocarbon parts of the polar headgroup on surfactant aggregates in gemini and bola surfactant solutions

Cationic gemini surfactant homologues alkanediyl-alpha,omega-bis(dodecyldiethylammonium) bromide, [C12H25(CH3CH2)2N(CH2)SN(CH2CH3)2C12H25]Br2, where S = 4, 6, 8, 10, or 12, referred to as C12CSC12(Et), and cationic bolaamphiphiles BPHEAB (biphenyl-4,4'-bis(oxyhexamethylenetriethylammonium) bromide), PHEAB (phenyl-4,4'- bis(oxyhexamethylenetriethylammonium) bromide) were synthesized, and their aggregation behaviors in aqueous solution were studied and compared by means of dynamic light scattering, fluorescence entrapment, and transmission electron microscopy. Spherical vesicles were found in the aqueous solutions of these gemini and bola surfactants, which can be attributed to the increase of the hydrocarbon parts of the polar headgroup of the surfactants. In combination with the result of the other gemini with headgroup of propyl group, the increase of the hydrophobic parts of the surfactant polar headgroup will be beneficial to enhance the aggregation capability of the gemini and bola surfactants. Both of the vesicles formed in the gemini and bola systems showed good stabilities with time and temperature, but different stability with salt due to the different membrane conformations of surfactant molecules in the vesicles.     

Micellization of dissymmetric cationic gemini surfactants and their interaction with dimyristoylphosphatidylcholine vesicles

The micellization process of a series of dissymmetric cationic gemini surfactants [CmH2m+1(CH3)2N(CH2)6N(CH3)2C6H13]Br2 (designated as m-6-6 with m = 12, 14, and 16) and their interaction with dimyristoylphosphatidylcholine (DMPC) vesicles have been investigated. In the micellization process of these gemini surfactants themselves, critical micelle concentration (cmc), micelle ionization degree, and enthalpies of micellization (DeltaHmic) were determined, from which Gibbs free energies of micellization (DeltaGmic) and entropy of micellization (DeltaSmic) were derived. These properties were found to be influenced significantly by the dissymmetry in the surfactant structures. The phase diagrams for the solubilization of DMPC vesicles by the gemini surfactants were constructed from calorimetric results combining with the results of turbidity and dynamic light scattering. The effective surfactant to lipid ratios in the mixed aggregates at saturation (Resat) and solubilization (Resol) were derived. For the solubilization of DMPC vesicles, symmetric 12-6-12 is more effective than corresponding single-chain surfactant DTAB, whereas the dissymmetric m-6-6 series are more effective than symmetric 12-6-12, and 16-6-6 is the most effective. The chain length mismatch between DMPC and the gemini surfactants may be responsible for the different Re values. The transfer enthalpy per mole of surfactant within the coexistence range may be associated with the total hydrophobicity of the alkyl chains of gemini surfactants. The transfer enthalpies of surfactant from micelles to bilayers are always endothermic due to the dehydration of headgroups and the disordering of lipid acyl chain packing during the vesicle solubilization.     

Encapsulation of Hb into unsaturated lipid vesicles and γ-ray polymerization

A carbonyl hemoglobin (HbCO) solution was stirred with a mixed powder of polymerizable 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphocholine (DODPC), cholesterol and stearic acid (7/7/2 by mol). The mixture was extruded through polycarbonate membrane filters (final pore size = 0.2 μm Ø). The average diameter of the resulting vesicles was 203 ± 39 nm. The [Hb]/[Lipid] ratio (the weight ratio of Hb in vesicle to lipid) increased with the Hb concentration, and decreased with the NaCl concentration. A maximum [Hb]/[lipid] ratio was observed at pH 6.9, which was the same as the isoelectric point of Hb. The vesicles were stabilized by γ-irradiation (⁶⁰Co) because the bilayer lipids bound each other to yield polyphospholipids. Denaturation of Hb by γ-irradiation was not detected. These polyphospholipid vesicles encapsulating Hb were stable even against the freeze–thaw cycles and the freeze-drying procedure.     

Structural effects of a basic peptide on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine membranes: a fluorescent resonance energy transfer study

We studied the effect of a model basic peptide, hexalysiltryptophan, on the organization of dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine unilamellar vesicles by means of fluorescent resonance energy transfer (FRET) between fluorescently labeled phospholipids. Several FRET theoretical models assuming different bilayer geometries and probe distributions were fitted to the time-resolved data. The experiments were carried out at two temperatures in different regions of the lipid mixture phase diagram. At 45 degrees C, the expected gel/fluid phase separation was verified by model fitting in peptide-free vesicles, which from the FRET approach means that domains are larger than approximately 200 A. No noticeable alteration of membrane organization was detected upon increasing the peptide concentration. At variance, for the single fluid phase at 60 degrees C, there was a large increase in FRET efficiency upon peptide addition to the lipid vesicles, mainly caused by peptide-induced vesicle aggregation. The system gradually changed from unilamellar lipid vesicles to a multibilayer geometry, and a limit lamellar repeat distance of approximately 57 A was recovered. Furthermore, no evidence for lateral domain formation on the FRET length scale was found at this temperature, the cationic peptide being only able to induce local lipid demixing, causing a short-range sequestration of 2-3 acidic lipids around each surface-adsorbed peptide.     

Adsorbed layer structure of cationic gemini and corresponding monomeric surfactants on mica

We report a comprehensive study of the adsorbed layer morphologies of cationic gemini surfactants of the type dodecanediyl-alpha,omega-bis(dimethylalkylammonium bromide) and their corresponding monomers, dimethyldodecylalkylammonium bromide, on mica using atomic force microscopy soft-contact imaging. As in the bulk, aggregate curvature of the adsorbed geminis is found to increase with increasing spacer length, but the adsorbed aggregate curvature also increases in the presence of CsCl and CsBr. The monomeric surfactants exhibit an unexpected transition from globular adsorbed aggregates to a bilayer when the alkyl side chain reaches butyl, and this transition is also sensitive to added electrolyte.     

Characteristics of pH-sensitive hydrogel microsphere of poly(acrylamide-<Emphasis Type="Italic">co</Emphasis>-methacrylic acid) with sharp pH–volume transition

The forming process and characteristics of monodispersed hydrogel microspheres of poly(acrylamide–methacrylic acid) with sharp pH–volume transition were studied. pH-/ion-sensitive and thermosensitive behaviors of microspheres sampled at various stage of polymerization were evaluated by using the dynamic light scattering. It was observed that the sharpness of pH–volume transition increased with the increase in monomer conversion. Both thermo- and ion-sensitive behaviors were affected by pH. At pH 4.3, the hydrodynamic diameter of microspheres monotonically and slightly decreased with the increase in temperature, whereas at pH 3.5 and 3.8, the curves of thermo–volume transition were similar to those of pH–volume transition with a maximum temperature at 25 and 20 °C, respectively. Increasing the [CaCl₂] was to decrease the hydrodynamic diameters of microspheres, irrespective of pH. However, a region at lower [NaCl] was found, where the diameter increased with the increase in [NaCl]. Moreover, the range of diameter increasing extended to higher [NaCl] as pH increased.     

混合胶束中Gemini阳离子表面活性剂14-s-14与常规表面活性剂的协同作用:连接臂及反离子效应(英文)

The mixed micelle formation of binary cationic 14-s-14 gemini with conventional single chain surfactants was studied by conductivity measurements.The critical micelle concentration(cmc) and the degree of counterion binding values(g) of the binary systems were determined.The results were analyzed by applying regular solution theory(RST) to calculate micellar compositions(X),activity coefficients(f1,f2),and the interaction parameters(β).The synergistic interactions of all the investigated cationic gemini+conventional surfactant combinations were found to be dependent upon the length of hydrophobic spacer of the gemini surfactant.The excess Gibbs free energy of mixing was evaluated,and it indicated relatively more stable mixed micelles for the binary combinations.     

Evaluation of different photosensitizers for use in photochemical gene transfection

Many potentially therapeutic macromolecules, e.g. transgenes used in gene therapy, are taken into the cells by endocytosis, and have to be liberated from endocytic vesicles in order to express a therapeutic function. To achieve this we have developed a new technology, named photochemical internalization (PCI), based on photochemical reactions inducing rupture of endocytic vesicles. The aim of this study was to clarify which properties of photosensitizers are important for obtaining the PCI effect improving gene transfection. The photochemical effect on transfection of human melanoma THX cells has been studied employing photosensitizers with different physicochemical properties and using two gene delivery vectors: the cationic polypeptide polylysine and the cationic lipid 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP). Photochemical treatment by photosensitizers that do not localize in endocytic vesicles (tetra[3-hydroxyphenyl]porphyrin and 5-aminolevulinic acid-induced protoporphyrin IX) do not stimulate transfection, irrespective of the gene delivery vector. In contrast, photosensitizers localized in endocytic vesicles stimulate polylysine-mediated transfection, and amphiphilic photosensitizers (disulfonated aluminium phthalocyanine [AlPcS2a] and meso-tetraphenylporphynes) show the strongest positive effect, inducing approximately 10-fold increase in transfection efficiency. In contrast, DOTAP-mediated transfection is inhibited by all photochemical treatments irrespective of the photosensitizer used. Neither AlPcS2a nor Photofrin affects the uptake of the transfecting DNA over the plasma membrane, therefore photochemical permeabilization of endocytic vesicles seems to be the most likely mechanism responsible for the positive PCI effect on gene transfection.     

Flocculation of microgel particles with sodium chloride and sodium polystyrene sulfonate as a function of temperature

The flocculation behavior of poly(N-isopropylacrylamide) (PNIPAM) microgel particles, containing surface sulfate groups, has been studied as a function of sodium chloride [NaCl] concentration, between 0.1 and 800 mM NaCl and over the temperature range 25-60 degrees C. The critical flocculation temperature (CFT) of the particles was determined as a function of NaCl concentration. Three regions of NaCl concentration were established. First, at very low values of [NaCl] (< approximately 25 mM), no CFT value could be determined; this implies that the interparticle electrostatic repulsion is sufficient to prevent any flocculation occurring. This remains the case even at temperatures well in excess of the lower critical solution temperature for PNIPAM in solution, where the particles are essentially deswollen. Second, at intermediate [NaCl] (approximately 25-100 mM), the CFT decreased strongly with increasing [NaCl]. In this region, the electostatic forces are weakened sufficiently for the van der Waals forces to cause flocculation. Third, at higher [NaCl] (> approximately 100 mM), the electrostatic repulsion is screened out, and the CFT decreases linearly with [NaCl]. The reason for this decrease is the fact that aqueous solutions of NaCl become increasingly poorer solvent environments for PNIPAM with increasing [NaCl]. These trends are apparent also in the values determined for the hydrodynamic size of the stable PNIPAM particles as a function of [NaCl] and temperature. It is shown that the flocculation of the PNIPAM particles is consistent with a weak, reversible flocculation model. This is apparent, for example, from the fractal dimensions of the flocs (approximately 2.0), determined from the power law used to fit the time evolution of the hydrodynamic size of the flocs, and also from the estimated depth of the mimimum in the interparticle pair potential, based on the critical size of the primary particles where flocculation just begins to occur. The effect of adding sodium poly(styrene sulfonate) [PSS] to the PNIPAM dispersions, in the absence of NaCl, was also investigated. The minimum amount of PSS required to induce flocculation was found to decrease with increasing temperature.     

Lipid composition-dependent incorporation of multiple membrane proteins into liposomes

Membrane proteins from bacteria Pasteurella multocida were used as a model for studying its incorporation into liposomes. An important step to achieve efficient high yield protein incorporation in proteoliposomes is the study of the more suitable lipid composition. To this end, we compared the amount of total protein, reconstituted by co-solubilization methods, into liposomes of phospholipids with different polar head groups and acyl chain lengths. The liposomes and proteoliposomes were characterised by isopycnic centrifugation in sucrose gradient and by dynamic light scattering. Experimental and theoretical results were compared considering the effects exerted through the hydrocarbon chain length, volume, and optimal cross-sectional area of the phospholipid (combined in the geometrical critical packing parameter, lipid–protein matching), critical spontaneous radius of curvature of the bilayer vesicle, phase transition temperature of the lipid and ratio of lipid–protein molecules present in the vesicles. The highest incorporation of multiple proteins was found with dipalmitoylphosphatidylcholine (DPPC), reaching a yield of 93% compared to the lower relative amounts incorporated in proteoliposomes of the other lipids. The incorporation of multiple proteins induces a proportional enhancement of vesicular dimension, since DPPC–proteoliposomes have an average diameter of 1850 Å, compared to the 1430 Å for pure DPPC vesicles.     

Microenvironmental differences and changes in bilayers of unilamellar vesicles probed by spectroscopic and kinetic parameters

The UV/Vis absorption band maximum lambdamax of trans-4,4'-nitrophenylaminoazobenzene, the thermal isomerization rate constant kiso of its cis-isomer, the fluorescence intensity ratio of monomer and excimer, and the fluorescence lifetime of the excimer, respectively, of 1,3-di(1-pyrenyl)propane were determined as probes for polarity, water content, and viscosity, respectively, in unilamellar vesicles of di-n-alkyl-dimethylammonium bromides and 1,2-acyl-sn-glycero-3-phosphocholines. The dependence on vesicle size, the solvent (water or HEPES buffer/NaCl solution, each with H2O or D2O), and the temperature (20-60 degrees C) was studied. Apparent Arrhenius activation energies and kinetic solvent isotope effects (KSIE = kiso,H2O/kiso, D2O) were derived. Size and stability of the vesicles prepared by extrusion were controlled by dynamic light scattering. The probe properties clearly indicate the reversibly decreasing size of didodecyldimethylammonium bromide vesicles with increasing temperature but are insensitive against vesicles size variation in most other cases. In the temperature range of the main phase transition of the bilayers, changes of the microenvironment of the probes, and their changing position in the bilayer, respectively, are reflected by characteristic changes of their properties. Buffer/NaCl solution causes vanishing influence of the lipid chain but remaining difference between cationic and zwitterionic headgroups probed by means of kiso.     

Effect of surfactant counterion and organic modifier on the properties of surfactant vesicles in electrokinetic chromatography

Counterion and organic modifier are two parameters in EKC that can be varied in order to obtain improved solubility, selectivity, and efficiency. The effect of changing surfactant counterion and/or organic modifier on the chromatographic and electrophoretic properties of cetyltrimethylammonium bromide (CTAB)/sodium octyl sulfate (SOS) vesicles is examined in EKC. The vesicles are prepared in a 1:3.66 cationic/ anionic mole ratio for a total surfactant concentration of 69 mM. The cationic CTAB is replaced by cetyltrimethylammonium chloride (CTAC) and the first use of CTAC/SOS vesicles is reported. The mean diameter of the CTAC/SOS vesicles is 96 nm while that of the CTAB/SOS vesicles is 85 nm. A class I modifier (2-amino-1-butanol) and a class II modifier (acetonitrile) have similar effects on the EOF, elution range, methylene selectivity, and the efficiency of the CTAB/SOS vesicles and the CTAC/SOS vesicles. Upon addition of 10% ACN, there is roughly a 10-fold increase in the efficiency of heptanophenone, a model hydrophobic compound, compared to the efficiency using unmodified vesicles. Linear free energy relationship (LFER) analysis using the Abraham solvation model is employed to characterize solute-vesicle interactions. The results suggest that organic modifier-vesicle interactions depend somewhat on the counterion.