Effect of concentric multilamellae on the depolarized light scattering from phospholipid vesicles

An expression of the depolarization ratio is presented for multilayered spherical shells using the Rayleigh-Gans theory of light scattering. The effect of multilamellar bilayers of a phospholipid vesicle on the depolarization is investigated numerically. It is shown that the depolarization ratio decreases markedly with increasing number of bilayers.     

Theory of light scattering from vesicles

The scattering and depolarization of light from spherical membrane systems, vesicles, is calculated in a new succinct formalism appropriate to spherical systems and the rotations of anisotropic elements composing them. The results of Tinker (1972) and Mishima (1980, 1981) are readily derived within this formalism and there is a considerable saving in numerical computation. The problem of fluctuations in the molecular packing of the component phospholipid molecules is also addressed.     

Interactions of novel, nonhemolytic surfactants with phospholipid vesicles

PEG-12-acyloxystearates constitute a novel class of pharmaceutical solubilizers and are synthesized from polyethylene glycol and 12-hydroxystearic acid, which has been esterified with a second acyl chain. The hemolytic activity of these surfactants decreases drastically with increasing pendant acyloxy chain length, and surfactants with an acyloxy chain of 14 carbon atoms or more are essentially nonhemolytic. In this paper, the interactions of PEG-12-acyloxystearates (acyloxy chain lengths ranging from 8 to 16 carbon atoms) with phosphatidylcholine vesicles, used as a model system for erythrocyte membranes, were studied in search of an explanation for the large variations in hemolytic activity. Surfactant-induced alterations of membrane permeability were investigated by studying the leakage of vesicle-entrapped calcein. It was found that all of the surfactants within the series interact with the vesicle membranes and cause slow leakage at elevated surfactant concentrations, but with large variations in leakage kinetics. The initial leakage rate decreases rapidly with increasing pendant acyloxy chain length. After prolonged incubation, on the other hand, the leakage is not a simple function of acyloxy chain length. The effect of the surfactants on membrane integrity was also investigated by turbidity measurements and cryo-transmission electron microscopy. At a surfactant/lipid molar ratio of 0.4, the vesicle membranes are saturated with surfactant. When the surfactant/lipid molar ratio is further increased, the vesicle membranes are progressively solubilized into mixed micelles. The rate of this process decreases strongly with increasing acyloxy chain length. When comparing the results of the different experiments, it can be concluded that there is no membrane permeabilization below saturation of the vesicle membranes. The large variations in the kinetics suggest that several steps are involved in the mechanism of leakage induced by PEG-12-acyloxystearates and that their relative rates vary with acyloxy chain length. The slow kinetics may in part be explained by the low critical micelle concentrations (CMCs) exhibited by the surfactants. The CMCs were found to be in the range of 0.003-0.025 microM.     

Nucleation and growth of silver nanoparticles monitored by titration microcalorimetry

The nucleation and the growth of silver nanoparticles were studied by spectrophotometry and transmission electron microscopy, with simultaneous recording of the concomitant enthalpy changes. Silver nanoparticles were stabilized by sodium citrate and reduction was brought about by the addition of hydroquinone in aqueous medium. It was established that nucleation is an exothermic process and heat effects are basically determined by the ratio of silver ions to hydroquinone. The process of nanoparticle formation was divided to three phases: the nucleation phase is exothermic, the growth phase is endothermic and further addition of the reducing agent results in the aggregation of silver nanoparticles, which produces a second exothermic heat effect.     

Interactions and two-phase coexistence in nonionic micellar solutions as determined by static light scattering

We suggest semi-phenomenological approaches for the pair interaction potential in aqueous C(m)E(n) solutions. These expressions are non-linear least squares fitted to static light scattering data from solutions of C(m)E4 and C(m)E8 surfactants in the long wavelength limit. From the resulting interaction parameters we calculate the location of the liquid/liquid two phase coexistence curves, which are in very good agreement with experimental data reported by others.     

Comparison between neutron and quasielastic light scattering by polyacrylamide gels

The authors describe small-angle neutron scattering measurements of the screening length ζ in polyacrylamide-water gels. Although these are inhomogeneous systems, the screening length is clearly observable and is in good numerical agreement with the relation E = 3kT/4πζ³, where E is the longitudinal elastic modulus of the gel obtained from measurements of the intensity of qu-asielastically scattered light. Static light scattering observations reveal a larger-scale (ca. 30 nm) superstructure in the gel.     

Extruded vesicles of dioctadecyldimethylammonium bromide and chloride investigated by light scattering and cryogenic transmission electron microscopy

Combined dynamic and static light scattering (DLS, SLS) and cryogenic transmission electron microscopy (cryo-TEM) were used to investigate extruded cationic vesicles of dioctadecyldimethylammonium chloride and bromide (DODAX, X being Cl(-) or Br(-)). In salt-free dispersions the mean hydrodynamic diameter, D(h), and the weight average molecular weight, M(w), are larger for DODAB than for DODAC vesicles, and both D(h) and M(w) increase with the diameter (varphi) of the extrusion filter. NaCl (NaBr) decreases (increases) the DODAB (DODAC) vesicle size, reflecting the general trend of DODAB to assemble as larger vesicles than DODAC. The polydispersity index is lower than 0.25, indicating the dispersions are rather polydisperse. Cryo-TEM micrographs show that the smaller vesicles are spherical while the larger ones are oblong or faceted, and the vesicle samples are fairly polydisperse in size and morphology. They also indicate that the vesicle size increases with phi and DODAB assembles as larger vesicles than DODAC. Lens-shaped vesicles were observed in the extruded preparations. Both light scattering and cryo-TEM indicate that the vesicle size is larger or smaller than phi when phi is smaller or larger than the optimal phi approximately 200 nm.     

Slow insertion kinetics during interaction of a model antimicrobial peptide with unilamellar phospholipid vesicles

The mechanism of interaction between a model antimicrobial peptide and phospholipid unilamellar vesicle membranes was studied using fluorescence spectroscopy, fluorescence lifetime measurements, and light scattering. The peptide, a mellitin mutant, was labeled at position K14 with the polarity-sensitive probe AlexaFluor 430. The kinetics of the interaction of this derivative with various concentrations of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) vesicles was examined. Our work unveiled two novel aspects of peptide-lipid interactions. First, the AB plot or phasor analysis of the fluorescence lifetime studies revealed at least three different peptide states, the population of which depended on the lipid to peptide (L:P) concentration ratio. Second, complex fluorescence kinetics were observed over extended time-scales from 30 s to 2 h. The extended kinetics was only observed at particular lipid concentrations (L:P ratios 20:1 and 10:1) and not at others (30, 40, 50 and 100:1 L:P ratio). Analysis of the complex kinetics revealed several intermediates. We assign these to distinct states of the peptide formed during helix insertion into the vesicle membrane that are intermediate to lytic pore formation.     

Low angle laser light scattering and photon correlation spectroscopy in surfactant vesicles

Using low angle laser light scattering, weight averaged molecular weights, M?_w, diffusion coefficients, and hydrodynamic radii, R_H, have been determined for completely synthetic surfactant vesicles, prepared by ultrasonic irradiation of dioctadecyldimethylammonium chloride. DODAC, and dihexadecylphosphate. DHP dispersion. Both the M?_w and R_H values were found to decrease exponentially as a function of sonication time. At the limit, M?_w and R_H for DODAC vesicles are 12.6 × 10⁶ dalton and 396 Å, and those for DHP vesicles are 23 × 10⁶ dalton and 595 Å. Calculations indicate both vesicles to be prolates.     

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.     

Structural and morphological transition of long-chain phospholipid vesicles induced by mixing with short-chain phospholipid

Effects of a short-chain phospholipid, dihexanoylphosphatidylcholine (DHPC), on the structure and morphology of membrane assemblies of a long-chain phospholipid, dimyristoylphosphatidylcholine (DMPC), were examined by fluorescence spectroscopy, differential scanning calorimetry (DSC), and cryogenic transmission electron microscopy (cryo-TEM). It was found by fluorescence measurements that DHPC affects on the gel and liquid crystalline state of DMPC vesicle membranes in different ways. Further, the result of DSC suggested that, along the transition process from DMPC vesicle to DMPC–DHPC mixed micelle, there are at least three different concentration regions which are characterized by the individual variation pattern of the transition temperature and enthalpy change. The cryo-TEM micrographs demonstrated the formation of thread-like assemblies in the second region and the coexistence of the assemblies and spherical micelles in the third region. Thus, it was concluded that the structural transition from DMPC vesicle to DMPC–DHPC mixed micelle could occur in a stepwise manner through the formation of the thread-like assembly, which cannot be described by the three-stage model of vesicle to micelle transition.     

Aggregation and fusion of phospholipid vesicles

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

Interactions between cationic gemini/conventional surfactants with polyvinylpyrrolidone: Specific conductivity and dynamic light scattering studies

The interaction between bis(hexadecyldimethylammonium)hexane dibromide (16-6-16), bis(tetradecyldimethylammonium)hexane dibromide (14-6-14), their conventional counterparts cetyltrimethylammonium bromide (CTAB) and tetradecyltrimethylammonium bromide (TTAB) with polyvinylpyrrolidone (PVP) was investigated using the conductivity technique. The results show that gemini surfactants interact strongly with PVP as compared to conventional surfactants. The results also reveal that the surfactants with shorter hydrocarbon chain interact weakly as those of longer hydrocarbon chain. The interactions of 16-6-16 and 14-6-14 and their conventional counterparts with PVP were also studied using dynamic light scattering (DLS) measurements. We have also highlighted the effect of surfactant–polymer interactions on the dispersion force in the solution. Critical aggregation concentration (cac) and critical micelle concentration (cmc) were obtained using the conductivity data. The degrees of micelle ionization and free energies associated with aggregation, micellization, and transfer have also been evaluated and discussed.     

Interactions between sodium dodecyl sulphate and non-ionic cellulose derivatives studied by size exclusion chromatography with online multi-angle light scattering and refractometric detection

The novel approach described allows to characterise the surfactant-polymer interaction under several sodium dodecyl sulphate (SDS) concentrations (0-20 mM) using size exclusion chromatography (SEC) with online multi-angle light scattering (MALS) and refractometric (RI) detection. Three different cellulose derivatives, hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC) and hydroxyethyl cellulose (HEC), have been studied in solution containing 10 mM NaCl and various concentrations of sodium dodecyl sulphate. It is shown that this approach is well suited for successful application of both Hummel-Dreyer and multi-component light scattering principles and yields reliable molecular masses of both the polymer complex and the polymer itself within the complex, the amount of surfactant bound into the complex as well as appropriate values of the refractive index increment (dn/dc)micro, of both the complex and the polymer in question. The more hydrophobic derivatives HPC and HPMC adsorbed significantly more SDS than HEC. The inter-chain interactions close to critical aggregation concentration (cac) were clearly seen for HPC and HPMC as an almost two-fold average increase in polymer molecular mass contained in the complex.     

Interactions between a nonionic gemini surfactant and cyclodextrins investigated by small-angle neutron scattering

The microstructure of complexes between hydroxypropyl-cyclodextrins (HPCDs) (alpha, beta, and gamma) and a novel gemini surfactant has been investigated by small-angle neutron scattering (SANS). This nonionic hetero-gemini surfactant (denoted NIHG750) contains two hydrophobic groups and two hydrophilic groups. One is a methyl-capped polyoxyethylene chain with 16 oxyethylene units and the other is a secondary hydroxyl group. Various form factor models have been considered for fitting the SANS data. Spherical aggregates (25 to 40 A) with a size slightly larger than that of NIHG750 micelles (about 23 A) appear in mixed systems. These could be micellar aggregates partly covered with a few cyclodextrin molecules. In addition, the results indicate rod formation (r approximately 8 A, L approximately 70 A) for the NIHG-HPCD complexes. This result is consistent with the threading of HPCDs onto NIHG750 to such an extent that the surfactant molecule takes an extended conformation at high levels of HPCD. Also, the results indicate that HPCDs may interact with the oxyethylene groups of the spherical micellar aggregates leading to an increase in micelle size and a gradual transformation to rod-shaped aggregates. The tendency to form rods increases in the order gamma-CD相似文献   

Formation of vesicles in diluted aqueous solutions of surfactant investigated by direct analysis of light scattering

Formation of aggregates in the binary systems of double-tailed surfactant, sodium 4-(1-pentylheptyl)benzenesulfonate, and water in the dilute regime was supposed to occur within 5.0–8.5% of surfactant concentration. The size of particles was determined by light scattering. In addition, the samples were observed at room temperature using an Axiovert 35 Zeiss polarized light microscope operated with differential interference contrast optics. The observed aggregates could, in theory, belong a vesicle phase. All the histograms obtained by light scattering showed a bimodal distribution of particles. Weight factors including intensity, volume and number distribution indicate 97–100% of small aggregate sizes, since the peaks for the big sizes indicate only a small number of the aggregate population. Small aggregates have shown monodispersity with diameters of the aqueous core amounting to 38.94 and 54.94 nm relating to the surfactant concentration of 6.0 and 8.0%, respectively. Hydrodynamic radii determined by the Cumulant method, by the inverse Laplace transformation, and using a plot 1/τ vs. q², showed values within the usual precision limits.     

Interactions between light and circadian rhythms in plant photoperiodism

Abstract— The timing mechanism in plant photoperiodism seems to involve two endogenous circadian rhythms: a light-on (dawn) rhythm and a light-off (dusk) rhythm. Following a period of darkness light may affect flowering without affecting the rhythms, or it may affect flowering by rephasing the rhythms. A hypothesis is presented concerning the mechanism of the interaction between illumination and endogenous rhythms based upon correlations between leaf movements and flowering responses in various photoperiodic treatments. The possible role played by phytochrome in the response is considered in relation to the effects of light quality on the responses.     

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles

We describe a method to embed phospholipid vesicles into polyelectrolyte multilayers built up by the alternate deposition of polyanions and polycations. Before deposition, the vesicles are rigidified by polycation adsorption onto their surface avoiding their fusion once deposited on the multilayer surface. The vesicles adsorb to form a compact and "hard" monolayer as imaged by atomic force microscopy. The thickness of the adsorbed vesicle layer, of the order of 250 nm, is very close to the diameter of the vesicles in solution. This work should open the route to the buildup of multilayer films containing phospholipid vesicles that could act as "reservoirs" for drugs or enzymatic nanoreactors.     

Shape fluctuations of large unilamellar lipid vesicles observed by laser light scattering: influence of the small-scale structure

In the present paper, we apply the dynamic laser light scattering technique to investigate the dependence of the characteristic times of thermally induced shape fluctuation of large unilamellar vesicles (LUVs) on bilayer composition. After addressing single-component LUVs made of two common phospholipids, dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC), we investigate the changes in vesicle shape fluctuation times due to the presence of cholesterol and gangliosides (GM1), added in small amounts. The experimental results show that the addition of a second component, even in small amount, to DMPC vesicles induces a change in membrane fluctuation times. Moreover, in the case of ganglioside addition, also the disposition of GM1 within the bilayer is of importance. Quite unexpectedly, the symmetric or asymmetric disposition of GM1 has opposite effects on bilayer dynamics, the first resulting in a "hardening" and the second in a "softening" of the membrane. Those results support that the small-scale structure of the bilayer is important in determining the overall dynamics of the vesicle. They also suggest that the physiological disposition of GM1 in the outer leaflet of real cells has a significative result in mechanical terms, positively affecting the dynamics of the membrane.     

Layer by layer self-assembled polyelectrolyte multilayers with embedded phospholipid vesicles obtained by spraying: integrity of the vesicles

In a previous paper (Michel, M.; Vautier, D.; Voegel, J.-C.; Schaaf, P.; Ball, V. Langmuir 2004, 20, 4835), we showed that phospholipid vesicles can be incorporated into poly(glutamic-acid)/poly(allylamine) (PGA/PAH) multilayered polyelectrolyte films built by the alternated dipping of a surface in polyanion and polycation solutions. AFM imaging, quartz crystal microbalance, and ellipsometry suggested that the vesicles remain intact when adhering on the surface. In the present paper, we show that such films can also be realized by spraying both the polyelectrolyte solutions and the vesicles onto the surface. Using such vesicles filled with ferrocyanide ions, we prove by cyclic voltammetry that the sprayed vesicles remain intact when embedded in the multilayers. We show that multilayers containing two distinct layers of intact vesicles separated by several polyanion/polycation bilayers can also be constructed. Polyelectrolyte multilayers containing layers of phospholipid vesicles could act as reservoirs for drug or other biologically active molecules in controlled release bioactive coatings.