The Influence of Ca2+ and the Interdigitated Bilayer on the Colloidal Stability of DPPC and F-DPPC Liposomes (original) (raw)
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On the Effect of Ca 2+ and La 3+ on the Colloidal Stability of Liposomes
Langmuir, 2005
This work deals with the effect of Ca 2+ and La 3+ on the colloidal stability of phosphatidylcholine (PC) liposomes in aqueous media. As physical techniques, nephelometry, photon correlation spectroscopy, electrophoretic mobility, and surface tension were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersity values were observed as La 3+ concentration increases, suggesting that this cation induces the aggregation of liposomes. However, changes in polydispersity were not observed with Ca 2+ , suggesting a coalescence mechanism or fusion of liposomes. The stability factor (W), calculated from the nephelometry measurements indicated that aggregation/fusion occurs at a critical concentration (c.c.) of 0.3 and 0.7 M for La 3+ and Ca 2+ , respectively. To gain a better insight into the interaction mechanism between the liposomes and the studied ions, the interaction between PC monolayers and Ca 2+ and La 3+ was studied. Changes in the surface area per lipid molecule (A0) in the monolayer at the c.c. values were found for both ions, with a more pronounced effect in the case of Ca 2+ . This corresponds with a larger reduction of the steric repulsive interaction between the headgroups at the phospholipid membrane (πhead). The experimental result validates the hypothesis made on the liposome fusion in the presence of Ca 2+ and liposome aggregation in the presence of La 3+ . These aggregation mechanisms have also been confirmed by transmission electron microscopy.
Aggregation of liposomes induced by calcium: A structural and kinetic study
Physical Review E, 2007
In this work, the calcium-induced aggregation of phosphatidylserine liposomes is probed by means of the analysis of the kinetics of such process as well as the aggregate morphology. This novel characterization of liposome aggregation involves the use of static and dynamic light-scattering techniques to obtain kinetic exponents and fractal dimensions. For salt concentrations larger than 5 mM, a diffusion-limited aggregation regime is observed and the Brownian kernel properly describes the time evolution of the diffusion coefficient. For slow kinetics, a slightly modified multiple contact kernel is required. In any case, a time evolution model based on the numerical resolution of Smoluchowski's equation is proposed in order to establish a theoretical description for the aggregating system. Such a model provides an alternative procedure to determine the dimerization constant, which might supply valuable information about interaction mechanisms between phospholipid vesicles.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2006
Upon storage of phospholipid liposome samples, lysolipids, fatty acids, and glycerol-3-phosphatidylcholine are generated as a result of acid-or base-catalyzed hydrolysis. Accumulation of hydrolysis products in the liposome membrane can induce fusion, leakage, and structural transformations of the liposomes, which may be detrimental or beneficial to their performance depending on their applications as, e.g., drug delivery devices. We investigated in the present study the influence of phospholipid hydrolysis on the aggregate morphology of DPPC/DSPE-PEG 2000 liposomes after transition of the phospholipid membrane from the gel phase to liquid crystalline phase using high performance liquid chromatography (HPLC) in combination with static light scattering, dynamic light scattering, and cryo-transmission electron microscopy (cryo-TEM). The rates of DPPC hydrolysis in DPPC/DSPE-PEG 2000 liposomes were investigated at a pH of 2, 4, or 6.5 and temperatures of 22°C or 4°C
Biophysical Journal, 2003
Poly(ethylene glycol) (PEG) decorated lipid bilayers are widely used in biomembrane and pharmaceutical research. The success of PEG-lipid stabilized liposomes in drug delivery is one of the key factors for the interest in these polymer/lipid systems. From a more fundamental point of view, it is essential to understand the effect of the surface grafted polymers on the physical-chemical properties of the lipid bilayer. Herein we have used cryo-transmission electron microscopy and dynamic light scattering to characterize the aggregate structure and phase behavior of mixtures of PEG-lipids and distearoylphosphatidylcholine or dipalmitoylphosphatidylcholine. The PEG-lipids contain PEG of molecular weight 2000 or 5000. We show that the transition from a dispersed lamellar phase (liposomes) to a micellar phase consisting of small spherical micelles occurs via the formation of small discoidal micelles. The onset of disk formation already takes place at low PEG-lipid concentrations (\5 mol %) and the size of the disks decreases as more PEG-lipid is added to the lipid mixture. We show that the results from cryo-transmission electron microscopy correlate well with those obtained from dynamic light scattering and that the disks are well described by an ideal disk model. Increasing the temperature, from 258C to above the gel-to-liquid crystalline phase transition temperature for the respective lipid mixtures, has a relatively small effect on the aggregate structure.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2007
Complexation between linear poly-L-lysine (PLL) and negatively charged phosphocholine unilamellar liposomes has been investigated by means of dynamic light scattering, microelectrophoresis, and differential scanning calorimetry. It is found that complexation results in charge inversion (vesicle coating/stabilization) or vesicle aggregation depending on various experimental conditions. Complexation in dependence on PLL concentration and molecular mass, lipid phase state, rate and order of liposome and PLL mixing and time evolution of complexes are investigated and discussed. Aggregation profiles are determined and size distribution of the aggregates formed is studied, leading to the possibility of aggregation control. The time evolution of vesicle aggregation shows particle enlargement consisting in particle growth up to the irreversible formation of thermodynamically stable aggregates of about 2 μm in diameter. The formation of stable aggregates is in agreement with theoretical predictions of colloid particles aggregation by an interplay of long range electrostatic repulsion and short range attraction. Differential scanning calorimetry reveals that physical adsorption occurs exclusively on the vesicle surface and the lipidic organization is not significantly disturbed. The present study describes multivariable aspects of the complexation process between liposomes and polyions which results in the formation of a new class of still poorly defined colloids. These results allow establishing and optimization of a procedure for fabrication of polycation-stabilized vesicles to be used for various applications such as drug delivery.
Effect of Gd^{3+} on the colloidal stability of liposomes
Physical Review E, 2006
Lanthanide ions such as La 3+ and Gd 3+ are well known to have large effects on the structure of phospholipid membranes. Unilamellar vesicles of dipalmitoylphosphatidylcholine ͑DPPC͒ were prepared by sonication method and confirmed by transmission electron microscopy. The effects of concentration of gadolinium ions Gd 3+ on DPPC unilamellar vesicles in aqueous media were studied by different techniques. As physical techniques, photon correlation spectroscopy, electrophoretic mobility, and differential scanning calorimetry were used. The theoretical predictions of the colloidal stability of liposomes were followed using the Derjaguin-Landau-Verwey-Overbeek theory. Changes in the size of liposomes and high polydispersities values were observed as Gd 3+ concentration increases, suggesting that this cation induces the aggregation of vesicles. Electrophoretic mobility measurements on unilamellar vesicles as a function of Gd 3+ ion concentration show that the vesicles adsorb Gd 3+ ions. Above Gd 3+ concentrations of 0.1 mol dm −3 , the potential and light scattering measurements indicate the beginning of aggregation process. For comparison with similar phospholipids, the zeta potential of phosphatidylcholine interacting with Gd 3+ was measured, showing an analogous behavior. Differential scanning calorimetry has been used to determine the effect of Gd 3+ on the transition temperature ͑T c ͒ and on the enthalpy ͑⌬H c ͒ associated with the process.
Chemistry and Physics of Lipids, 2011
The cationic large unilamellar mixed liposomes from 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) and didodecyldimethylammonium bromide (DDAB) or dioctadecyldimethylammonium bromide (DODAB) were prepared. The influence of the addition of Triton X-100 (TX-100) or octaethylene glycol mono-n-dodecylether (C12E8) on the membrane integrity was investigated turbidimetrically. The stability of the liposomal systems was estimated by monitoring fluorimetrically at 25 °C the rate of spontaneous and surfactant-induced release of entrapped 5(6)-carboxyfluorescein (CF). In order to evaluate the interaction of the cationic DODAB guest with the host POPC membrane, the main phase transition temperatures (Tm) were determined by electron paramagnetic resonance spectroscopy (EPR). All the results obtained show that the presence of DODAB and DDAB stabilizes the POPC liposomes. The extent of stabilization depends on the concentration and nature of the cationic guest.► The kinetic stability of cationic mixed liposomes was investigated fluorimetrically. ► Rate constants for spontaneous and induced breakdown of liposomes have been obtained. ► EPR has been used to monitor the molecular dynamics of liposomal membranes. ► The insertion into the bilayer of the cationic guest enhances liposome stability. ► The stabilization depends on the concentration and nature of the cationic guest.
Biophysical Journal, 1997
Phospholipids with covalently attached poly(ethylene glycol) (PEG lipids) are commonly used for the preparation of long circulating liposomes. Although it is well known that lipid/PEG-lipid mixed micelles may form above a certain critical concentration of PEG-lipid, little is known about the effects of PEG-lipids on liposome structure and leakage at submicellar concentrations. In this study we have used cryogenic transmission electron microscopy to investigate the effect of PEG -PE on aggregate structure in preparations of liposomes with different membrane compositions. The results reveal a number of important aggregate structures not documented before. The micrographs show that enclosure of PEG-PE induces the formation of open bilayer discs at concentrations well below those where mixed micelles begin to form. The maximum concentration of PEG-lipid that may be incorporated without alteration of the liposome structure depends on the phospholipid chain length, whereas phospholipid saturation or the presence of cholesterol has little or no effect. The presence of cholesterol does, however, affect the shape of the mixed micelles formed at high concentrations of PEG-lipid. Threadlike micelles form in the absence of cholesterol but adapt a globular shape when cholesterol is present.