Salt-induced aggregation and fusion of dioctadecyldimethylammonium chloride and sodium dihexadecylphosphate vesicles (original) (raw)
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Biochimica Et Biophysica Acta-biomembranes, 2000
Fusion of vesicles with the air–water interface and consequent monolayer formation has been studied as a function of temperature. Unilamellar vesicles of DMPC, DPPC, and DODAX (X=Cl−, Br−) were injected into a subphase containing NaCl, and the surface pressure (tension) was recorded on a Langmuir Balance (Tensiometer) using the Wilhelmy plate (Ring) method. For the zwitterionic vesicles, plots of the initial surface pressure increase rate (surface tension decrease rate) as a function of temperature show a peak at the phase transition temperature (Tm) of the vesicles, whereas for ionic ones they show a sharp rise. At high concentrations of NaCl, ionic DODA(Cl) vesicles seem to behave like zwitterionic ones, and the rate of fusion is higher at the Tm. The influence of size was studied comparing large DODA(Cl) vesicles with small sonicated ones, and no significant changes were found regarding the rate of fusion with the air–water interface.
Spontaneous Vesicles Formed in Aqueous Mixtures of Two Cationic Amphiphiles
Langmuir, 2000
The spontaneous formation of vesicles was detected in aqueous mixtures of two cationic amphiphiles: the double-tailed didodecyldimethylammonium bromide, DDAB, and the single-tailed dodecyltrimethylammonium chloride, DTAC. These aggregates appear in a high-dilution region of the system, intermediate between those where monomers and micelles prevail, and are most easily formed at a considerable excess of the single-chained surfactant (DTAC/DDAB molar ratios ≈ 2-20). Vesicles were characterized at 25°C by cryogenic transmission electron microscopy (cryo-TEM) and dynamic light-scattering (DLS) measurements: they present a well-defined contour, are mostly spherical and unilamellar, but show a large size polydispersity, with the more frequent population distributions of diameters from ≈40-50 to ≈500-600 nm. Apart from intact vesicles (and in most cases coexisting with them), vesicles with ruptured membranes, small bilayer disks (probably discoidal micelles, rarely found), and globular micelles were also visualized by cryo-TEM. Ruptured vesicles and disks were assigned to intermediate structures between intact vesicles and globular micelles. We propose that the main factor which drives the appearance of vesicles in this bicationic system is the difference in the spontaneous curvature (or packing parameter) of the two long-chained surfactant ions.
Cationic, O-alkylphosphatidylcholines, recently developed as DNA transfection agents, form bilayers indistinguishable from those of natural phospholipids and undergo fusion with anionic bilayers. Membrane merging (lipid mixing), contents release, and contents mixing between populations of positive vesicles containing O-ethylphosphatidylcholine (EDOPC) and negative vesicles containing dioleolylphosphatidylglycerol (DOPG) have been determined with standard fluorometric vesiclepopulation assays. Surface-charge densities were varied from zero to full charge. All interactions depended critically on surface-charge density, as expected from the adhesion-condensation mechanism. Membrane mixing ranged from zero to 100%, with significant mixing (>10 <70%) occurring between cationic vesicles that were fully charged and anionic vesicles that had fractional surface charges as low as 0.1. Such mixing with membranes as weakly charged as cell membranes should be relevant to transfection with cationic lipids. Unexpectedly, lipid mixing was higher at high than at low ionic strength when one lipid dispersion was prepared from EDOPC plus DOPG (in different proportions), especially when the other vesicles were of EDOPC; this may somehow be a consequence of the ability of the former mixture to assume non-lamellar phases. Leakage of aqueous contents was also a strong function of charge, with fully charged vesicles releasing essentially all of their contents less than 1 min after mixing. EDOPC was more active in this regard than was DOPG, which probably reflects stronger intermolecular interactions of DOPG. Fusion, as measured by contents mixing, exhibited maximal values of 10% at intermediate surface charge. Reduced fusion at higher charge is attributed to multiple vesicle interactions leading to rupture. The existence of previously published data on individual interactions of vesicles of the same composition made it possible for the first time to compare pairwise with population interactions, confirming the likelihood of population studies to overestimate rupture and hemifusion and underestimate true vesicle fusion.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1981
We have investigated the contribution of various phospholipids to membrane fusion induced by divalent cations. Fusion was followed by means of a new fluorescence assay monitoring the mixing of internal aqueous contents of large (0.1 #m diameter) unilamellar liposomes. The rate and extent of fusion induced by Ca 2÷ in mixed phosphatidylserine/phosphatidylcholine vesicles were lower compared to those in pure phosphatidylserine vesicles. The presence of 50% phosphatidylcholine completely inhibited fusion, although the vesicles aggregated upon Ca 2÷ addition. When phosphatidylserine was mixed with phosphatidylethanolamine, however, rapid fusion could be induced by Ca 2÷ even in mixtures that contained only 25% phosphatidylserine. Phosphatidylethanolamine also facilitated fusion by Mg 2÷ which could not fuse pure phosphatidylserine vesicles. In phosphatidylserine/phosphatidylethanolamine/phosphatidylcholine mixtures, in which the phosphatidylcholine content was kept at 25%, phosphatidylethanolamine could not substitute for phosphatidylserine, and the fusogenic capacity of Mg 2÷ was abolished by the presence of merely 10% phosphatidylcholine. The initial rate of relelase of vesicle contents was slower than the rate of fusion in all the mixtures used. The presence of phosphate effected a considerable decrease in the threshold concentration of Ca 2÷ and also enhanced
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1988
Large unilamellar vesicles (REV) containing phosphatidylserine and phosphatidylethanolamine at a ratio of 1:3 were induced to fuse by adding calcimn (4 mM). The kinetics of fusion was monitored by fluorometry using terbium or dipicolinic acid-containing vesicles. The morphology and the states of vesicle aggregation and fusion were examined at approx. 2, 30, 60, 150 and 900 s after calcium addition, by rapid quenching and freeze-fracture electron microscopy. The size and the state of aggregation of vesicles are quantitated from 4000 randomly selected vesicles. The aggregation and fusion kinetics as assayed by fluorescence volume mixing is very well simulated and predicted by the mass action model. The model essentially predicts the time course of the distribution of the aggregates and the increase in size of fused particles as measured by electron microscopy, although in some cases the predicted fusion rate exceeds that by morphometric measurement. No morphological features can be defined as fusion intermediates, although bead-Uke and rim-like materials may be attributed to the remnants of broken diaphragms between fusion partners.
Effect of Hydration and Packing Order on Large Unilamellar Vesicle Fusion: The Role of Cholesterol
Journal of the Chilean Chemical Society, 2017
Several studies examining vesicle fusion have been reported in last decades and have established a number of factors favoring the process of vesicle fusion. To determine whether variations to the physicochemical properties of the membrane affect the process of vesicle fusion, we worked with binary and ternary mixtures of large unilamellar vesicles (LUVs). The selected binary models were dioleoyl phosphocholine-cholesterol (DOPC-chol) and disteraroyl phosphocholine-cholesterol (DSPC-chol), and the tertiary mixtures were phosphatidylcholine-phsophatidylethanolamine-cholesterol (PC-PE-Chol); phosphatidylcholine-sphingomyelincholesterol (PC-SM-Chol); and phosphatidylcholine-phosphatidylserine-cholesterol (PC-PS-Chol). For all these models, the effect of cholesterol content on the lamella physicochemical properties was determined using 1,6-diphenyl-1,3,5-hexatriene (DPH) anisotropy, generalized polarization of 2-dimethylamino-6-lauroylnaphthalene (Laurdan), and DPH fluorescence lifetime. To determine whether fusion of these vesicles varied according to lipid composition, the % mixing content and the % leakage were determined. Examining membrane incorporation using fluorescence steady-state and time-resolved probe assays in the models indicated that cholesterol content affected packing order and lamellar hydration. In most of the models, nonmonotonic variations were observed for these parameters, and these variations could be interpreted as increases in the proportion of ordered microdomains. When the proportion of these domains is higher, the packing order increases, and the lamellar water decrease. Similarly, the % mixing, which was assessed as a fusion parameter, also exhibited nonmonotonic behavior, indicating that the fusion process is enhanced at these concentrations of cholesterol. However, DSPC vesicles do not merge, so more than the presence of microdomains is required to stabilize fusion.
Counterion Effects on Properties of Cationic Vesicles
Langmuir, 1998
The effect of counterion nature and concentration on phase transition, bilayer structure, vesicle size, vesicle internal volume per mole of amphiphile, and surface potential is evaluated for cationic vesicles composed of dioctadecyldimethylammonium (DODA) acetate, chloride, or bromide. Over a range of ionic strengths (0-5 mM monovalent salt), no interdigitation was detected in the bilayer structure for the three DODA counterions. The preferential type of aggregate formed from self-assembly of DODA salts is a large vesicle composed of a single traditional and noninterdigitated bilayer. Vesicle size and zeta-potentials were inversely related, i.e., an increase in zeta-potential was accompanied by a decrease in vesicle size. The largest zeta-potentials and smallest sizes were obtained for bilayer vesicles of DODA acetate which have the largest and more hydrated counterion. The effect of ionic strength (0-5 mM NaAc, NaCl, or NaBr as monovalent salt) was a slight decrease followed by a significant increase in vesicle size as a function of salt concentration. The results for counterion effects on vesicle size agree with predictions from the selfassembly model by Israelachvili and co-workers.