Preparation and Characterization of Vesicles from Mono- n -alkyl Phosphates and Phosphonates (original) (raw)
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The effects of chain flexibility on the properties of vesicles formed from di-n-alkyl phosphates
Recueil des Travaux Chimiques des Pays-Bas, 1996
This paper describes a study of the effects of chain flexibility and chain packing on the properties of vesicles formed from sodium di-n-alkyl phosphates. Three di-n-alkyl phosphates with a constant chain length but with different degrees of unsaturation have been synthesized: dioleyl phosphate (DOP) and dielaidyl phosphate (DEP) having, respectively, a cis and a trans double bond at C-9 and the saturated distearyl phosphate (DSP). These surfactants form vesicles, as confirmed by transmission electron microscopy (EM). The gel to liquid-crystalline phase transition was studied using both fluorescence polarization and differential scanning microcalorimetry. According to fluorescence polarization, using trans,trans,trans-1,6-diphenyl-hexa-1,3,5-triene (DPH) as a probe, DSP vesicles undergo a cooperative transition from a gel to a liquid-crystalline state at 72°C. The polarization of the probe captured in DOP or DEP vesicles decreased gradually in the temperature range 0-40°C, indicating a non-cooperative phase transition. It appears that the vesicle bilayer is in a gel state below 0°C and in a liquid-crystalline state above 40°C. A temperature-dependent 31 P NMR study failed to identify the exact phase-transition temperature. The effect of the fusogenic cation Ca 2+ was studied qualitatively using EM. Calcium induces fusion of DEP vesicles but, within a short time, tubules are formed which are most probably anhydrous crystals of the calcium salts. For DSP vesicles the latter process is extremely fast and fused vesicles cannot be detected. In contrast, DOP vesicles fuse under the influence of calcium, but no crystallization takes place. The fused DOP vesicles are stable for more than one week in the presence of 4 mM Ca 2+ stored at room temperature or at 60°C. Addition of EDTA to DOP vesicles leads to chelation of the calcium ions and to a transition to multilamellar vesicles.
Journal of Polymer Science Part A: Polymer Chemistry, 1995
Certain phosphate bipolar amphiphiles, both monomeric (I and 11) and polymeric or rather oligomeric (poly-I and poly-11), were used as basic materials for the preparation of simple and mixed vesicles. Specifically, it was found that oligomeric phosphate bipolar amphiphiles form stable vesicles in aqueous media. The same oligomeric bolaamphiphiles in mixture with their monomeric counterparts also form stable mixed vesicles with sonication; they are relatively less stable with the "thin film method." Furthermore, it was shown that the method of spanning the membrane of didodecylphosphate vesicles with the dipolar amphiphile I1 is not effective for enhancing stability.
pH effects on properties of dihexadecyl phosphate vesicles
The Journal of Physical Chemistry, 1991
Vesicle size (S) and bilayer structure and function as well as colloidal stability were assessed in large dihexadecyl phosphate (DHP) vesicles on the basis of turbidity spectra, phase transition temperature (T,) determinations, water permeation rates (us), and initial flocculation rates (uo), respectively, for a range of NaCl concentration (C) or pH. S, T,, uI, and log l/vo display a bell-shaped profile as a function of pH for a given C. In water, the maximal S, T,, us, and log l/uo values occur at pH 6.0-7.0, i.e., around the apparent pK value of the phosphate polar head at the membranelwater interface. The results emphasize the importance of the extent of surface hydration in determining the total surface area of the dispersion and thereby the vesicle size and phase behavior.
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.
Chemical communications (Cambridge, England), 2014
This article deals with artificial vesicles and their membranes as reaction promoters and regulators. Among the various molecular assemblies which can form in an aqueous medium from amphiphilic molecules, vesicle systems are unique. Vesicles compartmentalize the aqueous solution in which they exist, independent on whether the vesicles are biological vesicles (existing in living systems) or whether they are artificial vesicles (formed in vitro from natural or synthetic amphiphiles). After the formation of artificial vesicles, their aqueous interior (the endovesicular volume) may become - or may be made - chemically different from the external medium (the exovesicular solution), depending on how the vesicles are prepared. The existence of differences between endo- and exovesicular composition is one of the features on the basis of which biological vesicles contribute to the complex functioning of living organisms. Furthermore, artificial vesicles can be formed from mixtures of amphiph...
Aggregation of phospholipid vesicles by water-soluble polymers
Biophysical Journal, 1996
Water-soluble polymers such as dextran and polyethylene glycol are known to induce aggregation and size growth of phospholipid vesicles. The present study addresses the dependence of these processes on vesicle size and concentration, polymer molecular weight, temperature, and compartmentalization of the vesicles and polymers, using static and dynamic light scattering. Increasing the molecular weight of the polymers resulted in a reduction of the concentration of polymer needed for induction of aggregation of small unilamellar vesicles. The aggregation was fully reversible (by dilution), within a few seconds, up to a polymer concentration of at least 20 wt %. At relatively low phosphatidylcholine (PC) concentrations (up to -1 mM), increasing the PC concentration resulted in faster kinetics of aggregation and reduced the threshold concentration of polymer required for rapid aggregation (C4). At higher PC concentrations, CA was only slightly dependent on the concentration of PC and was approximately equal to the overlapping concentration of the polymer (C*). The extent of aggregation was similar at 37 and 40C. Aggregation of large unilamellar vesicles required a lower polymer concentration, probably because aggregation occurs in a secondary minimum (without surface contact). In contrast to experiments in which the polymers were added directly to the vesicles, dialysis of the vesicles against polymer-containing solutions did not induce aggregation. Based on this result, it appears that exclusion of polymer from the hydration sphere of vesicles and the consequent depletion of polymer molecules from clusters of aggregated vesicles play the central role in the induction of reversible vesicle aggregation. The results of all the other experiments are consistent with this conclusion.
Biochimica et Biophysica Acta (BBA) - Biomembranes, 2002
We tested the ability of saturated n-monocarboxylic acids ranging from eight to 12 carbons in length to self-assemble into vesicles, and determined the minimal concentrations and chain lengths necessary to form stable bilayer membranes. Under defined conditions of pH and concentrations exceeding 150 mM, an unbranched monocarboxylic acid as short as eight carbons in length (n-octanoic acid) assembled into vesicular structures. Nonanoic acid (85 mM) formed stable vesicles at pH 7.0, the pK of the acid in bilayers, and was chosen for further testing. At pH 6 and below, the vesicles were unstable and the acid was present as droplets. At pH ranges of 8 and above clear solutions of micelles formed. However, addition of small amounts of an alcohol (nonanol) markedly stabilized the bilayers, and vesicles were present at significantly lower concentrations (V20 mM) at pH ranges up to 11. The formation of vesicles near the pK a of the acids can be explained by the formation of stable RCOO 3 THOOCR hydrogen bond networks in the presence of both ionized and neutral acid functions. Similarly, the effects of alcohols at high pH suggests the formation of stable RCOO 3 THOR hydrogen bond networks when neutral RCOOH groups are absent. The vesicles provided a selective permeability barrier, as indicated by osmotic activity and ionic dye capture, and could encapsulate macromolecules such as DNA and a protein. When catalase was encapsulated in vesicles of decanoic acid and decanol, the enzyme was protected from degradation by protease, and could act as a catalyst for its substrate, hydrogen peroxide, which readily diffused across the membrane. We conclude that membranous vesicles produced by mixed short chain monocarboxylic acids and alcohols are useful models for testing the limits of stabilizing hydrophobic effects in membranes and for prebiotic membrane formation. ß 2002 Elsevier Science B.V. All rights reserved.
Langmuir, 2005
Two N-acyl amino acid surfactants, sodium N-(11-acrylamidoundecanoyl)-glycinate (SAUG) and L-alaninate (SAUA), were synthesized and characterized in aqueous solution. A number of techniques, such as surface tension, fluorescence probe, light scattering, and transmission electron microscopy were employed for characterization of the amphiphiles in water. The surface and interfacial properties were measured. The amphiphiles have two critical aggregation concentrations. The results of surface tension and fluorescence probe studies suggested formation of bilayer self-assemblies in dilute aqueous solutions of the amphiphiles. The magnitudes of free energy change of aggregation have indicated that bilayer formation is more favorable in the case of SAUG. Steady-state fluorescence measurements of pyrene and 1,6-diphenyl-1,3,5-hexatriene (DPH) were used to study the microenvironment of the molecular selfassemblies. Temperature-dependent fluorescence anisotropy change of DPH probe revealed phase transition temperature of the bilayer self-assemblies. The effects of pH on the structure of the self-assemblies of SAUG and SAUA have been studied. The role of intermolecular hydrogen bonding between amide groups upon aggregation toward microstructure formation in solution has been discussed. Circular dichroism spectra suggested the presence of chiral aggregates in an aqueous solution of SAUA. The transmission electron micrographs revealed the presence of closed spherical vesicles in aqueous solutions of the amphiphiles. Dynamic light scattering measurements were performed to obtain average size of the aggregates. Kawasaki, H.; Souda, M.; Tanaka, S.; Nemoto, N.; Karlsson, G.; Almgren, M.; Maeda, H.
Size, Electrophoretic Mobility, and Ion Dissociation of Vesicles Prepared with Synthetic Amphiphiles
Journal of Physical Chemistry, 1990
Vesicles prepared with synthetic amphiphiles (dioctadecyldimethylammonium bromide and chloride, dihexadecyl phosphate and its sodium salt) were obtained by sonication, ethanol injections, and chloroform injections. The hydrodynamic diameter of vesicles (Dh), estimated from the diffusivity measured by quasielastic light scattering, ranged from 230 to 3000 A. The electrophoretic mobility (U,) was measured by free-flow electrophoresis. The zeta potential (l) and the degree of counterion dissociation (a) of the vesicles were calculated from U, and conductivity data. a decreased with increasing Dh of the vesicles, probably due to the decreasing headgroup area and the increasing counterion association needed to relax the surface electrostatic potential. The electrophoretic mobility was also calculated (U,) according to an impenetrable, nonconducting sphere model with a spherically symmetric charge distribution approximation. Within the limits of the experimental error(s) of the (different) methods employed and the assumptions made in the calculations, the fact that the UJU, ratio ranged from 1.3 to 7.5 was considered to be a good agreement between the calculated and the experimental values.