The effects of chain flexibility on the properties of vesicles formed from di-n-alkyl phosphates (original) (raw)
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Preparation and Characterization of Vesicles from Mono- n -alkyl Phosphates and Phosphonates
The Journal of Physical Chemistry B, 1997
The aggregation properties of different linear, single-chain alkyl phosphates and phosphonates in water were investigated at concentrations of up to 50 mM as a function of pH, focusing in particular on spontaneous vesicle formation. Under conditions where about half the molecules are monoionic and half the molecules are completely protonated (pH ≈ 2), n-dodecylphosphoric acid, n-decylphosphonic acid, and n-dodecylphosphonic acid spontaneously form vesicles at room temperature. For n-hexadecylphosphoric acid, stable vesicles only form above ∼40°C. The presence of vesicles was evidenced by light and electron microscopy and in the case of n-dodecylphosphoric acid by entrapment experiments using as water soluble probes glucose, dextran, and pepsin. The phase-transition temperature of vesicles of n-dodecylphosphoric acid was 2.3°C, as determined by differential scanning calorimetry. For n-hexadecylphosphoryladenosine evidence for micelle formation has been obtained with a cmc of 20-50 µM at 25°C. In an experimental extension of the vesicle selfreproduction principles to phosphoamphiphiles, results are also presented on the alkaline hydrolysis of the water-insoluble din -decyl-4-nitrophenyl phosphate, which led to the formation of 4-nitrophenol and di-ndecyl phosphate, the latter being a known vesicle-forming amphiphile.
Biophysical Journal, 1986
Small dioctadecyldimethylammonium chloride (DODAC) vesicles prepared by sonication fuse upon addition of NaCl as detected by several methods (electron microscopy, trapped volume determinations, temperaturedependent phase transition curves, and osmometer behavior. In contrast, small sodium dihexadecyl phosphate (DHP) vesicles mainly aggregate upon NaCl addition as shown by electron microscopy and the lack of osmometer behavior. Scatter-derived absorbance changes of small and large DODAC or DHP vesicles as a function of time after salt addition were obtained for a range of NaCl or amphiphile concentration. These changes were interpreted in accordance with a phenomenological model based upon fundamental light-scattering laws and simple geometrical considerations. Short-range hydration repulsion between DODAC (or DHP) vesicles is possibly the main energy barrier for the fusion process.
Journal of the American Chemical Society, 1997
Three different stereoisomers of a phosphatidic acid analog bearing two phosphate groups have been synthesized from tartaric acid and erythritol. The obtained diastereomeric gemini surfactants differ in their aggregation behavior due to the different spatial orientations of their functional groups. The most remarkable difference in aggregation behavior is encountered when calcium ions are added to vesicle suspensions of the respective isomers. The vesicles formed from the (S,S) and (R.R) isomer undergo fusion, whereas those of the (R,S) isomer show vesicle fission. This remarkable behavior can be explained by a change in the molecular packing of the lipid molecules upon the complexation with calcium ions. An analysis of physical data obtained prior to and after the addition of the calcium ions reveals that the head groups of the diastereomeric surfactants respond differently to these ions: those of the (S,S) isomer increase in size, whereas those of the {R,S) isomer decrease in size. This phenomenon accounts for occurrence of fusion and fission of the vesicles, respectively.
Characterization of DODAB/DPPC vesicles
Chemistry and Physics of Lipids, 2008
Dioctadecyldimethylammonium bromide (DODAB)/dipalmitoylphosphatidylcholine (DPPC) large and cationic vesicles obtained by vortexing a lipid film in aqueous solution and above the mean phase transition temperature (T m ) are characterized by means of determination of phase behaviour, size distribution, zeta-potential analysis and colloid stability. The effect of increasing % DODAB over the 0-100% range was a nonmonotonic phase behaviour. At 50% DODAB, the mean phase transition temperature and the colloid stability were at maximum. There is an intimate relationship between stability of the bilayer structure and colloid stability. In 1, 50 and 150 mM NaCl, the colloid stability for pure DPPC or pure DODAB vesicles was very low as observed by sedimentation or flocculation, respectively. In contrast, at 50% DODAB, remarkable colloid stability was achieved in 1, 50 or 150 mM NaCl for the DODAB/DPPC composite vesicles. Vesicle size decreased but the zeta-potential remained constant with % DODAB, due to a decrease of counterion binding with vesicle size. This might be important for several biotechnological applications currently being attempted with cationic bilayer systems.
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.
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
The Role of Lateral Tension in Calcium-Induced DPPS Vesicle Rupture
Langmuir, 2012
We assess the role of lateral tension in rupturing anionic dipalmitoylphosphatidyserine (DPPS), neutral dipalmitoylphosphatidylcholine (DPPC), and mixed DPPS−DPPC vesicles. Binding of Ca 2+ is known to have a significant impact on the effective size of DPPS lipids and little effect on the size of DPPC lipids in bilayer structures. In the present work we utilized laser transmission spectroscopy (LTS) to assess the effect of Ca 2+ -induced stress on the stability of the DPPS and DPPC vesicles. The high sensitivity and resolution of LTS has permitted the determination of the size and shape of liposomes in solution. The results indicate a critical size after which DPPS single shell vesicles are no longer stable. Our measurements indicate Ca 2+ promotes bilayer fusion up to a maximum diameter of ca. 320 nm. These observations are consistent with a straightforward free-energy-based model of vesicle rupture involving lateral tension between lipids regulated by the binding of Ca 2+ . Our results support a critical role of lateral interactions within lipid bilayers for controlling such processes as the formation of supported bilayer membranes and pore formation in vesicle fusion. Using this free energy model we are able to infer a lower bound for the area dilation modulus for DPPS (252 pN/nm) and demonstrate a substantial free energy increase associated with vesicle rupture.