Dipalmitoylphosphatidylcholine Vesicles Research Articles

Mechanism of Action of Thymosinα1: Does It Interact with Membrane by Recognition of Exposed Phosphatidylserine on Cell Surface? A Structural Approach.

Thymosinα1 is a peptidic hormone with pleiotropic activity, which is used in the therapy of several diseases. It is unstructured in water solution and interacts with negative regions of micelles and vesicles assuming two tracts of helical conformation with a structural flexible break in between. The studies of the interaction of Thymosinα1 with micelles of mixed dipalmitoylphosphatidylcholine and sodium dodecylsulfate and vesicles with mixed dipalmitoylphosphatidylcholine/dipalmitoylphosphatidylserine, the latter the negative component of the membranes, by (1)H and natural abundance (15)N NMR are herewith reported, reviewed, and discussed. The results indicate that the preferred interactions are those where the surface is negatively charged due to sodium dodecylsulfate or due to the presence of dipalmitoylphosphatidylserine exposed on the surface. In fact the unbalance of dipalmitoylphosphatidylserine on the cellular surface is an important phenomenon present in pathological conditions of cells. Moreover, the direct interaction of Thymosinα1 with K562 cells presenting an overexposure of phosphatidylserine as a consequence of resveratrol-induced apoptosis was carried out.

250 - Calcium-Independent Phospholipase A2y Is Reversibly Activated by H2O2 Following Direct Thiol Oxidation

The fusion of sonicated dipalmitoylphosphatidylcholine (DPPC) vesicles was studied by gel-exclusion chromatography as a function of temperature, permeable and impermeable solute concentration, and cholesterol content of the bilayer membrane. Fusion is faster at lower temperatures: there is no fusion at or above 35.5°C (0.10 M DPPC/0.1 M K2SO4/0.01 M Hepes buffer (pH 7.4)/0.02% NaN3). There is about 10% fusion after 1 week at 30°C, and about 60% fusion after 2 days at 13–25°C. Between 13 and 8°C, the fusion product changes from 700-Å-diameter vesicles to the 950-Å vesicles previously reported by Wong et al. (Wong, M., Anthony, F.W., Tillack, T.W. and Thompson, T.E. (1982) Biochemistry 21, 4126–4132). At 1°C, fusion is about 90% complete after 1 day. Membrane-impermeable solutes (NaCl, trehalose and glucose) inhibit fusion in a manner reflecting the total particle concentration. There is no detectable fusion after 3 days (22°C) in either 1.0 M NaCl or 2.0 M sugar, the highest concentrations studied. A suggested explanation is that impermeable solutes osmotically inhibit the influx of solution that accompanies the fusion of vesicles to form a larger vesicle, and, could conceivably thereby inhibit the fusion reaction. By contrast, membrane-permeable solutes (glycerol, ethylene glycol, propylene glycol and ethanol) dramatically increase the fusion rate. 1.0 M ethanol causes 100% fusion in 15–30 min at 22°C. The simultaneous presence of 0.15 M NaCl entirely negates the fusion-promoting effect of 1.0 M ethanol. 1 mol% cholesterol completely inhibits fusion in 0.1 M KCl (20°C), and greatly slows it down either in 1.0 M ethanol at 20°C or in 0.1 M KCl at 4°C. A suggested mechanism is that cholesterol might concentrate in and stabilize bilayer lattice defect sites that are critical for the fusion reaction. The trapping efficiency of vesicles formed by the fast ethanol fusion conditions in the presence of the water-soluble markers, chromate and arsenazo III, ranged from 9.0 to 12.7% of the marker captured in the vesicles, corresponding to trapped volumes of 1.8 to 2.5 1/mol DPPC. Bromophenol blue gave anomalously high values of 67% and 13 1/mol DPPC, which presumably reflect binding, in addition to encapsulation.

Properties of calcium carbonate precipitated in the presence of DPPC liposomes modified with the phospholipase A2

Small unilamellar dipalmitoylphosphatidylcholine (DPPC) vesicles of average diameter 119 nm were used as a template for calcium carbonate precipitation. The precipitation was carried out from CaCl2 and Na2CO3 solutions at room temperature 25 °C and physiological temperature 37 °C. Moreover, liposomes modified with the enzyme phospholipase A2 (PLA2) were also used as an organic matrix. The influence of liposomes on the precipitation of calcium carbonate has been investigated using scanning electron microscopy and X-ray diffraction measurements. The particles surface modification was evidenced using zeta potential measurements and thermogravimetric analysis. The obtained results show that the effect of DPPC vesicles on the CaCO3 precipitation is concentration dependent. The stability of vaterite increases with the increase in liposomes content in the solutions. At the liposomes concentration equal to 100 mg L−1 a mixture of precipitated vaterite and calcite is observed. The temperature increase weakens this effect. Moreover, the enzyme PLA2 action has no significant influence on the calcium carbonate precipitation in the presence of DPPC liposomes at both investigated temperatures.

Trehalose-Induced Variation in Mechanical Properties of Vesicles in Aqueous Solution.

The effect of the trehalose incorporation on the nanomechanical properties of dipalmitoylphosphatidylcholine vesicles was studied using atomic force microscope (AFM) on mica surface. The vesicles were prepared only with the variation in the trehalose concentration and adsorbed on the mica surface. After the morphology of the adsorbed vesicles was characterized, the behavior of an AFM tip into the vesicle was monitored using the plot of the tip displacement versus the tip deflection. It was observed that the breakthrough of the tip into the vesicles occurred two times. Each breakthrough represented each penetration of the tip into each layer. Force data prior to the first breakthrough fitted well with the Hertzian model to estimate Young's modulus and bending modulus of the vesicles. It was found that the Young's modulus and bending modulus decreased proportionally to the increase in the trehalose concentration up to 0.5 of trehalose to lipid. However, above 0.5, the moduli were a little varied with the increase. In the identical measurements at glucose, just a slight change in the moduli was observed with the increase in the glucose composition from 0 % glucose up to even 2:1 ratio of glucose:lipid. These results in the mechanical properties seem attributable to the osmotic and volumetric effects on the headgroup packing disruption.

GUVs Melt Like LUVs: The Large Heat Capacity of MLVs Is Not Due to Large Size or Small Curvature

The excess heat capacity functions (ΔCp) associated with the main phase transition of large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs) are very different. Two explanations are possible. First, the difference in vesicle size (curvature) results in different gel-fluid interactions in the membrane; those interactions have a large effect on the cooperativity of the phase transition. Second, there is communication between the bilayers in an MLV when they undergo the gel-fluid transition; this communication results in thermodynamic coupling of the phase transitions of the bilayers in the MLV and, consequently, in an apparent increase in the cooperativity of the transition. To test these hypotheses, differential scanning calorimetry was performed on giant unilamellar vesicles (GUVs) of pure dipalmitoylphosphatidylcholine. The ΔCp curve of GUVs was found to resemble that of the much smaller LUVs. The transition in GUVs and LUVs is much broader (half-width ∼1.5°C) than in MLVs (∼0.1°C). This similarity in GUVs and LUVs indicates that their size has little effect on gel-fluid interactions in the phase transition. The result suggests that coupling between the transitions in the bilayers of an MLV is responsible for their apparent higher cooperativity in melting.

Influence of Vesicle Size and Aqueous Solvent on Intact Phospholipid Vesicle Adsorption on Oxidized Gold Monitored Using Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy

Through attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), we have studied the adsorption characteristics of small unilamellar vesicles (SUVs) of a gel-phase phospholipid, dipalmitoylphosphatidylcholine (DPPC), on an oxidized gold substrate. By monitoring the frequencies and intensities of vibrational absorption modes due to phosphate and methylene functional groups in the head and tail regions of the phospholipid, we differentiated the adsorption state of the precursor vesicles (i.e., intact vs ruptured vesicles) as a function of vesicular size in the SUV limit and the properties of the aqueous-phase solvent. We found that on oxidized gold, vesicles of DPPC in ultra pure water remained intact for all sizes tested (viz., 65, 80, and 160 nm) with varying degree of deformation. In contrast, when phosphate-buffered saline (PBS) was used as a bathing medium, all vesicles remained intact, were more distorted than the same size in pure water, and appeared to be nearly fully collapsed. Taken together, these results provide a rough guide for controlling vesicular behavior at the oxidized gold surface.

Synergistic effect of thermal and pulsed electric field (PEF) treatment on the permeability of soya PC and DPPC vesicles

The permeability change of vesicles composed of soya phosphatidylcholine (PC) and dipalmitoyl phosphatidylcholine (DPPC) encapsulated with 5(6)-carboxyfluorescein (CF) suspended in isotonic sucrose solution under separate or combined thermal and continuous pulsed electric field (PEF) treatments was investigated, respectively. The electric field intensity applied was in the range from 20 to 40kV/cm with total treatment time of 1.2ms, and thermal treatments were conducted at temperatures from 15 to 65°C. Results indicated that the permeability behavior between DPPC vesicles and soya PC vesicles were quite different under the experimental conditions. Compared to soya PC vesicles, DPPC vesicles were more sensitive to thermal treatments. As the temperature increased to 41°C, the 5(6)-CF release percentage (R) reached almost 100% abruptly. On the other hand, the synergetic effects of PEF and temperature on permeability of two vesicles were apparently different. For soya PC vesicles, the increment degree of R value was almost the same under various combination treatment conditions. However, for DPPC vesicles, it was demonstrated that the lipid bilayers membrane was almost impermeable to CF when the PEF strength was lower than 30kV/cm at 20°C and 20kV/cm at 30°C. As the temperature increased to 38°C, the permeability of DPPC vesicles was significantly increased with increasing PEF intensity, reaching 25.25% at 40kV/cm. Results from the current study could be helpful for better understanding of the underlying mechanism on PEF inactivation as affected by the lipid composition of cell membrane and its phase transition temperature.

Chain interdigitation in DPPC bilayers induced by HgCl2: Evidences from continuous wave and pulsed EPR

Continuous-wave electron paramagnetic resonance (CW-EPR) spectroscopy and electron spin echo methods of pulsed EPR of phosphatidylcholine spin-labeled at different positions, n, in the sn-2 chain (n-PCSL, n=5, 7, 10, 12, 14, and 16) are used to study the interaction of inorganic mercury chloride HgCl2 with multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC). For temperatures through the gel phase of DPPC multilayers, the CW-EPR spectra show that an increase of HgCl2 content in the dispersion medium slightly increases the rotational mobility of 5-PCSL and markedly restricts the motion of 16-PCSL. Mercury chloride at 100mM (HgCl2/lipid molar ratio=2:1) removes the gradient of increasing mobility along the chain found in DPPC bilayers in the gel phase. In contrast, HgCl2 does not influence the DPPC chain flexibility profile in the fluid phase. It also suppresses the pre-transition and moderately downshifts the main transition temperature of DPPC membranes. These findings indicate that HgCl2 affects the lipid chain packing of DPPC bilayers and are consistent with the induction of an interdigitated gel phase. Further, D2O-electron spin echo envelope modulation spectroscopy indicates that in the interdigitated phase a higher water permeation is favored at any chain position and the sigmoidal transmembrane water accessibility profile of DPPC bilayers is abolished. Accordingly, the positional dependence of 14N-hyperfine splitting, 2Azz, shows that the typical hydrophobic barrier of DPPC is significantly altered in the interdigitated phase and all the segments of the lipid chains result to be in a more polar environment.

Amphiphilic adducts of myrcene and N-substituted maleimides as potential drug delivery agents

The title drug delivery compounds with pharmacophoric moieties were synthesized, and their interaction with model biomembranes (dipalmitoylphosphatidylcholine vesicles) was examined.

Surface Dilution Kinetics of Phospholipase A2 Catalyzed Lipid-Bilayer Hydrolysis

Phospholipase A2 (PLA2) enzymes catalyze hydrolysis of phospholipids in membranes. Elucidation of the kinetics of interfacial enzymatic activity is best accomplished by investigating the interface substrate concentration dependence of the activity for which appropriate diluents are required. PLA2 is stereoselective toward the L_enantiomers of phospholipids. A novel approach employing D_phospholipids as diluents to perform surface dilution kinetic studies of PLA2 is presented. Activity of bee venom PLA2 at mixed L+D_DPPC (dipalmitoylphosphatidylcholine) bilayer interfaces was measured as a function of substrate L_DPPC mole fraction and vesicle concentration using a sensitive fluorescence assay. A model for interface enzymatic activity based on the three-step kinetic scheme of (i) binding of PLA2 to the bilayer interface, (ii) binding of a lipid to PLA2 at the interface, and (iii) hydrolysis was applied to the hydrolysis data. Activity profiles showed that D_enantiomers also bind to the enzyme but resist hydrolysis. Activity dependences on vesicle and substrate concentrations could be disentangled, bringing resolution to an outstanding problem in membrane hydrolysis of separating the effects of the three steps. Individual values of the kinetic parameters of the model, including the vesicle-PLA2 equilibrium dissociation constant of step (i), interface Michaelis-Menten-Henri constant for L and D_DPPC of step (ii), and the rate constant for interface hydrolysis, step (iii), were obtained as solutions to equations resulting from fitting the model to the data.

Formulation, characterization and optimization of liposomes containing eicosapentaenoic and docosahexaenoic acids; a methodology approach.

Omega-3 fatty acids (FAs) have been shown to prevent cardiovascular disease. The most commonly used omega-3 fatty acids like eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are highly vulnerable to oxidation and therefore, have short shelf life. Recent advances in nanoliposomes provided a biocompatible system for stabilizing omega-3 FAs. Several methods could be implemented to prepare nanoliposomes. To the best of our knowledge, the performances of these methods in preparation omega-3 FAs have not been examined. Nanoliposomes were prepared by thin film hydration followed by one of the following methods: 1- extrusion, ultrasonic irradiation; 2- bath sonication; 3- probe sonication; or 4- combined probe and bath sonication. The size of liposomes obtained from methods 1 to 4 were 99.7 ± 3.5, 381.2 ± 7.8, 90.1 ± 2.3, and 87.1 ± 4.10 nm with zeta potential being -42.4 ± 1.7, -36.3 ± 1.6, -43.8 ± 2.4, and 31.6 ± 1.9 mV, respectively. The encapsulation efficiency (EE) for DHA was 13.2 ± 1.1%, 26.7 ± 1.9%, 56.9 ± 5.2% and 51.8 ± 3.8% for methods 1 to 4, respectively. The corresponding levels for EPA were 6.5 ± 1.3%, 18.1 ± 2.3%, 38.6 ± 1.8%, and 38 ± 3.7%, respectively. The EE for DHA and EPA of liposomes for both methods 3 and 4 increased significantly (p<0.05). Propanal, as the major volatile product formed during liposomal preparations, amounts from 81.2 ± 4.1 to 118.8 ± 2.3 μg/Kg. The differential scanning calorimetry (DSC) study showed that DHA and EPA influence the phase transition temperature of small unilamellar vesicles (SUVs) of dipalmitoyl phosphatidyl choline (DPPC). Transmission electron microscopy (TEM) images of liposomes stained with uranyl acetate showed that the liposomes were spherical in shape and maintain high structural integrity. In conclusion, probe ultrasound of pre-formed liposomes facilitates significant loading of DHA and EPA into the nanoliposomal membrane.

Morphological and physicochemical characterization of liposomes loading cucurbitacin E, an anti-proliferative natural tetracyclic triterpene

Cucurbitacin E (Cuc E), an oxygenated triterpene molecule, has demonstrated anti-proliferative effect on various cancer cells. Here, we examined the effect of Cuc E on the membrane morphology and properties using differential scanning calorimetry, transmission electron microscopy and atomic force microscopy techniques. Dipalmitoylphosphatidylcholine vesicles were prepared by the thin film hydration method in the absence and presence of Cuc E at molar ratios 100:12 and 100:20. The loading efficiency of Cuc E was found to be higher than 98% upon HPLC analysis. The thermodynamic parameters suggest that Cuc E does not penetrate into the bilayers and interacts with the polar/apolar interface of the lipid membranes. Blank and Cuc E loaded liposomes prepared from a mixture of DPPC/DPPE/DPPG/Cho were imaged by TEM and AFM. Images obtained by TEM revealed unilamellar liposomes for blank and Cuc E loaded liposomes. AFM images showed that the size and the height of Cuc E loaded liposomes were respectively smaller and higher than blank ones. Results suggest that Cuc E produces modifications in the lipid membrane structures.

Hyaluronan and Phospholipid Association in Biolubrication

It is becoming increasingly clear that the outstanding lubrication of synovial joints is achieved by a sophisticated hierarchical structure of cartilage combined with synergistic actions of surface-active components present in the synovial fluid. In this work we focus on the association of two components of the synovial fluid, hyaluronan and dipalmitoyl phosphatidyl choline (DPPC), in bulk solution and at interfaces. We demonstrate that hyaluronan associates with DPPC vesicles and adsorbs to supported DPPC bilayers. The association structures formed at the interface are sufficiently stable to allow sequential adsorption of DPPC and hyaluronan, whereby promoting the formation of thick composite layers of these two components. The lubricating ability of such composite layers was probed by the AFM colloidal probe technique and found to be very favorable with low friction coefficients and high load bearing capacity. With DPPC as the last adsorbed component, a friction coefficient of 0.01 was found up to pressures significantly above what is encountered in healthy synovial joints. Hyaluronan as the last added component increases the friction coefficient to 0.03 and decreases the load bearing capacity somewhat (but still above what is needed in the synovial joint). Our data demonstrate that self-assembly structures formed by hyaluronan and phospholipids at interfaces are efficient aqueous lubricants, and it seems plausible that such self-assembly structures contribute to the exceptional lubrication of synovial joints.

Kinetics of reduction of cis-bis(dodecylamine)bis(1,10-phenanthroline)cobalt(III) perchlorate and cis-bis(dodecylamine)bis(2,2′-bipyridine)cobalt(III) perchlorate by Fe(II) in dipalmitoylphosphatidylcholine vesicles

The kinetics of reduction of the surfactant complex ions cis-[Co(phen)2(C12H25NH2)2]3+ and cis-[Co(bpy)2(C12H25NH2)2]3+ (phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine, C12H25NH2 = dodecylamine) in solutions of unilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) were studied at different temperatures by a spectrophotometric method under pseudo-first-order conditions using an excess of the reductant. Experimentally the reaction was found to be second order and the electron transfer postulated as outer sphere. Below the phase temperature of DPPC the second-order rate constant for this electron-transfer reaction of both the complexes was found to decrease with increasing concentration of the liposome. Above the phase transition temperature the reaction rate was found to increase with increasing concentration of DPPC. Below the phase transfer temperature the results have been explained on the basis of electrostatic double layer repulsion which prevents the vesicles from accumulation of surfactant–cobalt(III) complexes in the interior of the vesicle membrane. Above the phase transfer temperature the release of the surfactant–cobalt(III) complex molecules from the interior core to the exterior surface of the vesicle through the strong hydrophobic effect increases the reaction rate. The main driving force for this phenomenon is considered to be the intervesicular hydrophobic interaction between the surfactant complexes and vesicles surface. We could observe isokinetic relationships for both the complexes of the present study at the phase transfer temperature of DPPC.

Structural Characterization of More Potent Alternatives to HAMLET, a Tumoricidal Complex of α-Lactalbumin and Oleic Acid

HAMLET is a complex of human α-lactalbumin (hLA) with oleic acid (OA) that kills various tumor cells and strains of Streptococcus pneumoniae. More potent protein-OA complexes were previously reported for bovine α-lactalbumin (bLA) and β-lactoglobulin (bLG), and pike parvalbumin (pPA), and here we explore their structural features. The concentration dependencies of the tryptophan fluorescence of hLA, bLA, and bLG complexes with OA reveal their disintegration at protein concentrations below the micromolar level. Chemical cross-linking experiments provide evidence that association with OA shifts the distribution of oligomeric forms of hLA, bLA, bLG, and pPA toward higher-order oligomers. This effect is confirmed for bLA and bLG using the dynamic light scattering method, while pPA is shown to associate with OA vesicles. Like hLA binding, OA binding increases the affinity of bLG for small unilamellar dipalmitoylphosphatidylcholine vesicles, while pPA efficiently binds to the vesicles irrespective of OA binding. The association of OA with bLG and pPA increases their α-helix and cross-β-sheet content and resistance to enzymatic proteolysis, which is indicative of OA-induced protein structuring. The lack of excess heat sorption during melting of bLG and pPA in complex with OA and the presence of a cooperative thermal transition at the level of their secondary structure suggest that the OA-bound forms of bLG and pPA lack a fixed tertiary structure but exhibit a continuous thermal transition. Overall, despite marked differences, the HAMLET-like complexes that were studied exhibit a common feature: a tendency toward protein oligomerization. Because OA-induced oligomerization has been reported for other proteins, this phenomenon is inherent to many proteins.

Photophysical Behavior of 8-Anilino-1-Naphthalenesulfonate in Vesicles of Pulmonary Surfactant Dipalmitoylphosphatidylcholine (DPPC) and Its Sensitivity toward the Bile Salt–Vesicle Interaction

The photophysical behavior of 8-anilino-1-naphthalenesulphonate (ANS) in vesicles of dipalmitoylphosphatidylcholine (DPPC), a pulmonary surfactant, has been carried out in a detailed manner. ANS shows notable variations in fluorescence intensity, lifetime, and anisotropy parameters as it gets into the vesicle. It was found that ANS partitions well into the DPPC bilayer membrane with an estimated partition coefficient of ~2.0 × 10(5). Among the various fluorescence parameters of ANS, fluorescence anisotropy was found to be most responsive to the temperature induced phase change of the bilayer membrane. These interesting fluorescence parameters of ANS were then used to study the hydration of lipid bilayer membrane by submicellar concentration of bile salts. From the steady-state fluorescence intensity and dynamic fluorescence lifetime analyses it is clear that ANS is able to probe the submicellar concentration (≤1 mM) of bile salt induced hydration of lipid bilayer membrane that accompanies expulsion of ANS from the bilayer to the aqueous bulk phase. Lower-temperature shift in the phase transition of DPPC bilayer indicates that fluorescence anisotropy of ANS is sensitive enough to the bile salt induced perturbation in the packed acyl chains of DPPC bilayer and modification in the membrane fluidity. In presence of sodium deoxycholate (NaDC) and sodium cholate (NaC) in DPPC vesicles, ANS experiences restriction in rotational mobility which is evident from the variation in steady-state fluorescence anisotropy and fluorescence anisotropy decay parameters.

Combined Use of Several Fluorescent Membrane Probes to Study the Subgel Phase and the Effects of Cholesterol Thereon

Several lines of evidence have indicated that a highly ordered and condensed phase occurs for phosphatidylcholines at low temperatures. Generation of this lamellar subgel phase (Lc) with dipalmitoylphosphatidylcholine (DPPC) depends on the thermal history of the sample; vesicles must be cooled at ∼4°C for at least 48 h. Similarly, samples exit the Lc phase slowly at the sub-transition temperature (18°C). Consequently, vestiges of Lc behavior can still be detected at 25°C within 15 min of passing through the transition temperature. We used this hysteresis as an assay for detecting the Lc phase with fluorescent membrane probes by recording the emission spectrum or anisotropy on refrigerated vesicles at 15, 25, 35, and 45°C followed by a return to 35, 25, and finally 15°C. In each case, samples were incubated for 10 min at the new temperature before data acquisition. The presence of a Lc or Lc-like phase was recognized by lack of reversal of fluorescent properties at 25 or 15°C. The Lc phase was observed for pure DPPC vesicles by merocyanine 540, prodan, and patman but not laurdan or diphenylhexatriene; fluorescence properties of the latter two probes were completely reversible at all temperatures. Inclusion of cholesterol in the vesicles up to 20 mol% caused a linear reduction in the magnitude of the difference between membrane properties detected by these probes before and after vesicle heating. In contrast to previous reports using other techniques, the lack of reversal was not completely eradicated by cholesterol in the range of 20 to 45 mol% suggesting that an Lc-like phase exists for DPPC/cholesterol mixtures. The distinction between probes that can and cannot detect these phases has implications for interpreting the nature of them.

Effect of simulation temperature on phospholipid bilayer–vesicle transition studied by coarse-grained molecular dynamics simulations

Computer simulations of the spontaneous self-assembly of phospholipid vesicles (liposomes) have found a bicelle intermediate which curls up into a vesicle. Although coarse-grained (or intermediate-resolution) molecular dynamics simulation studies have been conducted at intermediate (323 K) to high (360 K) temperatures, it is unclear if there exists an ‘optimal’ temperature that accelerates the bicelle-to-vesicle transition process yet do not cause large deviation of structural properties as observed from simulations at the lower temperatures. Further, use of a lipid mixture that includes a certain fraction of lipids preferring negative curvature (such as dipalmitoylphosphatidylethanolamine or DPPE) has been shown to again accelerate the transition. But what is the interplay of the temperature and DPPE ratio in accelerating the transition? By conducting a systematic study of simulation temperatures (323 K, 330 K, 340 K, 350 K, 360 K, 380 K, and 400 K) and DPPE percentages (0, 10, 20, and 27) for dipalmitoylphosphatidylcholine (DPPC) bicelles, I found that 340 K and 10% DPPE would give a DPPC : DPPE vesicle (<20 nm diameter) with structural properties close to the reference state (pure DPPC vesicle at 323 K) but at a much reduced computational cost (about 12× lower). This simulation finding has implications in the acceleration of coarse-grained molecular dynamics simulations of large unilamellar vesicles (50–500 nm diameter) for drug delivery applications.

The role of lateral tension in calcium-induced DPPS vesicle rupture.

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.

Self-Assembly of a Peptide Amphiphile Containing l-Carnosine and Its Mixtures with a Multilamellar Vesicle Forming Lipid

The self-assembly of the peptide amphiphile (PA) hexadecyl-(β-alanine-histidine) is examined in aqueous solution, along with its mixtures with multilamellar vesicles formed by DPPC (dipalmitoyl phosphatidylcholine). This PA, denoted C(16)-βAH, contains a dipeptide headgroup corresponding to the bioactive molecule L-carnosine. It is found to self-assemble into nanotapes based on stacked layers of molecules. Bilayers are found to coexist with monolayers in which the PA molecules pack with alternating up-down arrangement so that the headgroups decorate both surfaces. The bilayers become dehydrated as PA concentration increases and the number of layers in the stack decreases to produce ultrathin nanotapes comprised of 2-3 bilayers. Addition of the PA to DPPC multilamellar vesicles leads to a transition to well-defined unilamellar vesicles. The unique ability to modulate the stacking of this PA as a function of concentration, combined with its ability to induce a multilamellar to unilamellar thinning of DPPC vesicles, may be useful in biomaterials applications where the presentation of the peptide function at the surface of self-assembled nanostructures is crucial.