Two-component Vesicles Research Articles

Budding and fission dynamics of two-component vesicles

We studied the shape deformation induced by the phase separation of two-component vesicles using a dissipative particle dynamics simulation. Two types of amphiphiles, which have the same architecture but segregate from each other, are modeled by connecting particles representing the hydrophilic head and hydrophobic tail groups. After vesicle formation using a single component system, some of the amphiphiles are replaced by a second component, and then phase separation on the vesicle is simulated. Under appropriate conditions, typical shape deformations of a vesicle, such as crenated and invaginated shapes, are observed. We demonstrate that the budding and the fission are facilitated by lateral phase separation upon vesicle coupling to an asymmetric transversal distribution of amphiphiles in the bilayer. Moreover, simulations show a possible new pathway for the fission process triggered by cleavage along the domain boundary. Mesoscopic simulation based on a simple coarse-graining model reveals the dynamics of amphiphiles in vesicles that is essentially unpredictable using a conventional continuum model or full atomistic simulation.

Effect of acyl chain mismatch on the contact mechanics of two-component phospholipid vesicle during main phase transition

It has been recently demonstrated that acyl chain mismatch of phospholipid bilayer composed of a binary lipid mixture induces component formation on the lateral plane of the bilayer [Biophys. J. 83 (2002) 1820–1883]. In this report, the contact mechanics of unilamellar vesicles composed of binary dimyristoyl-phosphatidylcholine (DMPC)/dipalmitoyl-phosphocholine (DPPC) mixtures on fused silica and amino-modified substrates is simultaneously probed by confocal-reflectance interference contrast microscopy (C-RICM) and cross-polarized light microscopy during gel to liquid crystalline transition of the lipid bilayer. C-RICM results indicate that the average degree of vesicle deformation for DMPC-rich and DPPC-rich vesicles adhering on fused silica substrate is increased by 30% and 14%, respectively, in comparison with that in pure DMPC and DPPC vesicles. Also, lateral heterogeneity induced by acyl chain mismatch increases the average magnitude of adhesion energy in DMPC-rich and DPPC-rich vesicles of all sizes by 6.4 times and 2.3 times, respectively. Similar modulation of adhesion mechanics induced by carbon chain difference is obtained on amino-modified substrate. Most importantly, the thermotropic transition of the mixed bilayer from gel (below T m ) to fluid phase (above T m ) further exemplifies the effect of acyl chain mismatch on the increases of degree of vesicle deformation and adhesion energy.

Association of a Fluorescent Amphiphile with Lipid Bilayer Vesicles in Regions of Solid–Liquid-Disordered Phase Coexistence

The effects of solid-fluid phase separations on the kinetics of association of a single-chain fluorescent amphiphile were investigated in two different systems: pure DMPC (dimyristoylphosphatidylcholine) and a 1:1 mixture of DMPC and DSPC (distearoylphosphatidylcholine). In pure DMPC vesicles, solid ( s) and fluid ( ℓ d) phases coexist at the phase transition temperature, T m, whereas a 1:1 mixture of DMPC and DSPC shows a stable s- ℓ d phase separation over a large temperature interval. We found that in single-component bilayers, within the main phase transition, the experimental kinetics of association are clearly not single-exponential, the deviation from that function becoming maximal at the T m. This observation can be accounted for by a rate of desorption that is slower than desorption from either fluid or solid phases, leaving the rates of insertion unchanged, but a treatment in terms of stable fluid and solid domains may not be adequate for the analysis of the association of an amphiphile with pure DMPC vesicles at the T m. In DMPC/DSPC mixtures with solid-fluid phase coexistence, association occurs overall faster than expected based on phase composition. The observed kinetics can be described by an increase in the rate of insertion, leaving the desorption rates unchanged. The fast kinetics of insertion of the amphiphile into two-phase bilayers in two-component vesicles is attributed to a more rapid insertion into defect-rich regions, which are most likely phase boundaries between solid and fluid domains. A two-component mixture of lipids that shows a stable phase separation between ℓ d- s phases over a large temperature interval thus behaves very differently from a single-component bilayer at the T m, with respect to insertion of amphiphiles.

Theory of Fission for Two-Component Lipid Vesicles

The coupling between Gaussian curvature and local lipid composition for two-component lipid vesicles can destabilize the narrow neck in a budding transition. Such a coupling reduces the Gaussian rigidity of a membrane and enhances the fission transition if the minor component lipids prefer to stay at regions with large positive Gaussian curvature. On the other hand, it increases the Gaussian rigidity and a fusion transition is enhanced if the minor component lipids are expelled from regions with large positive Gaussian curvature.

Shape deformation and phase separation dynamics of two-component vesicles.

We numerically study dynamics of shape deformations of vesicles resulting from intramembrane phase separation. At off-critical compositions resultant vesicle shapes closely resemble the echinocytosis of red blood cells, while at the critical composition a bicontinuous domain structure emerges. The coarsening process of domain structures on such a deformable surface is considerably slower than that on a rigid surface. The total length of domain boundaries ${N}_{\mathrm{DB}}$ decreases as ${t}^{\ensuremath{-}1/3}$ for the rigid sphere, whereas in the present system ${N}_{\mathrm{DB}}\ensuremath{\simeq}{t}^{\ensuremath{-}z}$ $(z\ensuremath{\simeq}0.1)$ at the late stage due to the coupling between the shape and the phase separation.

Molecular-Thermodynamic Modeling of Mixed Cationic/Anionic Vesicles.

Vesicles are widely used as model cells in biology and medicine and are also potentially useful as drug carriers and other industrial encapsulating devices. To facilitate the practical implementation of vesicles, as well as to gain a fundamental understanding of the process of vesicle formation, we have developed a molecular-thermodynamic theory to describe the formation of two-component mixed vesicles in aqueous solutions. The central quantity in this theory is the free energy of vesiculation, which is calculated by carefully modeling the various free-energy contributions to vesiculation. In particular, we (i) estimate the surfactant-tail packing free energy using a mean-field approach that accounts for the conformations of the surfactant tails in the vesicle hydrophobic region, (ii) adopt a more accurate equation of state in the calculation of the surfactant-head steric repulsions, and (iii) utilize the nonlinear Poisson-Boltzmann equation to calculate the electrostatic interactions in the case of mixed cationic/anionic charged vesicles. Particular attention has also been paid to issues such as the location of the outer and inner steric-repulsion surfaces in a vesicle and the curvature correction to the interfacial tensions at the outer and inner hydrocarbon/water vesicle interfaces. By knowing only the molecular structures of the surfactants involved in vesicle formation and the solution conditions, our theory can predict a wealth of vesicle properties, including vesicle size and composition distribution, surface potentials, surface charge densities, and compositions of vesicle leaflets. More importantly, this theory enables us to gain an understanding of (i) the underlying mechanisms of stabilization in mixed cationic/anionic vesicular systems, (ii) the effect of the interplay between the various intravesicular free-energy contributions on vesiculation, and (iii) the role of the distribution of surfactant molecules between the two vesicle leaflets in vesicle formation. As an illustration, the theory has been applied to describe vesicle formation in an aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium octyl sulfate (SOS). In this system, the vesicles are found to be stabilized entropically, with a predicted mean radius of about 1200 Å for a mixture containing 2 wt % surfactant and a CTAB/SOS weight ratio of 3/7, a value which compares well with the experimentally measured value of 1300 Å. In addition, the predicted outer surface potential of -72 mV is consistent with the measured ζ potential value. The effect of added salt on vesicle properties has also been studied using this theory, and the predicted results conform well to experimental observations.

Curvature-induced lateral phase segregation in two-component vesicles.

Shape transformations of two-compoent vesicles in which the spontaneous curvature depends on the local compositions in the two monolayers are investigated theoretically. Even if the two components do not exhibit lateral phase separation in spherical vesicles, temperature-induced shape transformations cause lateral phase segregation. Vesiculation of smaller buds is favored in the two-component system. In two-component vesicles with intermonolayer phase separation, thermal budding is dominated by the different thermal expansion coefficients of the two monolayers.

Persistence at low temperature of the P β′ ripple in dipalmitoylphosphatidylcholine multilamellar vesicles containing either glycosphingolipids or cholesterol

The disappearance and reappearance of the P β′ ripple in multilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) has been examined by freeze-etch electron microscopy. The presence of less than 10 mol% of various glycosphingolipids or cholesterol in the liposomes markedly increases the time required for ripple disappearance when the vesicles are cooled from 38°C to 30°C, as compared to the pure phospholipid. Once the ripples have begun to disappear in the two-component vesicles, they do not uniformly reappear until the system is heated above the main transition of DPPC and allowed to cool into the pertransition region. These results suggest that the long time for ripple disappearance in the two-component systems reflects a slow molecular reorganization process which occurs when the systems are forced to change from the P β′ gel to the L β′ gel by a temperature downshift.

Geometric and thermodynamic restrictions for the self-assembly of glycosphingolipid-phospholipid systems

The thermodynamic and geometrical features of possible self-assembled structures of a series of chemically related glycosphingolipids differing in the complexity of their polar headgroup, and of their mixture with phospholipids, have been predicted according to the theory of self-assembly of hydrocarbon amphiphiles of Israelachvili et al. ((1980) Q. Rev. Biophys. 13, 340–357). The type and number of carbohydrate residues in the oligosaccharide chain of the polar headgroup are of paramount importance to determine the characteristics and thermodynamic stability of the possible self-assembled structure. In single component systems, the general prediction of the theory is that smaller aggregates may form as the polar headgroup of the glycosphingolipid is more complex and as the lateral surface pressure is smaller. In noninteracting two-component glycosphingolipid-phospholipid systems, the thermodynamic stability and the overall geometry of the possible aggregate appear to be determined by the proportion and type of glycosphingolipid present. Large and abrupt changes of the possible free energy per molecule, radius of curvature, and predicted asymmetry ratio for a particular glycosphingolipid may be triggered by relatively small changes of the molecular parameters, lipid composition, lateral surface pressure or vice-versa. If intermolecular interactions are takne into account with respect to the predictions for an ideal, noninteracting system, the theory indicates that two-component bilayer vesicles of polysialoganglioside-phosphatidylcholine may be thermodynamically and geometrically more stable. On the other hand, for systems constituted by phosphatidylcholine and neutral glycosphingolipids or monosialogangliosides, the possible bilayer vesicle is predicted to be less stable than in the ideal, noninteracting case. The results emphasize the general validity of the theory as applied to glycosphingolipid-containing systems.