Studies of Physical Properties and Fusion Mechanism of Vesicles by Dissipative Particle Dynamic Simulations
Date Issued
2011
Date
2011
Author(s)
Lin, Chun-Min
Abstract
The size-dependent behavior of small unilamellar vesicles is explored by dissipative particle dynamics, including the membrane characteristics and mechanical properties. The spontaneously formed vesicles are in the metastable state and the vesicle size is controlled by the concentration of model lipids. As the vesicle size decreases, the bilayer thickness is getting thinner and the area density of heads declines. Nonetheless, the area density in the inner leaflet is higher than that in the outer. The packing parameters are calculated for both leaflets. The result indicates that the shape of lipid in the outer leaflet is like a truncated cone but that in the inner leaflet resembles an inverted truncated cone. Based on a local order parameter the orientation order is found to decay with reducing vesicle size and this fact reveals that the thinner bilayer is mainly attributed to the orientation disorder. The membrane tension can be obtained through the Young-Laplace equation. The tension is found to grow with reducing vesicle size. Therefore, small vesicles are less stable against fusion. Using the inflation method, the area stretching and bending moduli can be determined and those moduli are found to grow with reducing size. Nonetheless, the general relation among the bending modulus, area stretching modulus, and bilayer thickness are still followed with a small numerical constant. Finally, a simple metastable model is proposed to explain the size-dependent behavior of bilayer thickness, orientation, and tension.
The swelling of small unilamellar vesicles is explored by dissipative particle dynamics, including the growth, rupture, healing, and fusion process. The spontaneously formed vesicles are inflated by adding water into the inner water region. As the water is added into the vesicle, the vesicle size increases and the bilayer thickness decreases. It is also found that the area density of the tail group in the outer leaflet is greater than that of the inner leaflet during the swelling process; however, the area density of the outer head is less than that of the inner head. The results of the packing parameters for both leaflets reveal that the shape of lipid in the inner leaflet resembles an inverted truncated cone and in the outer leaflet is like a truncated cone. The outcome of local order parameter points out the orientation order is found to decay as the vesicle is inflated demonstrating that the bilayer becomes thinner partly attributed to the orientation disorder. The distribution of the lipid tail beads for the inner and outer leaflets indicates an interdigitated formation takes place within the bilayer of an inflated vesicle. The membrane tension can be obtained through the Young-Laplace equation is applied to estimate the membrane tension and the tension is found to grow with the inflation of the vesicle. Therefore, inflated vesicles are less stable against fusion. Finally, it is found that spontaneously formed vesicles of different sizes rupture at distinct degree of inflation. However, certain membrane properties, such as packing parameter, order parameter, and membrane thickness need to reach the same critical values before rupture.
It is found that only surfactants with suitable hydrophobicity are able to solubilize vesicles by forming small mixed micelles. Surfactants with inadequate hydrophobicity tend to stay in the bulk solution and only a few of them enter into the vesicle. Consequently, the vesicle structure remains intact for all surfactant concentrations studied. On the contrary, surfactants with excessive hydrophobicity are inclined to incorporate with the vesicle and thus the vesicle size continues to grow as the surfactant concentration increases. Instead of forming discrete mixed micelles, lipid and surfactant are associated into large aggregates taking the shapes of cylinders, donuts, bilayers, etc. For addition of surfactant with moderate hydrophobicity, perforated vesicles are observed before the formation of mixed micelles and thus the solubilization mechanism is more intricate than the well-known three-stage hypothesis. As the apparent critical micellar concentration is attained, pure surfactant micelles form and the vesicle deforms because the distribution of surfactant within the bilayer is no longer uniform. When the surfactant concentration reaches medium concentration, the vesicle perforates. The extent of perforation grows with increasing surfactant concentration. The solubilization process begins at excess of concentration and lipids leave the vesicle and join surfactant micelles to form mixed micelles. Eventually total collapse of the vesicle is observed.
Subjects
Dissipative Particle Dynamic
Vesicle
Fusion
Type
thesis
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