Size-Dependent Properties of Small Unilamellar Vesicles Formed by Model Lipids

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 gets 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, our simulations indication that the orientation order of lipid molecules decreases as the size of the vesicle reduces and this fact reveals that the bilayer becoming thinner for smaller vesicle is mainly attributed to the orientation disorder of the lipids. 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, a general equation with a single numerical constant can relate bending modulus, area stretching modulus, and bilayer thickness irrespective of the vesicle size. Finally, a simple metastable model is proposed to explain the size-dependent behavior of bilayer thickness, orientation, and tension.

Size-dependent ultrafast structural dynamics inside phospholipid vesicle bilayers measured with 2D IR vibrational echoes

“…Other SANS experiments, supported by computer simulations (7), reported curvature dependent structural changes, with a decrease of the bilayer thickness with increasing vesicle size (8,9). In contrast, other calculations showed that bilayers become thicker with increasing vesicle size, but with no difference in thickness of inner and outer leaflets for vesicles of different sizes (10). Other research reported an asymmetrical geometry and that the thickness of the outer leaflet is greater than that of the inner (11,12).…”

“…Artificial lipid bilayers are frequently used as models of cell membranes for investigation of membrane properties and in studies of transmembrane proteins. Although a good deal is known about the structure of artificial membranes (6)(7)(8)(9)(10)(11)(12) and orientational and translational diffusion of lipids and other molecules (7,(13)(14)(15)(16), much less is known about the fast interior structural dynamics of the bilayers, which serve as the dynamic bath modes that can impact membrane processes.…”

Hybrid, Nanoscale Phospholipid/Block Copolymer Vesicles

“…500 nm particles are rapidly filtered out of the blood stream [13]). Indeed, membrane characteristics and the mechanical properties of the bilayer may vary significantly with vesicle size, an effect that is especially relevant for nanoscale vesicles [22].…”

Kinetic of Phospholipid Oxidation in Liposomal Suspensions

“…This was also perceived in a previous work [17] although the authors did not expose any explanation for this behavior. This behavior was unexpected since, in unilamellar liposomes, a higher content of the lipid phase leads to the formation of larger liposomes, which results in an increased interfacial area [18], and this should favor the oxidation in higher lipid content liposomal suspensions.…”

“…If this is the case, either the transition metal ions and oxygen or the hydroxyl radicals should penetrate into the membrane bilayer and a higher water permeability of the latter would definitely facilitate this process. Numerous physical studies of the acyl chain structure of liposome suggest that acyl chain packing partly depends on the radius of curvature of the liposome [18][19][20]. With a higher surface curvature, the unsaturated fatty acyl chains of small liposomes should have a greater exposure to water, resulting in a higher accessibility of these potential targets of oxidation reactions to the water soluble oxidants (Figure 2).…”