The Influence of Polymer Molecular Weight in Lamellar Gels Based on PEG-Lipids

Abstract

We report x-ray scattering, rheological, and freeze-fracture and polarizing microscopy studies of a liquid crystalline hydrogel called Lalpha,g. The hydrogel, found in DMPC, pentanol, water, and PEG-DMPE mixtures, differs from traditional hydrogels, which require high MW polymer, are disordered, and gel only at polymer concentrations exceeding an "overlap" concentration. In contrast, the Lalpha,g uses very low-molecular-weight polymer-lipids (1212, 2689, and 5817 g/mole), shows lamellar order, and requires a lower PEG-DMPE concentration to gel as water concentration increases. Significantly, the Lalpha,g contains fluid membranes, unlike Lbeta' gels, which gel via chain ordering. A recent model of gelation in Lalpha phases predicts that polymer-lipids both promote and stabilize defects; these defects, resisting shear in all directions, then produce elasticity. We compare our observations to this model, with particular attention to the dependence of gelation on the PEG MW used. We also use x-ray lineshape analysis of scattering from samples spanning the fluid-gel transition to obtain the elasticity coefficients kappa and B; this analysis demonstrates that although B in particular depends strongly on PEG-DMPE concentration, gelation is uncorrelated to changes in membrane elasticity.

RETRACTED: Flexible bilayers with spontaneous curvature lead to lamellar gels and spontaneous vesicles

“…1 A shows a freeze-fracture TEM (FFTEM) image of 7:3 wt:wt CTAT:SDBS in 40% water. The image is typical of gently undulating but overall flat bilayers similar to other lipid and surfactant lamellar phases and thermotropic smectic phases (21,(29)(30)(31). The structure correlates well with the resolution-limited SAXS peaks, implying large lamellar domains.…”

“…For water fractions between 50% and 75%, an opaque lamellar phase gel forms from 7:3 (wt:wt; 64:36 mol:mol) CTAT:SDBS mixtures that has a highly defected, spherulite texture, similar to lamellar gels of dimyristoyl phosphatidylcholine, pentanol, and polyethylene glycol lipids (19)(20)(21). At 40% water volume fraction, the lamellar phase becomes transparent and fluid, and the bilayers are flat and well aligned (Fig.…”

“…1). A simple defect-based theory of this structural progression requires that ϳ k B T and that a bilayer spontaneous curvature exists (19)(20)(21). Adding pentanol to the lamellar gel phase eliminates the defects in the bilayers and the lamellar phase becomes fluid.…”

“…To evaluate quantitatively, we present Caillé line shape analysis of the small-angle x-ray scattering (SAXS) (21)(22)(23)(24)(25) that confirms ϳ k B T for two different CTAT:SDBS ratios. From this analysis, it appears that surfactant mixing is sufficient to lead to low values of the bending elasticity.…”

The origins of stability of spontaneous vesicles

“…There are a limited number of measurements of for vesicle-forming membranes; double-tailed phospholipid bilayers range from 10k B T Ͻ Ͻ 40k B T (27)(28)(29)(30)(31)(32). For highly swollen surfactant lamellar (10,(33)(34)(35) and microemulsion (36,37) phases, Ϸ k B T. There are even fewer measurements of the saddle-splay modulus (10) and the spontaneous curvature (22).…”