Floret-Shaped Solid Domains on Giant Fluid Lipid Vesicles Induced by pH

Abstract

Lateral lipid phase separation of titratable PS or PA lipids and their assembly in domains induced by changes in pH are significant in liposome-based drug delivery: environmentally responsive lipid heterogeneities can be tuned to alter collective membrane properties such as permeability (altering drug release) and surface topography (altering drug carrier reactivity) impacting, therefore, the therapeutic outcomes. At the micrometer scale fluorescence microscopy on giant unilamellar fluid vesicles (GUVs) shows that lowering pH (from 7.0 to 5.0) promotes condensation of titratable PS or PA lipids into beautiful floret-shaped domains in which lipids are tightly packed via hydrogen-bonding and van der Waals interactions. The order of lipid packing within domains increases radially toward the domain center. Lowering pH enhances the lipid packing order, and at pH 5.0 domains appear to be entirely in the solid (gel) phase. Domains phenomenologically comprise a circular "core" cap beyond which interfacial instabilities emerge resembling leaf-like stripes. At pH 5.0 stripes are of almost vanishing Gaussian curvature independent of GUVs' preparation path and in agreement with a general condensation mechanism. Increasing incompressibility of domains is strongly correlated with a larger number of thinner stripes per domain and increasing relative rigidity of domains with smaller core cap areas. Line tension drives domain ripening; however, the final domain shape is a result of enhanced incompressibility and rigidity maximized by domain coupling across the bilayer. Introduction of a transmembrane osmotic gradient (hyperosmotic on the outer lipid leaflet) allows the domain condensation process to reach its maximum extent which, however, is limited by the minimal expansivity of the continuous fluid membrane.

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“…Examples, as summarized in Ref. [5], are an assembly of viral capsids [6], filament bundle packings [7], formation of molecular monolayers [8], functionalization of nanoparticles [9], and growth of solid domains on vesicles [10][11][12].…”

Vesicle adhesion visualized with total internal reflection imaging ellipsometry biosensor

“…It is a weak polyelectrolyte (only partially charged at moderate pH near its pK of 9.36), which interacts with lipids through NH3 + groups. The pH and the ionic strength environments greatly influence poly-l-lysine dissociation, and, consequently the electrostatic interaction between the lipid membrane of the vesicles and the coated substrate [27]. Two concentrations of poly-l-lysine solutions are used to coat the gold surface of the substrates, c 0 = 1 mg/ml and 1:100 dilution c 0 /100, referred hereafter as c 0 and c 0 /100, respectively.…”

Growth Inhibition of Triple-Negative Breast Cancer: The Role of Spatiotemporal Delivery of Neoadjuvant Doxorubicin and Cisplatin

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