Glass Bead Probes of Local Structural and Mechanical Properties of Fluid, Supported Membranes

Abstract

Nonspecific interactions between microscopic particles and a nearby “soft” surface report on the dynamical-mechanical properties of the surface. A realization of this principle is afforded by diverse systems exhibiting an interplay between energies of adhesion and local elastic deformation of the fluid. Within this framework, the behavior of supported, fluid lipid membranes interacting with microscopic colloids is particularly interesting because it offers a physical model for many biologically important mechanisms, such as particle engulfment, membrane fusion, and viral budding. Previous theoretical efforts establish that the elastic response of the membrane is determined by a finely coordinated balance between the adhesion, in-plane tension, and bending energies. 8] Model experiments have focused primarily on the interaction between “free” membranes of giant phospholipid vesicles ACHTUNGTRENNUNG(>10 mm) and latex microspheres. 10] Notable limitations of giant vesicles in modeling particle adhesion at biological membranes include their vanishingly low lateral tension and the presence of large, out-ofplane undulations. At cellular surfaces, however, the cytoskeleton constraints, the presence of transmembrane proteins, and even compositional asymmetries “renormalize” membrane tension and limit the cocperativity in the natural dynamics of the two leaflets. In this regard, supported membranes provide an attractive model system. They are typically formed by rupture of lipid vesicles onto planar surfaces. Depending on the substrate’s hydrophilic or hydrophobic nature, vesicles rupture to form either bilayers or monolayers respectively. Bilayers are separated from the substrate surface by a thin water cushion 1–3 nm thick, which is believed to preserve their essential fluidity and their elastic structural properties. Lipid monolayers, on the other hand, experience a rigid hydrophobic interface with the substrate limiting the elastic response to a single leaflet. This provides a limiting case for decoupling of dynamics between the two membrane leaflets. In both cases, the lateral restrictions on membrane conformations imposed by the proximal surface reduce the configurational entropy and increases lateral tension. We presented microscopic, bare glass beads to binary patterns comprising alternate regions of 1) lipid bilayers and glass and 2) lipid monolayers and bilayers. Representative epifluorescence and brightfield data, shown in Figures 1a and 1b, display gravitationally settled beads on a patterned POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) bilayer (see Experimental Section). Beads settle uniformly over the entire surface. Physically perturbing the system via shaking or pipette pumping, selectively removes beads from the bare surface. Time-lapse bright-field images (Figures 1b–1 f) reveal the formation of a visually striking patterned assembly of beads (a