Controlling spatial distribution of functional lipids in a supported lipid bilayer prepared from vesicles

Abstract

Conjugating biomolecules, such as antibodies, to bioconjugate moieties on lipid surfaces is a powerful tool for engineering the surface of diverse biomaterials, including cells and nanoparticles. We developed supported lipid bilayers (SLBs) presenting well-defined spatial distributions of functional moieties as models for precisely engineered functional biomolecular-lipid surfaces. We used quartz crystal microbalance with dissipation (QCM-D) and atomic force microscopy (AFM) to determine how vesicles containing a mixture of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[azido(polyethylene glycol)-2000] (DSPE-PEG-N₃) form SLBs as a function of the lipid phase transition temperature (T_m). Above the DPPC T_m, DPPC/DSPE-PEG-N₃ vesicles form SLBs with functional azide moieties on SiO₂ substrates via vesicle fusion. Below this T_m, DPPC/DSPE-PEG-N₃ vesicles attach to SiO₂ intact. Intact DPPC/DSPE-PEG-N₃ vesicles on the SiO₂ surfaces fuse and rupture to form SLBs when temperature is brought above the DPPC T_m. AFM studies show uniform and complete DPPC/DSPE-PEG-N₃ SLB coverage of SiO₂ surfaces for different DSPE-PEG-N₃ concentrations. As the DSPE-PEG-N₃ concentration increases from 0.01 to 6 mol%, the intermolecular spacing of DSPE-PEG-N₃ in the SLBs decreases from 4.6 to 1.0 nm. The PEG moiety undergoes a mushroom to brush transition as DSPE-PEG-N₃ concentration varies from 0.1 to 2.0 mol%. Via copper-free click reaction, IgG was conjugated to SLB surfaces with 4.6 nm or 1.3 nm inter-DSPE-PEG-N₃ spacing. QCM-D and AFM data show; 1) uniform and complete IgG layers of similar mass and thickness on the two types of SLB; 2) a higher-viscosity/less rigid IgG layer on the SLB with 4.6 nm inter-DSPE-PEG-N₃ spacing. Our studies provide a blueprint for SLBs modeling spatial control of functional macromolecules on lipid surfaces, including surfaces of lipid nanoparticles and cells.

Keywords: Atomic force spectroscopy; Quartz crystal microbalance; Supported lipid bilayer.