Influence of phase separating lipids on supported lipid bilayer formation at SiO2 surfaces

Abstract

The importance of the lipid phase on the formation of supported lipid bilayers (SLBs) via vesicle fusion and on the resulting SLB homogeneity at SiO₂ surfaces has been studied by the quartz crystal microbalance with dissipation (QCM-D) monitoring technique. Physiologically relevant lipid compositions were chosen to correspond to different regions (l_d, l_o and coexistence of phases) in established phase diagrams of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), N-palmitoyl-D-erythro-sphingosylphosphorylcholine (PSM) and cholesterol. For most compositions, SLBs formed through vesicle rupture in a critical-surface-coverage dependent manner. Inclusion of PSM and cholesterol into POPC vesicles significantly impaired the vesicle rupture process such that a higher critical concentration of vesicles on the surface was needed before the rupture process started. When increasing the cholesterol content the vesicles formed SLBs containing more defects in the form of intact vesicles adsorbed on the surface up to a point (l_o phase) where vesicles did not break at all but formed supported vesicular layers. The hampering of vesicle rupture is interpreted in terms of the ability of cholesterol to accommodate vesicle deformation. Experiments using elevated temperatures to alter the lipid membrane into a more fluid phase significantly improved the quality of the SLB showing the importance of both cholesterol content and the lipid phase on SLB homogeneity.