A detailed investigation of the formation kinetics and layer structure of poly(ethylene glycol) tether supported lipid bilayers

Abstract

Supported lipid bilayers formed from liposomes containing phospholipids with covalently attached poly(ethylene glycol) (PEG-SLBs) potentially circumvent two current limitations of membrane sensor platforms, namely air-stability and limited aqueous space between the sensor substrate and the membrane. However, questions regarding the formation kinetics through self assembly and the presence of a PEG cushion underneath the lipid bilayer are still unanswered. Quartz crystal microbalance with dissipation monitoring and fluorescence recovery after photobleaching (FRAP) measurements show that PEG-SLB formation through self-assembly is possible in the mushroom regime while it is hindered in the brush regime. The observed dependence of the kinetics on grafting density and molecular weight of the PEG-lipids is attributed to the electrostatic and steric shielding effect of the enveloping PEG layer. In the mushroom phase, non-densely packed PEG does not completely screen lipid-surface interactions while in the brush phase, densely packed non-interacting PEG chains stabilise liposomes and prevent overall attractive interactions. Force–distance measurements are used to directly measure the thickness of the PEG-SLB structure and demonstrate the presence of PEG chains on both sides of the lipid bilayer. FRAP measurements support this finding by showing increased lipid mobility for increased PEG layer thickness through decoupling of the membrane from the substrate. Force–distance and FRAP measurements further reveal the mobility of the PEG-lipids and the novel detailed behavior of laterally mobile PEG mushrooms under mechanical compression.