Visualization of Chemical and Physical Properties of Calcium-Induced Domains in DPPC/DPPS Langmuir−Blodgett Layers

In this study, we explored the calcium-induced formation of phosphatidylserine-enriched lipid domains in Langmuir−Blodgett (LB) monolayers composed of the binary mixture of the two saturated lipids 1,2-dipalmitoyl- sn -glycero-3-phosphocholine (DPPC) and 1,2-dipalmitoyl- sn -glycero-3-phosphoserine (DPPS) in a molar ratio of 4:1 by means of surface analysis tools. We employed fluorescence microscopy in conjunction with pressure−area isotherms, time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging, scanning electron microscopy (SEM), and lateral force microscopy to visualize and analyze lipid domains by their chemical and physical properties. TOF-SIMS and SEM images of DPPC/DPPS LB monolayers transferred onto gold surfaces clearly reveal that small amounts of calcium ions in the aqueous phase are sufficient to trigger the formation of circular DPPS-enriched domains, but domain formation is abolished in the presence of ethylene glycol-bis(β-aminoethyl) ether- N,N,N ‘,N ‘ -tetraacetic acid (EGTA). Lateral force microscopy of solid supported lipid bilayers transferred from the air/water interface to mica surfaces and imaged in water also allowed visualization of these DPPS-enriched domains despite the absence of a topographic contrast.