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 phosphatidylser ine-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 (DPP S) in a molar ratio of 4:1 by means of surface analysis tools. We employed fluorescence microscopy in conjunction with pressure-area isotherms, time-o f-flight secondary ion mass spectrometry (TOF-SIMS) imaging, scanning elect ron 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 re veal that small amounts of calcium ions in the aqueous phase are sufficient to trigger the formation of circular DPPS-enriched domains, but domain for mation is abolished in the presence of ethylene glycol-bis(beta -aminoethyl ) ether-N,N,N',N'-tetraacetic acid (EGTA). Lateral force microscopy of soli d supported lipid bilayers transferred from the air/water interface to mica surfaces and imaged in water also allowed visualization of these DPPS-enri ched domains despite the absence of a topographic contrast.