Atomic Force Microscopy Observation of the Phase Separation in Acidic Phospholipid/Neutral Phospholipid Supported Bilayers

Phase separation induced by Ca2+ in the DPPS (dipalmitoyl phosphatidylserine)/DPPC (dipalmitoyl phosphatidylcholine) or DPPS/DOPC (dioleoyl phosphatidylcholine) system was observed by atomic force microscopy (AFM). Lateral modulation frictional force microscopy (LM-FFM) revealed phase separation in supported bilayer membranes prepared by the transfer of the monolayers of DPPS/DPPC with a small amount of Ca2+ in the subphase, while the phase separation was not shown in their topographic images obtained by contact mode AFM. Phase separation induced by Ca2+ in DPPS/DOPC mixed systems was observed in topographic images and the time course of the appearance and development of DPPS/DOPC (1/9) phase separation after the addition of Ca2+ (0.1 mM) was revealed.