Effect of Ca²⁺ on the morphology of mixed DPPC-DOPS supported phospholipid bilayers

The morphology of supported phospholipid bilayers (SPBs) containing mixtures of phospholipids in gel (dipalmitoyl phosphatidylcholine, DPPC) and fluid (dioleoyl phosphatidylserine (DOPS) or -choline (DOPC)) states at room temperature was investigated by atomic force microscopy (AFM). Fluid-gel phase separation was clearly detectable on topography images of mixed SPBs. The presence or absence of Ca²⁺ in the subphase was found to have a dramatic effect on the organization of the gel phase when the fluid phase was composed of pure DOPS: In the absence of Ca²⁺, large, well-defined DPPC domains were found in both the DPPC/DOPC and DPPC/DOPS mixtures, while in its presence small, isolated DPPC domains were found in the DPPC/DOPS mixture. Ca²⁺ had no effect on the organization of DPPC in DPPC/DOPC mixtures, and its effect was abolished by adding DOPC to DPPC/DOPS mixtures.