In a solution of phosphate-buffered saline (PBS), unilamellar bilayers with saturated phosphatidylcholines in one leaflet and negatively charged, unsaturated phospholipids in the other leaflet were observed in the ripple phase at room temperature using atomic force microscopy (AFM). This is the first observation of the ripple phase in asymmetric bilayers. Sodium and phosphate, components of PBS, were found to be necessary for the formation of the ripple structure in the asymmetric bilayers composed of dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylglycerol (POPG), demonstrating a dependency for specific ions for this phase. These results indicate that the two leaflets of a bilayer are closely coupled to give rise to such a long range and complicated morphology.
Ions Lipid Bilayers