Blistering of Langmuir-Blodgett bilayers containing anionic phospholipids as observed by atomic force microscopy

Asymmetric bilayers of different phospholipid compositions have been prepar ed by the Langmuir-Blodgett (L-B) method, and imaged by atomic force micros copy (AFM). Such bilayers can function as a model for biological membranes. The first leaflet consisted of zwitterionic phospholipids phosphatidylchol ine (PC) or phosphatidylethanolamine (PE). The second leaflet consisted of the anionic phospholipid phosphatidylglycerol (PG), in either the condensed or liquid phase or, for comparison, of PC. Different bilayers showed diffe rent morphology. In all bilayers defects in the form of holes were present. In some bilayers with a first leaflet consisting of PC, polygonal line-sha ped defects were observed, whereas when the first leaflet consisted of PE, mainly round defects were seen. Not only the shape, but also the amount of defects varied, depending on the condition and the composition of the secon d leaflet. In most of the PC-containing systems the defects were surrounded by elevations, which reversibly disappeared in the presence of divalent ca tions. This is the first time that such elevations have been observed on ph ospholipid bilayers. We propose that they are induced by phospholipid excha nge between the two leaflets around the defects, leading to the presence of negatively charged phospholipids in the first leaflet. Because the substra te is also negatively charged, the bilayer around the edges is repelled and lifted up. Since it was found that the elevations are indeed detached from the substrate, we refer to this effect as bilayer blistering.