Defects in ordered aggregates of cardiolipin visualized by atomic force microscopy

The formation and the nature of defects in ordered aggregates of cardiolipin (tetra acyl diphosphatidylglycerol) supported on solid substrates have been investigated by atomic force microscopy (AFM). The experiments were performed on two model systems, i.e. three-dimensional liquid crystals dispersed in water and partially de-hydrated on a hydrophilic surface, and two-dimensional films of molecules self-assembled onto an isotropic hydrophobic surface. Defects were induced both by varying the preparation temperature and by treatment with specific chemicals known to modify the order parameters in natural and artificial membranes, specifically: 2,4-dinitro-phenol (DNP) and pentachloro-phenol (PCP). The effect of lipid oxidation on the nanocrystalline order was also investigated. The images obtained by AFM allow to characterize the type of defects and their local density at nanoscale level. They also provide additional information to differentiate the specific role of acyl chains and polar heads in the process of lipid self-organization.