Molecular structure of dipalmitoylphospatidylcholine Langmuir-Blodgett monolayers studied by atomic force microscopy.

Monolayers of dipalmitoylphosphatidylcholine (DPPC) on the air-water interface have been transferred at various surface pressures onto quartz substrates using the Langmuir-Blodgett (LB) technique. The topography of these layers, on a molecular scale, has been examined by atomic force microscopy (AFM). Monolayers transferred at a surface pressure corresponding to the liquid-condensed phase at the air-water interface show a very regular and densely-packed structure with the methyl ends of the phospholipid chains, which can be seen separately, sticking out from the surface. The incorporation of 5 mol% of the fluorescent probe molecule 2-(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine (DPHpPC) in the DPPC monolayer does not affect this structure. The LB monolayers obtained at a surface pressure corresponding to the liquid-condensed/liquid-expanded coexistence phase show ordered and less ordered regions. No regular structure is found in the LB monolayers transferred at a surface pressure corresponding to the liquid-expanded phase at the air-water interface.