ATOMIC FORCE MICROSCOPY (AFM) STUDY OF PHOSPHOLIPID MONOLAYERS AND BILAYERS

Abstract

Lipid monolayers and bilayers have proved to be efficient models for biological membranes. In order to acquire a better knowledge of the structure of these membranes, 1,2-dipalmitoyl-sn-glycero-3-phospb.ocholine (DPPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-di [ cis-9-octadecenoyl]-sn-glycero3-phosphocholine (DOPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphoethanomine (DPPE) Langmuir-Blodgett (LB) films were formed on freshly cleaved mica surface, and subsequently imaged by atomic force microscopy (AFM). AFM observations show that the surface pressure at which the films were transferred plays an important role in the morphology of both LB monolayers and bilayers of saturated DPPC. The DPPC LB films transferred at higher surface pressure (40mN/m) exhibit flat and featureless except a few small grains on the DPPC monolayer and depressions on DPPC bilayer, whereas those transferred at lower surface pressure (15 mN/m) show phase separation either monolayers in air or bilayers in water, The chain unsaturation and head group type of phospholipids significantly affect the morphology and stability of the LB films in water. DPPC/POPC (with one double bond) bilayers in water exhibit phase separation with the height difference of 1.1 nm between two domains, while the DPPC/ DOPC (with two double bonds) bilayers have defects which may be caused by missing of the top monolayer. DPPC/DPPE bilayers show the domain structures with the height difference of about 0.7 nm, which may be the result of the acyl chain interdigitation induced by the interaction between the two leaflets of bilayers. The lateral force microscopy (LFM) images obtained from all bilayers studied show the same feature as the AFM topographical images but with clearer boundary, except the DPPC/DOPC bilayer. The LFM image of DPPC/DOPC bilayer presents a higher friction coefficient in hydrophobic tail region than that in hydrophilic head region. This suggests that LFM images are complementary to distinguish the features of the thin films especially for the films that the height difference is very small and the chemical composition of the films. In conclusion, AFM allows us to have a better insight into the relation between morphology of LB films and experimental conditions such as surface pressure at which the films were transferred, as well as chemical structure of phopholipids such as chain unsaturation and head group type.