Atomic force microscopy study of thick lamellar stacks of phospholipid bilayers

We report an atomic force microscopy (AFM) study on thick multilamellar stacks of approximately 10 mu m thickness (about 1500 stacked membranes) of 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine deposited on silicon wafers. These thick stacks could be stabilized for measurements under excess water or solution. From the force curves we determine the compressional modulus B and the rupture force F-r of the bilayers in the gel (ripple), in the fluid phase, and in the range of critical swelling close to the main transition. We observe pronounced ripples on the top layer in the P-beta' (ripple) phase and find an increasing ripple period Lambda(r) when approaching the temperature of the main phase transition into the fluid L-alpha phase at about 24 degrees C. Metastable ripples with 2 Lambda(r) are observed. Lambda(r) also increases with increasing osmotic pressure, i.e., for different concentrations of polyethylene glycol.