THE STRUCTURE AND STABILITY OF PHOSPHOLIPID-BILAYERS BY ATOMIC-FORCE MICROSCOPY

Atomic force microscopy (AFM) was used to investigate the structure, s tability, and defects of the hydrophilic surfaces of Langmuir-Blodgett bilayer films of distearoylphosphatidylcholine (DSPC) and dipalmitoyl phosphatidylethanolamine (DPPE) in the solid phase, and dilinoleoylpho sphatidylethanolamine (DLPE) in the fluid phase. Their relative resili ence to external mechanical stress by the scanning tip and by fluid ex change were also investigated. DPPE monolayers showed parallel ridges at the surface with a period of 0.49 nm, corresponding to the rows of aligned headgroups consistent with the known crystallographic structur e. DSPC and DLPE monolayers did not show any periodic order. The solid DSPC and DPPE monolayers were stable to continued rastering by the AF M tip; however, the stability of DLPE monolayers depended on the pH of the aqueous environment. Structural defects in the form of monolayer gaps and holes were observed after fluid exchange, but the defects in DLPE monolayer at pH 11 were stable during consecutive scanning. At pH 9 and below, the defects induced by fluid exchange over DLPE monolaye rs were more extensive and were deformed easily by consecutive scannin g of the AFM tip at a force of 10 nN. The pH dependence of resilience was explained by the increasing bending energy or frustration due to t he high spontaneous curvature of DLPE monolayers at low pH. The tangen tial stress exerted by the AFM tip on the deformable monolayers eventu ally produced a ripple pattern, which could be described as a periodic buckling known as Shallamach waves.