Hui S W, Viswanathan R, Zasadzinski J A, Israelachvili J N
Department of Biophysics, Roswell Park Cancer Institute, Buffalo, NY 14263.
Biophys J. 1995 Jan;68(1):171-8. doi: 10.1016/S0006-3495(95)80172-4.
Atomic force microscopy (AFM) was used to investigate the structure, stability, and defects of the hydrophilic surfaces of Langmuir-Blodgett bilayer films of distearoylphosphatidylcholine (DSPC) and dipalmitoylphosphatidylethanolamine (DPPE) in the solid phase, and dilinoleoylphosphatidylethanolamine (DLPE) in the fluid phase. Their relative resilience to external mechanical stress by the scanning tip and by fluid exchange 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 structure. DSPC and DLPE monolayers did not show any periodic order. The solid DSPC and DPPE monolayers were stable to continued rastering by the AFM 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 monolayers were more extensive and were deformed easily by consecutive scanning 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 the high spontaneous curvature of DLPE monolayers at low pH. The tangential stress exerted by the AFM tip on the deformable monolayers eventually produced a ripple pattern, which could be described as a periodic buckling known as Shallamach waves.
原子力显微镜（AFM）用于研究固相二硬脂酰磷脂酰胆碱（DSPC）和二棕榈酰磷脂酰乙醇胺（DPPE）以及液相二亚油酰磷脂酰乙醇胺（DLPE）的Langmuir-Blodgett双层膜亲水表面的结构、稳定性和缺陷。还研究了它们对扫描探针和流体交换引起的外部机械应力的相对弹性。DPPE单分子层在表面呈现出周期为0.49 nm的平行脊，这与已知晶体结构中排列的头基行相对应。DSPC和DLPE单分子层未显示任何周期性有序结构。固态DSPC和DPPE单分子层对AFM探针的持续光栅扫描具有稳定性；然而，DLPE单分子层的稳定性取决于水环境的pH值。流体交换后观察到以单分子层间隙和孔洞形式存在的结构缺陷，但pH为11时DLPE单分子层中的缺陷在连续扫描过程中是稳定的。在pH为9及以下时，DLPE单分子层上流体交换引起的缺陷更为广泛，并且在AFM探针以10 nN的力连续扫描时容易变形。弹性的pH依赖性可以通过低pH下DLPE单分子层的高自发曲率导致的弯曲能增加或受挫来解释。AFM探针施加在可变形单分子层上的切向应力最终产生了一种波纹图案，这种图案可以描述为一种称为沙拉马赫波的周期性屈曲。
