Lipid Bilayer Characterization - AFM

AFM

Atomic force microscopy (AFM) has been used in recent years to image and probe the physical properties of lipid bilayers. AFM is a promising technique because it has the potential to image with nanometer resolution at room temperature and even underwater, conditions necessary for natural bilayer behavior. These capabilities have allowed direct imaging of the subtle ripple phase transition in a supported bilayer. Another AFM experiment performed in a tapping mode under aqueous buffer medium allowed (1) to determine the formation of transmembrane pores (holes) around nanoparticles of approximately 1.2 to 22 nm diameter via subtraction of AFM images from series recorded during the lipid bilayer formation and (2) to observe adsorption of single insulin molecules onto exposed nanoparticles. Another advantage is that AFM does not require fluorescent or isotopic labeling of the lipids, as the probe tip interacts mechanically with the bilayer surface. Because of this, the same scan can reveal information about both the bilayer and any associated structures, even to the extent of resolving individual membrane proteins. In addition to imaging, AFM can also probe the mechanical nature of small delicate structures such as lipid bilayers. One study demonstrated the possibility of measuring the elastic modulus of individual nano-scale membranes suspended over porous anodic alumina.

Although AFM is a powerful and versatile tool for studying lipid bilayers, there are some practical limitations and difficulties. Because of the fragile nature of the bilayer, extremely low scanning forces (typically 50pN or less) must be used to avoid damage. This consideration is particularly important when studying metastable systems such as vesicles adsorbed on a substrate, since the AFM tip can induce rupture and other structural changes. Care must also be taken to choose an appropriate material and surface preparation for the AFM tip, as hydrophobic surfaces can interact strongly with lipids and disrupt the bilayer structure.