Surface-charge differentiation of streptavidin and avidin by atomic force microscopy-force spectroscopy

Chemical information can be obtained by using atomic force microscopy (AFM) and force spectroscopy (FS) with atomic or molecular resolution, even in liquid media. The aim of this paper is to demonstrate that single molecules of avidin and streptavidin anchored to a biotinylated bilayer can be differentiated by using AFM, even though AFM topographical images of the two proteins are remarkably alike. At physiological pH, the basic glycoprotein avidin is positively charged, whereas streptavidin is a neutral protein. This charge difference can be determined with AFM, which can probe electrostatic double-layer forces by using FS. The force curves, owing to the electrostatic interaction, show major differences when measured on top of each protein as well as on the lipid substrate. FS data show that the two proteins are negatively charged. Nevertheless, avidin and streptavidin can be clearly distinguished, thus demonstrating the sensitivity of AFM to detect small changes in the charge state of macromolecules.