Non-Contact Electrostatic Surface Force Imaging of Single Protein Filaments using Intermolecular Force Microscopy

An electrostatic force image from a single protein filament was measured and visualized in pure water using Intermolecular Force Microscopy (IFM). Refining atomic force microscopy in combination with very flexible cantilevers and an optical feedback system that controlled the position of the cantilever allowed these non-contact measurements to be made with piconewton and nanometer accuracy.Cantilever probes with positively charged ZnO whisker crystals as the scanning stylus were used to probe the electrostatic features of myosin filament surface. In contrast to topographical images obtained by a conventional atomic force microscope (AFM), non-contact force images revealed two areas on the myosin filament with different charge densities. A central bare zone had the greatest negative charge in the filament, which neutralized the repulsive interaction between the charged probe and a charged glass surface. The remainder of the filament was less negatively charged, because positively charged heads decreased the net charge density of the filament. This interpretation was supported by the observation that electrostatic repulsive forces exist between the S1 self-assembled monolayer formed on gold and the positively charged whiskers in a low salt solution. Thus, it is the electrostatic features of the protein surface, rather than the surface topography, that are measured and visualized at the molecular level.