Molecular Dynamics Simulations Of The Indentation Of A Crystalline Surface By An Atomic Force Microscope Tip

The indentation of protein crystals by an atomic force microscope tip is studied by molecular-dynamics simulations. This work is inspired by our previously reported experimental results, showing the occurrence of force jumps in the contact region of force-distance curves measured on ferritin crystals. These jumps were interpreted as due to the removal of molecules from the surface. Here we perform simulations of the indentation of topmost crystal layers with different tip sizes; special attention is devoted to the case corresponding to the experimental conditions, in which the tip size is twice the size of the crystal molecules. Force-distance curves are calculated. These curves show a first clear jump in correspondence of the rupture of the first layer; this jump is associated to the removal of a few molecules from the surface. The jump height, the curve shape, and the emission mechanism of the molecules from the crystal depend strongly on the indentation point. In particular, the amplitude of tip jumps resulting from crystal layer breakage is found to have a multimodal distribution. These results are in agreement with the interpretation previously given to our experimental results.