Force probing of protein crystals: An atomic force microscopy study

The atomic force microscope (AFM) was used for measuring force-distance cur ves on horse spleen ferritin crystals in liquid environment. In the region of the approach curve which corresponds to tip-surface contact, discrete ju mps were recorded, as predicted by molecular dynamics simulations in the ca se of low tip-sample interaction. The observed jumps can be related to the removal of individual molecules from the surface by the AFM tip. A simple s teric model, which takes into account tip and ferritin molecule size; can e xplain the displacements observed with excellent agreement. The elemental f orce jump resulting from the approach curves is a direct measure of the for ce required to remove a single molecule from the crystal face. Sire discuss the conditions under which the cantilever potential energy difference alon g the elemental force step provides the energy of extraction of a single mo lecule. The estimate of the intermolecular binding energy turns out to be i n good agreement with the value calculated independently from the surface f ree energy of ferritin crystals.