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Figure 2. How the atomic force microscope generates force–distance curves for a cell receptor and ligand. (a) A representative force–distance curve between a ligand, which is bound to the tip of an atomic force microscope, and a receptor molecule, which is bound to a solid surface. (b) A schematic illustration of the molecular interactions observed. In both parts, from positions ‘1’ to ‘2’, the tip is approaching the surface, and at position ‘2’ contact is made. From positions ‘2’ to ‘3’, the cantilever bends until it reaches the specified force limit that is to be applied; it is then withdrawn during positions ‘4’ and ‘5’. At position ‘5’, the tip relinquishes contact with the surface that is being analysed; however, the ligand, which is coupled to the tip, remains bound to its receptor molecule on the surface of the sample and both molecules are extended. Following the further application of the retraction force, the molecule–ligand complex dissociates (at position ‘6’, which is referred to as ‘snap off’). Between positions ‘6’ and ‘7’, the cantilever returns to its resting position (i.e. position ‘1’) and is ready for another measurement. The maximum difference between the approach curve (i.e. the upper, solid line) and the retraction curves (i.e. the lower, dotted line), and the shape of the curve between positions ‘2’, ‘5’ and ‘6’ yield information on the interaction ‘binding’ force between the ligand on the tip of the microscope and the receptor molecule on the surface, and their physical properties. Abbreviations used: PEG = polyethylene glycol, a polymer that is used as a linker molecule between the tip of the atomic force microscope and the ligand that is used to probe the cellular receptor (fig002mhu).