Figure 1.

Single-molecule force measuring experiments by using AFM (a) and laser tweezers (b). In the AFM experiment (a), the sample is moved at a constant speed v relative to the cantilever with spring constant k. The position z_t = vt + δz_t of the cantilever tip with respect to the sample is recorded, where δz_t is the displacement of the cantilever tip. From repeated measurements of z_t, the free energy profile G₀(z) of the unperturbed system can be determined exactly (c).