FIGURE 3.

Calculated force-extension characteristics for the modified P5ab hairpin (G-A basepairs replaced by A-A) for a hard (λ = 0.2 pN/nm) and a soft (λ = 0.01 pN/nm) harmonic potential of the optical tweezer. (a) The average FEC, which resembles very much the FEC in Fig. 2 b. (b) The fluctuations of the extension as a function of the average extension. Although the curve is very similar to the inset of Fig. 2 b for a soft spring, it develops a pronounced minimum corresponding to an intermediate state for a hard spring. The arrows indicate the positions at which the x_m distributions shown in Fig. 4 were calculated.