Figure 3. The time-dependent force f_trap(t) on the bead (deduced from the position of the bead in the tweezers).

Apart from the (small) friction force onto the bead itself, f_trap(t) can be identified with the force f_P(L,t) exerted by the stretched polymer onto the bead. Experimental data were obtained from 36 independent single-molecule measurements (moving average over 5 ms) at five different KCl concentrations c=20, 50 mM (a, 10 traces), c=100 mM (b, 10 traces), c=500 mM (c, 8 traces), c=750 mM (d, 8 traces). Theory curves: f_trap(t) including the dynamic backbone tension of the recoiling polymer and the bead drag (solid line), using a concentration-dependent persistence length of , , , , (33), and a friction coefficient per length ; for comparison, the prediction for the hypothetical rigid-rod scenario from Fig. 1c (dotted) is also shown. At low salt concentrations (a) the surface charge of the capillaries has a significant influence on the ionic current34 and the observed relaxation characteristics exhibit a larger variability. Alternative plots with theory curves for (independent of the salt concentration) are shown in Supplementary Fig. S1. A replication of the experiment using a different measurement protocol is documented in Supplementary Fig. S2.