Fig. 2.

Force spectroscopy measurements of transition paths in DNA hairpin folding. (A) Schematic of assay: a single DNA hairpin is attached at each end via double-stranded handles to beads held in optical traps. (B, Left) Segment of extension trajectory for hairpin 20R100/T4, showing repeated transitions between the folded state (F) at extension x_f and the unfolded state (U) at x_u (dashed lines). (Right) Single transition from the trajectory, revealing the transition path across the barrier region (red). The transition time, t_tp, is measured between boundaries x₁ and x₂ bracketing the barrier region (dotted lines). (C) Sequences of hairpins studied with varying G:C content.