Fig. 1: Single-molecule quantification of the mechanical stability of 13-ab^zip.

a Left-top panel: design of the 13-a1a2Lb1b2-construct. Left-bottom panel: the expected structure of 13-ab^zip. Black circles and black arrows indicate the force-attaching points and force direction. Purple circles indicate the attachment points of the long linker. Components a and b are helix-hairpins that contains two helices a1&a2 (red), and b1&b2 (blue), respectively. Right panel: sketch of a single-molecule construct specifically tethered between a coverslip surface and a superparamagnetic bead with a 572-bp DNA handle (Supplementary Note 2). b Four representative force–height curves of the single-molecule 13-a1a2Lb1b2-construct at 1.0 ± 0.1 pN s⁻¹. The stepwise bead height jump indicates the force-induced rupturing (unlooping) of the 13-ab^zip. c The force–step size graph of the force-dependent rupturing (unlooping) transitions of the 13-ab^zip. The dash curves are the theoretically predicted. The dark gray line is predicted assuming that in the unlooped state a and b components are unfolded into an unstructured peptide chain; the light gray line is predicted assuming that the a and b components remain as folded helix-hairpin structures (Supplementary Note 6 and Supplementary Figs. 4 and 5). d Normalized histograms of the 13-ab^zip rupture forces. e Representative time traces of the bead height before and after the rupture of the 13-ab^zip at ~9 pN, where the rupturing events are indicated by the large bead height jump steps. f Force-dependent lifetimes of the 13-ab^zip. The solid squares represent the average lifetimes (τ) obtained by best-fitting of the lifetime histogram to a single-exponential decay function. The hollow gray circles represent individual lifetimes (N ~ 100 for each force). The vertical error bars indicate the standard errors of the lifetime obtained by \(\frac{\tau }{{\sqrt {N - 1} }}\). The horizontal error bars indicate 10% of relative force uncertainty resulted from the force-calibration method. For panels b–f: the experiments were performed at 23 °C. For panels b and e: 20-points FFT smooth (colored lines) of the raw data (gray lines) are shown. For panels c, d: the number of data points obtained from >5 independent tethers are indicated. Source data are provided as a Source Data file.