Figure 4.

Single-molecule observation of DNA G-quadruplex folding. (a) Representation of a folded G-quadruplex DNA sequence capturing two K⁺ cations, bound to an initial single-molecule probe on a nanotube. (b–c) Real-time trajectories with baseline correction, showing two-state fluctuations in conductance in the presence of 10 mM KCl or NaCl, respectively. (d) Survival probability plots for each conductance state (high-G/low-G) and each ion type (K⁺/Na⁺) are fitted with a double-exponential kinetic model. (e) Average lifetime ⟨τ_fast⟩ and error bars extracted from the survival plots in panel d for the high-G and low-G states as well as both K⁺ and Na⁺ ions. (f) High-G and low-G conductance states are attributed to the unfolded and folded DNA configurations, respectively. The folded configuration is found to have a longer lifetime in the presence of K⁺ cations than Na⁺.